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Full text of "A Handbook of descriptive and practical astronomy"

ASTKONOMY. 

CHAMBERS. 

I. 

THE SUN, PLANETS, AND COMETS. 



a 2 



Bonbon 
HENRY FROWDE 




OXFORD UNIVERSITY PRESS WAREHOUSE 
AMEN CORNER, E.G. 



f-a 



As 
c 



A HANDBOOK 



OF 



DESCRIPTIVE AND PRACTICAL 
ASTRONOMY. 



BY 

GEORGE F. CHAMBERS, F.R.A.S., 

OF THE INNER TEMPLE, BAHBI8TEB-AT-LAW I 

Author of "A Practical and Conversational English, French, and German Dictionary; 

"The Tourist's Pocket-Book; " "A Digest of the Law relating to Public 

Health;" "A Digest of the Law relating to Public Libraries 

and Museums;" "A Handbook for Public Meetings;" 

and other Works, 



' The heavens declare the glory of God ; and the firmament sheweth his handywork." 

Psalm xix. i. 



I. 

THE SUN, PLANETS, AND COMETS. 
FOURTH EDITION. 

FIG. 2. 




August 26. September 9. 

THE JIJRD SATELLITE OF JUPITER IN 1855. (StCcM.) 



AT THE CLARENDON PRESS. 

1889. 
[All right* referred.] 




PREFACE TO THE FOURTH EDITION. 



PT^HE remarks which appear in the Preface to the Third Edition 
(see post] apply almost word for word, so far as they go, to 
the Fourth Edition. Yet it is necessary for me to write an in- 
dependent Preface in order to call attention to the altered 
circumstances under which this work is now presented to the 
reader. If the development of Astronomy between 1867 and 
1877 was great, its development between 1877 and 1889 has 
been still greater. And besides this, there were important omis- 
sions in the ground-plan of the book which I have long been 
very desirous of making good, whenever time or opportunity 
became available. 

The last edition having reached to nearly 1000 pages it 
became quite clear that the now necessary additions would have 
swelled the work to a bulk and consequent price which probably 
the Public would not have regarded with favour. Accordingly 
when its division into two volumes became a necessity, I deter- 
mined to make the two into three, and to complete the under- 
taking as originally conceived twenty-nine years ago. 

The work will therefore henceforth be published in three 
divisions as follows : 

I. The Sun, Planets, and Comets. 
II. Instruments and Practical Astronomy. 
III. The Starry Heavens. 

It is intended that each volume shall be paged, indexed, and 
sold separately. 



viii PEEFACE TO THE FOUETH EDITION. 

This arrangement, whilst it will be financially more acceptable 
to the Public, will probably permit in after-years of new editions 
being brought out at lesser intervals of time than has hitherto 
been possible. 

Subject to the above explanations, it may be further stated 
that the whole work has been revised everywhere, and enlarged 
and rearranged wherever alterations seemed necessary or ex- 
pedient. 

A very large number of additional engravings have been 
prepared, and the list now includes a certain number selected 
from the various publications of the late Admiral W. H. Smyth. 
My grateful thanks are due to the surviving representatives of 
the lamented Admiral for their great kindness and liberality in 
regard to these engravings and other literary materials which 
they have placed at my disposal. Nor must I omit, in referring 
to engravings, to mention the kind help which I have received 
from the Secretaries of the Royal Astronomical Society, the 
Editor of the Observatory, and M. Gauthier Villars of Paris. 

The Second Volume will it is hoped be published in the 
Autumn of 1889, and the Third Volume in 1890. 

I have been glad to avail myself of the kind assistance of 
several astronomical friends in passing this volume through the 
press. To Mr. A. C. Eanyard, Mr. F. C. Penrose, and Mr. W. F. 
Denning especial thanks are due for particular chapters which 
are duly noted as they occur ; whilst the whole volume has been 
read for press by the Rev. /. B. Fletcher, M.A., of Trinity College, 
Dublin, and Vicar of All Souls, East-Bourne, and by Mr. W. T. Lynn, 
who has also made himself responsible for all calculations depend- 
ing on the new value of the Sun's parallax. It may be added that 
this has been taken at 8'8o", as probably a very close approxi- 
mation to the truth. 

It is now twenty-seven years since the first edition of this 
work was offered to the public, and from that time (December 
1861) to the present it has been, seemingly, a popular and 



PEEFACE TO THE FOUETH EDITION. ix 

appreciated book both in England and America, maintaining 
a steady sale from year to year. I am duly grateful for this, 
the more so as twenty-seven years ago I was a very young 
Author, with no reason to anticipate such a measure of success, 
and nothing to back me up in obtaining it. 

During this interval of more than a quarter of a century 
many things have happened in the World of Science, of which 
Astronomy is only one field. Many new and wholly unlooked- 
for discoveries have been made : new methods and processes 
have been introduced. Photography and Spectroscopy in their 
Astronomical applications may be said to be wholly the creatures 
of the period above named. New instruments have been in- 
vented, and the manufacture of old ones has been enormously 
developed. In 1860 the 1 2-inch refractor of the Greenwich 
Observatory was brought into use and was regarded as a grand 
advance. Now 12-inches counts for almost nothing in the race 
between different nations and different makers to obtain tele- 
scopes of large size for the exploration of the Heavens. 

Looking back on these years, the question forces itself upon 
our notice : ' Where are we now, in the effort to discover First 
Causes ? ' And the answer is : ' Very much where we were a 
quarter of a century ago.' The Theory of Evolution may be true 
or it may be false, but, be it one or the other, I agree very much 
with Professor Mivart, (who believes it,) when he says : " There is 
no necessary antagonism between the Christian Revelation and 
Evolution." Evolution is " an attempt to guess at a process ; 
it does not touch the Author of that process, and never will." 

a. jr. c. 

NOKTHFIELD GBANGE, 

EAST-BOURNE, SUSSEX : 
June, 1889. 



PREFACE TO THE THIRD EDITION. 

(EXTRACT.) 

ADVANTAGE has been taken of the call for a new edition 
-^- of this work to subject the whole, from the first page to 
the last, to a searching revision. This has proved to be a task 
of unusual difficulty and labour, in consequence of the astonishing 
developement which has taken place in the science of Astronomy 
during the last ten years. And moreover the demands on my 
time made by professional work have of late been such as to 
render it very difficult for me to give to Astronomical Studies 
that close attention which is indispensable if the author of an 
Astronomical Book would keep his pages up to date and so 
do justice alike to himself and his readers. It is not open to 
doubt that this is a matter which sits very lightly upon the con- 
sciences of some writers of Text-books. There is scarcely a 
single page which has not been, to a greater or less extent, 
dressed up, or in some way amended, with the object of making 
its statements more accurate in substance or intelligible in 
diction. 

I have to acknowledge a great amount of very useful advice 
and assistance from observers in all parts of the world, most 
of them total strangers to me, many of them being persons I 
had never heard of until the receipt of their letters. Indeed, 
the letters that I have received, especially from the United States 
of America, have been a very gratifying encouragement to me to 
persevere in improving this work in every possible way. 

0. jr. c. 

December, 1876. 



(EXTRACT.} 

ASTRONOMY is not cultivated in this country, either as a 
-*--*- study or as a recreation, to the extent that it is on the 
Continent of Europe and in America. And there is a lack of 
works in the English language which are at one and the same 
time attractive to the general reader, serviceable to the student, 
and handy, for purposes of reference, to the professional Astrono- 
mer ; in fact, of works which are popular without being vapid, 
and scientific without being unduly technical. 

The foregoing observations will serve to indicate why this 
book has been written. Its aim, curtly expressed, is, general 
usefulness. 

Preferring facts to fancies, I have endeavoured to avoid all 
those mischievous speculations on matters belonging to the 
domain of Recondite Wisdom, which have within the last few 
years borne such pernicious yet natural fruits. 

In regard to the matter of bringing up to date, it is believed 
that the present volume will compare favourably with any of 
its contemporaries. 

0. jr. c. 

March, 1867. 



PREFACE TO THE FIRST EDITION. 

(EXTRACT) 

ENGLISH literature, abundant though it may be in other 
respects, is undoubtedly very deficient in works on Astro- 
nomy. Our choice is limited either to purely elementary books, 
few in number, on the one hand ; or to advanced treatises, of 
which there is a similar paucity, on the other. The present 
work is designed to occupy a middle position between these two 
classes: to be attractive to the general reader, useful to the 
amateur, and ' handy ' also, as an occasional book of reference, 
to the professional astronomer. 

In pursuance of the plan laid down from the first, theoretical 
matter is, as a rule, excluded ; but in many cases it has been 
thought desirable not to abide with perfect strictness by this 
limitation. 

Finally, it is hoped that this book may be the means of in- 
ducing some, at least, to interest themselves in the study of that 
noble Science, which in so conclusive a manner shows forth the 
wonderful Wisdom, Power, and Beneficence of the Great Creator 
and Omnipotent Ruler of the Universe. 

0. jr. c. 

EAST-BOUKNE, SUSSEX : 
August, 1 86 1. 



CONTENTS. 

J 

BOOK I. 

THE SUN AND PLANETS. 

CHAPTEK I. 

THE SUN. 

Astronomical importance of the Sun. Solar parallax. The means of determining 
it. By observations of Mars. By Transits of Venus. Numerical data. 
Light and Heat of the Sun. Gravity at the Sun's surface. Spots. Descrip- 
tion of their appearance. How distributed. Their duration. Period of the 
Sun's Rotation. Effect of the varying position of the Earth with respect 
to the Sun. Their size. Instances of large Spots visible to the naked eye. 
The Great Spot of October 1865. Their periodicity. Discovered by Schwabe. 
Table of his results. Table of Wolfs results. Curious connexion between 
the periodicity of Sun-spots and that of other physical phenomena. The 
Diurnal variation of the Magnetic Needle. Singular occurrence in September 
1859. Wolfs researches. Spots and Terrestrial Temperatures and Weather. 
Ballot's inquiry into Terrestrial Temperatures. The Physical Nature of 
Spots. The Wilson-Herschel Theory. Luminosity of the Sun. Historical 
Notices. Scheiner. Faculae. Luculi. Nasmyth's observations on the cha- 
racter of the Sun's surface. Huggins's conclusions. Present state of our 
knowledge of the Sun's constitution. Tacchini's conclusions. Pages 1-53 

CHAPTER II. 

THE PLANETS. 

Epitome of the motions of the Planets. Characteristics common to them all. 
Kepler's Laws. Elements of a Planet's orbit. Curious relation between the 
distances and the periods of the Planets. The Ellipse. Popular illustration 
of the extent of the Solar system. Bode's law. Miscellaneous characteristics 
of the Planets. Curious coincidences. Conjunctions of the Planets. 
Conjunctions recorded in History. Different systems. The Ptolemaic 
system. The Egyptian system. The Copernican system. The Tychonic 
system. ... ... ... ... ... 54~74 



xiv Content*. 

CHAPTEK III. 
VULCAN (> . 

Le Verrier's investigation of the orbit of Mercury. Narrative of the Discovery of 
Vulcan. Le Verrier's interview with M. Lescarbault. Approximate elements 
of Vulcan. Concluding note by Le Verrier. Observations by Lummis at 
Manchester. Instances of Bodies seen traversing the Sun. Hind's opinion. 
Alleged Intra-Mercurial planets discovered in America by Watson and Swift 
on July J 9, 1878 75- 8 5 

CHAPTEK IV. 
MERCURY. $ 

Period, &c. Phases. Physical Observations by Schroter. Sir W. Herschel, 
Denning, Schiaparelli and Guiot. Determination of its Mass. When best 
seen. Acquaintance of the Ancients with Mercury. Copernicus and Mer- 
cury. Le Verrier's investigations as to the motions of Mercury. Tables of 
Mercury 86-92 

CHAPTEE V. 

VENUS. ? 

Period, &c. Phases resemble those of Mercury. Most favourably placed for 
observation once in 8 years. Observations by Lihou. By Lacerda. Daylight 
obervations. Its brilliancy. Its Spots and Axial Rotation. Suspected moun- 
tains and atmosphere. Its "ashy light." Phase irregularities. Suspected 
Satellite. Alleged Observations of it. The Mass of Venus. Ancient observa- 
tions. Galileo's anagram announcing his discovery of its Phases. Venus 
useful for nautical observations. Tables of Venus. ... ... 93-106 

CHAPTER VI. 

THE EARTH. 

Period, &c. Figure of the Earth. The Ecliptic. The Equinoxes. The Sol- 
stices. Diminution of the obliquity of the ecliptic. The eccentricity of the 
Earth's orbit. Motion of the Line of Apsides. Familiar proofs and illustra- 
tions of the sphericity of the Earth. Foucault's Pendulum Experiment. 
Madler's tables of the duration of day and night on the Earth. Opinions of 
ancient philosophers. English mediaeval synonyms. The Zodiac. Mass of 
the Earth. ... ... ... ... ... ... ... ... 107-17 

CHAPTER VII. 

THE MOON. ([ 

Period, &c. Its Phases. Its motions and their complexity. Libration. Evec- 
tion. Variation. Parallactic Inequality. Annual Equation. Secular ac- 
celeration. Diversified character of the Moon's surface. Lunar mountains. 
Seas. Craters. Volcanic character of the Moon. Bergeron's experiment. 
The lunar mountain, Aristarchus. Teneriffe. Lunar atmosphere. Re- 
searches of SchrOter, &c. Hansen's curious speculation. The Earth-shine. 
The Harvest Moon. Astronomy to an observer on the Moon. Luminosity 
and calorific rays. Historical notices as to the progress of Lunar Charto- 
graphy. Lunar Tables. Meteorological Influences. ... ... 118-41 



Contents. xv 

CHAPTER VIII. 
THE ZODIACAL LIGHT. 

General description of it. When and where visible. Sir J. Herschel's theory. 
Historical notices. Modern observations of it. Backhouse's Con- 
clusions. ... ., ... ... ... ... ... ... ... 142 7 

CHAPTER IX. 

MARS. <j 

Period. &c. Phases. Apparent motions. Its brilliancy. Telescopic appear- 
ance. Its ruddy hue. Schiaparelli's "Canals." General statement of the 
physical details of Mars. Map of Mars on Mercator's projection. Polar 
snow. Axial rotation. The seasons of Mars. Its atmosphere. The Satel- 
lites of Mars. Ancient observation of Mars. Tables of Mars. . . . 148-63 

CHAPTER X. 
THE MINOR PLANETS. 

Sometimes called Ultra-Zodiacal Planets. Summary of facts. Notes on Ceres. 
Pallas. Juno. Vesta. Olbers's theory. History of the search made for them. 
Independent discoveries. Progressive diminution in their size. 164-70 

CHAPTER XL 

JUPITER. I 

Period, &c. Jupiter subject to a slight phase. Its Belts. Their physical nature. 
First observed by Zucchi. Dark Spots. Luminous Spots. The great Red 
Spot. The great White Spot. Hough's observations. Alleged Connection 
between Spots on Jupiter and Spots on the Sun. Axial rotation of Jupiter. 
Centrifugal force at its Equator. Luminosity of Jupiter. Its Apparent 
Motions. Astrological influences. Attended by 4 Satellites. Are they visible 
to the Naked Eye? Table of them. Eclipses of the Satellites. Occupations. 
Transits. Peculiar aspects of the Satellites when in transit. Singular 
circumstance connected with the interior ones. Instances of 'all being 
invisible. Variations in their brilliancy. Observations of Eclipses for 
determining the longitude. Practical difficulties. Romer's discovery of the 
progressive transmission of light. Mass of Jupiter. The "Great Inequality." 
Tables of Jupiter J 73-99 

CHAPTER XII. 
SATURN, h 

Period, &c. Figure and Colour of Saturn. Belts and Spots. Observations of 
the Belts by Holden. By Ranyard. Bright spot recorded by Hall. Probable 
atmosphere. Observations of Galileo, and the perplexity they caused. 
Logogriph sent by him to Kepler. Huygens's discovery of the Ring. 
His logogriph. The bisection of the Ring discovered by Cassini. Sir 
W. Herschel's Doubts. Historical epitome of the progress of discovery. 
The "Dusky" Ring. Facts relating to the Rings. Appearances pre- 



xvi Contents. 

sented by them under different circumstances. Rotation of the Ring. 
Secchi's inquiries into this. The Ring not concentric with the Ball. 
Measurements by W. Struve. Other measurements. Miscellaneous par- 
ticulars. Theory of the Ring being fluid. Now thought to consist of an 
aggregation of Satellites. The "Beaded" appearance of the Ring. O. 
Struve's surmise about its contraction. Irregularities in the appearances of 
the ansae. Rings not bounded by plane surfaces. Mountains suspected on 
them. An atmosphere suspected. Physical observations between 1872 and 
1876 by Trouvelot. Observations by MM. Henry. By Keeler. Brightness 
of Rings and Ball. Bessel's investigations into the Mass of the Rings. 
Saturn attended by 8 Satellites. Table of them. Physical data relating to 
each. Elements by Jacob. Coincidences in the Rotation-periods of certain 
of them. Transits of Titan. Celestial phenomena on Saturn. Lockyer's 
summary of the appearances presented by the Rings. Peculiarity relative 
to the illumination of lapetus. Mass of Saturn. Ancient observations. 
Saturn ian Astronomy. ... ... ... ... 200-41 

CHAPTEE XIII. 

URANUS, y 

Circumstances connected with its discovery by Sir W. Herschel. Names pro- 
posed for it. Early observations. Period, &c. Physical appearance. Belts 
visible in large telescopes. Position of its axis. Attended by 4 Satellites. 
Table of them. Miscellaneous information concerning them. Mass of 
Uranus. Tables of Uranus. ... ... ... ... ... ... 242-51 

CHAPTER XIV. 

NEPTUNE. H* 

Circumstances which led to its discoveiy. Summary of the investigations of 
Adams and Le Verrier. Telescopic labours of Challis and Galle. The 
perturbations of Uranus by Neptune. Statement of these perturbations by 
Adams. Period, &c. Attended by i Satellite. Elements of its orbit. 
Mass of Neptune. Observations by Lalande in 1795. ... ... 252-60 



BOOK II. 

ECLIPSES AND ASSOCIATED PHENOMENA. 

CHAPTER I. 

GENERAL OUTLINES. 

Definitions. Position of the Moon's orbit in relation to the Earth's orbit. Con- 
sequences resulting from their being inclined to each other. Retrograde 
motion of the nodes of the Moon's orbit. Coincidence of 223 synodical periods 
with 19 synodical revolutions of the node. Known as the " Saros." State- 
ment of Diogenes Laertius. Illustration of the use of the Saros. Number of 
Eclipses which can occur. Solar Eclipses more frequent than Lunar ones. 
Duration of Annular and Total Eclipses of the Sun 261-9 



Contents. xvii 

CHAPTEE II. 

ECLIPSES OF THE SUN. 

Grandeur of a Total Eclipse of the Sun. How regarded in ancient times. 
Effects of the progress of Science. Indian Customs. Effect on Birds at 
Berlin in 1887. Solar Eclipses may be Partial, Annular, or Total. Chief 
phenomena seen in connexion with Total Eclipses. Change in the colour of 
the sky. The obscurity which prevails. Effect noticed by Piola. Physical 
explanation. Baily's Beads. Extract from Baily's original memoir. Prob- 
ably due to irradiation. Supposed to have been first noticed by Halley in 
1715. His description. The Corona. Hypothesis advanced to explain its 
origin. Probably caused by an atmosphere around the Sun. Remarks by 
Grant. First alluded to by Philostratus. Then by Plutarch. Corona visible 
during Annular Eclipses. The Red Flames. Remarks by Dawes. Physical 
cause unknown. First mentioned by Stannyan. Note by Flamsteed. 
Observations of Vassenius. Aspect presented by the Moon. Remarks by 
Arago. ... ... ... 270-85 

CHAPTER III. 

THE TOTAL ECLIPSE OF THE SUN OF JULY 28, 1851. 
Observations by Airy. By Hind. By Lassell 286-90 

CHAPTER IV. 

THE ANNULAR ECLIPSE OF THE SUN OF MARCH 14-15, 1858. 
Summary of observations in England 291-4 

CHAPTER V. 

THE TOTAL ECLIPSE OF THE SUN OF JULY 18, I860. 

Extracts from the observations of Sir G. B. Airy. Observations of the Red Flames 
by Bruhns. Meteorological observations by Lowe. ... ... 295-302 

CHAPTER VI. 

RECENT TOTAL ECLIPSES OF the SUN. 

Eclipse of August 18, 1868. Observations by Col. Tennant and M. Janssen at 
Guntoor. Summary of results. Observations of Governor J. P. Hennessy and 
Capt. Reed, R.N. Eclipse of August 7, 1869. Observations in America by 
Prof. Morton and others. Summary of results. Eclipse of December 22, 1870. 
English expedition in H. M. S. Urgent to Spain. Observations in Spain 
and Sicily. Eclipse of December n, 1871. Observed in India. Eclipse of 
April 16, 1 874. Summary by Mr. W. H. Wesley of the recent observations as 
to the Physical Constitution of the Corona 303-20 

CHAPTER VII. 

HISTORICAL NOTICES. 

Eclipses recorded in Ancient History. Eclipse of 584 B.C. Eclipse of 556 B.C. 
Eclipse of 479 B.C. Eclipse of 430 B.C. Eclipse of 309 B.C. Allusions in old 
English Chronicles to Eclipses of the Sun , 3 3I ~5 

b 



xviii Contents. 

CHAPTER VIII. 

ECLIPSES OF THE MOON. 

Lunar Eclipses of less interest than Solar ones. Summary of facts connected with 
them. Peculiar circumstances noticed during the Eclipse of March 19, 1848. 
Observations of Forster. Wargentin's remarks on the Eclipse of May 18, 
1761. Kepler's explanation of these peculiarities being due to Meteorological 
causes. Admiral Smyth's account of the successive stages of the Eclipse of 
Oct. 13, 1837. The Eclipse of Jan. 28, 1888. The Eclipse of Sept. 2, 1830, as 
witnessed in Africa by R. and J. Lander. Chaldaean observations of Eclipses. 
Other ancient Eclipses. Anecdote of Columbus. 326-33 

CHAPTER IX. 

A CATALOGUE OF ECLIPSES 334-6 

CHAPTER X. 

TRANSITS OF THE INFERIOR PLANETS. 

Cause of the phenomena. Lord Grimthorpe's statement of the case. Long 
intervals between each recurrence. Useful for the determination of the Sun's 
parallax. List of transits of Mercury. Of Venus. Transit of Mercury of 
Nov. 7, 1631. Predicted by Kepler. Observed byGassendi. His remarks. 
Transit of Nov. 3, 1651. Observed by Shakerley. Transit of May 3, 1661. 
Transit of Nov. 7, 1677. Others observed since that date. Transit of Nov. 9, 
1 848. Observations of Dawes. Of Forster. Transit of Nov. 1 1 , 1 86 1 . Observa- 
tions of Baxendell. Transit of Nov. 5, 1868. Transit of May 6, 1878. Transit 
of Nov. 7, 1 88 1. Summary by Jenkins of the main features of a Transit. 
Observations by Prince. By Langley. Transit of Venus of Nov. 24, 1639. 
Observed by Horrox and Crabtree. Transit of June 5, 1761. Transit of June 
3, 1769. Where observed. Singular phenomenon seen on both occasions. 
Explanatory hypothesis. Other phenomena. Transit of Dec. 8, 1874. 
Transit of Dec. 6, 1882. 337~54 

CHAPTER XL 

OCCULTATIONS. 

How caused. Table annually given in the "Nautical Almanac." Occultation 
by a young Moon. Effect of the Horizontal Parallax. Projection of Stars 
on the Moon's disc. Occultation of Jupiter, January 2, 1857. Occultation 
of Saturn, May 8, 1859. Occultation of Saturn, April 9, 1883. Historical 
notices. ... ... ... ... ... ... ... ... ... 355~6o 



BOOK III. 

PHYSICAL AND MISCELLANEOUS ASTKONOMICAL 
PHENOMENA. 

CHAPTER I. 

THE TIDES. 

Introduction. Physical cause of the Tides. Attractive force exercised by the 
Moon. By the Sun. Spring Tides. Neap Tides. Summary of the principal 
facts. Priming and Lagging. Diurnal Inequality. 361-5 



Contents. xix 

CHAPTEE II. 

LOCAL TIDAL PHENOMENA. 

Local disturbing influences. Table of Tidal ranges. Influence of the Wind. 
Experiment of Smeaton. The Tides in the Severn at Chepstow. Tidal phe- 
nomena in the Pacific Ocean. Remarks by Beechey. Velocity of the great 
Terrestrial Tidal wave. Its course round the earth, sketched by Johnston. 
Effects of Tides at Bristol. Instinct of animals. Tides extinguished in 
rivers. Instances of abnormal Tidal Phenomena. The "Mascaret" on the 
Seine. Historical notices. ... ... 366-73 

CHAPTEK III. 

PHYSICAL PHENOMENA. 

Secular Variation in the Obliquity of the Ecliptic. Precession. Its value. Its 
physical cause. Correction for Precession. History of its discovery. 
Nutation. Herschel's definition of it. Connexion between Precession and 
Nutation. 374~79 

CHAPTEK IV. 

ABERRATION AND PARALLAX. 

Aberration. The constant of Aberration. Familiar illustration. History of the 
circumstances which led to its discovery by Bradley. Parallax. Ex- 
planation of its nature. Parallax of the heavenly bodies. Parallax of the 
Moon. Importance of a correct determination of the Parallax of an Object. 
Leonard Digges on the distance of the Planets from the Earth. . . . 380-86 

CHAPTEE V. 

REFRACTION AND TWILIGHT. 

Refraction. Its nature. Importance of a correct knowledge of its amount. 
Table of the correction for Refraction. Effect of Refraction on the position 
of objects in the horizon. History of its discovery. Twilight. How caused. 
Its duration. ... 3^7-94 



BOOK IV. 

COMETS. 

CHAPTEE I. 

GENERAL REMARKS. 

Comets always objects of popular interest, and sometimes of alarm. Usual 
phenomena attending the development of a Comet. Telescopic Comets. 
Comets diminish in brilliancy at each return. Period of revolution. 
Density. Mass. Lexell's Comet. General influence of Planets on Comets. 
Special influence of Jupiter. Comets move in i of 3 kinds of orbits. 
Element of a Comet's orbit. For a parabolic orbit, 5 in number. Direction 
of motion. Eccentricity of an elliptic orbit. The various possible sections 
of a cone. Early speculations as to the paths in which Comets move. 

b a 



xx Content*. 

Comets visible in the daytime. Breaking up of a Comet into parts. 
Instance of Biela's Comet. Liais's observations of Comet iii. 1860. Comets 
probably self-luminous. Existence of phases doubtful. Comets with Plane- 
tary discs. Phenomena connected with the tails of Comets. Usually in the 
direction of the radius vector. Secondary Tails. Vibration sometimes 
noticed in tails. Olbers's hypothesis. Transits of Comets across the Sun's 
disc. Variation in the appearance of Comets exemplified in the case of that 
of 1769. Transits of Comets across the Sun. ... ... ... 395-414 

CHAPTEK II. 
PERIODIC COMETS. 

Periodic Comets conveniently divided into three classes. Comets in Class I. 
Encke's Comet. The resisting medium. Table of periods of revolution. 
Tempel's Second Comet. Winnecke's Comet. Brorsen's Comet. Tempel's 
First Comet. Swift's Comet. Barnard's Comet. D' Arrest's Comet. 
Finlay's Comet. Wolfs Comet. Faye's Comet. Denning's Comet. 
Mechain's Comet of 1790. Now known as Tuttle's Comet. Biela's Comet. 
Di Vico's Comet of 1844. List of Comets presumed to be of short periods 
but only once observed. Comets in Class II. Westphal's Comet. Pons's 
Comet of 1812. Di Vico's Comet of 1846. Olbers's Comet of 1815. Brorsen's 
Comet of 1847. Halley's Comet. Of special interest. Re"sumt$ of Halley's 
labours. Its return in 1759. Its return in 1835. Its history prior to 1531 
tra ;ed by Hind. Comets in Class III not requiring detailed notice. 415-45 

CHAPTER III. 
REMARKABLE COMETS. 

The Great Comet of 1811. The Great Comet of 1843. The Great Comet of 1858. 
The Comet of 1860 (iii.). The Great Comet of 1861. The Comet of 
1862 (iii.). The Comet of 1864 (ii.) The Comet of 1874 (iii.). The Comet 
of 1882 (iii.) 446-81 

CHAPTER IV. 

CERTAIN STATISTICAL INFORMATION RELATING TO COMETS. 

Dimensions of the Nuclei of Comets. Of the Comae. Comets contract and expand 
on approaching to, and receding from, the Sun. Exemplified by Encke's in 
1838. Lengths of the Tails of Comets. Dimensions of Cometary orbits. 
Periods of Comets. Number of Comets recorded. Duration of visibility of 
Comets. Unknown Comet found recorded on a photograph of the Eclipse of 
the Sun of May 17, 1882. 482-86 

CHAPTER V. 

HISTORICAL NOTICES. 

Opinions of the Ancients on the nature of Comets. Superstitious notions 
associated with them. Extracts from ancient Chronicles. Pope Calixtus III. 
and the Comet of 1456. Extracts from the writings of English authors of the 
i6th and i7th centuries. Napoleon and the Comet of 1769. Supposed 
allusions in the Bible to Comets. Conclusion ... 487-90 



Contents. 



xxi 



CHAPTER VI. 



SECTION 1. Preliminary. ... ... 49 1-4 

SECTION 2. On the proportioning of the Areas in the different Segments of the 

Projection 494-5 

SECTION 3. The Latitudes and the Inclination of the Plane of the Orbit. 496-7 
SECTION 4. To find a Parabola having its Focus at S and which shall coincide with 

two Points of the Orbit ... ... 498 

SECTIONS. The Measurement of the Areas in a Parabola 498-9 

SECTION 6. The Relations between the Time-intervals and the Longitude 

Lines ... ... ... ... ... ... ... ... ... 499-501 

SECTION 7- Checks available, derived from certain properties of Parabolic 

Orbits. ... ... ... ... 501 

SECTIONS. Examples of the Graphical Process 5J~9 

SECTIONS. To form an ephemeris of a Comet ... ... ... ... 509-10 

CHAPTER VII. 

A CATALOGUE OF ALL THE COMETS WHOSE ORBITS HAVE 

HITHERTO BEEN COMPUTED 5 11-47 

A Summary of the preceding Catalogue ... 548^9 

CHAPTER VIII. 

A CATALOGUE OF COMETS RECORDED, BUT NOT WITH SUFFICIENT 
PRECISION TO ENABLE THEIR ORBITS TO BE CATALOGUED. 550-88 



BOOK V. 

METEORIC ASTRONOMY. 

CHAPTER I. 

AEROLITES. 

Classification of the subject. Aerolites. Summary of the researches of Berzelius. 
Rammelsberg, and others. Celebrated Aerolites. Summary of facts. 
Catalogue of Meteoric Stones. Arago's Table of Apparitions. The Aerolite 
of 1492. Of 1627. Of 1795. The Meteoric Shower of 1803. The Aerolite of 
1876 (Rowton). The Aerolite of 1881 (Middlesborough). The Aerolite of 
1887 (Soko Banja). 589-98 

CHAPTER II. 
FIREBALLS. 

General Description of them. Fireball of Nov. 12, 1 86 1. Monthly Table of appar- 
itions. Dates of greatest frequency. Results of calculations with reference 
to these bodies. 6o1 ' 



xxii Contents. 

CHAPTER III. 

SHOOTING STARS. 

Have only recently attracted attention. Are visible with greater or less frequency 
every clear night. Summaries of the monthly and horary rates of apparition 
from observations by Coulvier-Gravier and Denning. Number of known 
meteor showers. Their distribution amongst the constellations. Monthly 
number of meteors catalogued. Early notices of great meteor showers. The 
showers of 1799, 1831, 1832, 1833, 1866, and following years. The shower 
of Aug. 10. Of Nov. 27, 1872, and Nov. 27, 1885. Nomenclature of meteor 
systems. Views of Olbers. Monthly summary of great meteoric dis- 
plays. 608-25 

CHAPTER IV. 

THE THEORY OF METEORS. 

Meteors are planetary bodies. Their periodicity. Meteoric orbits. Researches of 
Newton and Adams. Orbit of the meteors of November 13. Identity of the 
orbits of cornet^ and meteors. The meteor showers of Nov. 13 and 27. Recent 
progress of Meteoric Astronomy. Table of the chief radiant points. 626-38 

CHAPTER V. 

RADIANT POINTS. 
Explanation of Reference Letters in the List of Radiant Points. . . . 639-43 

CHAPTER VI. 

TELESCOPIC METEORS. 

Our knowledge of them limited. Observations. Probable heights in the 
atmosphere. Showers of telescopic meteors. Summary of Prof. Safarik's 
observations and deductions. Fireball observed in a telescope on Oct. 19, 
1863 644-50 



BOOK VI. 

TABLES OF THE PLANETS. 

The Major Planets ... ... .. ... 651-3 

The Minor Planets 654-71 

INDEX 672 



LIST OF ILLUSTRATIONS. 



Fig. Page 

1. Encke's Comet, 1848 : on Sept. 22 . . Plate I, Frontispiece. 

2. The Illrd Satellite of Jupiter in 1855 .... Title-page. 

3. General Telescopic appearance of the Sun .... 8 

4. Spot on the Sun, September 29, 1826. (Capocci.) , Plate II. n 

5. Spot on the Sun, May 23, 1861. (Birt.) . n 

6. Spot on the Sun, May 27, 1861. (Anon.) . . n 

7. Paths of Sun Spots at different times of the year . . .16 

8. Great Sun Spot visible on June 30, 1883. (Ricco.) . . . 18 

9. The same Sun Spot on July 2, 1883. (Ricco.) . . . .18 

10. Great Sun Spot visible on July 25, 1883. (Ricco.) . . .19 

11. The same Sun Spot on July 27, 1883. (Ricco.) . . . .19 

12. The Great Sun Spot of October 1865, Oct. n, ii A.M. (Brodie.) Plate III. 21 

13. The Great Sun Spot of October 1865, Oct. ii, 12-30 P.M. (Brodie.) 21 

1 4. The Great Sun Spot of October 1 865 , Oct. 1 2 , 9- 30 A. M. (Brodie. ) , , 21 

15. The Great Sun Spot of October 1865, Oct. 12, 10-30 A.M. (Brodie.) 21 

16. The Great Sun Spot of October 1865, Oct.- 1 2, 12-30 P.M. (Brodie.} 21 

17. The Great Sun Spot of October 1865, Oct. 12, 2-30 P.M. (Brodie.) 21 

18. The Great Sun Spot of October 1865. Pectinated edge 

visible on Oct. 12. (Brodie.) . . . . .23 

19. Diagram illustrating the connection between Aurorse, 

Terrestrial Magnetism, and Spots on the Sun . Plate IV. 30 

20. Change of form in Spots owing to the Sun's rotation . . -39 

21. Spot on the Sun, May 5, 1854, showing cyclonic action. (Secchi.) . 40 

22. Large Spot on the Sun in 1866 showing successive 

changes of form ....... 41 

23. Spot seen on the edge of the Sun in 1884 exhibiting 

itself as a depression ..... -4* 

24. Faculse on the Sun, December 3, 1865. (Tacchini.) . . -45 

25. Spot on the Sun, July 29, 1860, showing the "Willow- 

leaf" Structure. (Nasmyth.) . . . -47 

26. Spot on the Sun, January 20, 1865. (Secchi.) . . .48 

27. " Rice-like " particles seen on the Sun. (Stone.) ' . -49 

28. Ideal View of the "Granular" Structure of the Sun. 

(Huggins.) .....- 5 1 

29. Solar granules, 1866, showing cyclonic arrangement. (Hug<jin*. ja 

30. Solar granules, 1866, ordinary arrangement. (HttpffM.] ' 



xxiv List of Illustrations. 

Fig. Page 

31. Phases of an Inferior Planet . . . . . -55 

32. Apparent movements of Mercury, 1708-1715 .... 56 

33. Diagram illustrating Kepler's Second Law .... 58 

34. The Ellipse ........ 61 

35. Relative apparent size of the Sun, as viewed from the 

different Planets named ...... 62 

36. Comparative Sizes of the Planets . . . . .63 

37. Comparative Sizes of the Sun and Planets . . Plate V. 65 

38. Conjunction of Venus and Jupiter, July 2 1, 1859 ... 69 

39. Conjunction of Venus and Saturn, December 19, 1845 . . 70 

40. The Ptolemaic System . . . . . . .71 

41. The Egyptian System . . . . . . .72 

42. The Copernican System . . . . . .72 

43. The Tychonic System . . . . . . . 73 

44. House at Woolsthorpe where Newton was born ... 74 

45. Flight of Cranes seen crossing the Sun .... 80 

46. Mercury, Sept. 17, 1885. (Guiot.} ..... 89 

47. Mercury, Sept. 22, 1885. (Guiot.) ..... 89 

48. Venus near its Greatest Elongation. (Schroter,} 93 

49. Venus near its Inferior Conjunction. (Schroter.) . . . 94 

50. Venus, Nov. 10, 1885. (Lihou.} ..... 95 

51. Venus, Dec. 23, 1885. (Lihou.} ..... 95 

52. Venus, Sept. 8, 1884. (Lacerda.) . . . .96 

53. Venus, Sept. 9, 1884, (Lacerda.} ..... 96 

54. Venus, Oct. 8, 1884. (Lacerda.} ..... 97 

55. Venus, Oct. n, 1884. (Lacerda.} ..... 97 

56. Foucault's Pendulum experiment for demonstrating 

the Earth's Rotation .... Plate VI. 113 

57. View of a portion of the Moon's Surface. (Nasmyth.} . , . 1 23 

58. Imitation of the structure of the Moon's Surface, 

(Bergeron's experiment.) . . . . . .125 

59. The Lunar Mountain Aristarchus, illuminated . . .126 

60. The Lunar Mountain Aristarchus, waxing . . . .127 

61. The Lunar Mountain Aristarchus, waning . . . .127 

62. The Peak of Teneriffe . . . . . . .128 

63. The Lunar Mountain Archimedes . . . Plate VII. 129 

64. The Lunar Mountain Pico . . . . ,, 129 

65. The Lunar Mountain Copernicus. (Nasmyth.') . 129 

66. The Lunar Mountain Archimedes. (Weinek.) . Plate VIII. 131 

67. The Lunar Mountain Gassendi. (Weinek.} . . 131 

68. The Lunar Gulf Sinus Iridum. (Weinek.) . . 131 

69. The Lunar Mountains Kepler and Encke. (Weinek.}. 131 

70. The Lunar Mountain Frascatorius. (Weinek.} . ,, 131 

71. The Lunar Mountain Plato. (Weinek.} . . ,, 131 

72. The Lunar Mountain Eudoxus. (Trourelot.} .... 133 

73. The Gulf of Iris seen when the Moon is 10 days old . . . 133 

74. Mars, 1858. (Secchi.) ..... Plate IX. faces 148 

75. Mars, 1858. (Seccfii.) . . . . . ,,148 



List of Illustrations. 



Fig. 

76. Mars, 1856. (Brodie.) ..... 

77. Mars, on Mercator's projection. (N. E. Green.} 

78. The Polar Snows of Mars .... 

79. The Polar Snows of Mars .... 

80. The apparent Orbits of the Satellites of Mars. 

81. Jupiter, 1857. (Dawes.) .... 

82. Jupiter, 1858. (Lassell.) .... 

83. Jupiter, 1860. (Jacob.} .... 

84. Jupiter, 1860. (Baxendell) .... 

85. Jupiter, 1856. (De La Rue.} .... 

86. Jupiter, 1871. (LasseU.) .... 

87. Jupiter, 1857, October 6. (Sir W. K. Murray.) 

88. The Great Ked Spot on Jupiter, 1887. (Denning.) 

89. Jupiter and its Satellites .... 

90. Jupiter and its Satellites, seen with the Naked Eye, 

1863. (Mason.) ..... 

91. Jupiter and its Satellites, seen with a Telescope, 1863. 

(Mason.) ...... 

92. The IV th Satellite of Jupiter, 1873. (Roberts.) 

93. The III rd Satellite of Jupiter, 1860. (Dams.) 

94. The IV th Satellite of Jupiter, 1849. (Dawes.) 

95. Jupiter with the II nd Satellite in Transit, 1828 

96. Jupiter's I st Satellite in Transit, with a double shadow. 

(Trouvelot.) ...... 

97. Plan of the Jovian System .... 

98. Saturn, 1856. (De La Rue.) . . . . 

99. Saturn, 1883. (Holden.) .... 

100. Saturn, 1665. (Ball.) ..... 

101. Saturn, 1675. (Hevelius.) .... 

102. Saturn, 1676. (Cassini.) .... 

103. Saturn, 1853. (Dawes.) .... 

104. Saturn, 1848. (W. C. Bond.) .... 

105. Saturn, 1856. (Jacob.) .... 

106. Saturn, 1 86 1. (De La Rue.) 

107. Saturn, 1861. (Jacob.) . . 

108. Saturn, 1861. (Jacob.) ..... 

109. Saturn, 1861. (Anon.) .... 

no. Saturn, 1861. (Wray.) ..... 

in. Saturn, 1862. (Wray.) .... 

112. General View of the Phases of Saturn's Rings 

113. Phases of Saturn's Rings at the dates specified 

114. Saturn, 1883. (Ranyard.) .... 

115. Saturn, 1884. (Henry.) 

116. Saturn, 1887. (Terby.) 

117. Diagram illustrating the phenomenon of Saturn's 

Ring " Beaded "..... 

11 8. Diagram illustrating the phenomenon of Saturn's 

Ring " Beaded "..... 



Plate X. 



Plate XI. 



Plate XII. 



Plate XIII. 



Plate XIV. 



Plate XV. 



Plate XVI. 



XXV 

Page 
151 
153 
157 
157 
161 
172 
172 
172 
172 

174 
176 
177 
179 
183 

184 

184 
189 
189 
189 
190 

191 
193 

201 
204 
208 
208 
208 
210 
2IO 
210 
313 
313 
213 

"5 

"5 
25 
218 
219 
224 
224 
324 

226 

22$ 



XXVI 



List of Illustrations. 



Fig. 
I1 9 . 
1 2O. 
121. 

122. 

123- 
I2 4 . 

125. 
126. 

127. 
128. 
I2 9 . 
130. 

13I- 
132. 

'33- 
134- 
'35- 
I3. 
137- 
138. 

139- 

I 4 0. 

141. 
142. 

143- 
144. 

145- 

146. 
I 4 7. 

148. 
149. 
150. 
I 5 I. 
I 5 2. 
153. 
154. 
155- 
156. 
'57- 
158. 
159- 
1 60. 

161. 



General View of Saturn and its Satellites 

Plan of the Saturnian System . 

Diagram to facilitate the identification of the 

Satellites of Saturn, 1888 . 
Uranus, 1884. (Henry.) 
Plan of the Uranian System . 
Apparent Orbits of the Satellites of Uranus . 
Diagram illustrating the Perturbation of Uranus by 
Neptune .... 

Geometrical diagram of the Perturbation of Uranus 
by Neptune ...... 

Plan of the Orbit of Neptune's Satellite 
Orbit of the Satellite of Neptune 
Theory of a Total Eclipse of the Sun 
Theory of an Annular Eclipse of the Sun 
Theory of an Eclipse of the Moon 
" Baily's Beads "... 

the Red Flames. 

the Red Flames. 

the Red Flames, 

the Red Flames. 

the Red Flames. 

the Red Flames. 

the Annulus 



Plate XVII. 



Page 
232 

235 

240 
246 
249 

250 

256 



Eclipse of the Sun, 1851 
Eclipse of the Sun, 1851 
Eclipse of the Sun, 1851 
Eclipse of the Sun, 1851 
Eclipse of the Sun, 1851 
Eclipse of the Sun, 1851 
Eclipse of the Sun, 1858 
Eclipse of the Sun, 1860 
Eclipse of the Sun, 1 860 
Eclipse of the Sun, 1860 



257 
259 

259 

262 
262 
263 
277 

(Airy.') Plate XVIII. faces 286 
(Carrington.) ,, 286 

(Dawes.). 286 

(Hind.} . ,, 286 

(Sfeptenam.) ,, 286 

(<?. Williams.') ,, 286 

,, 291 



the Corona. (Feilitzsdi.') . Plate XIX. faces 297 



the Red Flames. (Bmhns.) 
the Red Flames. (Bruhns. ) 
Eclipse of the Sun, 1860 the Corona. (Tempel.) 
Diagram representing the Rays of the Corona, 1868. 
(Hennessey.") ...... 

Eclipse of the Sun, 1851, 1860, 1869: Diagrams of 
the Corona ...... 

Eclipse of the Sun, 1870: Diagram of the Corona 
Eclipse of the Sun, 1871 : Diagram of the Corona 



Plate XX. 



Eclipse of the Sun, 1874. 
Eclipse of the Sun, 1875 : 
Eclipse of the Sun, 1878 : 
Eclipse of the Sun, 1882 ; 
Eclipse of the Sun, 1883 : 
Eclipse of the Sun, 1885 
Eclipse of the Sun, 1886 
Eclipse of the Sun, 1887 
Eclipse of the Sun, 1887 : 
Conditions of Eclipses of the Moon 
Eclipse of the Moon, Oct. 13, 1837. (Smyth.). 
Mercury during its transit, Nov. 5, 1 868 
Venus during its transit in 1769 
Venus during its transit in 1769 



(Bright.) . 

Diagram of the Corona 
Diagram of the Corona 
Diagram of the Corona 
Diagram of the Corona 
Diagram of the Corona 
Diagram of the Corona 
Diagram of the Corona 
the Corona. (Khandrikoff.) , 



297 
297 
299 

305 

312 

313 

3^5 
316 

317 



319 
320 
320 

Plate XXI. faces 320 
327 
330 
343 
348 
349 





L/ist of Illustrations. 


XXV11 


Fig. 




Page 


162. 


Venus just before the commencement of its transit in 






1882. (Prince.) ..... 


35 


I6 3 . 


Venus during its transit in 1874. First formation of 






ligament. (Stone.) ..... 


Plate XXII. 351 


164. 


Venus during its transit in 1874. Apparent contact 






not perfect. (Stone.) .... 


35' 


I6 5 . 


Venus during its transit in 1874. Apparent contact. 






(Stone.) ...... 


351 


1 66. 


Venus during its transit in 1874. The ligament 






broad. (Stone.) ..... 


!! 35' 


167. 


Venus during its transit in 1874. The ligament 






broader. (Stone.) ..... 


35 1 


168. 


Venus during its transit in 1874. The ligament 






broadest. (Stone.) ..... 


35' 


169. 


Venus during its transit in 1882 


353 


170. 


Venus during its transit in 1882 


353 


171. 


Venus during its transit in 1882 


353 


172. 


Occultation of Jupiter, January 2, 1857. (Lassett.) . 


. . 358 


173. 


Occultation of Saturn, April 9, 1883. (Loomis.) 


359 


174. 


The " Mascaret " or '' Bore " on the river Seine 


37* 


'75- 


Diagram illustrating the phenomenon of Aberration 


. 381 


176. 


Diagram illustrating the phenomenon of Parallax 


384 


177. 


Diagram illustrating the phenomenon of Eefraction . 


388 


178. 


Telescopic Comet without a Nucleus . 


396 


179. 


Telescopic Comet with a Nucleus . 


39 6 


1 80. 


Comparative sizes of the Earth, the Moon's orbit, and 






certain Comets ..... 


Plate XXIII. 398 


181. 


Diagram illustrating the influence of Jupiter on 






Comets ...... 


. - 402 


182. 


The various Sections of a Cone 


. 406 


183. 


Comet I., 1847, visible at noon on March 30. (Hind.) 


47 


184. 


Biela's Comet in 1846. (0. Struve.) 


. 408 


185. 


Diagram illustrating the changes in the directions of 






the tails of Comets .... 


4" 


1 86. 


Encke's Comet in 1828. (W. Strure.) . 


. 418 


187. 


Encke's Comet in 1871. (Carpenter.) 


423 


188. 


Pons's Comet in 1884, Jan. 19. (Trepied.) 


436 


189. 


Halley's Comet, 1683, showing luminous Sector. 






(Hevelius.) .... 


. 438 


190. 


Plan of the orbit of Halley's Comet compared with the 






orbits of certain Planets 


438 


191. 


Halley's Comet, 1835 


44<> 


192. 


Halley's Comet, 1066 : (from the Bayeux Tapestry) . 


Plate XXIV. 442 


193- 


Halley's Comet, 684 : (from the Nuremburg Chron- 






icle) .... 


443 


194. 


The Great Comet of 1 8 1 1 


447 


J 95- 


Donati's Comet, 1858. (Pape.) . 


Plate XXV. 449 


196. 


Donati's Comet, 1858. -(Pape.) . 


Plate XXVI. 45' 



XXV111 



A <'."/ of Illustrations. 



Fig. 

197. 

198. 

I 99 . 

200. 

2OI. 

202. 

203. 

304. 

205. 

2O6. 

207. 

208. 

209. 

210. 

211. 

212. 

2I 3 . 

214. 

2I 5 . 

2l6. 

217. 

218. 

219. 

220. 

221. 
222. 
223. 
22 4 . 
22 5 . 
226. 
227. 
228. 
229. 
230. 

331. 
232. 

233- 
234- 
235- 
236. 

237- 



the Coma, 
the Coma. 



Donati's Comet, 1858. (Smyth.} , 

Donati's Comet of 1858 passing Arcturus 

Donati's Comet, 1858 : the Coma. (Pope.) 

Donati's Comet, 1858 

Donati's Comet, 1858 

Donati's Comet, 1858 : the Coma. 

Donati's Comet, 1858 : the Coma. 

Comet III. 1860. 

Comet III. 1860. 

Comet III. 1860. 



(Pope.) 
(Anon.} 
(Pope.) 
(Pope.) 

(CappeUetti and Rosa.} 
(Cappettetti and Rosa.} 
(CappeUetti and Rosa.} 



Plate XXVII. 



Plate XXVIII. 



Comet III. 1860. (CappeUetti and Rosa.} . . 

Comet III. 1860. (CappeUetti and Rosa.} . . 

Comet III. 1860. (CappeUetti and Rosa.} . . 

The Great Comet of 1861 : the Coma. (Welb.} . Plate XXIX. 

The Great Comet of 1861 : the Coma. (Brodie.} , 

The Great Comet of 1861 : naked-eye view. (Brodie.} 

The Great Comet of 1861 : naked-eye view. (Chambers.} 

The Great Comet of 1861. (0. Williams.} . . Plate XXX. 

Coggia's Comet of 1874 : skeleton outline. (Brodie.} . 
Comet III. 1862. (Chattis.} .... Plate XXXL 

Comet III. 1862. (Cliattis.) .... 

(CholUs.} .... 

(Oiallis.} .... 

(Chattis.} .... ,, 

(Chattis.} .... 

Coggia's Comet of 1874 : the Coma. (Brodie.}. . Plate XXXII. 

The Great Comet of 1882 : the Nucleus. (Prince.) 

(Hopkins.) ..... 

(Flammarion.) 

(WiUis.} . . .PlateXXXIII. 

the compound Nucleus 
the compound Nucleus 
Eclipse of the Sun of May 17, 1882, showing an unknown Comet 
Graphical determination of a cometary orbit : Relation of the 
Equator to the Ecliptic ...... 

Graphical determination of a cometary orbit : Scheme for adjusting 
the Subtended Areas ...... 

Graphical determination of a cometary orbit : Diagram for finding 

Points of Projection when Node and Inclination are given 
Graphical determination of a cometary orbit : Diagram for finding 

the Perihelion from given points on the Orbit 

Graphical determination of a cometary orbit : Diagram for com- 
prising Arcs ........ 

Graphical determination of a cometary orbit : Orbit of Comet III., 

1881 . . . Folding Plate, XXXIV. Faces 

Graphical determination of a cometary orbit : Orbit of Comet II., 

1881 .... Folding Plate, XXXV. Faces 

Meteorite of Sako-Banja, Oct 13, 1877. . 



Comet III. 1862. 
Comet III. 1862. 
Comet III. 1862. 
Comet III. 1862. 



The Great Comet of 1882. 
The Great Comet of 1882. 
The Great Comet of 1882. 
The Great Comet of 1882 
The Great Comet of 1882 



IV-r 

453 
454 
455 
455 
455 
455 
455 
459 
459 
459 
459 
459 
459 
463 
463 
463 
463 
465 
468 
469 
469 
469 
469 
469 
469 
47i 
475 
476 

477 
479 
481 
481 
486 

493 
495 
496 
498 



502 
506 



List of Illustrations. xxix 

Fig. Page 

238. Fireball of Aug. 18, 1783. (Saxby and Robinson.) . Plate XXXVI. 600 

239. Fireball of Aug. 1 8, 1783. (Saxby and Robinson.) . ,, . 600 

240. Fireball of June 7, 1878. (Denning.) ...,,. 600 

241. Fireball of Oct. 19, 1863. (Schmidt.) ...,,. 600 

242. Meteor of Nov. 12,1861. (Webb.) ..... 602 

243. Curious form of trail left by Fireball of October 19, 1877 . . 605 

244. Trail left by Fireball of November 13, 1888 .... 606 

245. Distribution of Meteor Streams in Right Ascension . . . 6ia 

246. Relative number of Meteors catalogued during the several months 

of the year ........ 615 

247. The Meteor Radiant Point in Leo : tracks of Meteors seen at 

Greenwich, Nov. 13, 1866 ...... 619 

248. Intersection of the plane of the Orbit of the Earth by the Shooting 

Stars of August 10 . . . . . . . 622 

249. Orbit of the Leonids of November 13 relatively to the Orbits of 

certain Planets ....... 632 

250. Positions of Biela's Comet at the time of the Meteor Showers of 

1798, 1838, and 1872 ...... 633 

251. Radiant Point of Geminids (Dec. 12) on Nov. 28- Dec. 9, 1864 . 636 

252. Radiant Point of Orionids (Oct. 18-21) on Oct. 20, 1865 . . 637 

253. Flight of Telescope Meteors. (Brooks.) .... 646 



ADDENDA ET COKKIGENDA. 

Page 

3, note e. Add : A further description of the principle of the method 
will be found in Challis's Lectures on Practical Astronomy, 
p. 301. 

1 6, Fig. 7. The dotted lines on these 4 discs have been somewhat 

exaggerated. The curves should neither be quite so sharp, nor 
the inclination of the straight lines quite so great, as the'engraver 
has made them. 

17, line 1 8. A good description of the details of the structure of a sun- 

spot is given by Janssen (Comptes Eendus, vol. cii. p. 80. 1886). 

56, line 3, for " Eastern " read " Western." 

line 5, for " Western " read " Eastern." 

line 6, for " motion " read " the apparent motion of revolution 
round the Sun." 

68, line 15, for "appendix" read "Book VI." 

78, line 15. For 0-132' read 0-132. 

126. In connection with Sir W. Herschel's supposition that he had seen 
a volcano in action, on the Moon, attention may be called to some 
remarks by Prof. Holden in The Observatory, vol. xi. p. 334, 
Sept. 1888. 

165, line 8. The minor planet Thule (279) is now the most distant one 
known. 

line 1 6, for "Massalia" read " Massilia." 

1 86, line 8. Add: Lord Stratford De Eedcliffe relates that on his 
voyage to America in September 1820 one night " at anchor on 
board ship I had occasion to observe the wonderful clearness of 
the atmosphere. From the Spartan's deck I saw with my 
naked eye the satellites of Jupiter." (Life of Stratford Canning, 
vol. i. p. 299, Lond. 1889.) 

1 89, note f. Add : Some useful information relating to the physical 

features of Jupiter's satellites will be found in E. Engelmann's 

Uber die Helligkeitsverhdltnisse der Jupiterstralanten. Leipzig, 

1871. 

200, line 5 of Chapter Contents, for " the brothers Ball " read "Cassini." 



Addenda et Corrigenda. xxxi 

Page 

233. For an account of some curiously mysterious circumstances con- 
nected with the discovery of the satellite Titan see a letter by 
Lynn in The Observatory, vol. xi. p. 338, Sept. 1888, and other 
letters in the numbers of that Magazine for March and April 
1889. 

250. Newcomb's mass of Uranus is 

259. Newcomb's mass of Neptune is 

320. The Total Eclipse of the Sun of Jan. i, 1889 was successfully 
observed in America. Professor Pickering noticed the corona to 
be longer and more irregular in its shape than usual, and that 
it exhibited great detail in its filaments. 

367. With regard to Wicklow Head, there is another reason why the 
rise and fall of the tide there is so small. That Head is only 
about 22 miles N. of Courtown, where the tide waves entering 
the Irish Sea by the South and by the North of Ireland nearly 
cancel each other. At Courtown the range of the tide is only 
1 8 inches, and that place is at the head of a bay, though a wide 
and shallow one. 

375, line 16, dele "periodical." 

376, lines 8 and 10, for " ecliptic" read " zodiac." 

377, line 10 from bottom, read Aristillus. 

line 3 from bottom, for " effect of" read " solar." 
385, line 14, after "these" insert "latter." 



THE GKEEK ALPHABET. 



*#* The small letters of this alphabet are so frequently 
employed in Astronomy that a tabular view of them, together 
with their pronunciation, will be useful to many unacquainted 
with the Greek language. 



a Alpha. 
ft Beta. 
y Gamma. 
8 Delta. 
Epsllon. 
^ Zeta. 
?/ Eta. 
Theta. 
t Iota. 
K Kappa. 
A Lambda, 
u Mu. 



v Nu. 

*Xi. 

o O-mlcron. 

TT Pi. 

p Rho. 

a- Sigma. 

T Tau. 

v Upsilon. 

Phi. 

X Chi. 

^ Psi. 

o) O-mega. 



BOOK I. 

THE STJIST AND PLANETS. 

CHAPTEK I. 

THE SUN. O 



" ye Sun and Moon, bless ye the LOKD : praise Him, and magnify 
Him for ever." Benedicite. 



Astronomical importance of the Sun. Solar parallax. The means of determining 
it. Sy observations of Mars. By Transits of Venus. Numerical data. Light 
andHeatofthe Sun. Gravity at the Sun's surface. Spots. Description of their 
appearance. How distributed. Their duration. Period of the Sun's Rota- 
tion. Effect of the varying position of the Earth with respect to the Sun. 
Their size. Instances of large Spots visible to the naked eye. The Great Spot 
of October 1865. Their periodicity. Discovered by Schwabe. Table of hit 
results. Table of Wolfs results. Curious connexion between the periodicity of 
sun-spots and that of other physical phenomena. The Diurnal variation of the 
Magnetic Needle. Singular occurrence in September 1 859. Wolf's researches. 
Spots and Terrestrial Temperatures and Weather. Ballot's inquiry into Ter- 
restrial Temperatures. The Physical Nature of Spots. The Wihon-Herschel 
Theory. Luminosity of the Sun. Historical Notices. Scheiner. Facula. 
Luculi. Nasmyth's observations on the character of the Sun's Surface. 
Hugffins's conclusions. Present state of our knowledge of the Sun's constitu- 
tion. Tacchini's conclusions. 

TF there is one celestial object more than another which may 
be regarded as occupying the foremost place in the mind of 
the astronomer, it is the Sun: for, speaking generally, there 
is scarcely any branch of astronomical inquiry with which, 
directly or indirectly, the Sun is not in some way associated. 
It will be only appropriate therefore to deal with this important 



2 The Sun and Planets. [BOOK I. 

body at the very commencement of a treatise on Descriptive 
Astronomy*. 

By common consent, the mean distance of the centre of the 
Earth from the centre of the Sun is taken as the chief unit of 
astronomical measurement. 

The most approved method of determining the value of this 
was at one time believed to be by the aid of observations of 
transits of the planet Venus across the Sun b (as was first pointed 
out by Halley). The problem is, for various reasons, an 
intricate one in practice, but when solved places us in possession 
of the amount of the Sun's equatorial horizontal parallax ; in other 
words, gives us the angular measure of the Earth's equatorial 
semi-diameter as seen from the Sun's centre, the Earth being at 
its mean distance from the Sun. With this element given, it 
is not difficult to determine, by trigonometry, the Sun's distance, 
expressed in radii of the Earth ; reducible thereafter to miles. 

Encke, of Berlin, executed an able discussion of the observations 
of the transits of Venus in 1761 and 1769, and deduced 8-571" as 
the amount of the angle in question . From this it was found 
that the mean distance of the Earth from the Sun is 24065-1 
times the equatorial radius of the former (3963 miles), equal 
to 95,370,000 miles ; but these results, excellent as they were 
once thought to be, have long ceased to command the acceptance 
of astronomers, the fact being that modern experience has dis- 
credited Halley's method. 

At a meeting of the Royal Astronomical Society, on May 8, 
1857, Sir G. B. Airy proposed to adopt a suggestion of Flam- 
steed's d for determining the absolute dimensions of the solar 
system, founded upon observations of the displacement of Mars 
in Right Ascension, when it is far E. of the meridian and far 
W. of the meridian, as seen from a single observatory; such 

Every one who wishes thoroughly b See Book II. post. 

to "get up" the Sun should read Young's c Der Venusdurchgang von 1769, 

Sun. Secchi's magnificent work Le Soleil, p. 108. Gotha, 1824. Followed by later 

of which a second and much enlarged and better results in the Berlin Abhand- 

edition was published in 1875, must not lungen for 1835, P- 2 95- 

be forgotten. d Baily, Life of Flams'eed, p. 32. 



CHAP. I.] The Sun. 3 

observations to commence a fortnight before and to terminate 
a fortnight after the Opposition of the planet. In consequence 
of the great eccentricity of the orbit of Mars, this method is only 
applicable to those Oppositions during which the planet is nearly 
at its least possible distance from the Earth. Airy pointed out 
the several advantages of this method, viz. : that Mars may 
then be compared with stars throughout the night ; that it 
has 2 observable limbs, both admitting of good observation ; that 
it remains long in proximity to the Earth ; and that the nearer 
it is, the more extended are the hours of observation ; in all of 
which matters Mars offers advantages over Venus for observations 
of displacement in Right Ascension. Airy also entered into 
some considerations relative to certain of the forthcoming 
Oppositions, and named those of 1860, 1862, and 1877, as favour- 
able for determining the parallax in the manner he suggested 6 . 

Le Verrier announced in 1861 f that he could only reconcile 
discrepancies in the theories of Venus, the Earth, and Mars, by 
assuming the value of the solar parallax to be much greater than 
Encke's value of 8^57 i". He fixed 8-95" as its probable value, 
though, as Stone pointed out, this conclusion taken by itself 
rests on a not very solid foundation . 

The importance of a re-determination was thus rendered more 
and more obvious, and Ellery, of Williamstown, Victoria, suc- 
ceeded in obtaining a fine series of meridian observations of 
Mars, at its Opposition in the autumn of 1862, whilst a corre- 
sponding series was made at the Royal Observatory, Greenwich. 
These. were reduced by Stone, and the mean result h was a value 
of 8-932" for the solar parallax, with a probable error of only 
003 2". This result was singularly in accord with Le Verrier's 
theoretical deduction. Winnecke's comparison of the Pulkova 
and Cape observations of Mars yielded 8- 964". 

8 Month. Not., vol. xvii., pp. 208-21. vol. iv., p. 101. Paris, 1861. 

May, 1857. Some practical hints on the e Month. Not.,\ol. xivii., p. 241. April 

conduct of observations are given by A. 1867. 

Hall in Ast. Nach., vol. Ixviii., No. 1623, h Month. Not., vol. xxiii., p. 185, April 

Jan. 16, 1867. 1863. 

' Annales de V Observatoire Imperial, 

B 2 



4 The Sun dti</ Planet*. [BooK I. 

The Opposition of 1877 was observed under favourable circum- 
stances by Gill at the Island of Ascension, and his observations 
yielded as their final result a parallax of 8-78", with a probable 
error of 0-012". This implies a mean distance of the Earth from 
the Sun of 93,080,000 miles 1 . 

Thus, though there may be some uncertainty in the amount of 
the correction, there is no doubt that the Sun is nearer than was 
formerly considered to be the case. 

The distance amended to accord with a parallax of 8-8" is 
about 92,890,000 miles, with an error not likely much to exceed 
150.000 miles k . 

Hansen contributed something towards the elucidation of the 
matter. As far back as 1854 that distinguished mathematician 
expressed his belief that the received value of the solar parallax 
was too small, and in 1 863 he communicated to Sir G. B. Airy 
a new evaluation, derived from his Lunar theory by the agency 
of the co-efficient of the parallactic inequality. The result was 
8-9 1 59", a quantity fairly in accord with the other values set 
forth above 1 . 

Such is a brief statement of the circumstances which caused 
such special interest to attach to the transits of Venus which 
were to happen on December 8, 1874, and December 6, 1882: 
for it was supposed, that, all things considered, transits of 
Venus were most to be relied on for the purpose of ascertaining 
the amount of the Sun's parallax. The particular circumstances 
of the transits in question will come under notice hereafter. 
Meanwhile it may be stated that Stone has deduced 8-823" 
as the general result of all the British observations of the 

1 Mem., R. A. S. xlvi., p. I, 1881 : put as the measurement of a ball one 

Month. Not., vol. xli., p. 323. April 1881. foot in diameter seen from a station nearly 

k C. A. Young in Sid. Mess., vol. vi., 4-4 miles distant from the ball. Unless 

p. n, Jan. 1887. the observer can "determine the diameter 

1 Month. Not., vol .xxiv., p. 8. Nov. of the ball so that he shall not be un- 

1863. The amount of the correction certain in his measure to the amount of 

to Encke's determination is about equal 0-03 of an inch, his work will not add 

to the apparent breadth of a human hair anything useful to present knowledge." 

seen from a distance of 125", or that of (Sid. Mesg., vol. vii., p. 101, March 
a sovereign at a distance of 8 miles. The 
whole amount of the parallax has been 



CHAP. I ] Th,e Sun. 5 

1882 transit. The Brazilian result by Wolf and Andre is 
8-808". 

It is almost needless to add that the acceptance of a new 
value for the solar parallax necessitates the recomputation of all 
numerical quantities involving the Sun's distance as a unit. 

The real mean distance of the Earth from the Sun being 
ascertained, it is not difficult to determine by trigonometry the 
true diameter of the latter body, its apparent diameter being 
known from observation m ; and, as the most reliable results 
show that the Sun at mean distance subtends an angle of 
32' 3-6", it follows that (assuming, as above, a parallax of 8-8") its 
actual diameter is 866,200 miles. It is generally accepted that 
there is no visible compression. The surface of this enormous 
globe therefore exceeds that of the Earth 1 1 ,900 times, whilst 
the volume is i ,306,000 times greater ; since the surfaces of two 
spheres are to each other as the squares of their diameters, and 
the volumes as the cubes. 

The linear value of i" of arc at the mean distance of the Sun 
is about 450 miles. 

The Sun's mass, and consequently its attractive power, is 
332,260 times that of the Earth, and (approximately) is 749 times 
the masses of all the planets put together. 

By comparing the volumes of the Sun and the Earth and 
bringing in the value of their masses, we obtain the relative 
specific gravity or density of the two. 

The Sun's volume is to that of the Earth in the ratio of 
1,306,000 to i ; the Sun's mass is to the Earth's in the lesser 
ratio of 332,260 to i. Therefore the density of the Sun is to 
the density of the Earth as 332,260 to 1,331,570, or approxi- 
mately as i to 4. Then taking Baily's value of the density of 
the Earth (5-67 times that of water), the density of the Sun is 
i -42 times that of water. 

Some interesting points may conveniently be noted here re- 

m Lindenau in 1809 and Secchi in 1872 to periodical change, but thoee ideas met 
propounded some strange ideas about the with no favour. (Auwers in Month. Not., 
visible diameter of the Sun being subject vol. xxxiv., p. 22, Nov. 1873.) 



6 The Sun and Planets. [BOOK I. 

specting the consequences which result from the stupendous 
magnitude and mass of the Sun. At the surface of the Earth 
a body set free in space falls i6a ft in the first second of time, 
with a velocity increasing during each succeeding second. A 
body similarly set free at the surface of the Sun would start with 
a velocity 27-4 times as great as that of a body falling at the 
surface of the Earth. This is equivalent to saying that a pound 
weight of anything on the Earth would, if removed to the Sun, 
weigh more than 27 lb . Liais has pointed out a singular conse- 
quence of this fact: "An artillery projectile would have on the 
Sun but very little movement. It would describe a path of 
great curvature, and would touch the surface of the Sun a few 
yards from the cannon's mouth." The centrifugal force due to the 
rotation of any body diminishes gravity at its surface. At the 
Earth's equator the total diminution is ^1^ pait ; whilst at the 
Sun's equator the centrifugal force is only about i^J^ P ar t f 
the force of gravity. It would be necessary that the Sun should 
turn on its axis 133 times quicker than it does, for the force of 
gravity to be neutralised. In the case of the Eaith, however, 
a speed of rotation 17 times as great as it is would suffice to 
produce the same result. The insignificance of centrifugal force 
at the Sun's equator, compared with the amount of the force of 
gravity, suffices to explain the absence of appreciable polar com- 
pression in the case of the Sun's disc. 

A consideration of the comparative lightness of the matter 
composing the Sun led Sir J. Herschel to think it "highly 
probable that an intense heat prevails in its interior, by which 
its elasticity is reinforced, and rendered capable of resisting [the] 
almost inconceivable pressure [due to its intrinsic gravitation] 
without collapsing into smaller dimensions n ." That the internal 
pressure exerted by the gases imprisoned within the luminous sur- 
face or photosphere of the Sun, must be absolutely stupendous, we 
have evidence of in the fact of the almost inconceivable velocity 
(100 to 200 miles per second) of the uprushes of incandescent gas 
and metallic vapours, which are almost constantly taking place 

n Outlines of Ast., p. 297. 



CHAP. I.] The Sun. 7 

at various parts of its surface. It would seem all but certain that 
the Sun is nearly wholly gaseous, and that its photosphere con- 
sists of incandescent clouds, in which the aqueous vapour of our 
terrestrial clouds is replaced by the vapours of metals. These 
considerations, however, introduce a difficulty of a precisely 
opposite character to that which Sir J. Herschel essayed to 
combat ; inasmuch as, in the light of our present knowledge, 
it seems hard to conceive how a mere shell of metallic vapour 
should be able to confine gases at the incomprehensible pressure 
at which those which rush out in the form of the now well- 
known "Red Flames" (see_po#f) must be confined. 

The Sun is to be regarded as a fixed body so far as we are con- 
cerned ; when therefore we say that the Sun " rises," or the Sun 
" sets," or the Sun moves through the signs of the zodiac once 
a year, we are stating only a conventional truth ; it is we that 
move and not the Sun, the apparent motion of the latter 
being an optical illusion. 

The Sun is a sphere, and is surrounded by an extensive and 
rare atmosphere ; it is self-luminous, emitting light and heat 
which are transmitted certainly beyond the planet Neptune, and 
therefore more than 2700 millions of miles. Of the Sun's heat, 
it has been calculated that only assiTnjTTTTTnr P ftr ^ reaches us , 
so that what the whole amount of it must be it passes human 
comprehension to conceive : like many other things in science. 
Our annual share would be sufficient to melt a layer of ice all 
over the Earth ioo ft in thickness, or to heat an ocean of fresh 
water 6o ft deep from 32 F. to 212 F., according to Herschel 
and Pouillet p . Another calculation determines the direct light 
of the Sun to be equal to that of 5563 wax candles of moderate 
size, supposed to be placed at a distance of one foot from the 

Ganot, Physics, p. 391, 7th Eng. ed. face, had their clothes burnt by coming 

1875. This was calculated on the old under the focus of the convex lenses 

value of the solar parallax. placed in the bell to let in the light. 

P To show the great power of the And houses have been set on fire by the 

calorific rays of the Sun, it may be men- Sun's rays. Langley puts the thickness 

tioned that in constructing the Plymouth of the layer of ice which could be melted 

Breakwater, the men, working in diving at i6o ft . (New Ast., p. 95.) 
bells, at a distance of 30" below the sur- 



8 



The Sun and Planets. 



[BOOK I. 



Fig. 3- 



observer. The light of the Moon being probably equal to that 
of only one candle at a distance of i2 ft , it follows, according to 
Wollaston, that the light of the Sun is 801,072 times that of the 
Moon. Zb'llner's ratio is 6 1 8,000 to i , and Bouguer's 300,000 to i . 
But all these results rest on a very weak foundation. 

If we represent the luminous surface of the Sun when the 
Earth is at its mean distance, by 1000, the numbers 967 and 

1035 will represent 
the same surface as 
it appears to us when 
the Earth is in Aphe- 
lion (July) and Peri- 
helion (January) re- 
spectively. 

When telescopically 
examined, there may 
frequently be seen 
in the equatorial re- 
gions of the Sun dark 
spots' 1 or macula*, each 
usually surrounded by 
a fringe of a lighter 
shade, called a penum- 
bra*, the two not passing into each other by gradations of tint, 
but abruptly. In the few cases in which a gradual shading has 




GENEKAL TELESCOPIC APPEARANCE OF THE SUN. 



* It will appear from what is stated 
further on that the familiar term "spot" 
is merely a conventional one used to con- 
vey a general idea of what is seen on 
viewing the Sun. In no precise sense 
are "spots on the Sun " truly " spots." 

r Lat. macula, a blemish. Dawes up- 
held a further classification : he applied 
to the ordinary black central portions 
the term umbra (shadow), on the highly 
probable ground that the blackness is 
mainly relative. Patches of deeper black- 
ness are occasionally noticed in the 
umbrae ; Dawes limited to these the 
designation nucleus, sometimes indiscri- 
minately applied to all the blackish area. 



This classification is adopted in the text. 
Mr. Langley of the Alleghany Observa- 
tory, however, viewing spots with the 
13-inch Equatorial of that institution, 
and a polarising eye-piece (which admits 
of the employment of the whole aperture), 
sees that the umbral structure is quite 
complex, and made up of sunken banks 
of " filaments " (see post). He further 
perceives that the nucleus which Dawes 
spoke of as "intensely black," is not 
black at all, nor even dark (save rela- 
tively), but is brilliant with a violet- 
purple light. (Month. Not., vol. xxxiv. 
p. 259. March 1874.) 

Pene, almost ; and umbra, a shadow. 



CHAP. I.] The Sun. 9 

been noticed, Sir J. Herschel believed that the circumstance 
may be ascribed to an optical illusion, arising from imperfect 
definition on the retina of the observer's eye. It is not how- 
ever always the case that each spot has a penumbra to itself, 
several spots being occasionally included in one penumbra. 
And it may further be remarked that cases of an umbra with- 
out a penumbra, and the contrary, are on record. Umbrae 
without penumbrae are exceptional, and may be considered as 
closely related to physical changes just commencing or termina- 
ting. A marked contrast subsists in all cases between the 
luminosity of the penumbra and that of the general surface 
of the Sun contiguous. Towards their exterior edges penumbrae 
aie (by contrast) usually darker than nearer the centre. They 
are frequently very irregular in their outlines (though often 
they conform somewhat closely to the general contour of the 
umbrae which they circumscribe), but the umbrae, especially 
in the larger spots, are frequently of regular form (compara- 
tively speaking, of course) ; and the nuclei of the umbrae still 
more noticeably exhibit a compactness of outline. 

Spots are for the most part confined to a zone extending 35, 
or so,, on each side of the solar equator, and are neither per- 
manent in their form nor stationary * in their position, frequently 
appearing and disappearing with great suddenness. 

The multitude of facts concerning them, accumulated from the 
journals of many observers extending over long periods of years, 
is so great as to bewilder one, and to marshal these in a suitable 
manner is a task of extreme difficulty : and howsoever per- 
formed it is certain that much will have been left out that 
might with advantage have been inserted. 

The general limits in latitude of the spots may be stated, as 
above, at 35, but instances of spots seen beyond these limits are 
on record. In 1871, B. Stewart saw one 43 distant from the 
solar equator; in 1858, Carrington one 44 53'; in 1826, Capocci 
one 46 ; in 1846, C. H. F. Peters one 50 55' ; and La Hire, in the 

* This is not said merely in view of the Sun's rotation ; spots sometimes possess 
an absolute motion of their own. 



10 The Sun and Planets. [BOOK I. 

last century, is said to have seen one in latitude 70. They are 
often confined to two belts on either side of the Sun's equator, 
being frequently absent from the equatorial regions except at 
particular epochs : from 8 to 20 is their most frequent range, 
or to be more precise still, their favourite latitude is 17 or 
18. They are often more numerous and of a greater general 
size in the Northern hemisphere; the zone between 11 and 
15 north is particularly noted for large and enduring spots. 
A gregarious tendency is very obvious, and where the groups 
are very straggling, the longer line joining extreme ends will 
pretty generally be found to be more or less parallel to the 
equator, and not only so, but extending across nearly the whole 
of the visible disc. 

Sir John Herschel remarked : " These circumstances .... 
point evidently to physical peculiarities in certain parts of the 
Sun's body more favourable than in others to the production of 
the spots, on the one hand ; and on the other, to a general 
influence of its rotation on its axis, as a determining cause in 
their distribution and arrangement, and would appear indicative 
of a system of movements in the fluids which constitute its 
luminous surface ; bearing no remote analogy to our trade- 
winds from whatever cause arising u ." In reference to the 
distribution in latitude of the spots, the observations of Carring- 
ton have placed us in possession of some important facts. That 
observer found that as the epoch of minimum approached, the 
spots manifested a very distinct tendency to advance towards 
the equatorial regions, deserting to a great extent their previous 
haunts above the parallels of 20 or so. After the minimum 
epoch had passed, a sudden and marked change set in, the 
equatorial regions becoming almost deserted by the spots, which 
on their reappearance showed themselves chiefly in parallels 
higher than 20. Wolf finds that the observations of Bohm 
reveal the fact that the same peculiarity was noticed by that 
observer in the years 1 833-6 v . Sir John Herschel remarked 
that if this should prove to be a general rule, " it cannot but 

Outlines ofAst., p. 251. v Month. Not., vol. xix., p. 325, July 1859. 



Figs. 4-6. 



Plate II. 




1826: September 29. (Capocci.} 




1861: May 21. (Birt.) 




1861 : May 27. (Anon.} 



SPOTS ON THE SUN. 



CHAP. I.] The Sun. 13 

stand in immediate and most important connexion with the 
periodicity itself, as well as with the physical process in which 
the spots originate." 

Confirming Carrington's results in a great measure, Sporer, 
who has devoted many years to assiduous observation of the Sun, 
finds that between the time of one minimum and another the 
region of greatest frequency gradually drifts downward from the 
zone of 30 25 of latitude to the immediate neighbourhood of 
the equator ; and that at the time of maximum its seat lies in 
about 17 or 18. As the next minimum approaches, spots more 
than 15 from the equator become more rare than spots of 35 
and upwards were at the time of maximum. But directly the 
minimum is past, spots begin to appear again in those higher 
latitudes where but very few have been seen for several years. 
This sudden transfer of the seat of energy from a zone where it 
has been manifested year after year to another and distant zone 
where pothing has been going on for a long time previously, is a 
remarkable fact, the import of which cannot at present be 
explained w . 

The duration of individual spots is a matter associated with 
extremes both ways. Some remain visible for several months, 
others scarcely for as many minutes ; but a few days or weeks 
will commonly be found the usual extent of permanency. Some 
are formed and vanish during the period of a single semi-rotation 
(rather more than I2| d ), others remain during several successive 
rotations ; for it will be readily understood that the Sun, being 
endued with an axial rotation, and the spots being fixed (or 
nearly so) on the Sun's surface, it will not be possible for any 
one spot to remain in sight continuously for longer than half the 
duration of the Sun's rotation. 

With respect to the distribution of spots in longitude there is 
little to be said, for it does not certainly appear that they have 
a preference for any one longitude more than another. Never- 
theless Kirkwood believes that this statement so far needs 

w Ast. Nach., vol. cvii., No. 2565. Dec. 31, 1883. See also V Astronomic, vol. i., 
p. 70, April 1882. 



14 The Sun and Planets. [BOOK I. 

modification that there is one particular longitude in which 
planetary influences (see post) are specially effective. Sporer also 
seems to think that there are special localities of disturbance. 

When observed for any length of time, a spot will first be 
noticed on the Eastern limb, disappearing in little less than a 
fortnight on the Western limb ; after an interval of nearly 
another fortnight, the spot, if still in existence, will reappear on 
the Eastern side, and in like manner traverse the disc as before. 
This phenomenon necessarily can only be accounted for on the 
supposition that the Sun rotates on its axis ; and observations 
specially conducted with that object in view will give the 
period of this rotation, which Laugier fixed at 2j d 8 h io m ; 
Carrington at 25 d 9 h 7 m ; and Sporer at 25 d 5 h 31 m results fairly 
in accord with Bianchini's determination of 25 d 7 h 48*, deduced 
in 1718, when the difficulties attending the observations due to 
the ever- varying forms and actual proper motions of the spots 
are taken into consideration. 

The entire period required by a spot to make a whole visual 
rotation (27 d 7 h ) is greater than that of the Sun's actual 
rotation, owing to the Earth's progressive movement in its 
orbit. 

On February 19, 1800, Sir W. Herschel was watching a group, 
but after looking away for a single moment, he could not 
find it again 1 . The same observer followed a spot, in 1779, 
for 6 months ; and, in 1840 and 1841, Schwabe observed one and 
the same group to return 1 8 times, though not in 1 8 consecu- 
tive rotations of the Sun y . 

In July, August, and September 1859, a large group was 
followed through several apparitions, and another very notice- 
able instance of the kind occurred in the autumn of 1865. 
Similar cases are by no means very rare. It has been sur- 
mised, and Sir J. Herschel thought " with considerable apparent 
probability," that some spots at least are generated again and 
again, at distant intervals of time, over the same identical 

1 Phil Trans., vol. xci., p. 293. 1801. 

* Ast. Nach., vol. xviii. No. 418, March 18, 1841. 



CHAP. I.] The Sun, 15 

points of the Sun's body. There appears to be some evidence 
to bear out this hypothesis 2 . Webb says: "Fritsch stated 
that he saw one stand nearly still for 3 days ; and Lowe 
that he even witnessed retrogradation but these assertions 
involve a suspicion of mistake. Schrb'ter and others have 
ascribed to them a more moderate locomotion. This was 
micrometically established in a lateral direction by Challis in 
1857 ; and Carrington has subsequently made known his very 
interesting discovery, that there appear to be currents in the 
photosphere, drifting the equatorial spots forward in comparison 
with those nearer to the poles, with deviations in latitude of 
smaller amount : the neutral line as to both these drifts lying in 
about 15 of latitude. With these shifting landmarks, it is not 
surprising that the Sun's period of rotation is still doubtful. 
. . . Hewlett and several others have found that spots near the 
limb require a different focus from those in the centre ; arising, 
no doubt, as Dawes says, from the effect on the retina of very 
different degrees of brightness*." According to Maunder a 
relative displacement amongst the members of the same group 
amounting to 7000 miles a day is not unusual. 

With respect to proper motion, Carrington found that most 
spots have an independent proper motion of their own (hence 
uncertainties in conclusions respecting the duration of the Sun's 
rotation), and not only so, but that the proper motion of spots 
varies systematically with the latitudes of the spots. 

The varying position of the Earth with reference to the Sun, 
combined with the inclination of the axis of the latter to the 
plane of the ecliptic (amounting to 82 45' according to Car- 
rington ; to 83 3' according to Sporer b ), gives rise to the fact 
that at no two periods of the year do the spots appear to traverse 
the Sun's disc exactly in the same way. About June 5 and 
December 6 the Earth is in the line of nodes of the spots or, in 

* Sir John seems afterwards to have b The longitude of the ascending node 
changed his opinion. In a Memoir in the for 1850 was 73 40' ; BO that the North 
Quart. Jour. Sc., vol. i. p. 225, April 1864, pole of the Sun's axis points nearly to it 
he says exactly the reverse. Draconis, and the South one to o Trian- 

* Celest. Objects, p. 33. (3rd ed.) guli Australia. 



The Sun and Planets. 



[BOOK I. 



other words, its longitude, as seen from the Sun, corresponds 
nearly with the points of intersection of the solar equator and 
the ecliptic and the paths of the spots are then inclined straight 
lines. In March the South pole is turned towards us, and the 
tracks are concave towards the South ; in September the condi- 
tions are precisely reversed in every respect, the North pole is 



Fig. 7. 




PATHS OF SUN SPOTS AT DIFFERENT TIMES OF THE YEAR. 

turned towards us and the tracks are concave towards the North ; 
at other intermediate periods (not being very near to June 5 or 
December 6) the paths are both inclined and curved at the same 
time. 

Individual spots also possess many peculiarities of their own. 
Dawes observed one on January 17, 1852, which, by the 23rd 
of that month, had rotated in its own plane through 90. Birt 



CHAP. I.] The Sun. 17 

believed that the same thing happened with a spot which he scru- 
tinised in February and March 1 859. Schwabe saw occasionally 
spots of a reddish-brown colour, under circumstances of contrast 
precluding the possibility of deception ; on one occasion 3 tele- 
scopes and several bystanders certified to this. In 1826, Capocci 
perceived a violet haze issuing from each side of the bright central 
streak of a great double umbra ; and during the eclipse of March 
15, 1858, Secchi saw a rose-coloured promontory in a spot visible 
to the naked eye. On April 24, 1886, Hopkins saw a spot with 
4 umbrae, 2 of which were black and 2 reddish-brown. The colour 
was very marked and was visible in different eyepieces, and a 
bystander confirmed the observation. The colour disappeared 
in 20 minutes after the observation was commenced d . Schwabe 
described the penumbrse as made up of a multitude of black dots, 
usually radiating in straight lines from the umbra ; Secchi with 
greater optical power, defined these radiations to be alternate 
streaks of the bright light of the photosphere and dark veins 
converging to the umbra 6 . 

Some Sun-spots are of such prodigious size, as to be visible 
to the naked eye. A few recent instances are here given. A 
spot measured by Pastorff on May 24, 1828, was computed to have 
an area about 4 times the entire surface of the Earth. In 
June 1843, Schwabe observed one 2' 47", or 75,000 miles in dia- 
meter. It was seen for an entire week without the aid of a 
telescope. On March 15, 1858, the day of the celebrated eclipse, 
a spot having a breadth from W. to E. of 4', or 108,000 miles, 
attracted considerable attention. On September 30, in the same 
year, one having a breadth from W. to E. of 5' 21", or 144,450 
miles, was observed f . On January 26, 1859, and during August 
1859, large spots were seen; one visible in the latter month 
measured nearly 58,000 miles, according to Newall, who saw it 
distinctly as a notch on the edge of the Sun's disc, the like of 

c Month. Not., vol. xix., p. 182, March authority of Webb, Celest. Objects, p. 25. 
1859. He gives no reference, so I am unable to 

d Month. Not., vol. xlvi., p. 393, May verify them. 
1886. ' Ast. Nach., vol. 1., No. 1 182, Feb. 25, 

e The preceding facts are given on the 1 859. 

C 



18 



Tlie Sun and Planets. 



[BOOK I. 



which he had only seen once before namely, on March 25, 1850*. 
During April and May 1870, and April 1882, several large spots, 

Fig. 8. 




GREAT SUN-SPOT VISIBLE ON JUNE 30, 1883. {HicCO.} 

easy to be seen by the naked eye, were visible. The last-named 
had on April 19 a length of 2' 15" and a breadth of i' 15". 

Fig. 9. 




THE SAME SUN-SPOT OX JULY 2, 1883. 



g Letter in the Times, Aug. 27, 1859. 
" An indentation on a globe will dis- 
appear in profile unless its breadth and 
depth are considerable : hence such ob- 
servations would be rare, but they are 
recorded by La Hire, 1 703 ; Cassini, 



1719; W. Herschel, 1800; Dollond and 
others, 1846 ; Lowe, 1849 '> Newall, 1850, 
1859 ; Observers at Kew and Dessau, 
1868." Webb, Celest. Objects, p. 28 
(n.). Of late years Indentations have 
been often recorded in photographs. 



CHAP. I.] 



19 



Violent magnetic storms accompanied its appearance. These 
storms continued from April 14 to April 2O h . 

Fig. 10. 







GREAT SUN-SPOT VISIBLE ON JULY 25, 1883. (JBlCCO.) 

Figs. 8 ii represent 2 large and important spots observed 
during the summer of 1883 by M. Ricco at Palermo. Their 

Fig. ii. 




THE SAME SUN-SPOT ON JULY 27, 1883. (B'-CCO.) 



" Hewlett, Month. Not., vol. xlii. p. 356, May 1882. 
C 2 



20 The Sun and Planets. [BOOK I. 

size relatively to the Earth will be realised generally by com- 
paring them with the shaded ball in the corner of each sketch 
marked " La Terre." 

One of the most interesting large spots ever subjected to careful 
scrutiny was that which was conspicuously visible in October 
1865. Many elaborate observations of it were made by astro- 
nomers, and a series of drawings by the Rev. F. Hewlett are well 
known. 1 here present copies of drawings by Brodie, exhibited 
at the Royal Astronomical Society 1 , which will be useful for com- 
parison with Hewlett's. Brodie furnished me with the following 
revised transcript of his notes : 

"OCTOBEB ii, 1865. The definition was fine enough to allow this spot to be 
examined with a power of 470 on an equatorial telescope of 8^ in. aperture and 
ii-J- ft. long. The shape of the spot was tolerably rectangular, the umbra being 
about 18,000 miles long and 9700 miles wide, or in measures of arc 41 -3" long 
and 22 - 3" wide. The penumbra 86'9" long and 73*5" wide. There was an exceed- 
ingly long promontory of luminous matter projecting over the umbra from one end 
of the spot, and running tolerably parallel to the side. Near the end of this promon- 
tory was an elongated portion of detached luminous matter of similar shape to that of 
the promontory itself, about 4000 miles long [see Plate III. Fig. 12]. This portion 
had elongated itself in a remarkable manner in the previous 15 minutes, for when first 
observed it was not more than 3000 miles long. The long promontory seemed 
drifting towards the penumbra, while the detached portion was moving rather away 
from it, indicating a cyclonic action of the forces in operation. 

" About i^ hours later I found that the detached portion of luminous matter had 
formed a junction with the long promontory [see Plate III. Fig. 13]. That side of 
the umbra opposite to this promontory was covered with a sort of ' mackerel sky ' 
formation of misty luminous matter, which extended more or less marked over the 
whole portion of the umbra. The Hack nucleus of the umbra first noticed by 
Mr. W. R. Dawes, as generally to be seen in spots, was absent in this umbra. This 
misty cloud-like appearance of the umbra can only be seen with large telescopes ; it 
seems to be formed by the nodules of luminous matter that break off from the 
pectinations which fringe the whole of the edge of the umbra ; these soon after 
become more and more diffused, until they become a sort of cloudy stratum floating 
over the umbra. These nodules invariably drift from the edge of the penumbra 
towards the centre of the umbra, which would seem to indicate a downward rush of 
gases from the surface of the sun. On October 1 2th there were five of these nodules, 
that had broken off from the ends of the small promontories or pectinations at the 
edge of the penumbra and had begun to drift on to the umbra, while one had not 
quite broken away, but was preparing to do so [see Fig. 18]. There was now also 
another change on the umbra at the end of the long promontory ; the misty cloud-like 
masses of luminous matter began to form into bridge-like formations [see Plate III] 

1 Month. Not., vol. xxvi., ]>. 21, Nov. 1865. 



Flys. 12-17. 



Plate III. 




October n ; n a.m. 




October n ; 12.30 p.m. 







October 1 2 ; 9.30 a.m. 




October 12 ; 10.30 a.m. 




October 12 ; 12.30 p.m. 




October 12; 2.30 p.m. 



THE GREAT SUN-SPOT OP OCTOBER 1865. 

(Drawn by Brodie.} 



CHAP. I.] 



The Sun. 



23 



Fig. 13] ; but these formations were not nearly so bright and defined as the long 
portion of the promontory : there was also another shorter promontory formed on the 
opposite side to that of the long one, or it might be termed an extreme lengthening 
of one of the pectinations. The rapidity of change in all parts of the umbra was re- 
markable, the cloudy strata seeming to condense and diffuse very similar to our earth 
clouds on a summer's day. 

" OCTOBER is. The shape of the umbra was very greatly altered, and its size was 
much increased. [See Plate III. Fig. 14]. Its length was nearly 29,000 miles, with 
a width in the greatest part of 10,400 miles, or 6^2" of arc long, and 23-6" wide, 
the penumbra being 50,000 miles long and 34,000 broad. The long promontory 
of yesterday had quite disappeared, and there was another formed at the opposite end 
of the spot of a serpentine form ; this was observed at 9.30 A.M. Within an hour 
another change took place, and at 10.30 this long serpentine promontory had broken 

Fig. i 8. 




THE GREAT SDK-SPOT OP OCTOBER 1865. PECTINATED EDGE VISIBLE ON 

OCTOBER 12. (Brodie.) 

into two portions, the shorter end floating on the penumbra. [See Fig. 15]. At 
12.30 P.M. the one end of that portion that had broken off had bodily floated towards 
the penumbra, and formed a junction, as seen in Fig. 16. At 2.30 P.M. the spot was 
again observed, and the portion originally broken off from the serpentine promontory 
of the morning had formed a complete bridge across the umbra, [see Fig. 17], while 
the part from which it was broken had bent round, forming nearly a semicircle. The 
outline of the spot did not seem to change perceptibly. The figure of the spot was 
thrown by the telescope on to a board and sketched from its own image. 

" OCTOBER 13. The shape of the spot slightly altered only, but the bridge across 
had quite disappeared, while the semicircular promontory had formed a junction with 
the penumbra." 

Schwabe said that good eyes would detect without optical aid 
any spots more than 50" in diameter, but this is very doubtful. 



24 The Sun and Planets. [BOOK I. 

Probably the minimum limit must be fixed in general at i' or 
even more. 

"The origin of a spot, when it can be observed, is usually 
traceable to some of those minute pores or dots which stipple the 
Sun's surface, and which begin to increase, to assume an umbral 
blackness, and acquire a visible and, at first, very irregular and 
changeable shape. It is not till it has attained some measur- 
able size that a penumbra begins to be formed a circumstance 
strongly favouring the origination of the spot in a disturbance 
from below, upward ; vice versa, as the spots decay they become 
bridged across, the umbrae divide, diminish in size, and close up, 
leaving the penumbne, which, by degrees, also contract and 
disappear. The evanescence of a spot is usually more gradual 
than its formation. According to Professor Peters and Mr. 
Carrington, neighbouring groups of spots show a tendency to 
recede from one another k ." And not only so, but neighbouring 
spots in the same group show the same tendency, particularly in 
longitude. The relative drift of members of the same group 
is far more noticeable than the relative drift of different 
groups. 

The most casual observer can hardly fail to be struck with 
the rapidity of the changes which take place in solar spots. 
Dr. Wollaston says: "Once I saw, with a 1 2-inch reflector, a 
spot burst to pieces while I was looking at it. I could not 
expect such an event, and therefore cannot be certain of the 
exact particulars ; but the appearance, as it struck me at the 
time, was lite that of a piece of ice when dashed on a frozen 
pond, which breaks in pieces, and slides on the surface in various 
directions. I was then a very young astronomer, but I think I 
may be sure of the fact." Their immense number is likewise 
very noticeable. On April 26, 1 846, Schmidt at Bonn counted 
upwards of 200 sing'le spots and points in one of the large groups 
then visible, and 180 in another cluster, in August 1845. On 
August 23, 1 86 1, 1 counted 70 distinct spots with a telescope of 
only 3 inches aperture charged with a power of 2 1 . Schwabe 

k Sir J.Hersehel, in Quart. Journ.Sc., vol. i. p. 225. April 1864. 



CHAP. I.] The Sun. 25 

found that the Western members of a group disappear first, and 
that at the Eastern end fresh ones are apt to form, where also 
the junior members are most numerous ; that the small points 
are usually arranged in pairs (much after the appearance of the 
" Dumb-Bell" Nebula); and that, when near the edge of the Sun, 
the penumbrse are much brighter on the side next the limb. 
Sir J. Herschel often noted the penumbrse to be least defined 
on the preceding side ; and Capocci found the principal spot of 
a group usually the leader. The same observer believed the 
umbrae to be better defined in their increase than in their 
decrease. The leader is usually the most black, symmetrical, 
and enduring of the group, according to Chacornac. 

Maunder disagreeing with Schwabe (as above) says that the 
leader of a group of spots, i.e. the most Westerly one, is the 
darkest and most enduring. He notes that the groups first begin 
to waste in the central members : then the Eastern members 
perish ; and last of all the Western members. 

Attention has now to be directed to one of the most curious 
and interesting discoveries of modern astronomy the periodicity 
of the solar spots. Schwabe, of Dessau, the hero of this l , shall 
be introduced to the reader in the words of the late Mr. M. J. 
Johnson, when, as President of the Royal Astronomical Society, 
he spoke on the award to him of the Society's Gold Medal in 
1857:- 

" What the Council wish most emphatically to express is their admiration of the 
indomitable zeal and untiring energy which he has displayed in bringing that 
research to a successful issue. Twelve years, as I have said, he spent to satisfy 
himself ; six more years to satisfy, and still thirteen more to convince, mankind. For 
thirty years never has the Sun exhibited his disc above the horizon of Dessau without 
being confronted by Schwabe's imperturbable telescope, and that appears to have 
happened, on an average, about 300 days a year. So, supposing that he observed 
but once a day, he has made 9000 observations, in the course of which he discovered 
4700 groups. This is, I believe, an instance of devoted persistence (if the word were 
not equivocal, I should say pertinacity) unsurpassed in the annals of astronomy. 
The energy of one man has revealed a phenomenon that had eluded even the sus- 
picion of astronomers for 200 years ." 

1 Wolf has pointed out that Chris- to a periodicity. (R. Wolf, GeschicMe 
tian Horrebow first suggested the idea dcr Astronomie, p. 654.) 
that the spots on the Sun were subject Month. Not.,\ol.xvn.p. 129. Feb. 1875. 



26 



The Sun and Planets. 



TABLE OF SCHWABE'S RESULTS n . 



Year. 


Days of 
Observation. 


Days of no 
Spots. 


New Groups. 


Mean diurnal 
Variation in 
Decimation of 
the Magnetic 
Needle. 


1826 


277 


22 


118 


9'-75 


1827 


2 73 


2 


161 


"33 


1828 


282 


O 


225 


11-38 


1829 


244 


O 


199 


14.74 


1830 


217 


I 


190 


12-13 


1831 


*39 


3 


149 


12*22 


1832 


270 


49 


84 




1833 


247 


139 


33 




1834 


273 


120 


5 




1835 


244 


18 


173 


9-57 


I8 3 6 


200 


O 


272 


J2-34 


1837 


168 


O 


333 


12-27 


1838 


202 


O 


282 


12-74 


1839 


205 





162 


11-03 


1840 


263 


3 


152 


9.91 


1841 


283 


15 


IO2 


7-82 


1842 


37 


64 


68 


7-08 


1843 


312 


149 


34 


7-i5 


1844 


321 


in 


52 


6-61 


1845 


332 


2 9 


114 


8-13 


1846 


314 


i 


'57 


8-81 


1847 


276 


O 


257 


9-55 


1848 


278 


O 


33 


11-15 


1849 


285 


O 


238 


10-64 


1850 


308 


2 


1 86 


1044 


1851 


3 08 


O 


J 5i 


8.32 


1852 


337 


2 


125 


8-09 


1853 


299 


3 


9i 


7-09 


1854 


334 


65 


67 


6-81 


1855 


313 


146 


79 


6-41 


1856 


321 


193 


34 


5-98 


1857 


3 2 4 


52 


98 


6-95 


1858 


335 


o 


188 


7.41 


1859 


343 





205 


xo-37 


i860 


332 


o 


211 


10-05 


1861 


322 


o 


204 


9.17 


1862 


37 


3 


1 60 


8-59 


1863 


33 


2 


124 


8-84 


1864 


325 


4 


130 


8-02 


1865 


307 


25 


93 


8-14 


1866 


349 


76 


45 


7-65 


1867 


312 


195 


25 


7.09 


1868 


301 


23 


101 


8-15 



Schwabe's observations, as published, end with 1868. The 
thread is not however absolutely broken, for Wolf had previously 



n Month. Not., vol. xvi., p. 63. Jan. 1856. Continued to 1868. 



CHAP. I.] 



The Sun. 



27 



started a series of his own. A table of his results, as prepared 
by himself for this work, at my request, is subjoined : 



Year. 


Days of 
Observation. 

* 


Days of no 
Spots. 


Relative 
Number. 


Mean diurnal variation in Mag- 
netic Declination at Prague. 


Observed. 


Calculated. 


1849 


313 


O 


95-9 


10-27 


IO-2I 


1850 


325 


7 


66-5 


9-97 


8-88 


1851 


3" 


o 


6 4-5 


8-32 


8-79 


1852 


322 


4 


54- 2 


8-09 


8-33 


1853 


332 


6 


39- 


7-09 


7-64 


1854 


348 


67 


2O-6 


6-Si 


6-82 


1855 


35 2 


223 


6-7 


6-41 


6-19 


1856 


356 


256 


4-3 


.r98 


6-08 


1857 


363 


70 22-8 


6-95 


6-92 


1858 


335 


2 54-8 


7.41 


8.36 


1859 


334 


o 93-8 


10-37 


1O-II 


1860 


363 


o 95-7 


10-05 


10-20 


1861 


364 


2 77-2 


9-17 


9-36 


1862 


359 


4 59- r 


8-59 


8-55 


1863 


361 


2 


44-0 


8-84 


7-87 


1864 


352 


7 


46-9 


8-02 


8-00 


1865 


361 


42 


3-5 


7-80 


7.26 


1866 


363 


85 


16-3 


6-63 


6-62 


1867 


360 


219 


7-3 


6-47 


622 


1868 


351 


37 


37-3 


7-27 


7-57 


1869 


34i 


2 


73-9 


9-44 


9.22 


1870 


354 


O 


J39- 1 


11.47 


12-15 


1871 


363 





III- 2 


1 1-60 


10-89 


1872 


365 





101-7 


10-70 


10.47 


1873 


363 


*4 


66-3 


9-5 


8-87 


1874 


363 


12 


44.6 


7.98 


7.90 


1875 


365 


132 


17.1 


6-73 


666 


1876 


366 


189 


"3 


6.47 


6-40 


1877 


365 


142 


123 


5-95 


6.44 


1878 


365 


28l 


3-4 


5-65 


6-04 


1879 


36.5 


217 


6-0 


5-99 


6-16 


1880 


366 


33 


32-3 


6-85 


7-34 


1881 


365 


5 


54-2 


7.90 


8-33 


1882 


365 


o 


59' 6 


7.92 


8-57 


1883 


365 


4 


63-7 


8-34 


8-76 


1884 


366 


o 


63-4 


8.27 


8-74 


1885 


365 


12 


52-2 


7-83 


8-24 


1886 


365 


62 


25-4 


7-40 


7-3 


1887 


39 ? 


86? 


13-5? 


6-72 


6-48? 



The gist of this discovery may be given in a few words: the 
spots are subject to a periodical variation in prevalence, extend- 
ing over about n y ; during this time their numbers follow 
a cycle which has a maximum and a minimum. At epochs of 



28 The Sun and Planets. [BOOK 1. 

minima, on many days absolutely no spots are to be seen, as was 
the case in 1 856. It has been hinted that at epochs of maxima, 
spots are more permanent in character, that is, can be more often 
watched through several rotations than is the case at epochs of 
minima : but the idea needs confirmation. 

A remarkable discovery has grown out of Schwabe's ; namely, 
that the diurnal variation in the declination of the magnetic 
needle is characterised by an n-year period, and (this is the 
singular circumstance) that the epoch of maximum variation 
corresponds with the epoch of the maximum prevalence of sun- 
spots, and vice versa, minimum with minimum. Lamont of 
Munich announced decisively, about 1850, the fact of the period, 
and General Sabine, in March 1851 , the fact of the coincidence ; 
Gautier and Wolf making the same deduction independently of 
Sabine and of each other. 

Perhaps it might be well just to explain here very briefly 
what the diurnal variation of the magnetic needle is. The needle 
is subject daily to a minute change of direction of an oscillatory 
character. The change is in the nature of an effort on the part 
of the needle to turn towards the Sun. When the Sun is on the 
meridian the needle has its mean position ; this happens twice 
in every 24 hours, corresponding to the upper and lower meridian 
passages of the Sun. Again, when the Sun is mid- way between 
these positions also of course twice in every 24 hours the 
needle has a mean position because its N. and S. ends make 
respectively equal efforts (so to speak) to direct themselves 
towards the Sun. Four times in the day then the needle has 
its mean position, or, in other words, is directed towards the 
magnetic meridian. But when the Sun is not in any one of the 
4 positions mentioned, that end of the needle which is nearest 
the Sun is slightly turned away from its mean position and 
towards the Sun. These diurnal vibrations are not uniform in 
amount from day to day during a succession of days and months 
and years: they vary in extent by gradual steps through 
a period of years, now recognised as being about i i y . And 

Phil. Trans., vol. cxlii. p. 103. '852. 



Fig. 19. 



Plate IV. 




CHAP. I.] The Sun. 31 

this fact underlies the coincidence mentioned in the previous 
paragraph. 

Two other curious discoveries have been made in close con- 
nection with the foregoing, and it is now accepted that aurorae 
and magnetic earth currents (currents of electricity which 
frequently travel below the surface of our globe, and interfere 
with telegraphic operations) likewise have an i i-year period, and 
that their maxima and minima are contemporaneous with those 
of the two phenomena dealt with above ; " so that," in the words 
of Balfour Stewart, " a bond of union exists between these four 
phenomena. The question next arises, What is the nature of 
this bond 1 Now, with respect to that which connects Sun-spots 
with magnetic disturbances, we can as yet form no conjecture ; 
but we may, perhaps, venture an opinion regarding the nature 
of that which connects together magnetic disturbances, aurorse, 
and earth-currents p ." The reality of the coincidences just 
adverted to will be best understood by an examination of the 
accompanying engraving of curves, which I copy from Loomis, 
who has investigated with great care the historical evidence 
available for drawing trustworthy conclusions in respect of these 
matters. Loomis points out that the discrepancies in the coinci- 
dences of critical periods in the three phenomena of Sun-spots, 
magnetic declination, and aurorse are both few and insignificant. 
His memoir will well repay attentive perusal q . 

Much more might be said on these matters, but a fuller 
elucidation of them would lead us into non-astronomical fields. 

I may here advert to a remarkable phenomenon seen on Sep- 
tember i, 1859, by two English observers whilst engaged in 
scrutinising the Sun. A very fine group of spots was visible at 
the time, and suddenly, at n h i8 m a.m., two patches of in- 
tensely bright white light were seen to break out in front of the 
spots. They were at first thought to be due to a fracture of the 
screen attached to the object-glass of the telescope, but such was 

P Proceedings of the Royal Inst., vol. p. 245. April 1873; vol. 50. (2nd s.) p. 
iv. p. 58. 1863. 153. Sept. 1870. 

i Silliman's Journal, vol. v. (3rd s.) 



32 The Sun and Planets. [BOOK I. 

not the case. The patches of light were evidently connected 
with the Sun itself; they remained visible for about 5 m , during 
which time they traversed a space of about 33,700 miles. The 
brilliancy of the light was dazzling in the extreme ; but the most 
noteworthy circumstance was the marked disturbance which (as 
was afterwards found) took place in the magnetic instruments 
at the Kew Observatory simultaneously with the appearance in 
question, followed in about i6 h by a great magnetic storm r , 
during which telegraphic communication was impeded, some 
telegraph offices were set on fire, and aurorse appeared. A storm 
on the sun not altogether unlike this, it would seem, was 
observed on September 7, 1871, in America by Professor C. A. 
Young. A prominence (or uprush of gas) which he was 
examining with a spectroscope suddenly burst into fragments 
with great violence. He calculated that the velocity of ascent 
was as great as 166 miles per second. A portion of the frag- 
ments of matter reached 200,000 miles from the Sun's surface 8 . 
An aurora occurred in the evening*. 

A more recent and extremely striking instance of the cor- 
relation of these physical forces occurred on April 16, 1882. A 
magnificent aurora, violent electrical disturbances, and numerous 
and large Sun-spots presented themselves simultaneously. The 
aurora was seen only in America, but the electrical disturbances 
and of course the Sun-spots were recorded in Europe also. No 
one can read Mr. H. C. Lewis's paper cited below without being 
convinced of the intimate association subsisting between these 
phenomena. Hardly less certain is their magnetic character. Mr. 
Lewis thus concludes his paper on the aurora in question: 
" The theory is not improbable that Sun-spots are the result of 
solar electrical or magnetic storms, and that auroras are the 
result of a disturbed electrical condition of the earth, caused by 

r Carrington and Hodgson, Month. Not., * For an account of 2 explosions on 

vol. xx. pp. 13-16. Nov. 1859. Se * ne Sun seen, the one by Rapin at Lau- 

also an account of a similar phenome- sanne, on Sept. 14, 1883, and the other 

non noted by Brodie, in vol. xxv. p. 21. by C. W. Irish, at Iowa (U. S.), on 

November, 1864. April 10, 1884, see IS Astronomic, vol. 

Nattire,\6l. iv. p. 488. Oct. 19,1871. iii. p. s8r. October 1884. 



CHAP. I.] 



The Sun. 



33 



induction from the Sun. The common cause for both phenomena 
is probably cosmigal u ." 

Wolf has tabulated all the observations of spots which he 
could collect. These date from 1611, but do not assume good 
regularity till 1749. Annexed is a copy of Wolf 's table w . He 
divides his materials into 2 groups, corresponding to the periods 
1610-1738, and 1745-1870, and his deductions as to the average 
duration of the sun-spot period are as follows : 



SEBIES I. 

Years. 

From Mimima, 11.20 + 2.11. 
.. Maxima, 11-20+2-06. 



SERIES II. 

Years. 
From Minima, 11-11 + 1.54. 

., Maxima, 10-94 + 2-52. 



Minima. 


Maxima. 


1 
Minima. 


Maxima. 


1610-8 


1615-5 


1745-0 


I750-3 


8-2 


10-5 


IO-2 


11*2 


1619-8 


1626-0 


1755-2 


1761-5 


15-0 


13-5 


"3 


8-2 


1634-8 


I639-5 


1766-5 


1769-7 


II-O 


9-5 


9-0 


8-7 


1645-0 


1649-0 


1775-5 


1778.4 


IO-O 


II-O 


9-2 


9-7 


1655-0 


1 660-0 


1784.7 


1788-1 


II-O 


15-0 


13-6 16-1 


1666-0 


1675-0 


I798-3 


1804-2 


13-5 


IO-O 


12-3 


12-2 


I<5 79-5 


1685-0 


1810-6 


1816-4 


IO-O 


8-0 


12-7 


13-5 


1689.5 


1693-0 


1823-3 


1829-9 


85 


12-5 


10-6 


7-3 


1698-0 


1705-5 


1833-9 


1837-2 


14-0 


12-7 


9-6 


10-9 


1712-0 


1718-2 


1843-5 


1848-1 


"1 


9-3 


12-5 


I2-O 


I 723-5 


I727-5 


1856-0 


1 860- 1 


10-5 


II-2 


II-2 


10-5 


17340 


I738-7 


1867-2 


1870-6 



u Proceedings Amer. Philos. Soc., vol. 
xx. p. 290, 1882. For further information 
on the connection between solar outbursts 
and magnetic storms see the Stonyharst 
College Observations for 1882, &c. 
(Observatory, vol. vi. p. 307, Oct. 1883.) 

w Mem. E. A. S., vol. xliii. p. 202, 
1877. Wolf's results, as recorded in 



his paper, will well repay careful study. 
His system of "relative numbers" to 
represent the monthly and annual energy 
displayed by the Sun is extremely in- 
teresting, and the preparation of his 
table to record this energy from July 
1749 to June 1876 must have involved 
incredible labour and research. 



34 The Sun and Planets. [BOOK I. 

The general result may be stated to be, that the period equals 
1 1 ii i years (u years 6 weeks,) but may vary as much as 
2 years either way from this average. 

Wolf has also considered himself warranted in asserting this 
law : " Greater activity in the Sun goes with shorter periods, 
and less with longer periods ''; and further, that there are grounds 
for the opinion that solar spots and variable stars are due to 
similar agencies. 

Generally speaking, there appears a tendency with maxima 
to anticipate the middle time between the consecutive minima, 
the interval ii'ii y being divided into two unequal sub-intervals 
of 4| y and 6J y , or, as it may be otherwise put, the maximum 
appears to fall about the 5 th year of the period comprised be- 
tween 2 minima x . Observations of various kinds discussed by 
De La Rue, Stewart, and Lowy confirm this inequality of interval, 
but make the sub-intervals 37 y and 7'4 y . or i to 2. As respects 
the law of increase and decrease in given spot-periods their con- 
clusion differs in an important respect from that of Wolf. He 
appears to consider that when the spot frequency has descended 
rapidly or slowly from a maximum value to the next minimum, 
it ascends with corresponding (relative) rapidity or slowness to 
the next maximum. De La Rue and his associates prefer to put 
it that when the spot frequency has passed rapidly or slowly 
from a minimum to the next maximum, it descends with corre- 
sponding (relative) rapidity or slowness to the next minimum y . 

Besides the irn y -period Wolf finds another period 5 times as 
long, and a third period 3 times the length of the second : in 
other words, that the activity of the Sun goes through a further 
series of changes every 55^ y and i66 y . He fancies that in 
adjacent or nearly adjacent n y -periods of unequal length, a 
greater activity during the shorter tends to compensate, in the 
total number of spots produced, for a less energy in the longer. 
The earlier observations are necessarily very imperfect z . 

Schwabe's original period was io y : but the ii-n y -period is 

* Herschel, Outlines of Ast., p. 253. z Mem. Soc. Phil, de Berne, 1852. 

y Month. Not., vol. xxxii. p. 177. Feb. The Table for 1749-1860 is given in 

1872. Month. .AW., vol. xxi. p. 77. Jan. 1861. 



CHAP. I.] The Sun. 35 

now considered preferable ; even Schwabe himself assented to 
it a , and the investigations of Hansteen and others have shown 
that it is also the average period of the variation in the magnetic 
declination. 

The examination by Fritsch of a large number of auroral 
observations enabled him to extend to auroral displays also the 
56 y -period, as preferable to the 65 y -period proposed by Olmsted 
without any reference to the spots. 

Another supposed coincidence has now to be adverted to. By 
carefully examining Schwabe's observations, Wolf thinks that 
he has detected the existence of minor periods of spot-prevalence, 
depending in some way upon the Earth, Venus, Jupiter, and 
Saturn b . " Thus he finds a perceptibly greater degree of apparent 
activity to prevail annually on the average of months of Sep- 
tember to January than in the other months of the year ; and 
again, by projecting all the results in a continuous curve, he 
finds in it a series of small undulations succeeding each other at 
an average interval of 7^65 months, or O'637 y . Now the periodic 
time of Venus (225*) reduced to the fraction of the year is 0-616, 
a coincidence certainly near enough to warrant some considerable 
suspicion of a physical connection c ." It is proper to state that 
Wolf does not appear to have made any use of Schwabe's obser- 
vations taken subsequent to 1 848 d . 

B. Stewart concurred in the opinion that Planetary influences 
on the Sun can be traced, and he thinks that Jupiter and 
Mercury, as well as Venus, are concerned. The general result as 
to Venus is that spots have a tendency to break out at that 
portion of the Sun which is nearest to Venus. "As the Sun 
rotates carrying the newly-born spot farther away from this 
planet, the spot grows larger, attaining its maximum at the 
point farthest from Venus, and decreasing again on its approach- 
ing this planet." 

Doubts must be deemed to attach to the influence assigned to 

a Ast. Nach., No. 1521, vol. Ixix. Ap. c Sir J. Herschel, Quart. Journ. Sc., 

3, 1865. vol. i. p. 238. April 1864. 

b Month. Not., vol. xix. p. 86. Jan. d Miltheilungen, No. 10. 

1859. 

D 2 



3(5 The S'ni <in<l Planet*. [BOOK I. 

Jupiter and Saturn. As Jupiter's period (i r8 y ) is nearly identical 
with the Sun-spot period, it has even been suggested that the 
prevalence of Sun-spots depends mainly on influence exerted by 
Jupiter in different parts of its orbit, in perihelion or aphelion, 
as the case may be, but the notion seems open to question for 
several reasons. 

Schwabe was disposed to find a connection between Sun-spots 
and meteoric showers. There is something of a coincidence 
between three Sun-spot periods and one shower period, but it is 
no doubt accidental e . 

Sir W. Herschel, considering that the prevalence of numerous 
spots on the Sun's disc was an indication that probably violent 
chemical action (with the extrication of an unusual amount 
of light and heat) was going on, was led to think that years of 
abundant spots would also be noted for high temperatures and 
good harvests, and years of few spots for low temperatures and 
bad harvests f . Wolf finds decisive evidence " that years rich in 
solar spots are in general drier and more fruitful than those of 
an opposite character, while the latter are wetter and stormier 
than the former &." This idea is supported by meteorological facts 
collected by an examination of the chronicles of Zurich from 
1000 to 1800 A.D. Gautier, from a discussion of 62 sets of 
observations, extending over i i y , and taken at various places in 
Europe and America, arrived at exactly the opposite conclusion h . 
A note of Arago's is highly appropriate here ; " In these matters 
we must be careful not to generalise until we have amassed a 
large number of observations." 

The general question of the influence of the Sun on the meteoro- 
logy of the Earth is a large and complex one, and it has re- 
ceived very little attention. I propose now to state what is at 
present known on this subject, though this will scarcely serve 
any more definite purpose than that of awakening a desire for 
further knowledge. 

Some relationship certainly seems to subsist between solar 

6 Month. Not., vol. xxvii. p. 286. June g Mitlheilungen,'&o. 10. 

1867. b Sibl. Univ. de Oentve, vol. li. p. 56. 

' PAH. Trans., vol. xci. p. 316. 1801. 1844. 



CHAP. I.] , The Sun. 37 

spots and terrestrial cloudiness and rainfall. Baxendell considered 
that diversities of solar activity are to be regarded as causing 
changes in the magnetic condition of the Earth, and so producing 
changes in the directions and velocities of the great currents of 
the atmosphere and in the distribution of barometric pressure, 
temperature, and rainfall. "The future progress of meteorology 
must depend to a much greater extent than has been generally 
supposed, upon the knowledge we may obtain of the nature and 
extent of the changes which are constantly taking place on the 
surface of the Sun'." 

M. Poey. from an elaborate catalogue of tropical storms, going 
back as far as i75> finds evidence of 12 storm cycles indicated 
by 12 epochs of frequent and severe storms: 10 of these epochs 
of maximum atmospheric disturbance correspond to maxima of 
Sun-spots. With respect to epochs of minima the coincidences 
are less noticeable; for in ir storm minima only 5 coincidences 
with Sun-spot minima are to be traced. M. Poey notes that 
years marked by storm maxima generally follow by one or two 
years the years of Sun-spot maxima k . 

A Canadian observer, Mr. A. Elvins, affirms that years in which 
maxima and minima of Sun-spots occur, are distinguished by 
general cloudiness, intermediate years being apparently much 
more free from clouds. He further states that records of the 
height of the water in Lake Ontario extending over 18 years 
indicate that a relation subsists between the changes in the Sun's 
surface and the height of the said water. This latter element is 
to be viewed of course as indicative of the amount of precipita- 
tion that has taken place. Mr. Elvins's general conclusions are 
that years of maxima and minima of Sun-spots are years of small 
rainfall and low temperature. He considers, however, that the 
year immediately preceding a maximum or minimum is usually 
a specially wet year. If future observations should confirm these 
ideas, it will (among other things) follow that the rainfall curve 

' See the statistics on which this is p. 249. Feb. 1873. 

based in Proc. Lit. and Phil. Soc. of k Comptes Rendut, vol. Ixxvii. p. 1226. 

Manchester, vol. xi. p. in. They are 1873. 
summarised in Month. Not., vol. xxxiii. 



38 The Sun and Planets. [BOOK I. 

is more abrupt than the Sun-spot curve. As regards there being 
a cycle for storms, Elvins confirms Poey l . 

Some investigations by an American meteorologist named 
Brocklesby, of observations extending over 60 years, have led 
him to consider that in 3 cases out of 5, years of maximum spot 
energy are years of excess of rainfall ; years of minimum spot 
energy to the number of 5 being, on the other hand, years notice- 
able in every case for deficiency of rainfall. He thinks that his 
inquiries justify the general deduction that "the rainfall tends to 
rise above the mean when the Sun-spot area is in excess, and to 
fall below when there is a deficiency of solar activity" 1 ." 

Professor C. P. Smyth is amongst those who have paid much 
attention to the subject of Sun-spot cycles and terrestrial tem- 
peratures. He considers that a great wave of heat passes over 
the Earth "every n years and a fraction, and nearly coincidently 
with the beginning of the increase of each Sun-spot cycle of the same 
1 1 -year duration. The last observed occurrences of such heat-wave 
(which is very short-lived, and of a totally different shape from 
the" Sun-spot curve), were in 1834*8, 1846-4, 1857-8, 1 868-8, whence, 
allowing for the greater uncertainty in the earlier observation, we 
may expect," he said, writing in 1872, " the next occurrence of the 
phenomenon in or about 1 880-0." Somewhat less pronounced 
than the foregoing is the extreme cold close on either side of the great 
heat-wave. Professor Smyth further said in 1872: "We may 
perhaps be justified in concluding that the minimum temperature 
of the present cold wave was reached in 1871-1, and that the 
next similar cold wave will occur in 1878-8." Finally, between 
the dates of these 2 cold- waves there are 3 "moderate" and nearly 
equi-distant heat-waves, with their 2 intervening and " very 
moderate " cold-waves n . Prince, however (a very experienced 
meteorologist as well as astronomer), says that he does not 
believe in any weather cycles whatever, though he admits that 
"a very cold wave was present in 1879," and that "1880 was 
above the average," and so in a measure confirms Smyth. 

1 Ast. Register, vol. x. pp. 171, 221, p. 447. Dec. 1874. 

and 265. 1872. Nature, vol. v. p. 317. Feb. 22, 1872. 

m Sillimuris Journal, 3rd Ser., vol. viii. 



CHAP. I.] 



The Sun. 



39 



Stone , making use of observations at the Cape of Good Hope, 
extending over 30 years, and Abbe p , of observations at Munich 
extending over 60 years, have both traced a connection between 
the Sun-spot period and terrestrial temperatures. Stone's con- 
clusion, based upon a comparison of curves, is thus expressed by 
himself: " I cannot but believe that the same cause which leads 
to an excess of mean annual temperature leads equally to a dissi- 
pation of solar spots." Abbe's conclusion is that there is "a 
decrease in the amount of heat received from the Sun during the 
prevalence of the spots." Observations at Oxford (1864-70) show 

Fig. 20. 




CHANGE OF FORM IN SPOTS OWING TO THE S0N*S BOTATION. 

that the mean azimuthal direction of the wind there varied year 
by year through a range of 58 on the whole, between maximum 
and minimum of Sun-spots, the tendency of the wind to a west- 
ward direction increasing with the increase of the spots. 

The only other observation which it appears necessary to cite 
here is by Ballot of Utrecht. He thinks he has established (by 
means of thermometric observations made at Haarlem, Zwanen- 
bourg, and Dantzig, during a great number of years) the fact that 

Proc. Eoy. Soc., vol. xix. p. 391. 1871. 

i* Silliman's Journal, and Ser., vol. 50. p. 345. Nov. 1870. 



40 



The Sun and Planets. 



[BOOK I. 



at each period of 27- 7 d (that of the Sun's visual axial rotation) 
there is in these localities a small elevation of temperature, and 
a depression at the intermediate epochs. 

Respecting the physical nature of the spots much uncertainty 
exists. Up to a comparatively recent period the generally re- 
ceived opinion, however, was that first enunciated by Professor 
Wilson of Glasgow in 1779, as modified by Sir W. Herschel 
namely, that the Sun is surrounded by two atmospheres, of which 
the outer one is luminous (thence usually termed, after Schroter, 
the photosphere], and the inner one, nearest to the Sun's surface, 

Fig. 21. 




SPOT ON THE SUN MAY 5, 1854, SHOWING CYCLONIC ACTION. 

non-luminous, and that the spots are rents or apertures in 
these atmospheres through which we see the solid body of the 
Sun, otherwise known to us as the "nucleus" of the spots. This 
idea is supported by the fact that, when near either limb, the 
spots are narrower (fore-shortened) than when seen directly in 
the centre of the disc. The lower stratum is assumed to receive 
some illumination from the photosphere, and thus to appear 
penumbral; to occupy, in the matter of luminosity, a medium 
position between the photosphere reflecting much light, and the 
solid matter reflecting little, or, perhaps, none at all. The tern- 



CHAP. I.] 



The Sun. 



41 



porary removal-of both the strata, but more of the upper than of 
the lower, he conceived to be effected by powerful- upward 
atmospheric currents, the origin of which is unknown. All, 
however, that now appears certain is that the nucleus of a spot 
is lower than the penumbra, and that both are beneath the level 

Fig. 22. 





July 3- 



June 30. 



June 29. 






July 8. 



July 7. July 6. 



July 5. 



July 4. 



LARGE SPOT ON THE SUN VISIBLE IN l886, AND SHOWING SUCCESSIVE 
CHANGES OF FORM OWING TO THE SUN'S ROTATION. 

of the Solar photosphere. Detached masses of luminous matter 
are seen actually to cross a spot without producing any alteration 
in it. It would seem also that the gases in the space occupied 
by a spot are at an appreciably lower temperature than those in 



42 



The Sun and Planets. 



[BOOK I. 



the brighter parts of the Sun, and this for the present represents 
practically the sum of our actual knowledge. That movements 
of a cyclonic character sometimes occur on the Sun, is sufficiently 
shown by a well-known drawing made by Secchi on May 5, 1854, 
of a spot in which a spiral motion is perfectly obvious. Above 
these atmospheres it is strongly believed that a thin and gaseous 
envelope exists, more nearly akin to what we understand by the 
word ' ; atmosphere " as applied to the envelope which surrounds 
the Earth ; and this supposition finds confirmation in the fact 

Fig- 23. 




A SPOT SEEN ON THE EDGE OF THE SUN EXHIBITING ITSELF AS A 
DEPRESSION IN THE SUN'S SURFACE. 

that the margin of the Sun's disc is in general less luminous 
than the centre a very obvious result on this hypothesis. 

Fig. 22 is a rough sketch of a large spot on the Sun seen in 
June and July, 1886, with the naked eye by various observers 15 . 

Fig. 23 is a representation obtained by photography at Dehra- 
Dun in India, in 1884, of a spot which, having arrived at the 
limb of the Sun, exhibited itself as a depression in the Sun's 
surface. 

As regards the luminosity of the Sun's disc at the edge and at 



P L'Astronomie, vol. v. p. 387, Oct. 1886. 



CHAP. I.] ^ . The Sun. 43 

the centre, Laplace gives the ratio at 30 to 48 ; Arago at 40 to 
41. The latter figures very greatly underrate the inequality. 
Secchi, taking the centre at i, said that the margin is only rd 
or j th as bright. He said that at times he found himself im- 
peded in his investigations by a ruddiness in the light near the 
limb. Vogel, the most recent, and, it may be added, the most 
methodical investigator of this subject, obtained by a photo- 
graphic expedient the following results ; taking the Sun's radius 
at 12 and the brightness at the centre at 100, the brightness was 
found to lessen thus : 

Centre = 100. 

4 = 96- 

8 = 77. 

i<J = 51. 

Edge = 13. 

Zollner's investigations indicate that an average black umbra 
of a Sun-spot is 4000 times as bright as an equal area of surface 
on a full Moon. This conclusion is supported by the spectro- 
scope, for even a very black umbra yields a spectrum exhibiting 
all the details of full sunlight q . 

Representing the general brightness of the Sun's disc by 1000, 
according to Sir W. Herschel that of the penumbrse is 469 and of 
the nuclei only 7. But it may well be doubted whether all 
these evaluations are not too fictitiously precise, however 
generally correct. 

The chemical rays given out by different parts of the surface 
of the Sun also appear to be of unequal power, but whether, like 
the rays of light, they vary regularly from centre to edge, seems a 
moot point. 

As regards the rays of heat, these likewise are radiated more 
from the centre than from the edges. The Polar regions, too, are 
colder than the Equatorial, and Secchi has shown that the heat 
radiated from the spots is less than that from the disc generally. 
Sir J. Herschel believed one hemisphere to be hotter than the 
other. That the luminous envelope of the Sun is an incandescent 
gas, Arago's Polariscope experiment is held to prove r . Sir John 

i Schellen, Spectrum Analysis, Eng. ed. p. 293. 
r See his Pop. Ast., vol. i. p. 419. 



44 The Sun and Planets. [BOOK I. 

Herschel showed that Arago's experiments were by no means 
conclusive, but spectroscopic observations have brought this 
matter more within reach of demonstration. 

Schwabe's observations seem to indicate that at epochs of 
minimum spot-display the Sun's surface is more uniformly 
bright than at other times ; that is to say, that there is less 
absorption or enfeeblement of the Solar light towards the 
margin of the Sun's disc than is usually the case. 

Spots on the Sun seem to have been discovered by J. 
Fabricius 8 and Galileo, independently, early in 1611, and by 
Harriot, also independently, in December of the same year. It 
will readily be understood that the observation of them was one 
of the first discoveries resulting from the invention of the 
telescope, though as spots large enough to be visible to the 
naked eye are now and then visible, they were occasionally seen 
before that event. Adelmus, a Benedictine monk, makes 
mention of a black spot on the Sun on March 17, 807*. It is 
also stated that a similar spot was seen by a Spanish Moor 
named Averroes, in the year 1 161 u . An instance of a solar spot 
is recorded by Hakluyt. He says, that in December 1590, the 
good ship "Richard of Arundell" was on a voyage to the coast 
of Guinea, and that her log states that " on the 7 at the going 
downe of the sunne, we saw a great blacke spot in the sunne, 
and the 8 day, both at rising and setting, we saw the like, 
which spot to our seeming was about the bignesse of a shilling, 
being in 5 degrees of latitude, and still there came a great 
billow out of the southerboard x ." The spot was also observed 
on the 1 6 th . 

The natural purity of the Sun seems to have been an article of 
faith with the ancients, on no account to be called in question ; 
so that we find that when Schemer (who was a Jesuit at 
Ingolstadt) reported to his Superior what he had seen, the idea 

8 An interesting account of Fabricius's n Commentary on the Almagest, quoted 

first observations of a spot on the Sun by Copernicus, De Stvol. Orb. Cel., 

will be found in Guillemin's Sun, p. 127, lib. x. 

Eng. Ed. * The Principal Navigations, Voiages, 

* Bede ; Polydorus Vergilius, Anglicce Traffiques, and Discoveries of the English 

Hi*tori(K. Nation, rf-c.,vol. ii. p. 131. London, 1599. 



CHAP. I.] 



The Sun. 



45 



was treated as a delusion. " I have read," replied the Superior, 
"Aristotle's writings from end to end many times, and I can 
assure you that I have nowhere found anything in them similar 
to what you mention. Go, my son, tranquillise yourself; be 
assured that what you take for spots in the Sun are the fault of 
the glasses or of your own eyes." Scheiner in the end, though 
permitted to publish his opinions 7 , was obliged to do so anony- 
mously, so great were the difficulties with which he had to 
contend as a member of the Church of Rome desiring to cultivate 

science. 

Fig. 24. 




FACUL.S: ON THE SUN, DEC. 3, 1865. (Tacckini.) 

In addition to spots, streaks of light may frequently be re- 
marked upon the surface of the Sun towards the equatorial 
margin of the disc. These are termed f acute*, and are generally 
found near spots (just outside the penumbrae) or where spots 
have previously existed or are soon about to appear; when 
near the limb of the Sun they are more or less parallel to it. 
They are of irregular form, and may be likened somewhat to 
certain kinds of coral, and are more luminous than the solar 



y In 3 letters addressed to Welser, 
chief magistrate at Augsburg. Printed 
copies of these letters were sent to 
Galileo and others. Schreiner's well- 
known Rosa Ursina, &c. was of later 
date (1630). Alluding to this enormous 
book, Delambre says : " There are few 



books so diffuse and so void of facts. It 
contains 784 pages ; there is not matter 
in it for 50 pages." Hist. Ast. Mod., 
vol. i. p. 690. Either printing must have 
been cheap or authors rich in those 
days. 

1 Latin facula, a torch. 



46 The Sun and Planets. [BOOK I. 

surface surrounding them. Secchi considered them to be not 
brighter than the centre of the Sun. They are elevations or ridges 
in the photosphere, as is proved by Dawes having seen one pro- 
ject above the limb in turning the (apparent) corner into the in- 
visible hemisphere 3 , and they have been seen on photographs 
projecting like a tooth from the limb. Sir W. Herschel saw a 
facula on December 27, 1 799, 2' 46" or 74,000 miles long b . Faculse 
are first alluded to by Galileo in his third letter to Welser c . 

Prominences give gaseous, i. e. bright line spectra ; faculse con- 
tinuous spectra. Faculse are seen in high latitudes much more 
frequently than spots are. 

Short, the optician, seems to have noticed during the eclipse 
of July 14, 1748 (o. s.), that the surface of the Sun was 
covered with irregular specks of light, presenting a mottled 
appearance not unlike that of the skin of an orange, but rela- 
tively much less coarse. The term lucr.H A has been applied to 
the constituent specks. This may perhaps only be an allusion, 
and the first recorded, to the " granulations " recognised in 
modern times. 

Schwabe found that faculge and luculi are usually absent at 
epochs of spot minima e . 

Of late years the Sun has received an unusual amount of 
attention from astronomers, and many interesting facts have 
been brought to light concerning its physical appearance f . In 
1 860 Nasmyth with his great reflector (alluded to hereafter) ascer- 
tained, it would seem for the first time, that the Sun's surface 
is covered with a tolerably compact agglomeration of entities, 
which he likened to willow leaves ; that is to say, they presented 
to his eye an appearance similar to that which a rather thin but 
flattened layer of willow leaves might be expected to exhibit. 

As an acrimonious controversy arose in regard to this alleged 
discovery, it may be fair to lay before the reader Nasmyth's own 
statement on the subject. 

* Month. JVo.,vol.xx. p. 56. 000.1859. ''Month. Not., vol. xxvii. p. 286. 

b Phil. Trans., vol. xci. p. 284. 1801. June 1867. 

c Istoria e Dimottrazioni intorno alle f See especially a paper by the Rev. 

Mncchie Solan, p. 131. Rome, 1613. S. J. Perry in Aat. Reg., vol. xxii. p. 257. 

d Latin lucus, a shining. Nov. 1884. 



CHAP. I.] The Sun. 47 

" In order to obtain a satisfactory view of these remarkable objects, it is not only 
requisite to employ a telescope of very considerable power and perfection of denning 
capability, but also to make the observation at a time when the atmosphere is nearly 
quite tranquil, and free from those vibrations which so frequently interpose most 




SPOT ON THE SUN, JULY 29, l86d, SHOWING THE " WILLOW-LEAF " 

STRUCTURE. (Nasmyth.) 

provoking interruptions to the efforts of the observer ; without such conditions as I 
allude to, it is hopeless to catch even a glimpse of these remarkable and delicate 
details of the solar surface. 

********** 

"The filaments in question are seen, and appear well defined, at the edges of the 
luminous surface, where it overhangs ' the penumbra,' as also in the details of the 
penumbra itself, and most especially are they seen clearly defined in the details of 
' the bridges/ as I term those bright streaks which are so frequently seen stretching 
across from side to side over the dark part of the spot. So far as I have as yet had 
an opportunity of estimating their actual magnitude, their average length appears to 
be about 1000 miles, the width about 100. 

' ' There appears no definite or symmetrical arrangement in the manner in which 
they are scattered over the surface of the Sun ; they appear to be across each other 
in all possible variety of directions. The thickness of the layer does not appear to be 
very deep, as I can see down through the interstices which are left here and there 



48 



The Sun and Planets. 



[BOOK I. 



between them, and through which the dark or penumbral stratum is rendered visible. 
It is the occurrence of the infinite number of these interstices, and the consequent 
visibility of a corresponding portion of the dark or penumbral stratum, that gives to 
the general solar surface that peculiar and well-known mottled appearance which has 
for a long time been familiar to the observers of the Sun. 

" When a solar spot is mending up, as was the case with the one represented, 
these luminous filaments or willow-leaf-shaped objects (as I term them) are seen to 

Fig. 26. 




SPOT ON THE SUN, JANUARY 2O, 1865. 



pass from the edges and extend across the spots, thus forming ' the bridges,' or 
bright streaks across the spots ; if these are carefully observed under favourable 
conditions, the actual form of these remarkable details, of which ' the bridges ' are 
composed, will be revealed to sight. 

" Subsequent observations and considerations of the subject have not caused me 
to desire to modify or alter the description in the letter above referred to e ; but only 
to confirm me in its general correctness. I have no desire to embark in any 
controversy on the subject, as I prefer to leave to the Sun itself, when carefully 
observed by adequate means and on favourable occasions, the complete confirmation 
of what I claim to be the first to discover, delineate, and accurately describe in 
reference to the structure of his entire luminous surface, as well as the precise form 



g Month Not., vol. xxiv. p. 66. Jan. 1864. 



CHAP. I.] The Sun. 49 

of the structural details, which, from their general similitude in respect to form, I at 
once compared with willow leaves h ." 

Nasmyth's views were much canvassed. Several eminent 
observers of unquestioned good faith, and possessed of first-class 
instruments and great experience, declared the alleged conforma- 
tion of the solar surface a myth, whilst others, equally entitled 
to be heard with respect, avouched their belief -in the reality of 
the discovery. I believe it to be an impartial summing up of the 
whole case pro and con to say that there is a very general agree- 
ment that innumerable detached (?) masses of unknown nature 
are scattered over the Sun's surface, and that whether " willow 
leaves," "rice grains," "granulations," or "shingle beach" be 
employed to designate them, is rather a matter of taste than 
evidence of substantial variance. Further, that in the main they 
do partake of an elliptic outline, and that the average ratio of 
the axes, whether it be 10 to i, as Nasmyth first had it, or 4, 3, 
or 2 to i, as other observers have since stated it, is, after all, the 
main point concerning which issue is joined, and even here 
apparent discrepancies may be ascribable to actual physical 
change in the bodies themselves. 

Writing from Greenwich under date of February 25, 1864, 
Stone made the following remarks : 

" At the first good opportunity I turned the telescope on the Sun. I may state 
that my impression was, and it appears to have 

been the impression of several of the assistants here, Fig. 27. 

that the willow leaves stood out dark against the 
luminous photosphere. On looking at the Sun I 
was at once struck with the apparent resolvability 
of its mottled appearance. The whole disc, as far 
as I examined, appeared to be covered over with 
relatively bright rice-like particles, and the mottled 
appearance seemed to be produced by the inter- 
lacing of these particles. I could not observe any 
particular arrangement of the particles, but they 
appeared to be more numerous in some parts than 
in others. 1 have used the words rice-like particles RICE . LIKE PARTICLES SEEN 
merely to convey a rough impression of their form ; QN THE SUN (Stone.) 

I consider them like the figure. 

h The preceding paragraphs are taken himself, with a brief supplementary note 
from a letter reproduced by Nasmyth appended. 

E 




50 The S'u, il Mntctx. [BOOK I. 

' 1 have seen these rice-like particles on two occasions since, but not so well as on 
the first day, when the definition was exceedingly good. Yesterday (Feb. 24) I saw 
them for a few minutes, but with great difficulty. I use the full aperture, 12 J 
inches, and a low power. On the first day I saw them [end of January 1864] I 
called Mr. Dunkin's attention to them. He appears to have seen them, and 
considers the figure above to represent them fairly. He says, however, that he 
should not have noticed them if his attention had not been called to them '." 

A valuable synopsis of the question was presented to the Eoyal 
Astronomical Society in 1866 by Huggins k . The following is 
a brief summary of its contents : 

1. Grannie is the best word to describe the luminous particles 
on the Sun's surface, as no positive form is thereby implied. 

2. The granules are seen all over the Sun, including (occasion- 
ally) the surfaces of umbrae and penumbne. More rarely they 
can be detected in faculse. 

3. With low powers " rice grains " is a very suitable expres- 
sion for these granules, but the regularity implied in this 
designation disappears to a great extent under high magnifiers. 
There is, however, undoubtedly, a general tendency to an oval 
contour. 

4. The average size of the more compact granules is i", of 
those more elongated \\", a few might be 3", many less than i". 
They appear to be not flat discs, but bodies of considerable 
thickness. 

5. The granules are sometimes packed together rather closely 
in groups of irregular and straggling outline ; at other times they 
are sparsely scattered. The well-known "mottling" arises 
wholly from the latter species of combination. 

6. The Sun's surface is by no means uniformly level The 
whole photosphere appears corrugated into irregular ridges and 
vales, and the granules are possibly masses of rather dense cloud- 
like matter floating about in the photosphere, considered as 
composed of more aeriform matter. If the granules really are 
incandescent clouds, their general oval form may be due to the 
influence of currents. 

1 Proceedings of Manchester Lit. and k Month. Not., vol. xxvi. p. 260. May 

Philos. Soc., vol. iii. p. 250, 1864. 1866. 



CHAP. L] 



The Sun. 



51 



The accompanying figure [28] shows some of the most charac- 
teristic modes of grouping of the bright granules noticed by 
Huggins on different occasions and on various parts of the 
Sun's surface, brought together, however, in one woodcut for 
convenience of comparison. 

Fig. 28. 




IDEAL VIEW OF THE "GRANULAR" STRUCTURE OF THE SUN. (HttffffinS.) 

Huggins has called attention to the fact that Janssen's photo- 
graphs of 1877 disclose, amongst other important features, a 
frequent tendency of the granules to arrange themselves in a 
spiral form, accompanied by more or less loss of distinctness of 
outline of the individual granules. The same observer has put 
on record the fact that a similar appearance was noticed by 
himself as long ago as 1866 : 

K" 2 



52 



The Sun and Planets. 



[BOOK I. 



" Saw distinctly the granules. A spiral band of closely associated granules, 
ending in one of larger size [fig. 26]. In one area near the centre of the Sun's disks 
the granules appeared more elongated than usual [fig. 30], rather sparsely scattered, 
and the larger diameters very nearly in the same direction. In neighbouring area, 
the granules smaller and less elongated. Amongst these no general direction was 
observed '." 

Fig. 29. 

Fig. SO- 





GKAKDLES 1 866, SHOWING CYCLONIC 
ARRANGEMENT. 



SOLAR GRANULES 1866, SHOWING 

ORDINARY ARRANGEMENT. 

(Hugging.) 



The present state of our knowledge respecting the physical 
constitution of the Sun, stated as shortly as possible, is, that 
the central solid or gaseous body of the Sun is surrounded by a 
series of concentric envelopes, the order of which reckoning out- 
wards is as follows : 

(1) The photosphere, the visible source of the solar light which 
reaches the Earth, defined by Young as a " shell of luminous 
clouds formed by the cooling and condensation of the conden- 
sible vapours at the surface where exposed to the cold of outer 
space." 

(2) The chromosphere, a thin casing of self-luminous gaseous 
matter, chiefly hydrogen gas in an incandescent state, and the 
seat of the solar prominences (formerly known as the " red flames " 
and seen only during total eclipses of the Sun until Lockyer and 
Janssen independently in 1868 conceived the idea that they 
might be rendered visible irrespective of the Sun being eclipsed). 

1 Month. Not., vol. xxxviii. p. 102. Jan. 1878. 



CHAP. I.] The Sun. 53 

(3) The corona, a vast shell of unknown vapours in a highly 
attenuated state, many thousands of miles thick, and oberved to 
extend to at least from what is ordinarily taken to be the 
visible edge of the Sun. 

Tacchini arrived at the following general ideas from obser- 
vations made by him on 281 days during 1880. 

As to the distribution of solar phenomena over the Sun's 
surface : The spots remain near the equator and present two 
maxima between the parallels 10 and 20 on either side. At the 
equator they are rare, or wholly absent. Faculse always occur 
at the equator ; they show maxima between + 20 and + 30, and 
come nearer the poles than the spots. Protuberances are rare 
near the equator ; they present two principal maxima between 
+ 50 and + 60, and two secondary ones in the latitudes of the 
faculae maxima. They reach further from the equator than the 
facutee, but the polar caps remain free of them. Of the two 
hemispheres the northern showed, during 1880, the greater 
activity. 

To the cloudy stratum giving rise to the penumbrae Petit assigns 
a depth exceeding 4000 miles. On the other hand, Phillips con- 
sidered 300 miles a probable amount. Neither estimate is primd 
facie entitled to much consideration. 

_ Sir W. Herschel supposed that one of the hemispheres of the Sun 
is by its physical constitution less adapted to emit light and heat 
than the other, but the grounds of this conclusion are not known. 

The study of the Sun has during the last few years taken a 
remarkable start, owing to the fact that by the aid of the spec- 
troscope we have been enabled to obtain much new information 
about its physical constitution. This subject being, however, a 
physical rather than an astronomical one, and involving a great 
amount of chemical and optical detail, it cannot conveniently be 
discussed at length in a purely astronomical treatise, though some- 
thing will be said concerning it later on in the portion of this 
work dedicated to spectroscopic matters. 



54 The Sun and Planet*. [BOOK I. 



CHAPTER II. 

THE PLANETS. 

Epitome of the motions of the Planets. Characteristics common to them all.- 
Kepler s laws. Elements of a Planet's orbit. Curious relation between the 
distances and the periods of the Planets. The Ellipse. Popular illustration 
of the extent of the Solar system. TfwJe's law. Miscellaneous characteristic* 
of the Planets. Curious coincidences. Conjunctions of the Planets. Conjunc- 
tions recorded in History. Different systems. The Ptolemaic system. 
The Egyptian system. The Copernican system. The Tychonic system. 

A ROUND the Sun, as a centre, certain bodies called Planets 8 
*"" revolve at greater or less distances 11 . They may be 
divided into two groups, (i) the "inferior" planets, or those 
whose orbits are within that of the Earth, namely Vulcan (?). 
Mercury, and Venus; and (2) the "superior" planets, or those 
whose orbits are beyond that of the Earth, namely Mars, the 
Minor Planets, Jupiter, Saturn. Uranus, and Neptune. 

If viewed from the Sun all the planets would appear to the 
spectator to revolve round that luminary in the order of the 
zodiacal signs ; such, however, cannot be the case when the 
observation is made from one of their number itself in motion, 
and therefore to us on the Earth the planets appear to travel in 
a capricious manner; and, further, the inferior and superior 
planets differ the one class from the other in their visible 
movements. 

The Inferior planets are never seen in those parts of the 
heavens which are in Opposition to the Sun ; in other words. 

" ir\avriTT)s, a wanderer. reckoned in all cases from the centre of 

" The distances of the planets are the Snn. and not from its surface. 



CHAP. II ] The Planets. 55 

they are never on the meridian at midnight, being always 
within a short angular distance of the Sun, to the E. or W. of it 
as the case may be. Twice in every revolution an inferior 
planet is in Con junction with the Sun [Fig. 31]; in Inferior 
Conjunction when it conies between the Earth and the Sun, and 
in Superior Conjunction when the Sun intervenes between the 
Earth and the planet. When it attains its greatest distance (as 
we see it) from the Sun, E. or W., it is said to be at its Greatest 
Elongation, E. or W., as the case may be. In the former case the 
planet is an "evening star," in the latter a "morning star." 




Inferior 6 . 
PHASES OF AN " INFERIOR" PLANET. 

Although a planet always truly moves in the order of the 
signs, yet there are periods when it appears stationary ; sometimes 
even periods when its motion appears retrograde or reversed. 
These peculiarities are owing to the fact that the Earth has 
simultaneously a motion of its own in its orbit ; and it will 
readily be understood that they are only apparent and not real. 
They also obtain with the superior planets. It sometimes 
(though very rarely) happens that an inferior planet, when 
in Inferior Conj unction, passes directly between the Earth and 
the Sun, and is consequently projected on the disc of the latter, 
which it crosses from E. to W. : this phenomenon is termed a 
transit*. Transits will be considered more particularly in Book 



Trattflre, to gr> across. 



The Sun and Planets. 



[BOOK I. 



A superior planet can have any angular distance from the 
Sun not greater than 180. After starting from Conjunction 
with the Sun it successively reaches its Eastern Quadrature (at 
an angular distance of 90) ; and its Opposition at 180. Pro- 
ceeding onwards it comes to its Western Quadrature, 270 from 




APPARENT MOVEMENTS OP MERCURY BETWEEN IfoS AND 1715. 

the Sun reckoned in the direction of its motion, but only 90 
reckoned in the other direction. Another stage of 90 brings it 
again into Conjunction. A planet cannot have a greater angular 
distance from the Sun than 180, because when that is attained 
it begins to approach the Sun again on the other side, for an 
obvious geometrical reason. 

An exhaustive account of the motions of the planets does not 
fall within my scope, but the books named in the note may 



CHAP. II.] The Planets. 57 

be consulted* 1 . How complicated these motions are will be 
readily understood by an inspection of Fig. 32, which represents 
the apparent movements of Mercury amongst the stars between 
the years 1708 and 1715. 

There are certain characteristics common to all the planets, 
which are thus enunciated by Hind : 

1. They move in the same invariable direction round the Sun; 
their course, as viewed from the north side of the ecliptic, being con- 
trary to the motion of the hands of a watch. 

2. They describe oval or elliptical paths rQund the Sun, not however 
differing greatly from circles. 

3. Their orbits are more or less inclined to the ecliptic, and inter- 
sect it in two points, which are the " nodes;" one half of the orbit lying 
north, and the other half south of the Earth's path. 

4. They are opaque bodies like the Earth; and shine by reflecting 
the light which they receive from the Sun. 

5. They revolve upon their axes in the same way as the Earth. 
This we know by telescopic observation to be the case with many 
planets, and, by analogy, the rule may be extended to all. Hence they 
will have the alternation of day and night, like the inhabitants of 
the Earth ; but their days are of different lengths to our own. 

6. Agreeably to the principles of gravitation, their velocity is 
greatest at those parts of their orbit which lie nearest the Sun, and 
least at the opposite parts which are most distant from it ; in other 
words, they move quickest in perihelion*, and slowest in aphelion*. 

From a long series of observations of the planet Mars, Kepler 
found that certain definite laws might be deduced relative to the 
motions of the planets, which may be thus stated : 

T. The planets move in ellipses, having the Sun in one of the foci. 

2. The radius vector of each planet describes equal areas in equal 
times. 

d Sir J. Herschel's Outlines of Ast., greater eccentricity of cometary orbits : 

p. 301 et seq. ; Hind's Introd. to Ast., thus the velocity of Donati's comet at 

p. 63 et seq. (very good). perihelion is 127,000 miles per hour, 

e *pl round, and T/AJOS the Sun. but at aphelion only 480 miles per 

' dwo from, and ^\tos. The fact here hour. (Hind, Letter in the Times, Oct. 

referred to is more strikingly manifest 25, 1858.) 
in the case of a comet, owing to the 



58 The Sun and Planets. [BOOK i. 

3. The squares of ike periodic titties of the planets are proportional 
to the cubes of their wean distances from the Sun. 

These laws hold good for all the planets and all their satellites. 
I have already referred in general terms to the I st law ; it may, 
however, be desirable to say that the orbit of a planet with re- 
ference to its form, magnitude, and position, is determined by 
the 5 following data or elements : 

1. The longitude of perihelion, or the longitude of the planet, 
when it reaches this point, denoted by the symbol 77. 

2. The longitude of the ascending node of the planet's orbit, as 
seen from the Sun. S3 . 

33- 




DIAGRAM ILLUSTRATING KEPLKR'.S SECOND L.V\V. 

3. The inclination of the orbit, or the angle made by the plane of 
the orbit with the ecliptic. i. 

4. The eccentricity. e. This is sometimes expressed by the 
angle <, of which e is the natural sine. 

5. The semi-axis-major, or mean distance. a. 

And in order to compute the place of a planet at any given 
moment, we further need to know : 

6. Its periodic time (obtainable from (5) by Keplei's 3 rd law) ; 
and : 

7. Its mean longitude, or place in its orbit, at a given epoch. 
Kepler's 2 nd law will readily be understood from the annexed 

diagram. Let P P 2 P 4 be the elliptic path of a planet, and let it 
move from P to P 1 , from P z to P 3 , and from P 4 to P'"' in equal 



CHAP. II.] 



Tin' 



59 



intervals of time ; then the 3 shaded areas, which are assumed to 
correspond with the movement of the radius vector, will all be 
equal in area 

The 3 rd law involves a curious coincidence, which may be thus 
expressed : If the squares of the periodic times of the planets be 
divided ly the cubes of their mean distances from the Sim, the 
quotients thus oltained are the same for all the planets. The follow- 
ing table exemplifies this: it should be remarked, however, that 
the want of exact uniformity in the fourth column 8 is owing to 
inexactness in the observations on which the calculations are 
based, as also to the perturbations which the planets mutually 
exercise on each other's orbits : 



Planot. 








i>" 
a 


Vulcan ' 


O-I41 


10*7 


I 32716 


Mercury 
Venus 


. ... 0-38710 
0-723^3 


87.969 
224-701 


J3.M- 21 
133413 


Earth 


I -OOOOO 


^65-2^6 


i 33408 


Mars . . 


1-52369 


686-970 


133410 


Ceres . . . 


2-77602 


1670-8^^ 


132210 


Jupiter . 


v 20277 


4332 58=1 


133294 


Saturn 
Uranus 


9-53858 
19-18239 


10759 220 
30686-821 


133375 
133422 


Neptune . .... 


30-03627 


60126-722 


i334'3 











This law also holds good for the satellites 11 , as will be seen 
from the following tables calculated for the purpose of exempli- 
fying it. 



THE SATELLITES OF MARS. 









P* 


Name. 


a 


/' 


=> 


De : mos 


2-50 


0-319 


736II 


Phoboa 6-00 


1-262 


73733 



' The decimal pointing is neglected in 
all cases in the 4th column, that the eye- 
appreciation of the coincidences may not 
be interfered with. 



h This is not rigorotisly true when the 
mass of the primary has an appreciable 
ratio to that of the Sun. 



60 



The Sun and Planets. 

THE SATELLITES OF SATURN. 



[BOOK I. 



Name. 


a 


P 


p2 

a3 


Mimas 


3-36 


0-94 


23295 


Enceladus 


. 4-31 


i-37 


23443 


Tethys 


5-34 


1-89 


23458 


Dione 


6-84. 


2. *!A 


2^460 


Rhea 


Q.CC 


A.S.2 


21457 


Titan 


22-14. 


I VQ^ 


23442 


Hyperion 


26-86 


21-30 


23412 


lapetus 


64-54 


79-33 


23409 



THE SATELLITES OF JUPITER. 



No. 


a 


p 


P 2 



I. 


6-05 


1.77 


14147 


II. 


9-62 


3-55 


14156 


III. 


15-35 


7- I 5 


14135 


IV. 


26-99 


16-69 


14168 


THE SATELLITES OF URANUS. 


No. 


a 


p ' 


p2 
a 


I. 


6-94 


2-51 


18848 


II. 


9-72 


4-14 


18664 


III. 


15-89 


8-70 


18909 


IV. 


21-27 


13.46 


18827 



Kepler's laws are the foundation of all planetary astronomy, 
and it was from them that Newton deduced his theory of 
gravitation. Arago says : " These interesting laws, tested for 
every planet, have been found so perfectly exact, that we do not 
hesitate to infer the distances of the planets from the Sun from 
the duration of their sidereal revolutions ; and it is obvious that 
this method of estimating distances possesses considerable ad- 



CHAP. II.] 



The Planets. 



61 



Fig. 34- 



vantages in point of exactness ; for it is always easy to determine 
precisely the return of each planet to a point in the heavens, 
while it is very difficult to determine exactly its distance from 
the Sun." 

Sir J. Herschel discussed the theoretical considerations con- 
nected with these laws with great perspicuity; and the reader 
will do well to consult his remarks 1 . 

A few definitions as to the properties of an ellipse will here be 
appropriate. 

In Fig. 34, S and S' are the foci of the ellipse ; A C is the 
major axis ; B D the minor or conjugate axis ; O the centre : or, 
astronomically O A is the 
semi-axis-major or mean dis- 
tance, O B the semi- axis - 
minor ; the ratio of O S to 
O A is the eccentricity ; the 
least distance, S A, is the 
perihelion distance ; the great- 
est distance, S C, the aphe- 
lion distance. SBO is the 
angle < referred to on p. 58. 
Where an eccentricity is 
stated in the form of a vulgar 

fraction, O S is the numerator and O A the denominator. A 
decimal expression is to the like effect. 

It will not be difficult to follow in the mind the additional 
characteristics of a planetary orbit. The orbit in the figure is 
laid down on a plane surface ; incline it slightly as compared to 
some fixed plane ring and the element of the inclination (as 
regards its amount) will present itself. (The astronomical fixed 
plane in this case is that of the ecliptic.) Imagine a planet 
following the inclined ellipse ; at some point it must rise above 
the level of the fixed plane : the point at which it begins to do 
so, measured angularly from some settled starting-point, gives 
the longitude of the ascending node. Then the planet's position in 
1 Outlines of Ast., pp. 322-7. 




THE ELLIPSE. 



62 



Tlif Snn an<i 



[BOOK I. 



the ellipse when it comes closest to the principal focus, gives us, 
when projected on the plane ring, the place of nearest approach to 



Ui 






the focus, in other words, the longitude of the perihelion. Follow- 
ing these steps, it is not a matter of much difficulty to form a 



CHAP. II.] 



general conception of a planetary orbit in space, for though the 
method is rather crude, it is so far strictly accurate. 



to 






The following scheme will assist the reader to obtain a fail- 
notion of the magnitude of the planetary system. Choose a 



64 The Sun and Planets. [BOOK I. 

level field or open common ; on it place a globe 2 feet in 
diameter, for the Sun ; Vulcan (?) will then be represented 
by a small pin's head, at a distance of about 27 feet from the 
centre of the ideal Sun ; Mercury by a mustard seed, at a 
distance of 82 feet; Venus by a pea, at a distance of 142 feet; 
the Earth also by a pea, at a distance of 215 feet; Mars by 
a small pepper-corn, at a distance of 327 feet ; the minor planets 
by grains of sand, at distances varying from 500 to 600 feet: 
if space will permit, we may place a moderate-sized orange 
nearly mile distant from the central point to represent Jupiter ; 
a small orange of a mile for Saturn ; a full sized cherry | mile 
distant for Uranus; and lastly a plum i miles off for 
Neptune, the most distant planet yet known. 

Extending this scheme, we should find that the aphelion 
distance of Encke's Comet would be at 880 feet ; the aphelion 
distance of Donati's Comet of 1858 at 6 miles; and the nearest 
fixed star at 7500 miles. 

According to this scale the daily motion of Vulcan (?) in its 
orbit would be 4f feet ; of Mercury 3 feet ; of Venus 2 feet ; 
of the Earth i| feet; of Mars i\ feet; of Jupiter 10^ inches; 
of Saturn 7^ inches ; of Uranus 5 inches ; and of Neptune 
4 inches. These figures illustrate also the fact that the orbital 
velocity of a planet decreases as its distance from the Sun 
increases. 

Connected with the distances of the planets, Bode of Berlin in 
1772 published the following singular 'law' of the numerical 
relations existing between them, which, although not discovered 
by him but by Titius of Wittemberg in 1766, usually bears his 
name. 

Take the numbers 

o 3 6 12 24 48 96 192 384; 

each of which (the second excepted) is double the preceding; 
adding to each of these numbers 4 we obtain 

4 7 10 16 28 52 100 196 388; 
which numbers approximately represent the distances of the 



MARS. EAKTH 



VENUS. MERCURY. 



COMPARATIVE SIZES OF THE SUN AND PRINCIPAL PLANETS. 



%* The diie on the left of the Sun's centre repretentt URANUS ; 
and that on the right, XEPTUNE. 



CHAP. II.] 



The Planets. 



planets from the Sun expressed in radii of the Earth's orbit, as 
exhibited in the following table : 



Planets. 


True Distance 
from (^\ 


Distance by 
Bode' a Law. 


Mercury ... 


3-87 


4-OO 


Venus . . 


7'23 


7-OO 


Earth .. .. 


IO-OO 


IO-OO 


Mars 


15*23 


16-00 


Ceres . . 


27-66 


28-00 


Jupiter ... 


52-03 


52-00 


Saturn 


05.30 


IOO-OO 


Uranus 


Kii-Sa 


106-00 


Neptune 


300-37 


388-00 









Bode having examined these relations, and noticing the void 
between 16 and 52 (Ceres and the other minor planets not being 
then known), ventured to predict the discovery of new planets ; 
and it may reasonably be believed that this conjecture guided or 
suggested the investigations of subsequent observers ; though some 
have disputed this k . In the above table the greatest deviation 
between the assumed and the true distance is in the case of 
Neptune. We may sum up Bode's law as follows : That the 
interval between the orbits of any two planets is about twice as great 
as the inferior interval, and only half the superior one 1 . 

Separating the major planets into two groups, if we take Mer- 
cury, Venus, the Earth, and Mars as belonging to the interior ; 
and Jupiter, Saturn, Uranus, and Neptune to the exterior group, 
we shall find that they differ in the following respects : 



k As far back as 450 B.C. Democritus 
of Abdera thought it probable that even- 
tually new planets would, perhaps, be 
discovered. (Seneca, Qucest. Nat.,\il>. vii. 
cap. 3 and 13.) Kepler was of opinion 
that some planets existed between the 
orbits of Mars and Jupiter, but too small 
to be visible to the naked eye. The same 
philosopher conjectured that there was 
another planet between Mercury and 
Venus. 



1 Many attempts have been made by 
ingenious dabblers in Astronomy to dis- 
cover other arithmetical coincidences 
formed after the spirit of Bode's law. 
The following is the only one I have met 
with which deserves reproduction. Take 
the series o, i, 2, 4, 8, 16, 32, and 64: 
add 4 to each, and the resulting figures 
represent with some approach to accuracy 
the relative distances of the satellites of 
Saturn from their primary. 

2 



68 The Sun and Planets. [BOOK I. 

1. The interior planets, with the exception of the Earth and 
Mars, are not, as far as we know, attended by satellites, while 
the exterior planets all have satellites. We cannot but consider 
this as one of the many instances to be met with in the universe 
of the beneficence of the Creator in other words, that the 
satellites of these remote planets are designed to compensate for 
the small amount of light which their primaries receive from the 
Sun, owing to their great distance from that luminary. 

2. The average density of the first group considerably exceeds 
that of the second, the approximate ratio being 5:1. 

3. The mean duration of the axial rotations, or mean length of 
the day, of the interior planets is much longer than that of the 
exterior m ; the average in the former case apparently being about 
24 h , but in the latter only io h . 

In the Appendix will be found a full tabular summary of 
information concerning the Sun, Moon, and Planets brought 
up to the latest possible date. 

The following coincidences may or may not deserve to be 
mentioned : 

1. Multiply the Earth's diameter (7912 miles) by 108, and we 
get 854,496 = + the Sun's diameter in miles. 

2. Multiply the Sun's diameter (852,584 miles) by 108, and 
we get 92,079,072 = + the mean distance of the Earth from the 
Sun. 

3. Multiply the Moon's diameter (2160 miles) by 108, and 
we get 233,280 = + the mean distance of the Moon from the 
Earth. 

A phenomenon of considerable interest, especially on account 
of its rarity, is the conjunction, or proximity, of two or more 
planets within a limited area of the heavens. A noticeable 
instance is depicted in Fig. 38. It occurred on the morning of 
July 21, 1859, when Venus and Jupiter came very close to each 

m This can only be presented as a except the Earth and Mars. It may be 

general conclusion the truth of which presumed, however, that size has more to 

seems probable ; for it cannot be said do with this than distance from the Sun. 

with any great confidence what are the (See a paper by Denning in the 06- 

rotation periods of any of the planets gervatory, vol. vii. p. 40, Feb. 1884.) 



CHAP. II.] The Planets. 69 

other; at 3 h 44 A.M. the distance between the two planets 
was only 1 3", and they accordingly appeared to the naked eye as 
one object. 

On Aug. 9, 1886, Venus, Saturn, and b Geminorum appeared 
in the same field of the telescope. 

During February, 1881, Venus, Jupiter, and Saturn were 
all in the constellation Pisces, and within a few degrees of one 
another. 

In Sept. 1878, Mercury and Venus were together in the same 

Fig. 38- 




VENDS AND JUPITER, July 21, 1859. 

field of the telescope for some hours. Venus looked like clean 
silver ; Mercury more like lead or zinc, according to Nasmyth. 

On Jan. 29, 1857, Jupiter, the Moon, and Venus were in a 
straight line with one another, though not within telescopic 
range. 

On Dec. 19, 1845, Venus and Saturn appeared in the same 
field of the telescope. [See Fig. 39.] 

On Oct. 3, 1801, Venus, Jupiter, and the Moon were in close 
proximity in Leo, and Saturn was not far off. 

On Dec. 23, 1 769, Venus, Jupiter, and Mars were very close 
to each other. 



70 



The Sun and Planets. 



[BOOK I. 



On March 17, 1725, Venus, Jupiter, Mars, and Mercury 
appeared together in the same field of the telescope. 

On Nov. n, 1544, Venus, Jupiter, Mercury, and Saturn were 
enclosed in a space of 10. 

On Nov. n, 1524, Venus, Jupiter, Mars, and Saturn were very 
close to each other, and Mercury was only 16 distant. 

In the years 1507, 1511, 1552, 1564, 1568, 1620, 1624, 1664, 
1669, 1709, and 1765, the three most brilliant planets Venus, 
Mars, and Jupiter were very near each other. 

Fig- 39- 




VENDS AND SATURN, Dec. 19, 1845. 

On Sept. 15, 1 1 86, Mercury, Venus, Mai's, Jupiter, and Saturn 
were in conjunction between the Wheat-ear of Virgo and Libra. 

The earliest record we possess of an occurrence of this kind 
is of Chinese origin. It is stated that a conjunction of Mars, 
Jupiter, Saturn, and Mercury, in the constellation S/ti, was 
assumed as an epoch by the Emperor Chuen-hio, and it has been 
found by MM. Desvignoles and Kirch that such a conjunction 
actually did take place on Feb. 18, 24463.0., between 10 and 18 
of Pisces n . Another calculator, De Mailla, fixes upon Feb. 9, 

n Bailly, Astron. Ancienne, p. 345. p. 166, and Kirch's in vol. v. p. 193 of 
Desvignoles's original memoir appears the game series. 
in Mem. de TAcad. de Berlin, vol. iii. 



CHAP. II.] 



The Planets. 



71 



2441 B.C., as the date of the conjunction in question ; and he 
states that the four planets named above, and the Moon besides, 
were comprised within an arc of 12, extending from 15 to 27 
of Pisces. It deserves mention that both the foregoing dates 
precede the usually received date of the Noachian deluge. It 
may therefore only be that the planetary conjunction in question 
was ascertained at some subsequent time. 
De Mailla gives the following positions : 



E.A. 



16 

12 
21 

47 
ii 



Fig. 40. 



Mercury ... ... ... ... 344 56 

Jupiter ... ... 347 2 

The Moon ... 353 1 8 

Saturn 354 39 

Mars ... 356 45 

A few general remarks on the different theories of the solar 
system which have at various times been current will appro- 
priately conclude this chapter. 

The Ptolemaic system claims the first place in consequence of 
its wide acceptance and the 
fame of the astronomer 
whose name it bears. It 
would, however, be more 
correct to say that Ptolemy 
reduced it into shape rather 
than that he actually origin- 
ated it. The Earth was 
regarded as the centre, and 
around this the Moon ( D ), 
Mercury ( $' ), Venus ( ? ), 
The Sun (0), Mars (<?), 
Jupiter ( 2/ ).and Saturn( fj ), 
all called planets, were as- 
sumed to revolve in the 
order in which I have here given them. 

More accurate ideas were, however, current even before 

Hist. Gen. de la Chine, vol. i. p. 155. 




THE PTOLEMAIC SYSTEM. 



72 



The Sun and Planets. 



[BOOK I. 



Fig. 41. 




THE EGYPTIAN SYSTEM. 



Ptolemy's time, but they found few supporters. Aristarchus 
of Samos, who lived about 280 B.C., supposed, according to 

Archimedes and Plutarch, 
that the Earth revolved 
round the Sun, for which 
'heresy' he was accused 
of impiety. Cleanthes of 
Assos, who flourished but 
20 years later, was, accord- 
ing to Plutarch, the first 
who sought to explain the 
great phenomena of the 
universe by supposing a 
motion of translation on the 
part of the Earth around 
the Sun, together with one 
of rotation on its own axis. 

The historian relates that this idea was so novel and so con- 
trary to the received no- 
tions that it was proposed 
to arraign Cleanthes also 
for impiety. 

The Egyptian system dif- 
fered from the Ptolemaic 
only in regarding Mercury 
and Venus as satellites of 
the Sun and not primary 
planets. 

A long period elapsed 
before any new theories 
of importance were started, 
but in the i6 th century 

THE COPKRN1CAN SYSTEM. ,, . . 

oi the Christian era Coper- 

nicug came forward and propounded his theory, which ulti- 
mately superseded all others, and is the one now (in substance) 
adopted. It places the Sun in the centre of the system as the 



Fig. 42. 




CHAP. II] 



The Planets. 



73 



point around which all the primary planets revolve It must not 
be supposed, however, that the Polish astronomer attained to our 
existing amount of knowledge on the subject. Far from it: his 
ideas were defective in more than one important particular. In 
order to account for the apparent irregularities in the motions 
of the planets, as seen from the Earth, he upheld theories which 
subsequent advances in the science showed to be unnecessary 
and to rest on no substantial basis. Amongst other things he 
retained the theory of Epicycles. The ancients considered that 
the planetary motions must be effected uniformly and in circles, 
because uniform motion appeared the most perfect kind of motion, 
and a circle the most perfect and most noble kind of curve. 
There is at any rate a reverential spirit in this idea which, not- 
withstanding our enlightenment, we need not despise. Copernicus 
announced his system in a treatise entitled De Revolutionibus Orbium 
ccelegtium, the actual publication of which, in 1543, he only just 
lived to see, for he died the same year ; for him this was perhaps 
fortunate rather than otherwise, because the work was condemned 
by the Papal ' Congregation of the Index.' Had it been possible 
for the reverend gentlemen who formed that body to have got 
the author within their clutches, 
it is more than likely that he 
would have suffered as well as 
his book ; as did Galileo after 
him. 

Tycho Brahe was the last great 
astronomer who ventured on any 
original speculations in this field. 
Influenced either by bond fide 
scruples resulting from an erro- 
neous interpretation of certain 
passages in Holy Scripture, or 
it may be. simply by a desire to 
perpetuate his name, he chose to 

regard the Earth as immoveable, and occupying the centre of the 
system : the Moon as revolving immediately round the Earth : 




THE TYCHONIC SYSTEM. 



74 



The Sun and Planets. 



[BOOK I. 



and, exterior to the Moon, the Sun doing the same thing the 
various planets revolving round the latter as solar satellites. 

Kepler and Newton finally set matters right by perfecting 
the Copernican system, and so negativing all the others ; yet 
down to quite recent times there have survived on the part of 
utterly ignorant people remnants of disbelief (real or professed) 
in the Copernican system, but even the most cursory examination 
of these remnants would be most unprofitable. 

Fig- 44- 




THE HOUSE AT WOOLSTHORPE, LINCOLNSHIRE, IN WHICH NEWTON WAS BORN, 
SHOWING THE SUNDIALS HE MADE WHEN A BOY. 

%* One of these dialt teas taken out of the wall about 1844, 
and pretented to the Royal Society. 



CHAP. III.] Vulcan. 75 



CHAPTEE III. 

VULCAN (?). 

Le Terrier's investigation of the orbit of Mercury. Narrative of the Discovery of 
Vulcan. Le Terrier's interview with M. Lescaroault. Approximate elements of 
Vulcan. Concluding note by Le Verrier. Observation* by Lummis at Man- 
chester. Instances of Sadies seen traversing the Sun. Hind' s opinion. Alleged 
Intra-Mercurial planets discovered in America by Watson and Swift on July 
29, 1878. 

"OEFOE.E entering upon the story of the supposed discovery of 
-*-^ a new planet to which this name has been given, a brief 
prefatory statement seems necessary. 

M. Le Verrier, having conducted an investigation into the 
theory of the orbit of Mercury, was led to the conclusion that a 
certain error in the assumed motion of the perihelion could only 
be accounted for by supposing the mass of Venus to be at least 
T V greater than was commonly imagined, or else that there 
existed some unknown planet or planets, situated between 
Mercury and the Sun, capable of producing a disturbing action. 
In laying his views before the scientific world in the autumn 
of 1859% Le Verrier suggested the latter theory as a probable 
solution of the difficulty b . 

On these views being made public, a certain M. Lescarbault, 
a physician at Orgeres, in the Department of Eure-et-Loire. 
France, came forward and stated that on March 26 in that year 
(1859), ne na d observed the passage of an object across the Sun's 

* Compt. Rend., vol. xlix. p. 379. in detail by Newcomb in Astron. Papers 
1859. for use of Amer. Naut. Almanack, vol. i. 

b Objections to this theory are stated p. 474. Washington, 1882. 



76 The Sun and Planets, [BOOK I. 

disc which he thought might be a new planet, but which he 
did not like to announce as such until he had obtained a con- 
firmatory observation ; he related in writing the details of his 
observation, and Le Verrier determined to seek a personal 
interview with him. 

The following account of the meeting will be read with 
interest. 

" On calling at the residence of the modest and unobtrusive medical practitioner, 
he refused to say who he was, but in the most abrupt manner, and in the most 
authoritative tone, began, ' It is then you, Sir, who pretend to have observed the 
intra- Mercurial planet, and who have committed the grave offence of keeping your 
observation secret for nine months. I warn you that I have come here with the 
intention of doing justice to your pretensions, and of demonstrat'ng either that you 
have been dishonest or deceived. Tell me then, unequivocally, what you have seen.' 
The doctor then explained what he had witnessed, and entered into all the particulars 
regarding his discovery. On speaking of the rough method adopted to ascertain the 
period of the first contact, the astronomer inquired what chronometer he had been 
guided by, and was naturally enough somewhat surprised when the physician pulled 
out a huge old watch with only minute hands. It had been his faithful companion in 
his professional journeys, he said ; but that would hardly be considered a satisfactory 
qualification for performing so delicate an experiment. The consequence was, that 
Le Verrier, evidently now beginning to conclude that the whole affair was an im- 
position or a delusion, exclaimed, with some warmth, ' What, with that old watch, 
showing only minutes, dare you talk of estimating seconds ? My suspicions are 
already too well founded.' To this Lescarbault replied, that he had a pendulum by 
which he counted seconds. This was produced, and found to consist of an ivory ball 
attached to a silken thread, which, being hung on a nail in the wall, is made to 
oscillate, and is shown by the watch to beat very nearly seconds. Le Verrier is now 
puzzled to know how the number of seconds is ascertained, as there is nothing to 
mark them ; but Lescarbault states that with him there is no difficulty whatever in 
this, as he is accustomed ' to feel pulses and count their pulsations,' and can with ease 
carry out the same principle with the pendulum. The telescope is next inspected, 
and pronounced satisfactory. The astronomer then asks for the original memoran- 
dum, which, after some searching, is found ' covered with grease and laudanum. 1 
There is a mistake of four minutes on it when compared with the doctor's letter, 
detecting which, the savant declares that the observation has been falsified. An 
error in the watch regulated by sidereal time accounts for this. Le Verrier now 
wishes to know how the doctor managed to regulate his watch by sidereal time, 
and is shown the small telescope by which it is accomplished. Other questions are 
asked, to be satisfactorily answered. The doctor's rough drafts of attempts to ascer- 
ta'n the distance of the planet from the Sun ' from the period of four hours 
which it required to describe an entire diameter ' of that luminary are produced, 
chalked on a board. Lescarbault's method, he being short of paper, was to make his 
calculations on a plank, and make way for fresh ones by planing them off. Not 
being a mathematician, it may be remarked he had not succeeded in ascertaining the 
distance of the planet from the Sun. 



CHAP. III.] Vnloa-n. 11 

" The end of it all was. that Le Verrier became perfectly satisfied that an intra- 
Mercurial planet had been really discovered. He congratulated the medical practi- 
tioner upon his discovery, and left with the intention of making the facts thus 
obtained the subject of fresh calculations c ." 

In March or April, 1860, it was anticipated that the planet 
would again pass across the Sun, which was carefully scrutinised 
by different observers on several successive days, but no trace of 
it was obtained then, and in a certain sense Lescarbault's obser- 
vation continues unconfirmed. However, this proves nothing, 
and many are prepared to regard the existence of this planet as 
a fact, to be fully demonstrated on some future occasion. 

The following approximate elements were calculated by Le 
Verrier from Lescarbault's rough observations: - 

Longitude of ascending node ... . . ... ... = 12 59' 

Inclination of orbit ... ... ... I2io' 

Semi-axis major ( = o) ... ... ... ... = o- 1 43 

Daily heliocentric motion ... ... ... ... = i8i6' 

Period ... ... ... ... ... n) d ij b 

Mean distance ... ... ... ... ... = 13,082,000 miles. 

Apparent diameter of from Vulcan ... ... ... = 3 36' 

Do. do. do. (=j) 6-79 

Greatest possible elongation ... ... ... =8 

The application of Kepler's third law yields, as has already 
been shown, a result sufficiently consistent with the results in 
the cases of the other planets to demand attention ; but, as will 
now be seen, some additional evidence can be adduced as to the 
reality of the discovery, much as it has been called in question. 

On March 20, 1862, Mr. Lummis, of Manchester, was examining 
the Sun's disc, between the hours of 8 and 9 A.M., when he was 
struck by the appearance of a spot possessed of a rapid proper 
motion. He called a friend's attention to it, and both remarked 
its sharp circular form. Official duties most unfortunately 
interrupted him, after following it for 20 ; but he had not the 
slightest doubt about the matter. The apparent diameter was 
estimated to be about 7", and in the 20 it moved over about 
12' of arc. The telescope employed was 2| inches in aperture. 

c Epitomised from the North British in Cosmos, vol. xvi. pp. 22-8, 1860; see 
Rfvieio, vol. xxxiii. pp. 1-20, August, also Cosmos, same vol. pp. 50-6. 
1860. A full account will also be found 



78 Tfte Sun and Planets. [BOOK I. 

and was charged with a power of 80. Mr. Lumniis communicated 
with Mr. Hind on the subject of what he had seen ; and the 
latter, by the aid of the diagram sent, determined that 1 2' was 
too great an estimate of the arc traversed by the spot in the 
time, and that 6' would be a nearer value d . 

Two French calculators deduced elements from Lummis's 
observations : the orbits which they obtained, though neces- 
sarily very imperfect, are fairly in accord both with each other, 
and with Le Verrier's earlier orbit. 

The first result is adopted from Valz's elements, the second 
from Radau's. 

I. II. 

Longitude of ascending node ... ... = 2 52' 

Inclination of orbit ... ... ... ... = io2i' 

Semi-axis major ( = I -o) ... ... ... 0-133' ... 0-144 

Daily heliocentric motion ... ... ... = 20 32' ... i85' 

Period = I7 d i3 h ... I9 d 22 h 

Mean distance in miles ... ... ... = 12,076,000 ... 13,174,000 

From the heliocentric position of the nodes, it appears that 
transits can only occur between March 25 and April 10 at the 
descending, and between September 27 and October 14 at the 
ascending node. 

Instances are not wanting of observations of spots of a 
planetary character passing across the Sun which may turn out 
to have been transits of Vulcan e . The following are a selection 
of these instances. 

On October 10, 1802, Fritsch, at Magdeburg, saw a round spot 
pass over the Sun. In 3 it had moved 2', and after a cloudy 
interval of 4 h had disappeared. 

On October 9, 1819, Stark, at Augsburg, saw a well-defined 
and truly circular spot, about the size of Mercury, which he 
could not find again in the evening. 

d Month. Not., vol. xxii. p. 232. April America and by Sporer in Europe. (Ast. 

1862. Lummis's observations were very Nach.,vol.xciv. No. 2253, April 16, 1879.) 

severely criticised by Prof. C. H.F.Peters, Certainly Peters's argument is strong, 
who claimed to have identified Lummis's e Month. Not., vol. xx. p. 100. Jan. 

"planet" beyond question with a par- 1860; also pp. 192-4. March, 1860; 

ticular Sun-spot recorded by himself in Webb, Celest. Objects, p. 40. 



CHAP. III.] Vulcan. 79 

On October 2, 1839, Decuppis, at Rome, saw a perfectly 
round and defined spot moving at such a rate that it would 
cross the Sun in about 6 hours f . 

On October n, 1847, Schmidt saw a small black point rapidly 
pass across the Sun. 

On March 12, 1849, Lowe and Sidebotham watched for half 
an hour a small round black spot traversing the Sun. 

On October 14, 1849, Schmidt saw a black body, about 15" 
in size, pass very rapidly from East to West before the Sun. 
"It was neither a bird nor an insect." 

In the works whence these instances are cited, others are 
given ; but, though suspiciously suggestive of planets, the dates 
do not come within the necessary limits for them to have been 
apparitions of Vulcan, so it is not worth while to transcribe 
them ; but nevertheless they are interesting, and worthy of 
attention s . 

Fig. 45 will be useful, if for no other purpose, as a warning to 
observers not to jump too hastily at conclusions as to what they 
see with their telescopes. On November 30, 1880, M. Ricco at 
Palermo, whilst making his customary daily observations of Sun- 
spots with a telescope of 3^ inches aperture, saw a swarm of 
black bodies slowly traverse the Sun's disc. He thought at first 
that he had the singular good fortune to be gazing on a shower 
of meteors, but sustained attention revealed the fact that the 
objects seen were evidently birds with wings. Subsequent con- 
sultation with certain zoologists rendered it tolerably clear that 
what M. Ricco saw was a swarm of cranes. Some calculations, 
the details of which need not be gone into here, imply that they 
were flying at an elevation of 5! miles h . 

It is right here to state that M. Liais asserts that being in 
Brazil he was watching the Sun during the period in which 
Lescarbault professes to have seen the black spot, and that he is 

f Complex Rendus, vol. ix. p. 809. E. Ledger's Lecture on Intra-Mereurial 

1839. Planets, Svo. Cambridge, 1879. 

B For an exhaustive summary of all h I? Astronomic, vol. vi. p. 66. Feb. 

the recorded observationa of black ob- 1887. 
jects seen on the Sun, see the Rev. 



80 



The Sun and Planet*. 



[BOOK I. 



positively certain that nothing of the kind was visible, though 
the telescope he employed was considerably more powerful than 
that of the French physician. He adds that parallax will not 
explain the discrepancy'. There is, however, in Liais's paper 

Fig. 45- 




FLIGHT OF CRANES SEEN CROSSING THE SUN AT PALERMO. NOV. 30, l88o. 

a malicious bitterness of tone, presumably intended to annoy 
Le Verrier, which greatly impairs the value of the writer's 
testimony. 

Though it is the fashion to repudiate the reality of Vulcan's 
existence, yet it is scarcely prudent to dogmatise on the subject 
as some have done, considering that an astronomer of Hind's 
experience leans to the affirmative side. He says : 

" It is a suspicious circumstance that the elements as regards the place of the node, 
or point of intersection of the orbit with the ecliptic, and it? inclination thereto, as 

1 A*f. Nach., vol. liv. No. 1281. Nov. I, 1860. 



CHAP. III.] Vulcan. 81 

worked out by M. Va'.z of Marseilles, from the data I deduced from a diagram 
forwarded to me by Mr. Lummis, are strikingly similar to those founded by M. Le 
Verrier upon the observations, such as they were, of Dr. Lescarbault. It is true if 
the place of the node and inclination were precisely as given by this astronomer, the 
object which was seen upon the Sun's disc on the 26th of March could not have been 
projected upon it as early as the 2Oth of March. But, considering the exceedingly 
rough nature of the observations upon which he had to rely, perhaps no stress need 
be placed upon the circumstance. Now the period of revolution assigned by M. Le 
Verrier from the observations of 1859 was 19-70 days. Taking this as an approxi- 
mate value of the true period, I find, if we suppose 57 revolutions to have been 
performed between the observations of Dr. Lescarbault and Mr. Lummis, there would 
result a period of 19-81 days. On comparing this value with the previous observa- 
tions in March and in October, when the same object might have transited the Sun 
at the opposite node, it is found to lead to October 9, 1819, as one of the dates when 
the hypothetical planet should have been in conjunction with the Sun. And on this 
very day Canon Stark has recorded the following notable observation, ' At this 
time there appeared a black, well-defined nuclear spot, quite circular in form, and as 
large as Mercury. This spot was no more to be seen at 4.37 P.M., and I found no 
trace of it later on the 9th, nor on the I2th, when the Sun came out again.' The 
exact time of this observation is not mentioned, but appears likely to have been about 
noon, one of Stark's usual hours for examining the solar disc. Hence I deduce a 
corrected period of 19-812 days." 

In the communication from which this is taken k Hind throws 
out suggestions for a scrutiny of the Sun at certain dates. It 
must be admitted that the scrutiny took place and that no 
planet was found, and here the matter rests. 

Notwithstanding, however, the strong negative evidence then 
existing against the existence of Lescarbault's planet Vulcan, 
Le Verrier, in December 1874, re-iterated his announcement 
that the orbit of Mercury is perturbed to an extent rendering it 
necessary to augment the movement of the perihelion. He put 
the amount at 31" in a century. "The consequence" (he said) 
" is very clear. There is, without doubt, in the neighbourhood 
of Mercury, and between that planet and the Sun, matter 
hitherto unknown. Does it consist of one, or several small 
planets? or of asteroids, or even of cosmic dust 1 Theory cannot 
decide this point 1 ." 

Le Verrier died in 1877, and the question had in great measure 
gone to sleep, when some observations made on the occasion of 
the eclipse of the Sun of July 29, 1878, brought the whole 

k Letter in the Times, Oct. 19, 1872. 

1 Compt. Send., vol. Ixxix. p. 1424. 1874. 

G 



82 The Sun and Planets. [BOOK I. 

matter again before the scientific world, though not precisely in 
the same shape. 

The total eclipse in question was visible over a large part of 
the western regions of North America. Two of the many 
American observers, Professor J. C. Watson and Mr. L. Swift, 
applied themselves to the task of searching for Intra-Mercurial 
planets, and with what result we shall now see. 

Professor Watson's observations, as described by himself, shall 
first be set out in full : 

" As soon as the total phase began, I commenced a systematic sweep for objects 
visible near the Sun. From my previous experience in work of this character I had 
determined not to undertake to sweep over too much space. Accordingly, I confined 
my search to a region of about 15 in Eight Ascension, and i in breadth. I had 
previously committed to memory the relative places of stars near the Sun down to 
the seventh magnitude, and the chart of the region was placed conveniently in front 
of me for ready reference whenever required. Before the totality began, I examined 
the regions distant from 8 to 15 on the E. side, and also on the W. side of the Sun, 
without finding any stars. As soon as the total phase had begun I placed the Sun 
in the middle of the field and began a sweep by moving the telescope slowly and 
uniformly towards the E. Then I retraced the path thus examined, moved the tele- 
scope one field further S., and again swept out and back over a distance of about 8. 
In the first of these sweeps I saw 5 Cancri and other known stars. Then I placed the 
Sun again in the field and swept in the same manner towards the W. Between the 
Sun and Cancri, and a little to the S., I saw a ruddy star whose magnitude I 
estimated to be 4^. It was fully a magnitude brighter than 6 Cancri, which I saw at 
the same time, and it did not exhibit any elongation, such as might be expected if it 
were a comet in that position. The magnifying power was 45 and the definition 
excellent. My plan did not provide for any comparison differentially with a neigh- 
bouring star by micrometric measurement, and hence I only noticed the relation of 
the star to the Sun and 6 Cancri. Its position I proceeded at once to record on my 
circles in the manner I have described ; and I recorded also the chronometer time of 
observation. This star was denoted by a. Previously to the commencement of the 
total phase I had recorded a place of the Sun in the same manner, which I designated 
by Sp Having made the record I assured myself that the pointing of the telescope 
had not been disturbed in the least, and I continued the search, sweeping out to 
about 8 W. from the Sun. Then I went back to the Sun, moved the telescope nearly 
one field S., and swept out again towards the W. In this sweep I came across a 
bright star, also ruddy in appearance, which arrested my attention, and for fear that 
the Sun might reappear before I could make an examination of its surroundings, 
I determined to make a record of its place upon my circles. This I next proceeded 
to do, and just as I had completed the record the Sun reappeared. This object was 
designated by b... 

" On September 15 I examined, with the same telescope and magnifying power 
used in the eclipse observations, the stars in this part of Cancer, with the moon 
in the western sky and the bright twilight in the E., so as to obtain as nearly as 



CHAP. III.] Vulcan. 83 

possible the conditions of sky-illumination which existed at the time of the eclipse. 
Having a very distinct recollection in respect to the brilliancy of the stars which I 
saw, and by observing when the approaching daylight had reduced the light of certain 
stars which were E. of the Sun at the time of the total eclipse, so as to be just 
visible in the telescope as they were then, I have been enabled to form a still more 
definite opinion of the relative brilliancy of Cancri, the two new objects which I 
observed, and f Cancri. The fainter of the two planets, that near Cancri, was cer- 
tainly brighter than f Cancri, and much more than a magnitude brighter than its 
neighbouring star. I am inclined to think that (a) should be classed as a good 4th 
magnitude, and that ( J) should be classed as a 3rd magnitude, at the time of the ob- 
servations on July 29. It is, of course, impossible to determine from these observa- 
tions the planetary character of the stars observed. They did not exhibit such 
appearances as might be expected if they were comets near the sun ; and since theory 
demonstrates the existence of such planets, I feel warranted in expressing the belief 
that the foregoing observations give places of two Intra-Mercurial planets. It is 
true that they were not so bright as might be expected if they were of size sufficient 
alone to account fur the outstanding perturbations of Mercury, but it should be re- 
membered that this expectation is based upon the assumption that the reflecting 
power of the surfaces of these planets is the same, or nearly the same, as that of Mer- 
cury. Now we know from actual observations that the intrinsic brilliancy of Mercury 
is scarcely ^th that of Venus when reduced to the same distance, and hence we 
cannot safely assume that the Intra-Mercurial planets must have the same relative 
brilliancy that they would have if their surfaces could reflect the light to the 
same extent as that of Mercury. I feel assured that by suitable devices these 
planets may be observed in full daylight near their elongations. Whether they are 
identical or not with moving spots which have been seen on the Sun's surface at dif- 
ferent times it does not yet seem possible to determine m ." 

Swift's account of his work runs as follows : 

" I reluctantly broke away from the wondrous scene [the Corona], and Immedi- 
ately essayed the well-nigh hopeless task which I had chosen the finding of an 
Intra-Mercurial planet. To my dismay I soon found that I had forgotten to untie 
the string holding the pole in place, and this prevented all search E. of the Sun, as if 
I attempted a move in that direction the lower end would plunge into the ground 
and against the little tufts of buffalo-grass. It is, perhaps, to this circumstance alone 
that I owe the discovery of Vulcan some 5 minutes after its detection by Professor 
Watson, totality having terminated at his station before its commencement at 
mine. 

" Almost the first sweep made to the westward of the Sun I ran across 2 stars 
presenting a very singular appearance, each having a red round disc and being free 
from twinkling. I at once resolved to observe these with great care. Time was 
precious and yet 6 questions demanded an immediate answer, viz : 

1 . What were their distances from the Sun ? 

2. What from each other ? 

3. What direction from the Sun ? 

4. What from each other ? 

m Washington Observations, 1876, App. Ill, " Reports on Total Solar Eclipses," 
pp. 119-23. 

G 2 



84 The Sun and Planets. [BOOK I. 

5. What the magnitude of each ? 

6. What stars were they ? 

" My telescope, though equatorially mounted, had no circles, and consequently 
no measurements were possible, but I endeavoured to be as accurate as existing 
circumstances would allow. My estimated answers were as follows : 

1 . About 3 from Sun's centre to midway between the stars. 

2. About 8'. 

3. South of West. 

4. They were both on a line with the Sun's centre. 

5. Equal, and of the 5th magnitude. 

6. Probably, one was Theta Cancri ; the other an Intra-Mercurial planet. 
"After completing these observations I resumed the quest, sweeping again southerly 

and W., but my fettered telescope behaved badly, and no regularity in the sweeps 
could be maintained, and I was surprised to find, in a few seconds, 2 stars in the field 
answering, in every particular, to the above description, and, sighting along the top 
of the tube on the outside, as in the first instance, I found they were the same objects. 
Again, I went through with the above comparisons, though I devoted only about 
one-fourth of the time given on the first occasion. Finding no necessity for modi- 
fying any of the above estimates, I, for the third time, renewed my sweeps, this time 
nearly along the ecliptic, though I feared to go too far to the W. lest I might not be 
able to get the glass back again to make a third and final observation of them, and 
also of the closing scenes of totality. I could place no dependence on the sweeps, 
and after a few seconds more (though it seemed longer) had them again in the field, 
This proved to be the last time. I again asked myself the already twice repeated 
questions, but found no appreciable change had taken place between the first and 
third observations an interval of probably i^ minutes. Again I searched, but saw 
nothing, and, recollecting that I had no more time to spare, I endeavoured to refind 
the stars for a last observation, but unfortunately a small cloud (the only one within 
50) passed over them, and I was unsuccessful. I saw no stars but these i, Hot even 
Delta, so near the Eastern limb of the Sun. As soon as totality was ended, I 
recorded in my note-book aa follows : ' Saw 2 stars about 3 S.W. of Sun, 
apparently of 5th magnitude some 12' apart, pointing towards Sun. Red.' On my 
homeward journey the thought occurred to me that the distance between the stars 
was, according to memory, a little greater than half that between Mizar and Alcor, 
whatever that might be. Consulting ' Webb's Celestial Objects,' I found they were 
but 1 1 ' apart, which would make the distance of the two stars not to exceed 8', instead 
of 12', as hastily written at the time. While scanning them, I asked the mental 
question, ' What star looks at night to the naked eye as bright as do these through 
the telescope now ? ' Instantly, I answered ' The Pole-star.' That one was Theta 
Cancri is in the highest degree probable, and the other a planet is beyond all ques- 
tion, for on the morning of the roth instant I observed Theta robbed of the com- 
panion I saw during the eclipsed Sun n ." 

These discoveries were hotly canvassed and their authenticity 
directly called in question, but not, I think, on fair or adequate 
grounds. It will be worth while, however, to examine the details 

" Washington Observations, 1876, App. Ill, "Reports on Total Solar Eclipses," 
p. 229. 



CHAP. III.] Vulcan. 85 

of the controversy. Watson's idea of what he saw may be thus 
expressed. He first noticed a star which he thought was 8 Cancri, 
then 6 Cancri, and near to an unknown body which he de- 
signated a ; then a second strange object (designated b] which 
he saw near to the place in which he expected to find 
Cancri, the discovery of which, because he presumed it to 
be Cancri, led him to search no further in that part of his 
field of view. 

The theory of the hostile critic, Professor C. H. F. Peters , is, 
that a was 9 Cancri and b was Cancri, and that some error in 
Watson's circles led to both his observations being vitiated in 
the same direction and to the same extent. This insinuation 
was however warmly repudiated by Watson p . Peters dealt with 
Swift's record in a still more simple fashion : charged him with 
describing objects which he did not see at all, and implied that 
he concocted his alleged discovery after the publication of a 
telegram from Watson ! Swift's reply to all this was as digni- 
fied as it was emphatic q . 

Swift's observations seem, in part at least, irreconcileable with 
Watson's, and if we assume the reality of Swift's 2 planets then 
Watson's a is a 3 rd object and perhaps his b a 4 th , so that in 
point of fact the 2 observers in question would seem to have 
discovered between them 4 Intra-Mercurial planets, which is in 
the highest degree improbable. Here the matter rests r , except 
that the observers of the Total Solar Eclipse of May 6, 1883, 
say that they saw no object which could have been a planet, 
although specially searching for the purpose of finding a planet, 
if possible. 

Ast. Nach., vol. xciv. No. 2253, evidence which appears in the Sidereal 
Apr. 16, 1879. Messenger (U.S.), vol. vi. p. 196 (May, 

P Ast. Nach., vol. xcv. No. 2263, 1887), seems to make the reality of some 

June 17, 1879. discovery perfectly clear ; on the contrary 

1 Ast. Nach., vol. xcv. No. 2277, side, reference maybe made to remarks 
Sept. 17, 1879. by Prof. Young in Sid. Mess., vol. vi. 

r A summary review by Colbert of the p. 21, Jan. 1887. 



86 The Sun and Planets. [BOOK I. 



CHAPTER IV. 



MERCURY. 

Period, tfc. Phases. Physical Observations by Schroter, Sir W. Herschel, Denning, 
ScJiiaparelli and Guiot. Determination of its Mass. When best seen. Ac- 
quaintance of the Ancients with Mercury. Copernicus and Mercury. Le 
Verrier's investigations as to the motions of Mercury. Tables of Mercury. 

1%/TERCURY is, of the old planets a , the one nearest to the 
-L*-'- Sun, round which it revolves in 87 d 23 h I5 m 43-9 i 8 , at a 
mean distance of 35,958,000 miles. The eccentricity of the orbit 
of Mercury amounting to 0-205, the distance may either extend 
to 43,347,000 miles, or fall as low as 28.569,000 miles. The 
apparent diameter of Mercury varies between 4-5" in superior 
conjunction, and 12-9" in inferior conjunction: at its greatest 
elongation it amounts to about 7". The real diameter may be 
about 3008 miles or less b . The compression, or the difference 
between the polar and equatorial diameters, has usually been 
considered to be too small to be measureable, but Dawes, in 
1 848, gave it at /y 

Mercury exhibits phases resembling those of the Moon. At 
its greatest Elongation (say W.) half its disc is illuminated, but 
as it approaches Superior Conjunction the breadth of the illu- 
minated part increases, and its form becomes gibbons ; and 
ultimately, when in Superior Conjunction, circular: at and near 
this point the planet is lost in the Sun's rays, and is invisible. 

a In case it should be thought that as it has been thought for several reasons 

these accounts of the planets are de- undesirable to encumber the Text of 

ficient in statistical data, it may here be Book I. with too many figures. 
remarked that they are intended to be b An American observer, D. P. Todd, 

read in connexion with the tabulated in 1880, put it at 2971 miles. 
statistics in the Appendix of this volume. 



CHAP. IV.] Mercury. 87 

On emerging therefrom the gibbous form is still apparent, but the 
gibbosity is on the opposite side, and diminishes day by day till 
the planet arrives at its greatest Elongation E., when it again 
appears like a half-moon. Becoming more and more crescented, 
it approaches the Inferior Conjunction; and having passed this, 
the crescent (now on the opposite side) gradually augments 
until the planet again reaches its greatest W. Elongation. 

Owing to its proximity to the Sun, observations on the 
physical appearance of Mercury are obtained with difficulty, and 
are therefore open to much uncertainty. The greatest possible 
elongation of the planet not exceeding 27 45' (and it being in 
general less), it can never be seen free from strong sunlight , 
under which conditions it may occasionally be detected with the 
naked eye during i| h or so after sunset in the spring (E. 
Elongation) and before sunrise in the autumn (W. Elongation), 
shining with a pale rosy hue. With the aid of a good telescope 
equatorially mounted, Mercury can frequently be found in the 
daytime. 

Mercury has not received much attention from astronomers in 
the present day, and the observations of Schrb'ter, at Lilienthal, 
and those of Sir W. Herschel, are the main sources of information. 
The former observer and his assistant Harding obtained what 
they believed to be decisive evidence of the existence of high 
mountains on the planet's surface : one in particular, situated in 
the Southern hemisphere, was supposed to manifest its presence 
from time to time, in consequence of the Southern horn, near 
Inferior Conjunction, having a truncated appearance, which it 
was inferred might be due to a mountain arresting the light of 
the Sun, and preventing it from reaching as far as the cusp 
theoretically extended 3 . The extent of this truncature would 
serve to determine the height of the mountain occasioning it, 

c When Mercury's Elongation is the greatest possible Elongation is a W. one 

greatest possible, the planet's position is which happens at the beginning of April, 

(in England) S. of the Sun, and there- The least (17 50') an Elongation (also 

fore the chances of seeing it are not so W.) which happens at the end of Sep- 

good as when an Elongation coincides tember. 

with a more Northerly position, albeit the d This has also been seen by Noble 

Elongation is less considerable. The (Ast. Reyit-ter, vol. ii. p. 106. May 1864). 



88 The Sun and Planets. [BOOK I. 

which has been set down at 10-7 miles, an elevation far ex- 
ceeding, absolutely, anything we have on the Earth, and in a still 
more marked degree relatively, when the respective diameters 
of the 2 planets are taken into consideration. Schroter, pursuing 
this inquiry, announced that the planet rotated on its axis in 
24 h 5 m 48". Sir W. Herschel was unable to confirm these re- 
sults either in whole or even in part 6 , and the alleged period 
of rotation we are justified in considering to be wholly a 
myth, so far at least as observation is concerned. Schiaparelli 
considers Schroter's rotation-period to be " very far from the 
truth." 

Denning and Schiaparelli think that Mercury is more easy to 
observe than Venus, and that its physical aspect resembles that 
of Mars more than any other planet. Schiaparelli 's most suc- 
cessful observations have been obtained with the planet near 
Superior Conjunction, when the defect of the diameter was 
compensated for by the fact that nearly the whole disc was to be 
seen. In such position it is then more strongly illuminated 
than at epochs of quadrature. 

Denning's observations above alluded to were made on 
November 6, 7, 9, 10, 1882, with a lo-inch reflector, power 212. 
He says: 

" Some dark, irregular spots were distinctly seen upon the planet ; also a small 
brilliant spot, and a large white area between the E. N. E. limb and terminator. 
The south horn was also much blunted, especially on the two first dates of observation. 
My results have led me to infer that the markings upon Mercury are far more 
decided and easily discernible than those of Venus ; and that the aspect of the 
former planet presents a close analogy to the physical appearance of Mars. The 
rotation-period given by Schroter seemed too short to conform with the relative 
places of the markings as I delineated them on the several dates referred to f ." 

Denning elsewhere g states that the large white area in ques- 
tion had in its centre a very brilliant small spot, " with luminous 
veins or radiations extending over the whole area." 

But it must not be forgotten in this r Month. Not., vol. xliii. p. 301. 

connection that Sir William was never March 1883. 

amicably disposed towards Schroter. g Observatory, vol. vii. p. 40. Feb. 

(See Holden's Life and Works of Sir 1884. 
W. Herschel, p. 91.) 



CHAP. IV.] 



Mercury. 



89 



Figs. 46-47 represent the planet Mercury as seen before sun- 
rise in the autumn of 1885. 

The observer remarked the truncated form of both of the 
horns on the former occasion, and of the Southern horn on the 
latter occasion. He makes no mention of any shading or spots. 

Fig. 46. Fig. 47. 




SEPT. 17, 1885, AT h 25 A.M. (Gwiot.) 



SEPT. 22, 1885, AT 5 h 30 A.M. (Guiot.) 



The phases of Mercury are noticeable, as it has sometimes 
been found that the breadth of the illuminated portion is less 
than according to calculation it should be. This does not rest 
on the testimony of Schrb'ter alone, but is supported by Beer 
and Miidler, from an observation made on September 29, 1832. 

Mercury is not known to be possessed of an atmosphere ; and 
if one exists, it must be very insignificant. Sir W. Herschel, 
contradicting Schroter and Harding, pronounced against its 
existence, and Zollner from photometric experiments on reflec- 
tion from the surface of Mercury generally, thinks that there 
cannot be any atmosphere sufficient to reflect the light of the 
Sun. [But see Book II., Chap. X., " Transits," post.~\ 

Mercury is, as far as we know, attended by no satellite, and 
the determination of its mass is a difficult and uncertain 
problem. However, the small comet of Encke has furnished the 



90 The Sun and Planets. [BOOK I. 

means of learning something, and from considerations based on 
the disturbances effected in the motion of this comet by the 
action of Mercury, it has been calculated by Encke that the 
mass of the latter is J^TTTTI ^ na ^ of the Sun. Le Verrier gives 
STnrsinRF ; Littrow sTnrHiir ; and Madler 1 w ^m ; but Newcomb 
has fixed on a fraction widely different from all these, namely, 



The ancients were not only acquainted with the existence of 
this planet h , but were able to ascertain with considerable accu- 
racy its period, and the nature of its motions in the heavens. 
" The most ancient observation of this planet that has descended 
to us is dated in the year of Nabonassar 494, or 60 years after 
the death of Alexander the Great, on the morning of the i9th 
of the Egyptian month Tkoth, answering to November 15 in the 
year 265 before the Christian era. The planet was observed to 
be distant from the right line joining the stars called /3 and 5 
in Scorpio, one diameter of the Moon ; and from the star /3 two 
diameters towards the North, and following it in Right Ascen- 
sion. Claudius Ptolemy reports this and many similar observa- 
tions extending to the year 134 of our era, in his great work 
known as the Almagest*" 

We have also observations of the planet Mercury by the 
Chinese astronomers, as far back as the year 118 A.D. These 
observations consist, for the most part, of approximations 
(appulses) of the planet to stars. Le Verrier tested many of 
these Chinese observations by the best modern tables of the 
movements of Mercury, and found, in the greater number of 
cases, a very satisfactory agreement. Thus, on June 9, 118 A.D. 
the Chinese observed the planet to be near the cluster of stars 
usually termed Praesepe, in the constellation Cancer; calcula- 
tion from modern theory shows that on the evening of the day 
mentioned Mercury was less than 1 distant from that group of 
stars. 

"Although the extreme accuracy of observations at the present 

h Pliny, Hist. Xat., lib. ii. cap. 7 ; Cicero, De Jfaltird Deoriini. lib. ii. cap. 20. 
Hind, Sol. Sy*t., p. 23. 



CHAP. IV.] Mercury. 91 

day renders it unnecessary to use these ancient positions of the 
planets in the determination of their orbits, they are still useful 
as a check upon our theory and calculations, and possess, more- 
over, a very high degree of interest on account of their remote 
antiquity k ." 

La Place said : " A long series of observations were doubtless 
necessary to recognise the identity of the two bodies, which were 
seen alternately in the morning and evening to recede from and 
approach the Sun : but as the one never presented itself until the 
other had disappeared, it was finally concluded that it was the 
same planet which oscillated on each side of the Sun." Arago 
considered that "This remark qf La Place's explains why the 
Greeks gave to this planet the two names of Apollo, the god of 
the day, and Mercury, the god of the thieves, who profit by the 
evening to commit their misdeeds." 

The Greeks gave Mercury the additional appellation of 6 2n'A- 
PO>V, "the Sparkling One." When astrology was in vogue, it 
was always looked upon as a most malignant planet, and was 
stigmatised as a sidns dolosum. From its extreme mobility 
chemists adopted it as the symbol for quicksilver. 

It is rather difficult, in a general way, to see Mercury, and 
Copernicus, who died at the age of 70, complained in his last 
moments that, much as he had tried, he had never succeeded in 
detecting it ; a failure due, as Gassendi supposes, to the vapours 
prevailing near the horizon on the banks of the Vistula where 
the illustrious philosopher lived. An old English writer, of the 
name of Goad, in 1686, humorously termed this planet "a 
squirting lacquey of the Sun, who seldom shows his head in 
these parts, as if he were in debt." 

When speaking on a previous page (see p. 75 ante) of the planet 
Vulcan, mention was made of Le Verrier's conclusion that the 
motion of Mercury's perihelion was influenced by some unknown 
cause of disturbance. Not to discuss this matter at length here 
it may be stated that Newcomb has given it as his opinion that 
the discordance between the observed and theoretical motions 

k Hind, Sol. *%/.. p. 23. 



92 The Sun and Planets. [BOOK I. 

of the perihelion of Mercury first pointed out by Le Verrier 
really exists, and is indeed larger than he supposed *. 

In computing the places of Mercury, the Tables of Baron De 
Lindenau, published in 1813, were long employed, but they are 
now superseded by the more accurate Tables of Le Verrier m . 

1 Astron. Papers for use of Amer. m Annales deTObs.de Paris, Memoires, 

Naut. Almanack, vol. i. p. 472 ; 1882. vol. v. p. I ; 1859. 



CHAP. V.] 



Venus. 



93 



CHAPTEB V. 

VENUS. ? 

Period, fyc. Phases resemble those of Mercury. Most favourably placed for obser- 
vation once in 8 years. Observations by Lihou. By Lacerda. Daylight 
observations. Its brilliancy. Its Spots and Axial Rotation. Suspected moun- 
tains and atmosphere. Its " ashy light." Phase irregularities. Suspected 
Satellite.- Alleged Observations of it. The Muss of Venus. Ancient observa- 
tions. Galileo's anagram announcing his discovert/ of its Phases. Venus useful 
for nautical observations. Tables of Venus. 

"JVTEXT in order of distance from the Sun, after Mercury, is 
Venus ; which revolves round the Sun in 224 d i6 h 49 8 s , 
at a mean distance of 67,190,000 miles. The eccentricity of the 
orbit of Venus amounting 
to only 0-007, the ex- 
tremes of distance are 
only 67,652,000 miles and 
66,728.000 miles. This ec- 
centricity is very small. 
No other planet, major or 
minor, has an eccentricity 
so small. The apparent 
diameter of Venus varies 
between 9-5" in Superior 
and 65-2" in Inferior Con- 
junction. At its greatest 

VENUS NEAK ITS GREATEST ELONGATION. 

(Schroter.} 



Fig. 48. 




Elongation its apparent 
diameter is about 25'' '. A 
numerous series of careful observations enabled Main to deter- 
mine that the planet's diameter (reduced to mean distance) is 



a Figs. 49-50 are copied, with an unimportant variation, from PI. xlii of Schroter's 
Selenotopographische Fragmente. 



94 



The Sun 



[BOOK I. 



I 7'55"y subject to a correction of 0-5" for the effects of 
irradiation. Stone, from an elaborate discussion of a large 
series of Greenwich observations, obtained 16-944", with a 
probable error of +0-08". Tennant in 1874 (during the Transit) 
obtained, as the mean of 68 measures, 16-9036" (reduced) with a 
probable error of 0-0016" only b . The real diameter corre- 
sponding to this latter evaluation is about 7500 miles, or, roundly, 
Venus is a planet almost as large as the Earth. The com- 
pression must be small, but Tennant thinks he found traces 
thereof. Great difficulty must ever remain in clearly detecting 
it, because the planet's diameter in Superior Conjunction is 
so small. 

Venus exhibits phases precisely identical in character with 
those of Mercury. 

Though under the most favourable circumstances Venus is 
never farther removed from the Sun than 47 15', and is there- 
fore always more or less under the influence of twilight, yet it 

is difficult to scrutinise this 
planet for a reason addi- 
tional to that which obtains 
with Mercury, namely, its 
own extreme brilliancy. 
This is such as to render 
the planet not unfrequently 
visible in full daylight and 
capable of casting a sen- 
sible shadow at night. This 
happened in January 1870, 
and indeed occurs every 
8 years, when the planet 
is at or near its greatest 
North latitude and about 
5 weeks from Inferior Conjunction. Its apparent diameter is 
then about 40", and the breadth of the illuminated part nearly 
10", so that rather less than -J of the entire disc is illuminated : 

fc Motk. Xof.. vol. XXTV. p. 347. May 1875. 



Fig. 49- 




VENUS NEAR ITS I.NFtKIuR CONJUNCTION. 



CHAP. V ] 



1V////X. 



95 



but this fraction transmits more light than do phases of greater 
extent, because the latter occur at greater distances from 
the Earth. A lesser maximum of brilliancy, due to the same 
circumstances less favourably carried out, occurs on either side 
of the Sun at intervals of about 29 months. The planet's 
angular distance from the Sun on these occasions is rather less 
than 40 (in the superior part of its orbit) ; its phase therefore 
corresponds with the phases of the Moon when n d and i7 d old. 
Figs. 50-1 are selected from some drawings by Lihou taken 
in the winter of 1885-6 with a refractor of 4^ inches aperture. 



Fig- 51- 




Nov. 10, 1885. 



Dec. 23, 1885. 



He makes c the following remarks on what he saw : 

Nov. 10, 1885. " With a telescope of about 4 inches aperture armed with a 
magnifying power of 100 I was able to distinguish a grey spot in the northern 
hemisphere. Spots on Venus being very difficult to see with small instruments, this 
observation merits attention." 

Dec. 8, 1885. "Sky very pure. The light of Venus is so bright as to fatigue 
the eye, but by making use of a coloured glass I am able to see the limbs sharply 
defined." 

Dec. 16, 1885. "Sky very pure. The image of Venus is extremely sharp. 
and the limbs well denned ; the northern cusp is sharply pointed, whilst the southern 
i- slightly truncated." 

Dec. 23, 1885. " The northern cusp of Venus is sharply pointed, and the southern 
cusp slightly truncated." 



' L' Aflronmfilf. vol. v. p. 148, April 1886. 



The Sun and Planet*. 



[BOOK I. 



Figs. 52-5 are intended to represent some drawings of Venus 
made in 1884 by M. Lacerda of Lisbon. Respecting these he 
writes as follows : 

"Sept. 8, 1884. The crescent of Venus appears sensibly more narrow towards the 
North Pole than towards the South Pole. With a magnifying power of 250 I 
cannot distinguish the Southern spots, which, however, were very visible with a 
magnifying power of 160. I notice that the Northern hemisphere is brighter than 
the rest of the planet. A very obscure and elongated spot is visible near the North 
Pole." 



Fig- 52- 



Fig. 53- 





Sept. 8, 1884. 



Sept. 9, 1884. 



VENUS. 



" Sept. 9, 1884. There is a very bright thread of light concentric with the 
Eastern limb of the planet ; perhaps some high clouds lying along a maritime shore 
of Venus. Two large spots are also visible on the crescent ; the one, oblong, stretched 
parallel to the bright spot ; the other, almost round, and much smaller, to the 
North of the first. The Southern horn is always longer than the Northern one. The 
elongated spot which hollows out the planet near the North Pole continues to be 
very visible. 

M. Lacerda says that on the following morning, Sept. 10, he 
was unable to distinguish any spots. 
His next observation is dated 

"Oct. 8, 1884. The 2 dark spots have sensibly shifted their positions towards the 
North. They disclose also a slight movement towards the West. The terminator 
which seemed shrunk up towards the North Pole is to-day almost perfect ; but the 
Southern horn continues to appear longer and more pointed than the Northern one. 
The lustre of the planet seems uniform. The dark spot which cut into the crescent 
near the North Pole is not visible." 



CHAP. V.] 



Venus. 



97 



" Oct. 13, 1884. There is a great depression near the Southern horn. 2 spots are 
visible on the planet ; one to the South ; the other, smaller, and to the North ; and 
a third was suspected under the equator, near the illuminated limb and concentric 
with it. The Northern horn is truncated." 

M. Lacerda concludes his observations by remarking that the 
most favourable time for observing Venus is between J an hour 
before sunrise and i hour after sunrise. He adds that he was 
never able to see any spots when the planet was in the west, at 
or near the time of sunset . 



Fig. 54- 



Fig- 55- 




Oct. 8, 1884. 



Oct. IT, 1884. 



Observations of Venus in the daytime were made at a very 
early period ; the following are the dates of a few instances : 
398 A.D., 984, 1008, 1014, 1077, 1280, 1363, 1715, 1750. "Bouvard 
has related to me," says Arago, " that General Buonaparte, upon 
repairing to the Luxembourg, when the Directory was about to 
give him a fete, was very much surprised at seeing the multi- 
tude which was collected in the Rue de Tournon pay more 
attention to the region of the heavens situate above the palace 
than to his person or to the brilliant staff which accompanied 
him. He inquired the cause, and learned that these curious 
persons were observing with astonishment, although it was 

c ISAstronomie, vol. iii. p. 462, Dec. 1884. 
H 



98 The Sun and Planets. [BOOK I. 

noon, a star, which they supposed to be that of the Conqueror 
of Italy; an allusion to which the illustrious general did not 
seem indifferent when he himself with his piercing eyes re- 
marked the radiant body. The star in question was no other 
than Venus d ." 

The dazzling brilliancy of this planet is such 6 that the 
daytime is to be preferred for observing it, but under the best 
of circumstances it is far too tremulous for physical observations 
to be conveniently made. J. D. Cassini attacked it in 1667, and 
some ill-defined dusky spots seen on various occasions during 
April, May, and June, enabled him to assign 23** I5 m for its axial 
rotation. Bianchini, at Rome, in 1726 and 1727, favoured by an 
Italian sky, observed spots with greater facility: thence he 
inferred a rotation performed in 24 (Jays 8 hours. Cassini's son 
came forward in defence of his father's observations, and assailed 
Bianchini's conclusions by alleging that the latter, only seeing 
Venus for a short time every evening by reason of the Barbarini 
Palace interrupting his view, and finding the spots night after 
night nearly in the same position, concluded that the planet had 
rotated through a very small arc during the previous 24 hours, 
whereas it had really made one complete rotation and part of 
a second. After the lapse of 24 days it would exhibit exactly 
the same portion of its surface, but in the 24-days' interval would 
really have made 25 revolutions instead of one, as Bianchini had 
supposed. Bianchini's observations thus interpreted imply a 
period of 23 h 2 m . 

Sir W. Herschel, desirous of arriving at some certain know- 
ledge on the subject, devoted much care to the matter ; but, 
failing to see any permanent markings on Venus, he was unable 
to assign a precise period beyond believing generally that 
Bianchini's statement was largely in excess of the true amount. 
Schroter claimed to have seen certain spots which enabled him to 
deduce a period of 23 b 2i m 7'98 8 , and Di Vico and his colleagues 

d Pop. Ast., vol. i. p. 701, Eng. ed. times -as bright as the brightest part of 

6 Lord Grimthorpe states that Venus the full moon. (Ast., 3rd ed.,p. 149.) 
has been experimentally found to be 10 



CHAP. V.] Venus 99 

at Rome, in 1840-2, rediscovering as they thought Bianchini's 
markings, assigned a period of 23 h 2i m 23'93 S . 

In spite of the seemingly circumstantial character of these 
evaluations it cannot be said that astronomers generally are 
satisfied to accept them, or to think that anything at all con- 
clusive is at present known as to the real duration of Venus's 
axial rotation. 

Sir W. Herschel saw a few transient spots, but his opinion 
was that they were in the atmosphere, and did not belong to 
the solid body of the planet. Di Vico, however, professed to 
have found the spots just as they had been delineated by 
Bianchini, with one exception. Of the several observers who 
worked with Di Vico the most successful were those who had 
most difficulty in catching very minute companions to large 
stars, the reason of which is obvious. A very sensitive eye, 
which would detect the spots readily, would be easily over- 
powered by the light of a brilliant star, so as to miss a very 
minute one in its neighbourhood. 

On Nov. 10, 1885, Lihou saw a gray spot in the Northern 
hemisphere of Venus as depicted in Fig. 50, ante. 

Mountains probably exist on Venus, though the testimony on 
which the statement must rest is not so conclusive as could be 
desired. In August 1700 La Hire, observing the planet in the 
daytime near its Inferior Conjunction, perceived in the lower 
region of the crescent inequalities which could only be produced 
by mountains higher than those in the Moon. Schroter asserted f 
the existence of several high mountains, in which he was con- 
firmed by Beer and Madler, but his details as to precise elevation 
measured by toises must be accepted with great reserve, amongst 
other reasons because it is doubtful whether his micrometers 
were of sufficient delicacy. Sir W. Herschel disbelieved him on 
some points, and attacked him in the Philosophical Transactions 
for I793 g : his reply was published in the volume for the year 
but one after h ; it was calm and dignified, and vindicated the 

' Phil. Trans., vol. Ixxxii. p. 337. 1792. h Phil. Trans., vol. Ixxxv. p. 117. 

g Phil. Trans., vol. Ixxxiii.p. 202. 1793. i?95- 

H 2 



100 The Sun and Planets. [BOOK I. 

mountains, if not the measurements. Di Vico, at Rome, in April 
and May 1841, appears to have noticed a surface-configuration 
akin to that of the Moon ; and Lassell, when at Malta in January 
1862, observed the same sort of thing. Browning, on March 14, 
1868, saw mottlings on the surface of Venus which reminded him 
of the look of the Moon as seen in a small telescope through a 
mist. A bluntness of the southern horn, referred to by Schroter, 
was also seen by the Roman astronomers, and often by Breen 
subsequently with the Northumberland telescope at Cambridge. 

That Venus has an atmosphere is almost certain ; that it is of 
considerable density is likewise an opinion apparently well 
founded. During the transits of 1761, 1769, and 1874, the planet 
was observed by several persons to be surrounded by a faint 
ring of light, such as an atmosphere would account for. 
Schroter, too, discovered what appeared to him to be a faint 
crepuscular light extending beyond the cusps of the planet into 
the dark hemisphere. From micrometrical measures of the 
space over which this light was diffused he considered the 
horizontal refraction at the surface of the planet to amount to 
30' 34", or much the same as that of the Earth's atmosphere. 
Sir W. Herschel confirmed the discovery as a whole h , and more 
recently Madler, in 1849, was able to do the same with the mere 
modification of making the amount somewhat greater, or equal 
to 43-7'. With this the Transit results of 1874 fairly agree ; e.g., 
Prof. C. S. Lyman, 44-5'. ' 

It is quite worth while to dwell upon the observations on 
which these conclusions rest, for the subject deserves much more 
telescopic investigation than has hitherto been given to it, and 
it is one within the reach of many amateurs. 

The observations must be made when the planet is very near 
Inferior Conjunction. Under such circumstances the limb of the 
planet which is farthest from the Sun is often to be seen 
illuminated, exhibiting a curved line of light ; this is a con- 

11 Phil. Trans., vol. Ixxxiii. p. 214. 54-4' and 53-5' respectively, an error 

1793. having crept in owing to an erroneous 

1 Neison suggests that Madler's and formula having been used. (Month. Not., 

Lyman's results must be increased to vol. xxxvi. p. 347, June 1876.) 



CHAP. V.] Venus. 101 

tinuation of the narrow crescent of the planet itself, and the 
result is, that the planet seems to be surrounded by a complete 
circle of light. " If only half the globe of the planet were 
illuminated by the Sun, this appearance could never present 
itself, as it is impossible for an observer to see more than half 
of a large sphere at one view. There is no known way in which 
the Sun can illuminate so much more than the half of Venus as 
to permit a complete circle of light to be seen, except by the 
refraction of an atmosphere k ." 

The existence of snow at the poles of Venus has been suspected 
by Webb and Phillips, but the idea awaits confirmation, though 
there is no prima facie reason why it should not be well founded ; 
indeed rather the reverse. 

A phenomenon analogous to the lumiere cendree, or ' ashy light,' 
of the Moon is well attested in observations of Venus when near 
Inferior Conjunction, having been first seen by Riccioli on 
Jan. 9, 1643. Many observers have noticed the entire contour 
of the planet to be of a dull grey hue beyond the Sun-illumined 
crescent. Webb used the expression "the phosphorescence of the dark 
side " ; this certainly is an objectionable phrase, for phosphor- 
escence notably conveys the idea that some inherent light is spoken 
of, whereas there can be little doubt that refraction and reflec- 
tion jointly are in some way or other the cause of what is seen 
in the case of Venus, though it may be difficult at present to 
specify the precise nature of it ] . Derham noticed this appear- 
ance, and refers to it in his book m ; and Schroter, Sir W. 
Herschel, Di Vico, and Guthrie n are amongst those that have 
seen it. Green, Winnecke, Noble, and others have repeatedly 
seen the unilluminated limb of Venus distinctly darker than the 
back-ground on which it was projected. The most recent 
observations in detail of this phenomenon are those made by 
Zenger, at Prague, in Jan. 1883. He speaks in strong terms of 

k Newcomb, Popular Astronomy, p. m Physics and Astro-theology, vol. ii. 

2 93- book v. ch. I. 

1 The supposition of the existence of Month. Not., vol. xiv. p. 169. March 

some such phenomenon as our Aurora 1854. 
Borealis rests on no foundation. 



102 The Sun and Planets. [BOOK I. 

the beauty of the spectacle when seen under favourable circum- 
stances as regards the planet's position and the condition of the 
Earth's atmosphere. He noticed, and considered the most im- 
portant point of all, a brownish red ring all round the planet's 
disc, " more pronounced on the illuminated side than on the dark 
part of the limb, but of a peculiar coppery hue, the close resem- 
blance of which to the coppery hue the Moon's disc assumes 
when totally eclipsed was very striking." He goes on to 
express the opinion that the two appearances owe their origin to 
precisely similar causes . 

The peculiarity about Mercury's phases already pointed out 
(the measured breadth being different from the calculated) 
obtains also with Venus. At the Greatest Elongations, the line 
terminating the illumination ought to be straight, as with a 
Half-Moon, but several observers have found an uncertainty 
varying between 3 d and 8 d in the first (or last) appearance of 
the dichotomisalion (according as to whether it was the E. or the 
W. Elongation that was in question). Thus, at the Western 
Elongation of August 1793, Schroter found the terminator 
slightly concave, and it did not become straight till 8 d after the 
epoch of Greatest Elongation. 

Previous to the present century testimony was not wanting 
that Venus had a satellite, but nothing has been ascertained 
about it in recent times, and Webb, with great propriety, called 
the matter "an astronomical enigma." On Jan. 25, 1672, J. D. 
Cassini saw, between 6 h 52 and 7 h 2 m A.M., a small star re- 
sembling a crescent, like Venus, distant from the Southern horn 
on the Western side by a space equal to the diameter of Venus. 
On Aug. 28, 1686, at 4 h 15 A.M., the same experienced observer 
saw a crescent-shaped light East of the planet at a distance of 
snhs o f fa diameter. Daylight rendered it invisible after ^ an 
hour. On Oct. 23, 1740 (o.s.), Short, the celebrated optician, 
with 2 telescopes and 4 different powers, saw a small star 
perfectly defined but less luminous than the planet, from which 

Zenger's paper should be consulted Month. Not., vol. xliii. p. 331. April 
by all who wish to study this subject. 1883. 



CHAP. V.] Venus. 103 

it was distant 10' 2". On 4 different occasions between May 3 
and u, 1761, Montaigne, at Limoges, saw what he believed to 
be a satellite of Venus. It presented the same phase as the 
planet, but it was not so bright. Its position varied, but its 
diameter appeared equal to " h that of the planet. The follow- 
ing extract is from the Dictionnaire de Physique, a French work 
published in 1789. "The year 1761 will be celebrated in 
astronomy in consequence of the discovery that was made on 
May 3 of a satellite circulating round Venus. We owe it to 
M. Montaigne, member of the Society of Limoges, who observed 
the satellite again on the 4 th and 7 th of the same month. 
M. Baudouin read before the Academy of Sciences of Paris a 
very interesting memoir, in which he gave a determination of 
the revolution and distance of the said satellite. From the 
calculations of this expert astronomer we learn that the new 
star has a diameter about % that of Venus, that it is distant 
from Venus almost as far as the Moon is from the Earth, that 
its period is 9 d 7 h , and that its ascending node is in the 22 nd 
degiee of Virgo." Wonderfully circumstantial! In March 1764 
several European observers, at places widely apart, saw a 
supposed satellite. Rb'dkier, at Copenhagen, on March 3 and 4, 
saw it : Horrebow, with some friends, also at Copenhagen, saw 
it on the io th and n th of the same month, and they stated that 
they took various precautions to make sure there was no 
optical illusion. Montbaron, at Auxerre, on March 15, 28, and 
29, saw the satellite in sensibly different positions p . 

This is the plaintiff's case, if I may be pardoned for using 
such an expression : on the other side it can only be said that 
no trace of a satellite has ever been found by any subsequent 
observer with larger telescopes. And with the care bestowed 
on Venus by Sir W. Herschel and Schroter during so many 
years, it is difficult to understand that, if a satellite existed, they 
should not have seen it at some time or other q . 

p Scheuten says he saw a satellite ac- letter by Lynn in The Observatory, vol. 

company Venus across the Sun during x. p. 73, March 1887. 
the transit of 1761. See Ast. Jahrbuck, 1 The question of the existence of a 

1778. Keference may also be made to a satellite of Venus is very fully discussed, 



104 The Sun and Planets. [BOOK I. 

Lambert combined all the observations in a very tolerable 
orbit r , but, as Hind points out 8 , notwithstanding its agreement 
with the observations, there is one fatal objection to it if it 
were correct, the mass of Venus would be 10 times greater than 
what other methods show it to be, namely 4^^2x1 * na ^ f the 
Sun. Encke gives TTr rV5jF> Littrow Ttnr V7T> Miidler TTTT V T ^, 
Le Verrier ^ r ^mF an d Newcomb T^^^S- There are several 
methods of ascertaining this quantity, the most obvious of which 
is based on the disturbing influence exerted by Venus on the 
Earth's annual motion. 

Venus has ever been regarded as an interesting and popular 
planet, and it is somewhat remarkable that it is the only one 
whose praises are sung by the great Greek bard, who thus 
apostrophises it: 

" "Eairepo j, Ss Ka\\urros tv ovpavy i<JT<nti dar^p*." 

This refers to it as the Evening Star, but elsewhere in the 
Iliad n we meet with it in its other function of the 'EaxrQopos, to 
which the Latin Lucifer corresponds. Some have thought, and 
perhaps not without reason, that it is the object referred to in 
Isaiah xiv. 12. 

The earliest recorded observations of Venus date from 686 B.C., 
and appear on an earthenware tablet now in the British 
Museum x . 

" Claudius Ptolemy has preserved for us in his Almagest many 
observations of Venus by himself and other astronomers before 
him, at Alexandria in Egypt. The most ancient of these obser- 
vations is dated in the 476 th year of Nabonassar's era and 13 th of 

and from a new standpoint, in a paper seen; and in one instance possibly it 
by M. Bertrand in IS Astronomic, vol. i. was Uranus which was seen and mistaken 
p. 201 , August 1882 ; but it does not seem for a satellite of Venus. 
worth while to go more fully into the sub- r Bode's Jahrbach, 1777. 
ject here. And see also M. Stroobant's 8 Sol. Syst., p. 27. 
very interesting Etude sur le satellite * Homer, Hind, lib. xxii. v. 318. 
tnigmatique de Venus published at u Lib. xxiii. v. 226. Pythagoras (or, 
Brussels in 1887. His researches show that according to others, Parmenides) deter- 
in almost all cases stars which can be iden- mined the identity of the two " stars." 
tified were mistaken for a satellite ; in a T Month. Not., vol. xx. p. 319. June 
few instances where the identity is 1860. 
doubtful possibly a minor planet was 



CHAP. V.] Venus. 105 

the reign of Ptolemy Philadelphia, on the night of the 17*'' of the 
Egyptian month Messori, when Timocharis saw the planet eclipse 
a star at the extremity of the wing of Virgo. This date answers 
to 271 B.C., Oct. 12 A.M. y " As this was not a telescopic observa- 
tion, it and all others recorded before telescopes came into use, 
are open to this uncertainty, that the two objects may merely 
have been in juxta-positon so as to have appeared as one without 
actual super-position taking place. The recorded occultation of 
Mercury by Venus on May 17, 1737, was no doubt an occultation 
in the strict sense of the word. 

The interesting discovery of the phases of Venus is due to 
Galileo z , who announced the fact to his friend Kepler in the 
following logogriphe or anagram a : 

" Haec immature, a me, jam frustra, leguntur. oy." 
"These things not ripe [for disclosure] are read, as yet in vain, by me." 

Or, as another interpretation has it 

" These things not ripe ; at present [read] in vain [by others] are read by me." 

The "me" in the former case being the ordinary reader ; in 
the latter, Galileo. 

This, when transposed, becomes 

"Cynthiae figuras aemulatur Mater Amorum." 
" The Mother of the Loves [Venus] imitates the phases of Cynthia [the Moon]." 

The letters ' o y ' are, it will be observed, redundant, so far that 
they cannot be made use of in the transposition. 

To the mariner, owing to its rapid motion, Venus is a useful 
auxiliary for taking lunar distances when continuous bad weather 
may have prevented observations of the Sun. 

In computing the places of Venus the tables of Baron De Lin- 
denau, published in 1810, were long in use. but they have now 

y Hind, Sol. Syst., p. 32. more distinctly, they would be found to 

1 It was one of the objections urged do so. Prof. De Morgan believes the 

to Copernicus against his theory of the anecdote to be apocryphal. (Month. Not., 

solar system that if it were true then the vol. vii. p. 290. June 1847.) But "se 

inferior planets ought to exhibit phases. non e vero, e ben trovato." 

He is said to have answered that if ever * Opere di Galileo, vol. ii. p. 42. Ed. 

men obtained the power of seeing them Padova, 1 744. 



106 The Sun and Planets. [BOOK I. 

been superseded by those of Le Verrier, for amongst other causes 
of error there existed a long inequality (first suspected by Sir G. 
B. Airy about 1828, and fully expounded in 1831 b ) affecting the 
heliocentric places of the Earth and the planet to a very sensible 
amount. This inequality goes through all its changes in about 
239 y , and when at a maximum displaces Venus by 3" and the 
Earth by 2", as viewed from the Sun. 

b Phil. Trans., vol. cxviii. p. 23, 1828 ; vol. cxxii. p. 67, 1832. 



CHAP. VI.] The Earth. 107 



CHAPTEK VI. 

THE EARTH. 



" let the Earth bless the Lord : yea, let it praise Him, and magnify Him 
for ever." Benedicite. 



Period, Sfc. Figure of the Earth. The Ecliptic. The Equinoxes. The Solstices. 
Diminution of the obliquity of the ecliptic. The eccentricity of the Earth's 
orbit. Motion of the Line of Apsides. Familiar proofs and illustrations of 
the sphericity of the Earth. Foucaulfs Pendulum Experiment. Madler's tables 
of the duration of day and night on the Earth. Opinions of ancient philosophers. 
English mediceval synonyms. The Zodiac. Mass of the Earth. 



^f^HE Earth is a planet which may perhaps be said to be in 
all essential respects similar to Venus and Mars, its nearest 
neighbours ; but as we are on it, it is needless to point out the 
impossibility of treating of it in the same way as we treat of the 
other planets. It revolves round the Sun in <$6$ d 6 h 9 m 9'6 S , at 
a mean distance of 92,890,000 miles. The eccentricity of its 
orbit amounting to 0-01679, this distance may either extend to 
94,450,000 miles or diminish to 91,330,000 miles ; and these 
differences involve variations in the light and heat reaching the 

I Earth which will be represented by the figures 966 and 1033, 
the mean amount being 1000. 
The Earth is not a sphere, but an oblate spheroid ; that is to 
say, it is somewhat flattened at the poles and protuberant at the 



108 



The Sun and Planets. 



equator; as is the case with probably all of the planets, 
following table gives the latest authentic measurements. 



[BOOK I. 
The 





Airy . 


Besselb. 


Polar Diameter 


Miles. 
78QQ-I7O 


Miles. 
78QQ.II4 


Equatorial Diameter 


7925-648 


7925-604 


Absolute Difference 


26-478 


26-490 


Excess of the Equatorial, ex- 
pressed as a fraction of its 
entire length 


i 

2 ..(8*330 


i 

2' 192 



The close coincidence between these results affords a good 
guarantee of the accuracy of both, and is noticeable as an illus- 
tration of the precision arrived at in the working out of such 
problems, the difference between the two values of the equatorial 
diameter being only 77 yards. If we represent the Earth by a 
sphere i yard in diameter, that diameter will make the polar 
diameter |- inch too long. 

Further, it has been suspected by General Schubert and 
Colonel A. R. Clarke that the equatorial section of the Earth is 
not circular, but elliptical. Colonel Clarke's conclusion is that 
the equatorial diameter, which pierces the Earth through the 
meridians 13 58' and 193 58' E. of Greenwich, is i mile longer 
than the equatorial diameter at right angles to it c . 

A consideration of the method in which such investigations 
are conducted does not fall within the scope of the present 
sketch, but in Airy's Popular Astronomy the subject of the Figure 
of the Earth is handled with much clearness d . 

The great circle of the heavens apparently described by the 
Sun every year (owing to our revolution round that body) is 
called the Ecliptic 6 , and its plane is usually employed by astro- 
nomers as a fixed plane of reference. The plane of the Earth's 
equator, extended towards the stars, marks out the equator of 
the heavens, the plane of which is inclined to the ecliptic at an 



Encycl Meirop., art. Fig. of Earth, 
vol. v. p. 220. 

b Ant. Nach.. vol. xiv. Nos. 333-5 ; vol. 
xix. No. 438. 



c Mem. R.A.S., vol. xxix. p. 39. 1861. 

d See p. 242 et seq. 

e " The line of eclipses." 



CHAP. VI.] The Earth. 109 

angle which, on Jan. i, 1880, amounted to 23 27' i7'55"; this 
angle is known as the Obliquity of the Ecliptic. It is this inclination 
which gives rise to the vicissitudes of the seasons during our 
annual journey round the Sun. The two points where the 
celestial equator intersects the ecliptic are called the Equinoxes { ; 
the points midway between these being the Solstices*. It is from 
the vernal (or spring) equinox that Right Ascensions are 
measured along the equator, and Longitudes along the ecliptic. 
The obliquity of the ecliptic is now slowly decreasing at the rate 
of about 46" in JOG years. "It will not always however, be on 
the decrease ; for before it can have altered 1 1 the cause which 
produces this diminution must act in a contrary direction, and 
thus tend to increase the obliquity. Consequently the change 
of obliquity is a phenomenon in which we are concerned only as 
astronomers, since it can never become sufficiently great to pro- 
duce any sensible alteration of climate on the Earth's surface. 
A consideration of this remarkable astronomical fact cannot but 
remind us of the promise made to man after the Deluge, that 
' while the earth remaineth, seedtime and harvest, and cold and 
heat, and summer and winter, and day and night shall not cease.' 
The perturbation of obliquity, consisting merely of an oscillatory 
motion of the plane of the ecliptic, which will not permit of its 
[the inclination] ever becoming very great or very small, is an 
astronomical discovery in perfect unison with the declaration 
made to Noah, and explains how effectually the Creator had 
ordained the means for carrying out His promise, though the way 
it was to be accomplished remained a hidden secret until the 
great discoveries of modern science placed it within human 
comprehension V 

It is stated by Pliny that the discovery of the obliquity of the 
ecliptic is due to Anaximander, a disciple of Thales, who was 

f From cequus equal, and nox a night; still ; because the Sun when it hag reached 

because when the Sun is at these points, these neutral points has attained its 

day and night are theoretically equal greatest declination N. or S. as the case 

throughout the world. In 1890 this oc- may be. In 1890 this occurs on June 

curs on March 20 at 4 h , and Sept. 22 at 21 at o h , and Dec. 21 at 9 h , G.M.T. 
I4 h , G.M.T. Hind, Sol. Syst., p. 33. 

* From .90? the Sun, and store to stand 



110 Tlie Sun and Planets. [BOOK I. 

bom in 610 B.C. Other authorities ascribe it to Pythagoras or 
the Egyptians, while Laplace believed that observations for the 
determination of this angle were made by Tcheou-Kong in 
China not less than noo years before the Christian era 1 . The 
accord between the various determinations ancient and modern 
is very remarkable, and indicates the great care bestowed by the 
astronomers of antiquity on their investigations. 

The eccentricity of the Earth's orbit amounts (to be more 
precise than above) to o - oi679i7, and it is subject to a very 
small diminution, not exceeding 0*000041 in the course of 100 
years. Supposing the change to go on continuously, the Earth's 
orbit must eventually become circular ; but we learn from the 
Theory of Attraction that this progressive diminution is only to 
proceed for a certain time. Le Verrier has shown that this 
diminution cannot continue beyond 24,000 years, when the 
eccentricity will be at its minimum of '0033 : it will then begin 
to increase again ; so that unless some external cause of pertur- 
bation arise, these variations may continue throughout all ages, 
within certain not very wide limits. They are due to the 
attractive influence of the Planets. The above value of the 
eccentricity is for i Sco'o A.D. 

The line of apsides is subject to an annual direct change of 
1 1 '7 7", independent of the effects of precession (to be described 
hereafter) ; so that, allowing for the latter cause of disturbance, 
the annual movement of the apsides may be taken at rather more 
than i'. One important consequence of this motion of the major 
axis of the Earth's orbit is the variation in the lengths of the 
seasons at different periods of time. In the year 3958 B.C., or, 
singularly enough, near the epoch of the Creation of Adam, the 
longitude of the Sun's perigee coincided with the autumnal 
equinox ; so that the summer and autumn quarters were of equal 
length, but shorter than the winter and spring quarters, which 
were also equal. In the year 1267 A.D. the perigee coincided 
with the winter solstice ; the spring quarter was therefore equal 
to the summer one, and the autumn quarter to the winter one, 

1 Conn, des Temps. 1811, p. 429. 



CHAP. VI.] T/ie Earth. Ill 

the former being the longest. In the year 6493 A - D - * ne perigee 
will have completed half a revolution, and will then coincide 
with the vernal equinox ; summer will then be equal to autumn, 
and winter to spring; the former seasons, however, being the 
longest. In the year 11719 A.D. the perigee will have completed 
three-fourths of a revolution, and will then coincide with the 
summer solstice ; autumn will then be equal to winter, but longer 
than spring and summer, which will also be equal. And finally 
in the year 16945 A.D. the cycle will be completed by the coinci- 
dence of the solar perigee with the autumnal equinox. This 
motion of the apsides of the Earth's orbit, in connection with 
the inclination of its axis to the plane of it, must quite obviously 
have been the cause of very remarkable vicissitudes of climate 
in pre- Adamite times k . 

One result of this position of things we may readily grasp at 
this moment. As a matter of fact, in consequence of our seasons 
being now of unequal length, the spring and summer quarters 
jointly extend to i86 d , whilst the autumn and winter quarters 
comprise only i78 d . The Sun is therefore a longer time in the 
Northern hemisphere than in the Southern hemisphere : hence 
the Northern is the warmer of the two hemispheres. Probably 
it may be taken as one result of this fact, that the North 
Polar regions of the Earth are easier of access than the 
South Polar regions. In the Northern hemisphere navigators 
have reached to 81 of latitude, whereas 71 is the highest 
attained in the Southern hemisphere. 

It is not a very easy matter in treating of the Earth to deter- 
mine where astronomy ends and geography begins ; but a brief 
allusion to the means available for deciding the form of the Earth 
seems all that it is now necessary to add here. We learn that 
the Earth is a sphere (or something of the sort) by the appear- 
ance presented by a ship in receding from the spectator : first 
the hull disappears, then the lower parts of the rigging, and 
finally the top-masts. The shadow cast on the Moon during a 

k See Papers by Croll, Phil. Mag,, 4th xxxvi. pp. 141 and 362, Aug. and Nov. 
Ser., vol. xxxv. p. 363, May 1868; vol. 1868; Geikie's Great Ice Age, &c. 



112 The Sun and Planets. [BOOK I. 

lunar eclipse, and the varying appearances of the constellations 
as we proceed northwards or southwards, are amongst the other 
more obvious indications of the Earth's globular form. 

Fig. 56, Plate VI, represents an experimental proof of the 
Earth's rotation on its axis. This particular form of proof excited 
no small interest in scientific (and unscientific) circles when it 
was first promulgated by the French savant Foucault in the year 
I85I 1 . If a pendulum, or its equivalent, a heavy weight sus- 
pended by a long wire, could be erected at either pole of the 
Earth, and be set swinging in any direction and a note of the 
direction taken, it is evident that if the plane of oscillation 
were observed to be perpetually shifting with regard to the 
terrestrial point noted at the beginning of the experiment, it 
would be a proof that either the terrestrial station was shifting 
with respect to the pendulum or the pendulum was shifting with 
respect to the station. The latter idea being contrary to reason 
tne former alternative must be adopted. It is evident that both 
poles of the Earth being inaccessible to us, the experiment 
cannot be carried out in the theoretically simple fashion sug- 
gested above ; but in a modified form it can be tried and will 
yield an intelligible result at a station on the Earth's surface 
between the Pole and the Equator, provided it be not very near 
the Equator. The rationale of the experiment is simply this, 
that the weight being made to oscillate in a straight line (and 
starting it by burning the thread which holds it should secure 
this) it will swing backwards and forwards in an invariable 
plane. If the building in which the experiment is tried were at 
rest, the plane of oscillation would be constantly parallel to a 
line joining any 2 points in the building if the pJane of oscilla- 
tion had been parallel to that line when the start was made. 
But if the building moves in consequence of an axial rotation 
of the Earth, the angle between the plane of oscillation and the 
line parallel thereto at the start will be continually varying and 
in the course of some hours will vary through an angular space 
of many degrees. Could the experiment be tried at the Pole the 

1 See Proc. Soy. Inxt., vol. i. p. 70: Arago, Pop. Ast., Eng. ed., vol. ii. p. 27. 



Fig. 56. 



Plate VI. 




FOUCAULT'S PENDULUM EXPERIMENT TO SHOW 

THE EARTH'S AXIAL ROTATION. 

I 



CHAP. VL] The Earth. 115 

angular variation would be the whole 360 of a circle, in the time 
24 hours, being the duration of the sidereal day. 

At the Equator there will be no visible effect, for the point of 
suspension will be carried round the Earth's axis equally with 
the ground beneath the weight ; on the other hand, because the 
point of suspension at the Pole was at the Pole it would have 
no motion at all and the plane of vibration would be telling its 
own tale every instant. For a station intermediate between the 
Pole and the Equator the effect will be, so to speak, of an 
intermediate character ; the ground will shift to a certain extent, 
but not through the angle of 360 in 24 hours. The extent of 
the shifting will vary with the latitude, so that it will not 
always be easy to obtain a covered building free from currents 
of air, and with an available point of suspension sufficiently 
elevated above the ground to insure the vibration going on long 
enough to enable the experiment to be readily visible to an 
audience. 

This experiment was first tried by Foucault at the Pantheon in 
Paris, and subsequently in London at The Russell, London, Poly- 
technic, and Royal Institutions and King's College, and at York, 
Bristol. Dublin, Aberdeen, New York, Ceylon, and other places. 
The angular deviation for i hour was found to be at Paris n-i; 
at Bristol uf; at Dublin nearly 12; and at Aberdeen about 
i2|, whilst at New York (Lat. 40) it was only 9! and in 
Ceylon (Lat. 7) only 1-8. 

Binet calculated that the time required for one revolution of 
the pendulum in the latitude of Paris would be 32 h 8 m . At 
Dublin a complete revolution was watched and observed to 
occupy 28 h 26 m . 

In the engraving the figures i, 2, 3, 4, 5, 6, are supposed to 
indicate the hours of the duration of the experiment after the 
pendulum has been set in motion by the severance by the candle- 
flame of the cord which held the weight at rest. 

O 

The following table of the greatest possible length of the day 
in different latitudes I cite from Madler 1 ": 

m Populare Astronomie, Berlin 1861, p. 30. 
I 2 



116 



The Sun and Planets. 



[BOOK I. 



Hours. 

O O 12 65 48 22 

16 44 13 66 21 23 

30 48 14 66 32 24 

41 24 15 67 23 I month. 

49 2 16 6q .51 2 

54 3 1 J 7 73 40 3 

58 27 18 78 ii 4 

61 19 

63 23 

64 50 

The 8646 hours which make up a year, are, according to 
Madler, thus distributed : 



Hours. 


o / 


12 


65 48 


13 


66 21 


14 


66 32 


15 


67 23 


16 


69 51 


17 


73 40 


18 


78 ii 


19 84 5 


20 


90 o 


21 





At the Equator. 

4348 hours Day, 

852 Twilight, 
3449 Night. 



At the Poles. 

4389 hours Day, 
2370 Twilight, 
1887 Night. 



Among the ancients, Aristarchus of Samos, and Philolaiis, 
maintained that not only did our globe rotate on its own axis, 
but that it revolved round the Sun in 12 months . Nicetas of 
Syracuse is also mentioned as a supporter of this doctrine . 
The Egyptians taught the revolution of Mercury around the 
Sun p ; and Apollonius Pergseus assigned a similar motion to 
Mars, Jupiter, and Saturn but I am digressing. 

Hesiod states that the Earth is situated exactly half-way 
between Heaven and Tartarus : 

" From the high heaven a brazen anvil cast, 
Nine days and nights in rapid whirls would last, 
And reach the Earth the tenth ; whence strongly hurl'd, 
The same the passage to th' infernal world." 

Theogonia, ver. 721. 

Our ancestors 300 or 400 years ago termed the ecliptic the 
"thwart circle"; the meridian, the "noonsteede circle"; the 
equinoxial, "the girdle of the sky"; the Zodiac, "the Bestiary," 



n Archimedes, In Arenario ; Plutarch, 
De Placit. Philos., lib. ii. cap. 24 ; Diog. 
Laert. In Philolao. 



Cicero, Acad. Quast., lib. ii. cap. 39. 
p Macrobius, Comment, in Somn. Scip., 
lib. i. cap. 19, and others. 



CHAP. VI.] The Earth. 117 

and "our Lady's waye." The origin of the division of the 
zodiac into constellations is lost in obscurity. Though often 
attributed to the Greeks, it now seems certain that the custom 
is of much earlier date ; and is possibly due to the Egyptians 
or even to the ancient Hindus or the Chinese, in whose behalf, 
however, a claim to prior knowledge is always put in, whenever 
we Europeans fancy that we have made a discovery. 

The following are recent values of the mass of the Earth com- 
pared with that of the Sun: Encke ssgVsT* Littrow -3-5^0^775 
Madler -s-g-s-f-g-g, an d Le Verrier -g^Vstf- Le Verrier, however, 
once seemed to consider that these values were all too small, but 
that in our state of uncertainty as to the Sun's parallax it was 
not possible to assign with confidence a definitive value q . 
Newcomb taking the Earth and the Moon together gives for 
their combined mass the fraction ^yVc^ or f r the Earth alone 



See Month. Not., vol. xxxii. pp. 302 and 323. 1872. 



118 The Sun and Planets. [Boon I. 



CHAPTER VII. 

THE MOON. <[ 

Period, if c. Its Phases. Its motions andtkeir complexity. Libration. Ececlion. 
Variation Parallactic Inequality. Annual Equation. Secular acceleration. 
Diversified character of the Moon's surface. Lunar mountains. Seas. 
Craters. Volcanic character of the Moon. Bergeron's experiment. The lunar 
mountain, Aristarchus. Teneriffe. Lunar atmosphere. Researches ofSchroter, 
&c. Hansens curious speculation. The Earth-shine. The Harvest Moon. 
Astronomy to an observer on the Moon. Luminosity and calorific rays. 
Historical notices as to the progress of Lunar Chartography. Lunar Tables. 
Meteorological Influences. 



rriHE Moon, as the Earth's satellite, is to us the most important 
of the " secondary planets," and will therefore receive a 
somewhat detailed notice. 

The Moon revolves round the Earth in 27 d / h 43 n-46n s , at 
a mean distance of 237,300 miles. The eccentricity of its orbit 
amounting to 0-0662, the Moon may recede from the Earth to a 
distance of 253,000 miles, or approach it to within 221,600 miles. 
Its apparent diameter 8 varies between 29' 21" and 33' 31". The 
diameter at mean distance is 31' 5". It will fix this in the 
memory to note that the apparent diameter is the same as the 
Sun's, and equals . The real diameter, according to Madler, is 
2159-6 miles; according to Wichmann 2162 miles. Recent re- 
searches shew that these values are too great ; and that a 
correction of about 2" (Airy) or 2-15" (De La Rue) must be 
applied to the measured visual diameter of the Moon, to allow 

These figures must be regarded as of the Moon will be found to vary con- 

geometrically rather than practically siderably. And the diameter at mean 

true, for under varying circumstances of distance is not the arithmetical mean of 

altitude above the horizon the diameter the extremes of apparent diameter. 



CHAP. VII.] The Moon. 119 

for the exaggeration of its dimensions by irradiation. This 
reduction amounts to about 2 miles. The most delicate measure- 
ments indicate no compression. 

The Moon has phases like the inferior planets ; and of the 
various influences ascribed to it, that which results in the tides of 
the ocean is the most important, and will hereafter be treated at 
some length. 

The motions of the Moon are of a very complex character : 
they have largely occupied the attention of astronomers during 
all ages, and it is only within a recent period that they can be 
said to have been mastered. 

Speaking roughly, we may say that the same hemisphere of the 
Moon is always turned towards us ; but although this is, in the 
main, correct, yet there are certain small variations at the edge 
which it is necessary to notice. .The Moon's axis, although 
nearly, is not exactly perpendicular to the plane of its orbit, 
deviating therefrom by an angle of i 32' 9" (Wichmann) ; 
owing to this fact, and to the inclination of the plane of the 
lunar orbit to that of the ecliptic, the poles of the Moon lean 
alternately to and from the Earth. When the North pole leans 
towards the Earth we see somewhat more of the region sur- 
rounding it, and somewhat less when it leans the contrary way ; 
this is known as librarian in latitude 10 . The extent of the dis- 
placement in this direction is 6 47'. In order that the same 
hemisphere should be continually turned towards us, it would 
be necessary not only that the time of the Moon's rotation on its 
axis should be precisely equal to the time of the revolution in its 
orbit, but that the angular velocity in its orbit should, in every 
part of its course, exactly equal its angular velocity on its axis. 
This, however, is not the case, for the angular velocity in its 
orbit is subject to a slight variation, and in consequence of this 
a little more of its Eastern or Western edge is seen- at one 
time than another : this phenomenon is known as the libration 
in longitude, and was discovered by Hevelius, who described it in 
1647. The extent of the displacement in longitude is 7 53'. 

'' Librans, balancing. c In his Selenoffraphia. 



120 The Sun and Planets. [BOOK I. 

The maximum total libration (as viewed from the Earth's centre) 
amounts to 10 24'. On account of the diurnal rotation of the 
Earth, we view the Moon under somewhat different circum- 
stances at its rising and at its setting, according to the latitude 
of the Earth in which we are placed. By thus viewing it in 
different positions, we see it under different aspects ; this gives 
rise to another phenomenon, the diurnal libration, but the 
maximum value of this is only i i' 24". 

This periodical variation in the visible portion of the Moon's 
disc seems to have been first remarked by Galileo a discovery 
very creditable to him when we consider the materials with 
which he worked. According to Arago, the various librations 
enable us to see altogether - T y ff of the Moon's surface, the portion 
always invisible amounting only to T Vtr of the same. 

The following account of the chief perturbations in the motion 
of the Moon is, in the main, abridged from that invaluable 
repertory of astronomical facts, Hind's Solar System. 

1. The Erection depends on the angular distance of the Moon 
from the Sun, and on the mean anomaly of the former. It 
diminishes the equation of the centre in the syzygies and in- 
creases it in the quadratures, increasing or diminishing the 
Moon's mean longitude by i 20' 29-9". Period, about 
3i d J9 h 3O m . Discovered by Ptolemy, but previously suspected 
by Hipparchus. 

2. The Variation depends solely on the angular distance of the 
Moon from the Sun. Its effect is greatest at the octants, and 
disappears in the syzygies and quadratures, the longitude of the 
Moon being altered thereby 35' 41-6" when at a maximum. 
Period, half a synodical revolution, or about I4 d i8 h . Its 
discovery is usually ascribed to Tycho Brahe, but Sedillot and 
others claim it for Abul Wefa, who lived in the 9th centur} 7 . It 
was the first lunar inequality explained by Sir I. Newton on the 
Theory of Gravitation. 

3. The Parallactic Inequality arises from the sensible difference 
in the disturbing influence exerted by the Sun on the Moon, 
according as the latter is in that part of its orbit nearest to, 



CHAP. VII.] The Moon. 121 

or most removed from, the Sun. At its maximum it alters the 
Moon's longitude by about 2'. Period, one synodical revolution, 
or 29 d i2 h 44 ra . 

4. The Annual Equation is that inequality in the Moon's 
motion, which results from the variation in the -velocity of the 
Earth, caused by the eccentricity of its orbit. At its maximum 
the Moon's longitude is altered by n' IJ>97". Period, one 
anomalistic solar year, or 365* 6 h 13 49'3 S . 

5. The Secular Acceleration of the Moon's mean motion had been 
supposed to be caused wholly by the diminution in the eccen- 
tricity of the Earth's orbit which has been going on for many 
centuries, as has already been pointed out; but in 1853 it was 
shewn by Professor Adams that the amount of this acceleration 
is just double that which such diminution per se would account 
for. At present the mean motion of the Moon is being increased 
at the rate of about 12" every 100 years. This inequality was 
detected by Halley in 1693 from a comparison of the periodic 
time of the Moon, deduced from Chaldsean observations of 
eclipses, made at Babylon in the years 720 and 719 B.C., and 
Arabian observations made in the 8th and 9th centuries A.D. 
Laplace first reasoned out and explained the theory of the 
inequality, and up to the date of Adams's researches his calcu- 
lations were supposed to be complete. It was, however, shewn 
by our great geometer that Laplace had neglected certain 
quantities in his calculations, and so estimated the accelerating 
effect of the increase of the minor axis of the Earth's orbit at 
double its true amount. It has been suggested by Delaunay and 
others that half of this seeming acceleration has its origin in the 
real increase in length of our terrestrial day, which has actually 
lengthened and continues to lengthen by a small fraction of a 
second annually ; and this slower rotation of the Earth (for that 
is what it amounts to) is conceived to have its origin in the 
friction of the tides, which act as a break on the Earth rotating 
beneath them. 

Hansen elucidated, a few years ago, two other inequalities in 
the Moon's motion, due, the one directly and the other indirectly 



122 The Sun and Planets. [BOOK I. 

to the influence of Venus d ; and it was hoped that when these 
were taken into account it would have been found possible to 
say that the position of the Moon deduced from theory is almost 
precisely the same as that obtained by direct observation, and 
therefore that our knowledge of the Moon's motion is almost 
perfect ; but further research by Sir G. B. Airy has cast a doubt 
on the matter. 

Some matters connected with the Moon's orbit which are of 
importance in relation to eclipses will be referred to when we 
come to deal with eclipses (Book II., post) ; but it is desirable 
to note here the fact that the line of nodes of the lunar orbit 
revolves round the ecliptic in a retrograde direction in 
i8 y 2i8 d ai h 22 m 46". "This retrogression of the nodes is 
caused by the action of the Sun which modifies the central 
gravity of the Moon towards the Earth. It is not, however, an 
equable motion throughout the whole of the Moon's revolution ; 
the node, generally speaking, is stationary when she is in 
quadrature, or in the ecliptic ; in all other parts of the orbit it 
has a retrograde motion, which is greater the nearer the Moon is 
to the syzygies, or the greater the distance from the ecliptic. The 
preponderating effect at the end of each synodic period is, 
however, retrocessive, and gives rise to the revolution of the line 
of nodes in between 18 and 19 years 6 ." 

This motion must not be confused with the motion of the line 
of apsides of the lunar orbit. " The line of apsides or major 
axis of the lunar orbit has, from a similar cause, a direct motion 
on the ecliptic, and accomplishes a whole revolution in 
8 y 310* i3 h 48 53 s , so that in 4 y I55 d the perigee arrives where 
the apogee was before. This motion of the line of apsides, like 
the movement of the nodes, is not regular and equable through- 
out the whole of a lunar month ; for when the Moon is in 
syzygies the line of apsides advances in the order of signs, but is 

d The statement in the text is not with the Earth. The second of these 

quite correct, so far that in the case of Hansen inequalities runs its course in 

one of these inequalities (the 239-year 273 years. See on the whole subject a 

one) what Hansen did was to trace the paper by Airy in Month. Not., vol. 

operation on the Moon of that influence xxxiv. p. i. Nov. 1873. 

of Venus which Airy connected only e Hind, Sol. Syst., p. 42. 



CHAP. VII ] 



The Moon. 



123 



retrograde in quadratures. But the preponderating effect in 
several revolutions tends to advance the apsides, and hence 
arises their revolution in between 8 and 9 years." 

Fig- 57- 




VIEW OF A PORTION OF THE MOON'S SURFACE ON THE 

S.E. OF TYCHO. (Nasmyth.} 

When viewed by the naked eye the Moon presents a mottled 
appearance ; this arises from our satellite being unequally 
reflective, a fact which the telescope teaches us to be due to 



124 The Sun and Planets. [BOOK I. 

numerous mountains and valleys on its surface, as was dis- 
covered by Galileo. The proof of the existence of these is found 
in the shadows cast by the high peaks on the surrounding 
plains, when the Sun shines obliquely ; these shadows disappear, 
however, at the full phase, as the Sun then shines perpendicularly 
on the Moon's surface. Between the times of New and Full 
Moon the boundary line of the illuminated portion (often called 
the "Terminator") has a rough jagged appearance: this is 
caused by the Sun's light falling first on the summits of the 
peaks, the surrounding valleys and declivities being still in 
shade ; thus a disconnected form is given to the whole edge, and 
so arises the jagged aspect above referred to. 

Most of the lunar mountains have received names, chiefly those 
of men eminent in science, both ancient and modern. Biccioli 
proposed this nomenclature as preferable to that of Hevelius, who 
adopted terrestrial geographical names. Beer and Madler, to 
whom we owe so much of our knowledge of the Moon, measured 
the heights of 1095 lunar elevations, several of which exceed 
2O,ooo ft . But the absence of water on the Moon makes the 
choice of a datum line difficult. 

Grey plains, or seas, analogous probably to our "steppes" and 
prairies, form another noticeable feature in the topography of 
the Moon. They were called " seas " from their supposed nature, 
but though the opinion is overthrown the appellation is retained, 
and specific names have been applied to several of them. 

The crater mountains are by far the most curious objects shewn 
by the telescope. These are apparently of volcanic origin, and 
usually consist of a basin with a conical elevation rising from 
the centre. Their outline is generally circular or nearly so, but 
oblique view will often give those in the neighbourhood of the 
limb an apparently elliptical contour. Their immediate formation 
is probably due to the escape of gases from the interior of the 
Moon when that body was in a semi-fluid state, as it is conceived 
once to have been. The effect of the passage of air through a 
semi-fluid substance may be seen in the case of lime slaked by 
builders for fine plastering, when the air-bubbles, having forced 



CHAP. VII.] 



The Moon. 



125 



their way upwards to the surface and burst, leave apertures 
rising in cones forming a good imitation of many lunar craters. 

Some further experimental proof is to be had of the soundness 
of this view. Bergeron, having noticed the manner in which 
gases or vapours, when they pass through a pasty mass, leave 
a series of funnel-shaped holes behind them, and struck with 
the analogy which these holes present to the craters of the 
Moon, tried to reproduce the phenomenon on a larger scale, and 
for that purpose caused a current of hot air to pass through 

Fig. 58. 



, 

! ^^''^^^^^ .:'. '& 



. js ' -:A 

K '\ : - 










IMITATION OF THE STRUCTURE OF THE MOON S SURFACE. 

(Bergeron's experiment?) 

a mass of molten metal. For the convenience of the experiment 
the metal chosen was an alloy fusible at a comparatively low 
temperature, Wood's alloy, which melts at about 158 F., being 
the first employed. A current of hot air was forced through 
the alloy, which was melted in a hot- water bath. Then, as the 
metal was allowed to cool slowly, the supply of air being kept 
up, a bubbling was created, which drove away the particles 
which were beginning to solidify from over a considerable area 
and a large ring was formed. The air still being blown through, 



126 



The Sun and Planets. 



[BOOK I. 



the edges of the ring rose little by little, and a perfect model 
of a crater was produced ; and as the process of cooling went 
on a cone was formed, and the crater at the same time grew 
deeper, its inner slopes shewing a much greater inclination than 
the outer. When the process of forcing the air through the alloy 
was interrupted, a second inner ring was formed, reminding the 
experimenter of the appearance presented by Copernicus, Archi- 
medes, and other lunar craters. M. Bergeron considers that his 
experiments throw much light on the past history of the Moon. 
Instead of air, various vapours may have given rise to the craters 
and ring-mountains. These vapours rose freely from the Moon 
when it was in a fluid state, but the exterior of the planet being 
cooled more rapidly than the interior, the latter, still fluid, 
continued to give off vapours when the surface had already become 
a pasty mass. These vapours passed through this envelope and 

found a vent at certain 

Fi - 59- points only, doubtless 

where the tendency to 
solidification was least f . 

Cassini, Sir W. Herschel, 
Kater, Smyth, and other 
observers have fancied a 
mountain named Aristar- 
chus to be a volcano in 
action. It is now generally 
understood that the faint 
illumination discerned on 
the summit is merely due 
to the " Earthshine " ; but, 
in the words of Sir J. 
Herschel, " decisive marks 
of volcanic stratification, 

arising from successive deposits of ejected matter, and evident 
indications of lava currents streaming outwards in all di- 
rections, may be clearly traced with powerful telescopes. In 

' Comptes Sendvs, vol. xcv. p. 324, 1882. 




THE LUNAR MOUNTAIN, AKI8TARCHU8, 
ILLUMINATED. 



CHAP. VII.] 



The Moon. 



127 



Lord Rosse's magnificent reflector the flat bottom of the crater 
called Albategnius is seen to be strewed with blocks, not 
visible in inferior telescopes, while the exterior ridge of another 
(Aristillus) is all hatched over with deep gullies radiating 
towards its centre 8 ." The accompanying engraving represents 
Aristarchus as seen by Smyth on Dec. 22, 1835, with its peak 
illuminated. Figs. 60 and 61 shew under opposite phases 



Fig. 60. 



Fig. 61. 







THE LUNAR MOUNTAIN, ARISTARCHUS. 

of illumination the streaky radiations surrounding Aristarchus 
which may or may not betoken streams of lava which have 
flowed away in various directions after being erupted from the 
crater. The external height of Aristarchus has been calculated 
to be 2500 ft , and its internal depth 7000 ft . Of Copernicus it 
may be remarked that it is near the Terminator and is seen 
under the most favourable conditions of illumination a day or two 
after the ist Quarter. 



Outlines of Ast., p. 283. 



128 



The Sun and Planets. 



[BOOK I. 



The Volcanic origin of the lunar craters cannot be more plainly 
demonstrated than by comparing an engraving such as Fig. 62, 
which represents a knoicn volcano Teneriffe with any good 
engraving of a lunar crater, e.g. Copernicus, Fig. 65. The simi- 
larity is too striking to admit of there being any doubts as to the 
identity of the physical causes which have originated each 
surface. 

Fig. 62. 



I** 




THE PEAK OF TENERIFFE. (C. P. Smyth.} 

A systematic topographical description of the Moon would be 
entirely beyond the compass of this work, and there is the less 
occasion for it as that by the Rev. T. W. Webb h is a very ex- 
haustive one. The works of Hind i and Arago k also contain 
briefer accounts. 

The question as to whether or not the Moon has an atmosphere l 



h Celest. Objects for common Tele- 
scopes. 

1 Sol. Syst., p. 48 et seq. 

k Pop. Ast., vol. ii. p. 258 et seq., 
Eng. ed. 

1 See an important memoir by Bessel 



in Ast. Nach., vol. xi. p. 411. July 
1 6, 1834. -^ n< i tne reader will do well 
to consult a paper by Prof. Challis in 
Month. Not., vol. xxiii. p. 231. June 
1863. And Neison has written on this 
subject. (The Moon, p. 19.) 



Figs. 63-65. 



Plate VII. 




ARCHIMEDES. (Schroter.') 




Pico. (Schroter.} 




COPERNICUS. (Nasmyth.'} 



LUNAR MOUNTAINS. 
K 



Figs. 66-71. 



Plate VIII. 




ARCHIMEDES. April 3, 1884. 
8 h 45 ra to 9 h 45 p.m. 




GASSENDI. April 6, 1884. 
9 h o ra to io h 45 ra p.m. 





SINUS IRIDUM. July 3, 1884. 
9 h 45 m to ii h 15 p.m. 



KEPLER AND ENCKE. Aug. 8, 1884. 
gh 2o ra to io h 15 p.m. 





FRASCATORIUS. Aug. 10, 1884. 
i4 h o ra to i h 41". 



PLATO. Nov. 10, 1884. 

I 8" Ira._ 1 gh -m_ 



LUNAR MOUNTAINS. 

(Dr L. WeinfJc. 
K 2 



CHAP. VII.] 



The Moon. 



133 



Fig. 72. 



must be answered in the negative, though some affirmative testi- 
mony is forthcoming. Schroter considered there is one, but 
he estimated the height at only 5376", and Laplace thought 
it to be more attenuated 
than the best attainable 
vacuum of an air-pump. 
Schroter arrived at his 
conclusion by following 
up a remark of Au- 
zout's m , that if the Moon 
had an atmosphere the 
phenomenon of twilight 
would in consequence 
present itself. He was at 
length able, he thought, 
to determine that when T HE LUNAR MOUNTAIN EUDOXUS, SHOWING WALL 

ACROSS THE CBATEK. (TrOUVelot.} 

the Moon exhibited a 

very slender crescent, a faint crepuscular light, extending from 

each of the cusps along the circumference of the unenlightened 




Tig. 73- 



portion of the disc to a distance 
of i' 20", could be perceived ; 
its greatest breadth being 2". 
He thence inferred the height 
of the atmosphere to be only 
o'94", corresponding to the 
5376" given above n . The 
Moon would describe this arc 
in less than 2 seconds of time, 
and this circumstance was 
adduced by Schroter as an 
explanation of the difficulty 
attending its direct detection THE GULF OF IRIS SEEN WHEN THE MOON 

, IS IO DAYS OLD. 

during eclipses and occulta- 

tions. Sir J. Herschel considered that we are entitled to conclude 




m Mem. Acad. des Sciences, vol. vii. p. 
1 06. 



11 Phil. Trans., vol. Ixxxii. p. 354. 
1792. 



134 The Sun and Planet*. [BOOK I. 

the non-existence of any atmosphere at the Moon's surface dense 
enough to cause a refraction of \" , i.e. having TT nro- the density 
of the Earth's atmosphere . Both Beer and Madler thought that 
the Moon has an atmosphere, but that it is of insignificant extent, 
owing to the smallness of our satellite's mass ; and they also 
say, " It is possible that this weak envelope may sometimes, 
through local causes, dim or condense itself," an idea which, if 
proved, would help to clear up some of the conflicting details of oc- 
cultation phenomena. The suddenness with which occultations of 
stars by the Moon take place is, however, commonly regarded as 
one of the best proofs that a lunar atmosphere does not exist. 
And the spectroscope supplies negative evidence of like import. 

" Professor Hansen has recently started a curious theory, from 
which he concludes that the hemisphere of the Moon which is 
turned away from the Earth may possess an atmosphere. Having 
discovered certain irregularities in the Moon's motion, which he 
was unable to reconcile with theory, he was led to suspect that 
they might arise from the centre of gravity of the Moon not 
coinciding with her centre of figure. Pursuing this idea, he found 
upon actual investigation that the irregularities would be almost 
wholly accounted for by supposing the centre of gravity to be 
situated at a distance of 33 \ miles p beyond the centre of figure. 
Assuming this hypothesis to be well founded, Professor Hansen 
remarks that the hemisphere of the Moon which is turned to- 
wards the Earth is in the condition of a high mountain, and that 
consequently we need not be surprised that [little or] no trace of 
an atmosphere exists ; but that on the opposite hemisphere, the 
surface of which is situated beneath the mean level, we have no 
reason to suppose that there may not exist an atmosphere, and 
consequently both animal and vegetable life q ." Professor New- 
comb however has disputed these conclusions of Hansen, which 
it is obvious must be very difficult of either proof or disproof. 

For a few day s, both before and after New Moon, an attentive 

Outlines of Ast., p. 284. This frac- erroneously so, though how the mistake 

tion is probably erroneous. Neison makes has crept in is not clear, 

it *$-$. i Note by translator, Arago's Pop. 

P " 1740 " in the English original, but Att., vol. ii. p. 276, Eng. ed. 



CHAP. VII.] The Moon. 135 

observer may often detect the outline of the unilluminated portion 
without much difficulty. This lustre is the light reflected on the 
Moon by the Earth " Earth-shine " in fact ; the French call it 
la lumiere cendree, following the Latin lumen incinerosum, or the 
" ashy light." In England it is popularly known as " the Old 
Moon in the New Moon's arms." This light is stronger during 
the waning of the Moon than at any other time ; as was noticed 
by Galileo, whose opinion was confirmed by Hevelius and other 
more modern astronomers. Hevelius remarked, moreover, that 
in the waning Moon the illumination is less intense than when 
the phases are increasing a fact which would seem to indicate, 
as Arago has pointed out r , that the Western part of the lunar 
disc is on the whole better adapted for reflecting the solar rays 
than the Eastern part ; assuming this to be true, an obvious 
explanation is furnished for the fact that the Earth-shine is more 
luminous before the New Moon than after it. Janssen, in 1881, 
succeeded in photographing the " Earth-shine " on the Moon when 
the latter was 3 days old. In the photographs the " continents " 
were plainly distinguishable from the " seas s ." 

The Harvest Moon is the name given to that full Moon which 
falls nearest to the autumnal equinox ; as our satellite then rises 
almost at the same time on several successive evenings, and at a 
point of the horizon almost precisely opposite to the Sun (so that 
the duration of its visibility is about the maximum possible), it 
is of much assistance to the farmer at that important period of 
the year. In the words of Ferguson, " The farmers gratefully 
ascribe the early rising of the full Moon at that time of the year 
to the goodness of God, not doubting that He had ordered it so 
on purpose to give them an immediate supply of moonlight after 
sunset, for their greater conveniency in reaping the fruits of the 
Earth*." Although this near coincidence in several successive 
risings of the Moon takes place in every lunation when our 
satellite is in the signs Pisces and Aries, yet the phenomenon is 

r Arago, Pop. Ast., vol. ii. p. 300, Eng. ed. 
8 Nature, vol. xxiii. p. 518. March 31, 1881. 
* Astronomy, p. 136. Ed. of 1757. 



136 The Sun and Planets. [BOOK I. 

only prominently noticeable when it is "full" in these signs, which 
only occurs at or near the autumnal equinox, and when the Sun 
is in Virgo or Libra. The rationale of the harvest Moon is this : 
Suppose the Moon to be full on the day of the autumnal equinox, 
the Sun is then entering Libra, and the Moon, Aries ; the 
former setting exactly in the West, the latter rising exactly 
in the East: the Southern half of the ecliptic is then entirely 
above the horizon, and the Northern half entirely below, and the 
ecliptic itself makes the least possible angle with the horizon. 
The Moon in then advancing 13, or one day's portion, in its 
orbit (which is but slightly inclined to the ecliptic) will become 
less depressed below the horizon, and will therefore have a less 
hour-angle to traverse by the diurnal motion after sunset in order 
that it may come into view the next night than at any other 
time u . That harvest Moon is (astronomically} most favourable 
which happens about Sept. 23, with the Moon in the ascending 
node of her orbit, which then coincides with the vernal equinox. 
Under such circumstances the Moon may rise for 2 or 3 nights, 
later, night by night, by no more than about io m . 

As a rule however, the variation between the times of two 
successive risings will seldom be less than about i7 m ; whilst 
the greatest possible variation is about i h i6 m ; this takes place 
when the Moon is in Libra, and at the same time at or near its 
descending node. 

The Moon next after the Harvest Moon is (or used to be) 
called the Hunter s Moon. 

It is in winter (just when it is most wanted, indeed) that 
there is most moonlight for dwellers in the Earth's Northern 
hemisphere. That is to say, the Moon is at its full at the same 
time that it is at its highest possible Northern altitude, and 
therefore longest above the horizon ; in other words, the Earth's 
Northern hemisphere experiences the maximum possible amount 
of exposure to Moonlight. All this is the necessary result of the 
fact that Full Moon happens when our satellite is 180 away 

u In Lockyer's Elementary Lessons in diagram and description dealing with 
Astronomy (p. 172) there is a good this matter. 



CHAP. VII.] The Moon. 137 

from the Sun, i. e. exactly opposite to it. At midwinter, the Sun 
being at its maximum depression, obviously the Moon is at its 
maximum elevation, with the result above stated. This recital 
will be complete by adding that the nights of short Moon in winter 
are also the nights before and after New Moon, when there is the 
smallest possible amount of Moonlight to lose. In summer, of 
course, in the Earth's Northern hemisphere the reverse of all this 
is the condition of things : the Moon's elevation above the 
horizon is the minimum possible, and the Earth's exposure to 
the Moon's rays is consequently also the minimum possible. 

As seen from the Sun, with the Earth in perihelion and the 
Moon in apogee, the Moon never departs more than 10' 42" from 
the Earth at its greatest Elongation. Since the axis of the Moon 
is very nearly perpendicular to the plane of her orbit, our 
satellite has of course scarcely any change of seasons. At its 
equator the mean solar day has a constant length of 354 h 22 m , 
or I4 d i8 h 22 m of our mean solar time; in other words, it is 
equal to half the period of the Moon's synodical revolution round 
the Earth. As is the case on the Earth, the length of the longest 
day on the one hand and of the shortest on the other increases 
and diminishes according as the assumed place of observation 
approaches the lunar poles : so that at the selenographic latitude 
of 45 these times become I4 d 2i b 19 and I4 d J5 h 26 ; and at 
the latitude of 88, i8 d i7 h 28 m and io d i9 h i6 m respectively. 

By an observer placed on the Moon some astronomical pheno- 
mena would be witnessed under circumstances widely different 
from those under which we see them. The apparent diameter 
of the Earth would be about 2, and its apparent superficial 
extent 13 times greater than the apparent superficial extent of 
the Moon as seen from the Earth. More than this: the Earth 
is almost a fixed object in the lunar heavens, only altering its 
place by the amount of the libration, or traversing backwards 
and forwards a space having an extent of 15 30' in longitude 
and 13 1 8' in latitude. The Earth exhibits to the Moon exactly 
the same kind of phases which the latter does to us, but in a 
reverse order. For when the Moon is Full, the Earth is invisible 



138 The Sun and Planets. [BOOK I. 

to the Moon ; and when the Moon is New, the Earth is Full to 
the Moon. These remarks apply only to those parts of the lunar 
surface which are turned towards our globe ; for a spectator 
on the opposite side would never see the Earth at all, and 
spectators located on the apparent borders of the lunar disc 
would only now and then obtain a glimpse of it in their horizon, 
for which they would be indebted to the librations in longitude 
and latitude already noticed. 

If the whole sky were covered with Full Moons they would 
scarcely make daylight, for Bouger's experiments 1 give the brilli- 
ancy of the full Moon as only 7!n nnnr that of the Sun. Wollaston's 
value is OTTTTT/ Zollner's ^rsVfr^ and G. P. Bond's TT 0Vw z 

The Moon's surface is supposed to be much heated, possibly,- 
according to Sir J. Herschel, to a degree much exceeding that 
of boiling water* ; yet we are not in a general way conscious of 
there being any heat at all available for warming the Earth. 
This need not however excite surprise, for it is probably very 
small in amount, and what there is of it is doubtless quickly 
absorbed in the upper strata of our atmosphere. Melloni, in 1 846, 
thought that he detected a sensible elevation of temperature by 
concentrating the rays of the Moon in a lens 3 ft in diameter. 
C. P. Smyth, in 1 856, also thought that he obtained evidence on 
Teneriffe b of the Moon's rays possessing calorific power, but his 
instrumental appliances were not very perfect. Professor Tyndall 
has stated that his experiments in 1861 seem to show that the 
Moon imparts to us, or at least to the Professor's thermometric 
apparatus, " rays of cold" More recently, however, the Earl of Kosse, 
M. Marie-Davy, and Prof. Langley have conducted experiments 
which seem to give conclusively affirmative results, and on the 
whole the balance of evidence leans to this view of the question . 

* Cited by La Place, Systeme du c See a summary of all the experi- 

Monde, Bk. I., cap. 4. ments hitherto made given by Carpenter 

y Phil. Trans., vol. cxix. p. 27. 1829. in Pop. Sc. Rev., vol. ix. p. i. January 

1 Month. Not., vol. xxi. p. 200. May 1870. Lord Rosse's experiments will be 

1 86 1. found described in Phil. Trans., vol. 

Outlines of Ast., p. 285. clxiii. p. 587. 1873. See also Month. 

b An Astronomers Experiment, &c., Not., vol. xxxiv. p. 197. Feb. 1874. 
p. 213. 



CHAP. VII.] The Moon. 139 

Prof. Langley's summary of his own observations and deduc- 
tions is as follows : " While we have found abundant evidence 
of heat from the Moon, every method we have tried, or that has 
been tried by others, for determining the character of this heat 
appears to us inconclusive ; and, without questioning that the 
Moon radiates heat earthward from its soil, we have not yet 
found any experimental means of discriminating with such 
certainty between this and reflected heat that it is not open to 
misinterpretation* 1 ." 

The first astronomer who paid much attention to the delinea- 
tion of the Moon's surface was Hevelius, who in his well-known 
Selenographia, published in 1647, gave a detailed description of it, 
. accompanied by one general and some 40 special charts ; which, 
taking into consideration the inferior optical means at his dis- 
posal, were very creditable to the industry of the illustrious 
observer of Dantzig. Four years later Riccioli brought out a 
map of the Moon, having proper names assigned to many of the 
principal localities: and this nomenclature, improved and en- 
larged, is still in general use. J. D. Cassini and T. Mayer of 
Gottingen published charts in the years 1680 and 1749 respec- 
tively, the latter of which was the only one used by observers 
for many years subsequent to the opening of the present century. 
In 1791 Schroter published a large work entitled Selenotopogra- 
phische Fragmenie, in which are given diagrams of many of the 
principal spots 6 . Schroter was an industrious observer, but his 
descriptions are not always satisfactory. 

In 1824, W. G. Lohrmann of Dresden published the first 4 of 
a series of 25 excellent lunar charts, but was prevented by 
failing sight from continuing the work. It was, however, taken 
up by others and completed in 1 878 f . Beer and Madler's elaborate 
Mappa Selenograpkica was published in 1837, and is undoubtedly 
the best of the kind yet published ; but the most generally 
useful and also most generally accessible map for the class of 

d Mem. Nat. Acad. Sciences, vol. iii. Plate XVI, and Pico from Plate XXII. 

p. 42. 1885. ' Month. Not., vol. xxxix. p. 267. 

e The two engravings on Plate VII are Feb. 1879. Published by J. A. Earth, 

copied from this work ; Archimedes from Leipzig ; price, with book, 50 marks. 



140 The Sun and Planets. [BOOK I. 

readers whom I address is the Rev. T. W. Webb's, reduced from 
Beer and Madler's. Undoubtedly, however, the most minutely 
accurate and elaborate lunar map yet made is the one of 7-67" 
in diameter, by Schmidt of Athens, published at the expense 
of the German Government in 1878. Maps by Russell and by 
Blunt are in circulation, but they are not of much value as 
regards details. 

The British Association for the Advancement of Science, 
through a sub-committee, began in 1866 the preparation of an 
entirely new map of the Moon, but this was eventually aban- 
doned by the Association. The late Mr. W. R. Birt, however, 
continued it for a time. 

A wax model of the whole lunar surface was executed many 
years ago by a Hanoverian lady named Witte, and Nasmyth has 
modelled in plaster of Paris several single craters 8 . Photo- 
graphy, too, has been called in by De La Rue, Rutherford, and 
others, with good results. 

In computing the places of the Moon the Tables of Burckhardt, 
published in 1812, were formerly used, but in 1862 the new and 
more perfect Tables of Hansen were introduced at the Nautical 
Almanac office ; and these have entirely superseded Burckhardt's. 
Damoiseau, Plana, Carlini, Pontecoulant, Lubbock, and after- 
wards Delaunay, in addition to Hansen, did much to improve 
the theory of the Moon. Delaunay's labours earned for him a 
foremost place in the rank of geometrical astronomers. More 
recently still, Sir G. B. Airy has been treating the subject by 
a new method. His memoir entitled the " Numerical Lunar 
Theory" was published in 1887. He is understood to be still 
investigating some points in it which need further elucidation 11 . 

According to a recent determination by Stone the Moon's 
mass is gx 1 ^ that of the Earth. 

To record a tithe of the influences ascribed to the Moon would 
be a herculean task ; nevertheless (in addition to the tides) one 

8 Fig. 65 is from a photograph of one h Month. Not., vol. xxxiv. p. 89. Jan. 

of these. But they are of little value, 1874. 
being very inexact. 



CHAP. VII.] The Moon, 141 

deserves notice. Evening clouds at about the period of Full 
Moon will frequently disperse as our satellite rises, and by the 
time it has reached the meridian a sky previously overcast will 
have become almost or quite clear. I first observed this in 1857, 
and subsequently found that Sir J. Herschel 1 had made the same 
remark. The idea has been disputed k , but I am firmly convinced 
of its truth. Humboldt speaks of it as well known in South 
America, and Arago indirectly confirms the theory when he 
shows that more rain falls at about the time of New Moon 
(cloudy period] than at the time of Full Moon (cloudless period 
according to the theory). According to Forster, Saturday new 
Moons result in 3 weeks of wet weather. He alleged that 
observations extending over 80 years showed this coincidence 1 . 
Bernadin asserts it as a fact that many thunderstorms occur 
about the period of New or Full Moon. With these possible 
exceptions it is safe to assert that " changes " of the Moon have 
no discoverable influence on the weather m . 

1 Outlines of Ast., p. 285. ! Month Not., vol. ix. p. 37. Dec. 1848. 

k Ellis, Phil. May., 4th Ser., vol. m See Nasmyth and Carpenter, Moon, 

xxxiv. p. 61. July 1867. p. 180. 



142 The Sun and Planets. [BOOK I. 



CHAPTEK VIII. 

THE ZODIACAL LIGHT. 

General description of it. When and where visible. Sir J. HerscheVs theory. 
Historical notices. Modern observations of it. Backhouse's Conclusion*. 

ASTRONOMICAL writers are not agreed as to the proper head 
-JL- under which to describe and discuss the Zodiacal Light. I 
deal with it here, because, whatever its origin, it is a matter of 
terrestrial cognizance, and therefore a description of it may, 
without any serious incongruity, be associated with what has to 
be said about the Earth. 

The Zodiacal Light is a peculiar nebulous light of a conical or 
lenticular form a , which may very frequently be noticed in the 
evening soon after sunset about February or March, and in the 
morning before sunrise about September. It extends upwards 
from the Western horizon in the spring and from the Eastern 
horizon in the autumn, and generally, though by no means 
always b , its axis is nearly in a line with the ecliptic, or, more 
exactly, in the plane of the Sun's equator. The apparent an- 
gular distance of its vertex from the Sun's plane varies, according 
to circumstances, between 50 and 70 ; sometimes it is more ; 
the breadth of its base, at right angles to the major axis, 
varies between about 8 and 30. During its evening apparition 
it usually reaches to a point in the heavens situated not far from 
the Pleiades in Taurus. It is always so extremely ill-defined at 

Lens, a lentil. b Month. Not., vol. xxx. p. 151. March 1870, ft infra. 



CHAP. VIII.] The Zodiacal Light. 143 

the edges that great difficulty is experienced in satisfactorily 
determining its limits. In Northern latitudes the Zodiacal Light 
is generally, though not always, inferior in brilliancy to the 
Milky Way; but in the Tropics it is seen to far greater ad- 
vantage. Humboldt said that it is almost constantly visible in 
those regions, and that he himself had seen it sufficiently 
luminous to cause a sensible glow on the opposite quarter of the 
heavens c . In the winter of 1 842-43 it was remarkably well seen 
in this country, the apex of the cone attaining a length of no less 
than 105 from the Sun d . Lassell also mentions having seen the 
light very conspicuous at Malta in January 1850. 

No satisfactory explanation has yet been given of this pheno- 
menon ; it is, however, very generally considered to be a kind of 
envelope surrounding the Sqn, and extending perhaps nearly or 
quite as far as the Earth's orbit. Sir J. Herschel's opinion was 
" that it maybe conjectured to be no other than the denser parts 
of that medium which we have some reason to believe resists the 
motion of comets ; loaded, perhaps, with the actual materials of 
the tails of millions of those bodies, of which they have been 
stripped in their successive perihelion passages [! !]. An atmosphere 
of the Sun, in any proper sense of the word, it cannot be ; since 
the existence of a gaseous envelope propagating pressure from 
part to part subject to mutual friction in its strata, and thereby 
rotating in the same, or nearly the same, time with the central 
body, and of such dimensions and ellipticity is utterly incom- 
patible with dynamical laws f ." In connexion with this specula- 
tion it may be mentioned that during the visibility of the great 
comet of 1843 in March of that year, the Zodiacal Light was 
unusually brilliant ; so much so, that by many persons it was 
mistaken for the comet. 

The Zodiacal Light is of a reddish hue, especially at its base, 

c But on this point see Humboldt's xiv. p. 16, Nov. 1853. Observations by 

later statement on p. 14$, post. Burr and Webb will be found at pp. 45, 

d Detailed particulars will be found in 83, and 181 of the same volume; and see 

the Greenwich Observations, 1842. a paper by T. Heelis in Mem. of the Lit. 

e For observations by E. J. Lowe, see and Phil. Soc. of Manchester, 3rd Ser., 

Month. Not., vol. x. p. 124, March 1850; vol. ii. p. 437, 1865. 

vol. xi. p. 132, March 1851; and vol. ' Outlines of Asf., p. 658. 



144 The Sun and Planets. [BOOK I. 

where also it is most bright, and where it effaces small stars. 
Undulations and likewise a sort of flashing have been noticed 
in it. 

It has been suggested that the Zodiacal Light is identical with 
what Pliny and Seneca call the "Trabes 8 ," but more likely this 
was the Aurora. 

The Zodiacal Light was treated of by Kepler ; afterwards by 
Descartes, about the year 1630 ; and then by Childrey, in i659 h ; 
it was not, however, till J. D. Cassini, who saw it first on March 
1 8, 1683, published some remarks on this phenomenon that much 
attention was paid to it '. 

In the year 1855, the Rev. G. Jones, Chaplain of the U. S. 
Steam- Frigate Mississippi, published some remarks on this 
phenomenon k , as brought under his notice during a cruise round 
the world in the 2 preceding years. He stated: "I was also 
fortunate enough to be twice near the latitude of 23 28' North, 
when the Sun was at the opposite solstice, in which position the 
observer has the ecliptic at midnight at right angles with his 
horizon, and bearing East and West. Whether this latter circum- 
stance affected the result or not, I cannot say ; but I there had 
the extraordinary spectacle of the Zodiacal Light simultaneously 
at both East and West horizons from 1 1 to i o'clock for several 
nights in succession." 

Mr. Jones concluded his very interesting letter as follows : 
" You will excuse my prolixity in stating these varieties of ob- 
servations, for the conclusion from all the data in my possession 
is a startling one. It seems to me that those data can be ex- 
plained only by the supposition of a nebulous ring tcith the Earth 
for its centre, and lying within the orbit of the Moon 1 ." 

On the publication of the foregoing, Humboldt transmitted to 

Hist. Nat., lib. II. cap. 26. distrustful remarks on this comrnunica- 

h Natural History of England, 1659. tion, to which the reader should refer, 

Srit. Bacon., p. 183. 1661. and at p. 47 is some account of J. F. J. 

1 Anc. Mem. de VAcad. des Sciences, Schmidt's work on the Zodiacal Light, 

vol. viii. p. 121. ' See Jones's original memoir in vol. 

k Gould's Astronomical Journal, No. iii. of the 4to. ed. of the U. S. Exploring 

84, May 27, 1855. In the Month. Not., Expedition Narrative. (Washington, 

vol. xvii. pp. 204-5, May 1857, are some 1856.) 



CHAP. VIII.] The Zodiacal Light. 145 

the Berlin Academy" 1 some unpublished observations made by 
him at sea in March 1803, to the effect that on one or two occa- 
sions he also saw a 2 nd light in the East contemporaneously with 
the principal beam in the West ; he, however, then thought that 
the 2 nd light was merely due to reflection. He concludes by 
saying that " the variations in the brightness of the phenomenon 
cannot, according to my experience, be accounted for solely by 
the constitution of our atmosphere. There remains much still to 
be observed relative to the subject." 

Jones seems in one sense to have been anticipated in his 
" double end " view of the Zodiacal Light, as will appear from 
the following extract, which is here cited for a twofold pur- 
pose : " The two extremities of the Zodiacal Light may be seen 
on the same night about the time of the solstices, particularly the 
Winter solstice, when the ecliptic makes, night and morning, 
nearly equal angles with the horizon, and these are sufficiently 
great to allow a considerable portion of the points of the light to 
appear above the line of the twilight. It is thus that it was ob- 
served by Cassini on Dec. 4, 1687, at 6 h 3o m P.M. and 4 h 4O m A.M. 
the following morning n ." 

Capt. C. Wilkes of the U.S. Exploring Expedition controverted 
Jones's views on many material points, and regarded the Zodiacal 
Light as the result of the illumination of that portion or section 
of the Earth's atmosphere on which the rays of the Sun fall 
perpendicularly in the Tropics . 

Jones's observations have been subjected to a very painstaking 
and searching review by Searle, whose conclusions, embodying 
as they do the observations of others besides Jones, may be thus 
brought to a focus: (i) The Secondary (or opposite) Light 
(called by the Germans " Gegenschein ") is an undoubted fact and 
its connection with the main Light highly probable ; (2) That 
the Zodiacal Light lies further to the N., near the Autumnal 

11 Monatsbericht der Kon. Preuss. ed., p. 106. 

Akademie der Wissenschaften, July 26, Theory of the Zodiacal Light, p. 12. 

^SS. P- 5 1 ?- Month. Not., vol. xvi. A Paper read at the Montreal Meeting of 

p. 16. Nov. 1855. the American Association for the Ad- 

n J. E. Jackson, What to Observe, 2nd vancement of Science, 1857. 



146 The Sun and Planets. [BOOK I. 

than it does near the Vernal Equinox, is also highly probable ; (3) 
Atmospheric absorption largely affects the apparent positions of 
the Zodiacal Light ; (4) The belt of sky occupied by the projec- 
tions of the first 237 Minor Planets presents certain peculiarities 
which correspond to those of the Zodiacal Light, and suggest 
that it may be partly due to minute objects circulating in 
planetary orbits p ; (5) The Light does not interfere with the 
visibility even of small stars q ; (6) The final disappearance of 
the Light occurs by its setting rather than by its fading q . 

Heelis considers that his observations, made in 1862 on board 
ship in the Tropics, point to the change of position in the Light 
as depending on the time of year more than on the observer's 
place of observation. 

The most extensive recent observations on this subject which 
are of value are those made in the years 1869-71 by Colonel 
Tupman in the Mediterranean. He COD firms on many points 
previous observers, but contradicts them on one very important 
point. He asserts that the plane of the Light does not pass 
through the Sun. He also remarks having noticed great want of 
uniformity in the position of the axis of symmetry with respect 
to the ecliptic. In August and September the axis. is frequently 
inclined as much as 20 to the ecliptic, whilst in the winter it is 
sensibly parallel to the ecliptic r . 

On December 19 and 20, 1870, when in Sicily, whither he had 
gone to observe the solar eclipse, Mr. A. C. Ranyard and some 
friends (Secchi amongst them) examined the Zodiacal Light 
through a Savart polariscope. His main conclusion is, that the 
Zodiacal Light consists of matter which reflects the Sun's light. 
He adds, that such matter either (i) exists in particles so small 
that their diameters are comparable with the wave lengths of 
light, or (2) is matter capable of giving specular reflection 8 . 

Some observations by Birt are not unworthy of attention. 
They were made chiefly in 1850, though a few of his notes refer 

P Mem. Am(r. Acnd., vol. xi. p. 157, r Month. Not., vol. xxxii. p. 74. Jan. 

1885. 1872. 

> Proc. Amer. Acafl., vol. xix. pp. 156, * Month. Not., vol. xxxi. p. 171. 

163. March 1871. 



CHAP. VIII.] The Zodiacal Light. 147 

to April 1871. Birt drew attention to two special points: 
(i) The fact that the greater portion of the Light always lies to 
the N. of the ecliptic ; and (2) That comparing the shape of the 
cone of light month by month from February to April it becomes 
progressively more and more blunt, so much so " as to lead to 
the suspicion that we view the phenomenon differently as the 
Earth advances in her orbit from the point at which we beheld 
it in the winter months*." 

Little or no progress has been made during recent years in 
elucidating the theory of the Zodiacal Light : and this is the more 
remarkable considering the development of all other branches of 
Astronomy. Backhouse published in 1881 the results of 418 
observations between 1867 and 1877, chiefly directed to a deter- 
mination of the Light's Inclination to the ecliptic u . His deduc- 
tions, though based on so large a series of data, are not very 
conclusive. He finds the average deviation of the axis of the 
Light from the plane of the ecliptic to be 2, and the Longitude 
of the Ascending Node, 35. 

A Dutch observer, Gronemann, after giving much attention to 
the matter, has pronounced against the solar theory of the 
Zodiacal Light ; he considers it to have a terrestrial origin. His 
main contention is that the affirmed connection between the 
evening and morning cones of light is not established, and that 
the participation of the cones in the daily motion of the heavens 
is likewise not proved to be a fact x . 

Serpieri, the Director of the Meteorological Observatory at 
Urbino, communicated to the Italian Spectroscopic Society 
in 1876 a very elaborate memoir on the Zodiacal Light, summing 
up all the results of previous observers 7 . He would see in 
the phenomenon an electrical origin. 

* Month. Not., vol. xxxi. pp. 177-82. * Archives Nterlandaises. 

April 1871. y Memorie degli Spettr. Italiani,vol. v. 

u Month. Not., vol. xli. p. 333. May, 1876. 
1881. 



L 2 



148 The Sun and Planets. [BOOK I. 



CHAPTEK IX. 

MARS *. <j 

Period, &c. Phases. Apparent motions. Its brilliancy. Telescopic appear- 
ance. Its ruddy hue. Schiaparelli's " Canals." General statement of the 
physical details of Mars. Map of Mars on Mercator's projection. Polar 
snow. Axial rotation. The seasons of Mars. Its atmosphere. The Satellites 
of Mars. Ancient observation of Mars. Tables of Mars. 

"1%/TARS is the first planet exterior to the Earth in the order 
^'-l- of distance from the Sun, and, as we shall presently 
see, bears a closer analogy to it than do any of the other 
planets. 

Mars revolves round the Sun in 686 d 23** 30 41", at a mean 
distance of 141,536,000 miles, which an orbital eccentricity of 
0-093 ma y augment to 154,714,000 miles, or dimmish to 
128,358,000 miles. The apparent diameter of Mars varies 
between 4-i" in conjunction and 30-4" in opposition; and 
owing to the great eccentricity of the orbit of Mars its 
apparent diameter as seen from the Earth will vary much at 
different oppositions. The diameter at mean distance of the 
planet from the Earth being 7-28" (Le Verrier), the real 
diameter is nearly 5000 miles. Very varying results have 
been arrived at as to the compression of Mars. Sir W. Herschel 
gave it at T V ; Schrb'ter contradicted this, and asserted that it 
must be less than $ '> Bessel merely decided that it was too 

a Observers interested in Mars should exhaustive account of the planet which 

consult a valuable memoir entitled Area- has ever appeared. A fine series of litho- 

graphie presented to the Academic graphic views by N. E. Green will be 

Royale de Belgique in June 1874 by found in Mem. R.A.S., vol. xliv. p. 123, 

F. Terby of Louvain. It is the most 1879. 



Figs. 74-5. 



Plate IX. 




1858 : June 3. 




1858 : June 14. 



MARS. 
(Draum by Secchi.*) 



CHAP. IX.] Mars. 149 

small for measurement with his great heliometer at Konigsberg b ; 
Arago from Paris observations extending over 36 years (from 
1811 to 1847) deduced -$ Hind considers that g^, and Main 
that -^g is not very far from the truth. Kaiser's T y T confirms 
Schroter. 

Mars exhibits phases, but not to the same extent as the 
inferior planets. In Opposition it is perfectly circular ; between 
this and the quadratures it is gibbous ; and at the minimum 
phase, which occurs at the quadratures, the planet resembles the 
Moon 3 d from the full. The character of these phases is a 
sufficient proof that Mars shines by the reflected light of the 
Sun. The phases of Mars were discovered by Galileo, who on 
Dec. 30, 1610 wrote to Castelli, "I dare not affirm that I can 
observe the phases of Mars ; however, if I mistake not, I think I 
already perceive that he is not perfectly round." 

After Conjunction, when Mars first emerges from the Sun's 
rays, it rises some minutes before the Sun, and has a direct or 
Easterly motion ; but since this motion is only half that of the 
Earth in the same direction, Mars appears to recede from the 
Sun in a Westerly direction, notwithstanding that its real motion 
among the stars is towards the East. This continues for nearly 
a year, and ceases when its angular distance from the Sun 
amounts to about 137; then for a few days it appears 
stationary. After that, its motion becomes retrograde, or 
Westerly among the stars, and continues so until the planet 
is 1 80 distant from the Sun, or in Opposition, and consequently 
on the meridian at midnight. At this period its retrograde 
motion is swiftest ; it afterwards becomes slower, and ceases 
altogether when the planet is again at a distance of about 137 
on the other side of the Sun. Its motion then again becomes 
direct, and continues so, till once more the planet is lost in the 
solar rays, when the phenomena are renewed, but with a 
considerable difference in the extent and duration of the move- 
ments. The retrogradation commences or finishes when the 
planet is at a distance from the Sun which varies from 128 44' 

b See his memoir in Ast. Nach., vol. xxxv. p. 351. Dec. 17, 1852. 



150 The Sun and Planets. [BOOK I. 

to 146 37', the arc described being from 10 6" to 19 35'; 
the duration of the retrograde motion in the former case is 
6o d i8 h , and in the latter 8o d i5 h . The period in which all 
these changes take place, or the interval between 2 Conjunctions 
and 2 Oppositions, constitutes the synodical period, which 
amounts to 78o d . Mars and the Earth come nearly to the 
same relative position every 32 y ; but several centuries elapse 
before precise coincidence occurs c . 

Mars when in Opposition is a very conspicuous object in the 
heavens, shining with a fiery red light, which from its striking 
character has led to the planet being celebrated throughout the 
historic period. It received from the Jews on this account an 
epithet equivalent to " blazing," and the Greek one (Trupo'eis) bears 
much the same meaning. Its name or epithet in many other 
languages is substantially the same. 

Its synodic period being 780 days, it comes to Opposition and 
therefore attains its (general) maximum brilliancy, once in rather 
more than 2 y . When in perihelion and in perigee at the same 
time, which occurs once in 7 synodical revolutions ( 1 4 y 1 1 J m ), 
Mars shines with a brilliancy rivalling that of Jupiter. In 
August 1719, the planet being only 2^ from perihelion, its 
brightness was such as to cause a panic d . The most favourable 
Oppositions are those which occur on or about August 26 ; and the 
least favourable those which occur about Feb. 22. Favourable 
Oppositions will occur in 1892 and 1909. 

With suitable optical assistance, Mars is found to be covered 
with dusky patches, which have been supposed, and with good 
reason, to be continents analogous to those of our own globe : 
these are of a dull red blue ; other portions, of a greenish hue, 
are believed to be tracts of water. The ruddy colour, which, 
overpowering the green, gives the tone to the whole of the 
planet, was believed by Sir J. Herschel to be due to " an ochrey 
tinge in the general soil, like what the red sandstone districts on 
the Earth may possibly offer to the inhabitants of Mars, only 

c Smyth, Cycle of Celest. Objects, vol. ' l)e Zach, Con: Astronomique, vol. 

i. pp. 151-2, abridged and corrected. ii. p. 293. March 1819. 



CHAP. IX.] 



Mars. 



151 



more decided 6 ." In a telescope Mars appears less red than to 
the naked eye, and according to Arago f the higher the power 
the less the intensity of the colour. Webb writes : " The disc, 
when well seen, is usually mapped out in a way which gives at 
once the impression of land and water, the outlines, under 
the most favourable circumstances, being extremely sharp : the 




MAKS, APRIL 18, 1856. (Brodie.} e 

bright part is orange, according to Secchi, sometimes dotted 
with red, brown, and greenish points ; sometimes found by 
Schiaparelli filled with a complete network of their lines and 
minute interspaces ; the darker regions, which vary greatly in 
depth of tone, are in places brownish, but more generally of a dull 
grey-green (or, according to Secchi, bluish tint), possessing the 
aspect of a fluid absorbent of the solar rays. If so, the pro- 
portion of land to water is considerably greater on Mars than 
on the Earth ; so that the habitable area may possibly be 

" Outlines of Ast., p. 339. e Month. Not., xvi. p. 205. June 

' Pop. Ast., vol. ii. p. 483. Eng. ed. 1856. 



152 The Sun and Planets. [Boox I. 

much more alike than the diameter of the planets. The water 
however (if such it be) is everywhere in communication, and 
long naiTow straits are more common than on the EarthV 

In 1877, when Mars was in a part of its orbit favourable for 
observation, Schiaparelli at Milan detected a number of minute 
dusky bands, for the most part very narrow and straight, 
traversing and cutting up the supposed continents in various 
directions. These markings are commonly spoken of as 
"Canals." They were seen again in 1879 and in 1882, in the 
latter year considerably more numerous and exhibiting a much 
more complex network. Though these markings have been 
seen by other observers it cannot be said that their existence in 
the sharply defined forms suggested by Schiaparelli is generally 
recognisable. 

The details of this planet are not readily seen with ah instru- 
ment of small aperture, yet there are several features which are 
well within the powers of a 4-inch refractor or 6-inch reflector. 

The general tone of the disc is a reddish orange, and on it 
there may be seen certain gray markings, the most important of 
these being the "Kaiser Sea" in longitude 285, sometimes 
called the " V " mark, from its resemblance to that letter. It 
commences above the equator on the Southern side, and extends 
half way to the N. pole. The Kaiser Sea is connected with two 
dark forms in the direction of the equator, that to the E. being 
called " Herschel II." Strait, and that on the W. Flammarion Sea. 
This large dark form cannot be mistaken, and if a telescope will 
show anything on the planet it will show this. 

It should be observed that the apparent form of the Kaiser 
Sea differs greatly at different oppositions of Mars, in conse- 
quence of the varying view we have of the poles. When the S. 
pole is towards the Earth, Kaiser Sea is considerably fore- 
shortened ; whereas when the N. pole is towards the Earth, it is 
elongated. 

Herschel II. Strait extends on the E. to the equator, where it 
terminates in a well-known mark, the a of Beer and Madler, from 

h Celest. Objects, 4th ed. p. 141. 



CHAP. IX.] Mars. 155 

which Martial longitudes are reckoned. This mark was dis- 
covered by Dawes to be composed of two points, as shown in the 
map, and it is appropriately named after that observer. 

Between Dawes's forked bay and the next dark point, Burton 
Bay, there is generally seen a space connecting the light portions 
of the equatorial region with Phillips Island to the S. ; but 
this was filled with shade during the opposition of 1877. 

When Burton Bay has passed the meridian, a large dark mark, 
called De La Rue Ocean, extends towards the S. pole, its Eastern 
extremity being Christie Bay. On the S.E. of De La Rue Ocean 
may be seen a well-defined round dark spot named Terby Sea in 
the map. This mark is difficult to observe during those oppo- 
sitions, when the N. pole is directed towards the Earth. 

When Terby Sea has passed, a long dark streak, called Maraldi 
Sea, comes into view, and continues till Flammarion Sea heralds 
Kaiser Sea, with which we started, thus completing the circuit 
of the planet. 

The polar snow-spots are seen with great distinctness when 
Mars is approaching Opposition ; from that time they decrease in 
size, till it requires sharp and educated vision to detect their 
presence. 

There is a round orange spot in the Southern hemisphere in 
longitude 300, called Lockyer Land. This was seen during the 
Opposition of 1873 to be white as though covered with snow. A 
similar, though smaller spot exists in the Northern hemisphere at 
210 of longitude, named Fontana Land. The details of the 
Northern hemisphere are not only less important than those of 
the Southern, but are the less known in consequence of the 
greater distance of Mars when the N. pole is turned towards the 
Earth. 

One point of contrast there is between Mars and the Earth. 
Whereas on the Earth the proportion of water to land is about 
ii to 4, on Mars the proportions are probably about equal. It is 
to be noted also that the water on Mars is for the most part dis- 
posed in long narrow channels ; of wide expanses of water, such 
as our Atlantic Ocean, there are few. 



156 The Sun and Planets. [BOOK I. 

In the vicinity of the poles brilliant white patches may be 
noticed, which are now considered by astronomers to be masses of 
snow an idea which is materially strengthened by the fact that 
they have been observed to diminish when brought under the 
Sun's influence at the commencement of the Martial summer, and 
to increase again on the approach of winter. 

The observation of these white patches appears to date from the 
middle of the 1 7th century, for they seem to be noticed in a figure 
of the planet by Huygens ; Maraldi, in 1 704, first gave specific 
representations of them. Sir W. Herschel 1 , who discovered the 
circumstances attending their variation in size, found that they 
were not always precisely opposite, both being sometimes visible 
or invisible at the same time. Madler noted the S. polar spot to 
undergo greater changes of magnitude than the Northern one, 
an observation harmonising with the fact that from the eccen- 
tricity of the planet's orbit it experiences a greater variety of 
climate. The same observer found (and herein he was con- 
firmed by Secchi) the N. patch concentric with the planet's 
axis, but the S. one considerably eccentric, which agrees sub- 
stantially with Sir W. Herschel's observation. It is not easy 
to understand why they are not exactly opposite ; if both were 
equally removed, and in opposite directions, from poles of 
rotation, it would occur, as with the Earth, that the poles of 
cold differed from those of rotation, but the subsisting facts are 
inexplicable. 

Figs. 78-79 represent the Polar snows of Mars as drawn by 
Mr. N. E. Green, an observer who has paid much attention to 
this planet J. 

It will be seen that in Fig. 78 there is on the west side of the 
Polar cap a detached point of light. Green regarded this as a 
patch of snow which rested on elevated ground after the snow 
had melted on the lower levels. This light was afterwards seen 
on Sept. 8 and 10. 

On Sept. 8, however, 2 patches were visible, and on Sept. 10 

' Phil. Trans., vol. Ixxiv. p. 2 et seg. 1784. 
> Mem. R.A.S., vol. xliv p. 126. 



CHAP. IX.] 



Mars. 



157 



a faint line of points concentric with the zone of snow. The 
observer thought that these alterations of form were in all 

Fig. 78. 




THE SOUTH POLE OF MARS, SHOWING SNOW. Sept. i, 1877. (Green.) 

probability due to perspective ; the single point of Sept. I 
appearing as two when less foreshortened, and that these when 

Fig. 79. 




THE SOUTH POLE OF MARS, SHOWING SNOW. Sept. 8, 1877. (Green.) 

still further separated appeared still further increased in numbers 
as they were seen nearer the central meridian of the disc. Green 
further suggests that 

" This brilliant appearance of the spots when most to the West of the pole, and 
their decrease in brilliance when passing the meridian, together with the most sig- 
nificant fact that they were not seen at all on the Eastern side, can best be explained 



158 The Sun and Planets. [BOOK I. 

by supposing the slopes of the hills that retained the snow to have a South-westerly 
aspect ; they would thus be sheltered from the Sun's rays during the greater part of 
a revolution, but fully exposed to its light, and therefore better seen, just as they 
were passing away towards the Western limb." 

Spots on the body of Mars led at an early period to attempts 
being made to ascertain the period of its axial rotation. J. D. 
Cassini, in 1 666, found this to be effected in 24 h 40 ; Hooke k . 
working contemporaneously, was unable to decide between 1 2 h 
and 24 h . Madler 1 fixed the time of revolution at 24 h 37 m 23", 
a result which singularly accords with Cassini 's, and says much 
for the accuracy and skill of the astronomer of Bologna. 
Drawings by Hooke and by Huygens more than 200 years old 
have been turned to account in modern times to throw light 
upon the rotation of Mars. Using some of Huygens's sketches, 
Kaiser was led to fix the period of Mars at 24 h 37 22'62 8 ; 
Proctor m , using some of Hooke's sketches, obtained as the 
result 24 h 37* 22'7i 8 . The most recent observations, resting 
on a prolonged basis, are those of Denning, who from 15 years' 
observations ending in 1884 obtained a period of 24 h 37 
22'34*. Sir W. Herschel's figures were 24 h 39 2 1 '67" ; he 
stated, though on wholly insufficient data, that the obliquity 
of the ecliptic on Mars was 28 42' an angle so close to that 
which obtains for the Earth, as, if confirmed, to warrant us 
in asserting that the seasons of Mars are not materially different 
from our own. 

The Martial year consists of 668 Martial days and 16 hours, 
the Martial day being longer than the terrestrial in the propor- 
tion of 100 to 97. Owing to the eccentricity of the planet's orbit, 
the summer half of the year in the Northern hemisphere con- 
sists of 372 days, and the winter half of 296 days. As a matter 
of course, the reverse state of things prevails in the Southern 
hemisphere ; there the winter half-year consists of 372 days and 
the summer of 296 days. Nevertheless, although the extremes 
of temperature may, and probably do, differ widely in the two 

k Phil. Trans., No. 14, p. 244. July 2, 1666. 
1 Att. Nock., vol. xv. No. 349. April 7, 1838. 
m Month. Not., vol. xxxiii., p. 558. 1873. 



CHAP. IX.] Mars. 159 

hemispheres, the mean temperatures of each may possibly differ 
but little. The duration of the seasons in Martial days in the 
Northern hemisphere is as follows: Spring 191, summer 181, 
autumn 149, winter 147. For the Southern hemisphere we 
must reverse the seasons: this being done, it will appear that 
spring and summer taken together are 76 days longer in the 
Northern hemisphere than in the Southern. 

The observations of Cassini led to the belief that Mars possessed 
a very extensive atmosphere : this has not been confirmed, and 
it is now only admitted that Mars has an atmosphere which is 
moderately dense. Sir J. South, who paid much attention to 
this subject, stated that he had seen one star in contact with the 
planet and 2 occulted without change ; thus overthrowing an 
opinion which resulted from an assertion of Cassini's that ty 
Aquarii (a star of the 5 th mag.) on one occasion, in Oct. 1672, 
disappeared in a 3~feet telescope when 6' from the planet's 
limb. But was the planet gibbous at the time ? 

In former editions of this work it was stated that '' Mars 
possessed no satellite, though analogy does not forbid, but 
rather, on the contrary, leads us to infer the existence of one ; 
and its never having been seen, in this case at least, proves 
nothing." 

In the year 1877 an able American observer, Asaph Hall 
disproved the first part of this statement, and confirmed the 
closing inference. The Opposition of Mars in 1877 promised 
by reason of the situation of the planet in the heavens to be 
a very favourable one, and Hall conceived the idea that, having 
the command of the fine refractor of the Washington Observatory 
(aperture, 26 inches), he might perhaps be fortunate enough to 
detect a satellite if Mare had one. Independently of the pro- 
mising circumstances just mentioned, Hall had hopes that some 
favourable result might come of his effort because, with the 
exception of an attempt made by D' Arrest at Copenhagen in 
1862 (or 1864), no systematic search for a Martial satellite had 
been made since Sir \V. Herschel's failure as far back as 1 783. 
Hall began his search early in August 1877. At first he found 



160 The Sun and Planets. [BOOK I. 

near the planet only some small stars ; but on the night of 
August 1 1 he detected a faint object on the nf. side of the planet 
which afterwards proved to be the outer satellite. Bad weather 
hindered him until August 16, when a small object was again 
seen which the observations of that night showed to be a satellite 
in motion with the planet and near one of its Elongations. On 
August 17, while waiting and watching for the satellite first 
seen (the outer one), he discovered a second (the inner one). 
Further observations on the following night placed beyond 
doubt the character of the two objects and their discovery was 
publicly announced. Nevertheless for several days Hall was 
much puzzled by the apparent motions of the inner moon. It 
seemed to appear on different sides of the planet the same night, 
and he at first thought there must be 2 or 3 satellites within 
the orbit of the outer one, since it seemed so unlikely that a 
satellite should revolve round its primary in less time than the 
primary rotated on its axis. In order to decide the point the 
inner satellite was watched throughout the nights of August 20 
and 21, by which means it was clearly ascertained that there 
was but one inner satellite, and that revolving round its primary 
in less than \ TA of the time of the primary's own axial rotation 
a case unique in the solar system. 

When the discovery of these satellites was made public 
various observatories took up the matter, and between August 
and the end of October 1877, that is to say, so long as Mars 
remained favourably placed for observation, the satellites were 
seen at several of the larger public observatories in Europe and 
America, and likewise at the private observatories of Mr. A. A. 
Common, Ealing, England, and Mr. W. Erck, Sherrington, near 
Bray, Ireland. At the Opposition of 1879 these satellites were 
both again observed in America, as also in 1881, but in the 
latter year observations were few, Mars not being very favourably 
placed for the purpose. 

At the suggestion of Mr. Madan. of Eton, the outer satelli te was 
named by the discoverer " Deimos " and the inner satellite 
" Phobos " ; these being the mythological names of the horses 



CHAP. IX.] 



Mars. 



161 



which drew the chariot of Mars, although by Homer personified 
and meaning the attendants of Mars. 

" He spake and summoned Fear and Flight to yoke 
His steeds, and put his glorious armour on n ." 

Considering the small size of these satellites it will not be 
expected that much information can be given respecting them. 

Phobos revolves round Mars in 7 h 39 at a distance of 
about 6000 miles. Hall thinks the orbit may have a slight 
eccentricity. The angular amount of the maximum distance 
from the planet is about 12"; and the brightness at Opposition is 
about that of a star of mag. 1 1. 

Deimos revolves round Mars in 3O h i8 ra at a distance of 
about 15,000 miles. The orbit is almost circular. The angular 
amount of the maximum distance from the planet is about 
32", and the brightness at Oppo- 
sition is about that of a star of 
mag. 134. 

The planes of the orbits of both 
satellites are very nearly coin- 
cident with the equator of Mars. 
The hourly areocentric motion of 
Phobos is 47, and on account 
of its rapid motion and its near- 
ness to the planet this satellite 
must present a very singular 
appearance to an observer on Mars. It will rise in the W. 
and set in the E. and will meet and pass Deimos, whose 
hourly areocentric motion is only ii'8. The semi-diameter 
of Mars being 2100 miles, the horizontal parallaxes of these 
satellites are very large, amounting to 21 for Phobos. The 
nearness of this satellite to the surface of the planet will pro- 
duce apparent singularities in its motion, and cause it to 
appear as a variable star. Some photometric observations by 

Bryant's 




THE APPARENT OKBITS OF THE 
SATELLITES OF MARS. 



n Homer, Iliad, lib. xv. 
Translation. 

The rationale of this is explained at 
length in the Rev. E. Ledger's The Sun 



and its Planets, p. 253 ; where will also 
be found some other speculations as to 
the phenomena connected with these 
satellites. 



M 



162 The Sun and Planets. [BOOK I. 

Pickering imply that Phobos has a diameter of 7 miles and 
Deimos of 6 miles p . 

It is interesting to note that there is extant a copy of a 
letter by Kepler to his friend Wachenfels, written shortly after 
the announcement of Galileo's discovery of the satellites of 
Jupiter, in which Kepler expresses his eagerness for a telescope 
wherewith to discover 2 satellites for Mars, that being the number 
which " proportion seems to require q ." 

Dean Swift, too, in Gullivers Travels 1 speaks of the astronomers 
of Laputa having done more than the astronomers of Europe, for 
" They have likewise discovered 2 lesser stars or satellites which 
revolve about Mars." And Voltaire, in his romance oiMicromegas, 
speaking of some of his characters says : " Us virent deux lunes 
qui servent a cette planete [Mars] et qui ont e'chappe' aux regards 
de nos astronomes." But of course these are nothing but happy 
" shots ; " there could have been no tradition of 2 Martial 
satellites as a historical fact. 

The want of a known satellite long prevented anything more 
than an approximation being arrived at of the mass of Mars. 
But the disturbing influence of this planet being insignificant, an 
extremely accurate determination of its mass is of no great con- 
sequence to science. The most trustworthy value appears to be 
A. Hall's, who by means of observations of the two satellites has 
obtained the figures 77777^. 

" The most ancient observation of Mars that has come to our 
knowledge is one reported by Ptolemy in his Almagest (lib. x. 
cap. 9). It is dated in the 52 nd year after the death of Alexander 
the Great, and 476 th of Nebonassar's era, on the morning of the 
21 st of the month Athir, when the planet was above but very near 
the star /3 in Scorpio. The date answers to B.C. 272, Jan. 17, at 
1 8 h on the meridian of Alexandria. An occultation 8 of the planet 

P The foregoing particulars are chiefly r Part III. ch. iii. 

from A. Hall's Observations and Orbits * Inasmuch as the apparent diameter 

of the Satellites of Mars, Washington, of Mars is (except under rare circum- 

1878, a memoir issued by the U. S. stances) less than that of Jupiter, the 

Naval Observatory. more correct expression would probably 

i Brewster, Life of Kepler. be "a transit of Mars across Jupiter," Sect 



CHAP. IX.] Mars. 163 

Jupiter by Mars on Jan. 9, 1591, is recorded. Such a phenomenon 
would be extremely interesting if viewed with the powerful tele- 
scopes so common at the present day *." 

In computing the places of Mars the tables of Baron De 
Lindenau, published in 1811, were generally used until recently, 
but they were superseded in 1861 by the more perfect tables of 
Le Verrier u . 

* Hind, Sol. Syst., p. 79. 

u Annales de VObservataire de Paris, Mem., vol. vi., Paris, 1861. 



M 2 



The Sun and Planets. [BOOK I. 



CHAPTER X. 
THE MINOR PLANETS". 

Sometimes called Ultra- Zodiacal Planets. Summary of f axis. Notes on Ceres. 
Pallas. Juno. Vesta. Olbers's theory. History of the search made for 
them. Independent discoveries. Progressive diminution in their size. 

BETWEEN the orbits of Mars and Jupiter there is a wide 
interval, which, until the present century, was not known 
to be occupied by any planet. The researches of late years, as 
previously intimated in Chapter II., have led to the discovery 
of a numerous group of small bodies revolving round the Sun, 
which are known as the Minor Planets b , and which have received 
names taken at the outset chiefly from the mythologies of ancient 
Greece and Rome, but in recent years from all sorts of sources c , 
many names being most fantastic and ridiculous. 

These planets differ in some respects from the other members 
of the system, especially in point of size, the largest being 
probably not more than, even if so much as, 200 miles or 300 
miles in diameter. Their orbits are also, as a general rule, much 
more inclined to the ecliptic than the orbits of the major planets, 
for which reason it was once proposed to term them the Ultra- 

tt The use of symbols has been discon- disuse. Such a designation was not very 

tinued, except for the four early ones, as appropriate; planetoids would have been 

follows: Ceres , Pallas \, Juno $, better. However, minor planets is pre- 

Vesta ( ; and even these are becoming ferable to either. 

obsolete. Gould's suggestion to adopt by c The names Lumen, Bertha, and Zelia, 

way of symbol the number in the order assigned by MM. Henry, are said to com- 

of discovery enclosed in a circle thus : memorate members of the family of the 

(jST), has been universally adopted. French astronomer Flammarion, a char- 

*> The old name of asteroids, proposed acteristic specimen of the French way of 

by Sir W. Herschel, has nearly fallen into doin g things. 



CHAP. X.] The Minor Planets. lf>5 

Zodiacal Planets: and many orbits are eccentric to a degree for 
which no parallel can be found amongst the major planets. 

It is needless to give any detailed account of each, but a short 
summary may not be out of place d . 

The nearest to the Sun is Medusa @, which revolves round that 
luminary in H39 d , or 3-i y , at a mean distance of 198,134,000 
miles. Next come Sita @, and Anahita. @. 

The most distant is Thule @, whose period is 322o d , or 8-8 y , 
and whose mean distance is 396,454,000 miles. Next come Hilda 
@, Ismene @, and Andromache @. The last-named recedes farthest 
from the Sun of any owing to the great eccentricity of its orbit. 

The least eccentric orbit is that of Philomela @, in which e 
amounts to only o-oi i. 

The most eccentric orbit is that of JEthra @, in which e 
amounts to 0-383. 

The least inclined orbit is that of Massalia @, in which t 
amounts to o4i'. 

The most inclined orbit is that of Pallas (T) 5 in which i 
amounts to 34 44'. 

The brightest, and, presumably, largest planet is by the con- 
current testimony of Argelander, Stone, and Pickering, Testa (7) 
The two former observers place Ceres Q second, and Pallas () 
third. 

The faintest cannot be specified. 

The more recently discovered planets are all so small that it is 
impossible to say which is the smallest. 

It has been thought that many of the minor planets (especially 
Vesta] are variable in their light. This may be nothing more 
than the result of, and a proof of their axial rotation 6 . Prof. 

d By far the most elaborate summary exhibit these changes are irregular cr 

which has yet appeared will be found in polyhedral in form, and show sometimes" 

an article by Niesten in the Annuaire de one and sometimes another face, or faces 

I'Observatoire Soy. de Itruxelles, 1881, (as the cases may be), seems sublime 

p. 226 ; and see Prof. D. Kirkwood's very fancy. But in the more modern form 

exhaustive little treatise The Asttroids, that probably these planets rotate on 

Philadelphia, 1888. their axes as do the major planets, his 

Littrow's idea that the planets which theory may be admissible. 



166 The Sun and Planets. [BOOK I. 

M. W. Harrington, on the assumption that the surfaces of all 
have the same reflecting power as Vesta, has estimated the volume 
of Vesta as T 5 T of the first 230 planets ; and that Ceres and Vesta 
together comprise about half the volume of the 230. Le Vender 
calculated that the total mass of the whole number could not 
exceed \ of the mass of the Earth. Even to approach this sum 
total Niesten considers there would have to be several thousand 
minor planets in all. 

Several of the minor planets have been found only to be lost 
again, and their positions cannot now be determined. Included 
in this category are Scylla @, Sylvia @, Dike, and Camilla @. 
Others (e.g. Hilda @, Lydia @, Sirona ("*)) have been found 
again after being lost. 

Under favourable circumstances Ceres has been seen with the 
naked eye, having then the brightness of a star of the 7 th mag- 
nitude ; more usually, however, it resembles an 8 th magnitude 
star. Its light is somewhat of a red tinge, and some observers 
have remarked a haziness surrounding the planet, which has 
been attributed to the density and extent of its atmosphere. 
Sir W. Herschel once fancied that he had detected 2 satellites 
accompanying Ceres ; but its mass can scarcely be sufficient for 
it to retain satellites around it large enough to be visible to us. 
Pallas, when nearest the Earth in Opposition, shines as a full 7 th 
magnitude star, with a decided yellowish light. Traces of an 
atmosphere have also been observed. Juno usually shines as an 
8 th magnitude star, and is of a reddish hue. Vesta appears at 
times as bright as a 6 th magnitude star, and may then constantly 
be seen without optical aid, as was the case in the autumn of 
1858. The light of Vesta is usually considered to be a pure 
white, but Hind considers it a pale yellow f . Hind found Victoria 
to possess a bluish tinge. 

The orbits most nearly alike are those of Fides and Maia, and 
Lespiault has remarked that when at their least distance from 
each other these planets are separated by a space which only 

' Sol. Sy*t., p. 85. 



CHAP. X.] The Minor Planets. 107 

amounts to -$ of the radius of the Earth's orbit, or about 4^ 
millions of miles. 

Sir J. Herschel once remarked : " A man placed on one of the 
minor planets, would spring with ease 6o ft , and sustain in his 
descent no greater shock than he does on the Earth from leaping 
a yard. On such planets giants might exist ; and those enormous 
animals which, on Earth, require the buoyant powers of water 
to counteract their weight, might there be denizens of the land g ." 
But to such speculations there is no end. 

Respecting the past history, so to speak, of the minor planets, 
little can be said. Olbers, in calculating the elements of the 
orbit of Pallas, was forcibly struck with the close coincidence 
he found to exist between the mean distance of that planet and 
Ceres. He then suggested that they might be fragments of some 
large planet which had, by some catastrophe, been shivered to 
pieces. When this theory was started it appeared a not wholly 
improbable one, but the discoveries of late years have upset it h . 
Nevertheless, a very close connection does apparently exist 
between these minute bodies, and on this subject D' Arrest 
writes : " One fact seems above all to confirm the idea of an 
intimate relation between all the minor planets ; it is, that, if 
their orbits are figured under the form of material rings, these 
rings will be found so entangled, that it would be possible, by 
means of one among them taken at hazard, to lift up all the rest." 

The circumstances which led originally to a search for planetary 
bodies in the space intervening between Mars and Jupiter, were 
these. In the year 1800, 6 astronomers, of whom Baron De 

* Outlines of Ast., p. 352. vinced that there had existed a planet 

h It may be shown mathematically, between Mars and Jupiter, in our own 

that if the disruption of a large planet system, of which the little asteroids, or 

ever did occur, its fragments (no matter planetkins, lately discovered, are indubit- 

how diverse their subsequent paths might ably fragments ; and ' Kemember,' said he, 

be) must, if continuing to revolve round ' that though they have discovered only 

the Sun, always pass through the point at 4 of these parts, there will be thou- 

which the explosion occurred, at one part sands perhaps 30,000 more yet dis- 

of their orbits. Sir W. Herschel thus covered.' This planet he believed to 

expressed himself on this subject to the have been lost by explosion." (Life and 

poet Campbell according to a letter Letters of T. Campbell, vol. ii. p. 234.) 
written by the latter : " He was con- 



168 The Sun and Planets. [BOOK I. 

Zach was one, assembled at Lilienthal, and there resolved to 
establish a society of 24. practical observers, to examine all the 
telescopic stars in the zodiac, which was to be divided into 24 
zones, each containing one hour of Right Ascension, for the 
express purpose of searching for undiscovered planets 1 . They 
elected Schroter their president, and the Baron was chosen their 
secretary. Such organisation was ere long rewarded by the 
discovery of 4 planets, but as no more seemed to be forthcoming, 
the search was relinquished in 1 8 1 6. 

It does not appear that any further labours in this field were 
prosecuted for some years, or till about the year 1830, when M. 
Hencke, an amateur of Driesen in Prussia, commenced the search 
for small planets, with the aid of the since celebrated Berlin Star 
Maps which contain all stars up to the 9 th or io th magnitudes 
lying within 15 of the equator. It is evident that a non-stellar 
body is much more likely to attract the notice of an observer 
possessing and using maps of this kind than of one not so provided, 
as a change of position virtually tells its own tale with com- 
paratively little trouble to the astronomer. This series of maps, 
one for each hour of R. A., was only completed in 1 859 ; therefore 
when Hencke commenced he had only a few at his command, 
and 15 years elapsed ere his zeal and perseverance produced any 
result: but when once one planet was found, the discovery of 
others quickly followed. 

Several of these small planets were discovered independently 
by two or more observers, each without a knowledge of what the 
other had done. For example, Irene was found by Hind on May 
19 1851, and by De Gasparis on May 23 ; Massilia by De Gasparis 
on Sept. 19, 1852, and by Chacornac on Sept. 20 ; Amphritrite by 
Marth on March i , 1 854, by Pogson on March 2, and Chacornac 
on March 3 (3 separate discoveries) ; Virginia by Ferguson on 
Oct. 4, 1857, and by Luther on Oct. 19; Eurynome by Watson 
on Sept. 14, 1863, and by Tempel on Oct. 3 ; Hecate by Watson 
on July n, 1868, and by Peters on July 14 ; Cassandra by Peters 
on July 23, 1871, and by Watson on August 6 ; &c. 

1 See p. 67, ante. 



CHAP. X.] The Minor Planets. 169 

Deducting duplicate discoveries, Palisa carries off the palm 
for the largest number, for (up to the end of 1888) he had detected 
68 minor planets. Then comes Peters with 47 ; Luther with 23 ; 
Watson with 22; Borelly with 15; Goldschmidt with 14; Hind 
with i o ; and so on. 

The want of telescopes suitable and available for looking after 
minor planets tends now to hinder new discoveries. All the 
brighter ones have evidently been found ; and, speaking gener- 
ally, each new one is fainter than its predecessors, and con- 
sequently small telescopes are now incapable of doing the work. 
The following table will show this better than any argument: 

Mean J* 

Star Mag. 

First Group : Planets to ... ... 8-5 

Second ... ... 9-6 

Third - ... ... 10-4 

Fourth ... ... n-o 

Fifth ... 10-9 

Sixth ., ... -. n-2 

Seventh., ... ... 11-3 

Eighth - 11-6 

Ninth - ... ... n-6 

Tenth - 11-4 

Eleventh @~ 11-5 

The above numbers are not, it is true, in perfect sequence, and 
it is not possible to complete the Table at present, but my 
meaning will be sufficiently clear. 

The figures in the column headed " Diameter " in the Table 
(see Book VI, post) are the results of calculations by Stone k . 
Photometric experiments made by Professor Stampfer of Vienna 
yielded somewhat similar results 1 . But both sets of figures are 
probably more relatively than absolutely accurate. Argelander 

k Month. Not., vol. xxvii. p. 302. June of certain of these planets will be found in 

1867. Mem. of the American Acad., vol. V.,N.S., 

1 See Bruhns's De Planetis Minoribus, pp. 1 23-35 : an abstract appears in Month. 

Berlin 1856, for details. Some physical Not., vol. xxi. pp. 55-7. Dec. 1860. 
investigations by Newcombinto the orbits 



170 The Sun and Planets. [BOOK I. 

published some suggestions for determining the brightness of 
these planets m . Pickering also has made a few endeavours in 
this direction . In Hornstein's opinion all the larger Minor 
Planets have now been found, and those having a greater diameter 
than 25 geographical miles are few in number. Omitting a few 
of comparatively larger size, he puts the general diameter of the 
bulk of them at from 5 to 15 miles . 

Below are given the names of the only minor planets for the 
determination of whose places we as yet possess Tables. It is 
not likely that this list will ever be much enlarged, for the in- 
crease of late years in the number of these planets has severely 
taxed the patience of astronomical computers. 

By Becker : Tables for Ampkitrile. 

By Brunnow : Tables for Iris, Flora, Victoria. 

By Hansen : Tables for Egeria. 

By Lesser: Tables for Metis, Lutetia, Pomona. 

By Leveau : Tables for Vesta. 

By Moller : Tables for Pandora. 

By Schubert: Tables for Parthenope, Eunomia, Melpomene, 
Harmonia. 

m Month. Not., vol. xvi. p. 206. June Sitzungsberichte der Math. Natur- 

1856. Ast. Nach., vol. xlii. No. 996. wissenschaftlichen Classe der Kaiserlichen 

Nov. 29, 1885. AJcademie, vol. Ixxxiv. pt. ii. p. 7. June 2, 

n Annals of the Observatory of Har- 1881. 
vard College, 1879. 



Fiffg. 81-4. 



Plate XL 




1857 : November 27. (Dawea.') 




1858: November 1 8. (Lassell.} 




1860: March 12. (Jacob.] 




1860: April 9. (Baxendell.} 



JUPITER. 



CHAP. XI.] Jupiter. 173 



CHAPTER XI. 

JUPITER a. l/ 



Period, fyc. Jupiter subject to a slight phase. Its Belts. Their physical nature. 
First observed by Zucchi. Dark Spots. Luminous Spots. The great Red 
Spot. The great White Spot. Sough's observations. Alleged Connection 
between Spots on Jupiter and Spots on the Sun. Axial rotation of Jupiter. 
Centrifugal force at its Equator. Luminosity of Jupiter. Its Apparent 
Motions. Astrological influences. Attended by 4 Satellites. Are they visible 
to the Naked Eye 1 Table of them. Eclipses of the Satellites. Occultations. 
Transits. Peculiar aspects of the Satellites when in transit. Singular cir- 
cumstance connected with the interior ones. Instances of all being invisible. 
Variations in their brilliancy. Observations of Eclipses for determining the 
longitude. Practical difficulties. Homer's discovery of the progressive trans- 
mission of light. Mass of Jupiter. The "Great Inequality." Tables of 
Jupiter. 

JUPITER, the largest planet of our system, revolves round the 
Sun in 4332'6 d or 11-86^, at a mean distance of 483,288,000 
miles. The eccentricity of its orbit is 0-048, so the planet may 
recede from the Sun to 506,563,000 miles, or approach it to within 
460,01 3,000 miles. The planet's apparent diameter varies between 
49-9" in opposition and 30-4" in conjunction, being 40-1 3" at its 
mean distance, according to very elaborate measurements by 
Main. The equatorial diameter is 88,400 miles or thereabouts. 
The compression is greater than that of any other planet except 
Saturn, and amounts, according to the trustworthy observations 
of Main, to r^ FT- All the values of this quantity are closely in 
accord: e.g. Lassell gave TT^T- 

* Important modern delineations of ing) ; vol. xxxiv. p. 235. March 1874 
Jupiter will be found as follows : Month. (the Earl of Rosse) ; vol. xxxiv. p. 403. 
Not., vol. xxxi. p. 34. Dec. 1870 (Brown- June 1874 (Knobel). 



174 



The Sun and Planets. 



[BOOK I. 



Jupiter is subject to a slight phase b : in quadratures it is 
gibbous : for reasons referred to in treating of Mars, the illu- 
minated portion always exceeds a semicircle, and in point of 
fact, owing to the greatly increased distance of Jupiter, the 
defalcation of light is very small, but perceptible nevertheless 
in the form of a slight shading off of the limb farthest from the 
Sun. Webb has noted that this is more easily seen in twilight 
than in full darkness. 

Fig. 85. 




JUPITER, OCTOBER 25, 1856. (De La Sue.) 

The principal telescopic feature of Jupiter its belts are well 
known, at least by name, to every one. They are dusky streaks 
of varying breadth and number, lying more or less parallel to 
the planet's equator . Various theories have been broached to 
explain the belts, but it is generally supposed that the planet is 
enveloped in dense masses of cloud, and that the belts are 
merely longitudinal fissures in these clouds, laying bare the 



b Sir J. Herschel says the contrary, 
but that is certainly an oversight. 



c A circumstance first remarked by 
Grimaldi in 1648. 



CHAP. XI.] Jupiter. 175 

solid body beneath d . The belts, or, as we should on this theory 
with more propriety call them, the atmospheric fissures, are 
constantly changing their features : occasionally only 2 or 3 
broad ones are seen ; at other times as many as 8, 10, or even a 
dozen narrow ones appear. They are not permanent, but change 
from time to time, and occasionally with extreme rapidity; even 
in the course of a few minutes. On this point, writing in 1877, 
Todd remarks : " I was much impressed on some nights with 
the sudden and extensive changes in the cloud belts, as though 
some tremendous storm was in progress on the planet's surface, 
changing the form and dimensions of the cloud belts in an hour 
or two, or even less 6 ." At other times the change they undergo 
is but gradual, and they retain nearly the same forms for several 
consecutive months. They are commonly absent immediately 
under the equator, but North and South of this there is usually 
one wide streak and several narrower ones. At each pole the 
luminosity of the planet is feebler than elsewhere. The belts, 
distinguished from the general hue of the planet (often rose- 
coloured), are usually greyish ; but superior optical power brings 
out traces of a brownish tinge, especially on the larger ones. 
Occasionally (as, for instance, during the years 1869-72, accord- 
ing to numerous observers) the belts are characterised by much 
colour; "copper," "deep purple," "claret/ 5 "red," "orange," 
"Roman ochre," are some of the terms employed by Browning 
and others. A sketch by Lassell is annexed. He described the 
colours recorded in the margin as " unmistakable f ." It is also 
to be remarked that they fade away towards the margin of the 
disc on either side a circumstance which it may be presumed is 
connected with the fact that the portions of the planet's atmo- 
sphere near the limbs are necessarily viewed by us obliquely. 
Sometimes, but rarely, oblique belts may be seen [Figs. 83-4] ; 
and with large telescopes sundry irregularities show themselves, 
which to smaller instruments are merged in fewer and simpler 

d I have used the word "clouds" in e Month. Not., vol. xxxvii. p. 285, 

the text, but their resemblance to the April, 1877. 

clouds of our own atmosphere must, for ' Month. Not., vol. xxxii. p. 82. Jan. 

many reasons, be only remote. 1872. 



176 The Sun and Planets. [BOOK I. 

outlines. Green has advanced various reasons for the opinion 
that the bright surface on Jupiter is at a higher elevation than 
the dark surface, thereby indeed supporting the theory already 
mentioned g . The belts of Jupiter were first observed by Zucchi, 
at Rome, on May 17, 1630, according to Riccioli h ; but a claim 
has been put in on behalf of Torricelli 1 . 



Fi 




JUPITEE, DEC. 30, 1871. (Lassell.} 

Spots are occasionally, but, with a special exception to be 
noted presently, not very frequently, visible on Jupiter. Hooke 
makes the first record of one in May i664 k . He watched it 
in motion for about 2 h , and it seems to have been sheer idle- 
ness that led him to neglect observations of it for determining 
the planet's axial rotation an honour reserved, as we shall 
presently see, for J. D. Cassini. Between Dec. n, 1834 and 
March 19, 1835, a remarkable spot was observed at Cambridge 

* Observatory, vol. vi. p. 121. April ' Moll, Jour. Koyal Inst., vol. i. p. 

1883. 494. May 1831. 

11 Almag. NOK., vol. i. p. 486. k Phil. Trans., No. i. 



CHAP. XI] 



Jupiter. 



Ill 



by Airy : during a portion of this interval a second was seen. 
In 1843 a very large black spot was observed by Dawes, and in 
Nov. and Dec. 1858 two oblong dark spots were noted by Lassell 
as interesting objects l . Luminous spots closely resembling 
satellites in transitu were detected for the first time in 1849 by 
Dawes m , and were seen in the following year by Lassell n . In 
the autumn of 1 857 Dawes again noticed some, and forwarded 

Fig. 87. 




SATELLITE. 



SPOTS ON JUPITER, OCTOBER 6, 1857. (Sir W. K. Murray.} 
drawings of them to the Royal Astronomical Society, which will 
repay examination. On Oct. 25 he counted no fewer than u, 
all clustered together in the Southern hemisphere n . In Nov. of 
the following year (1858) Lassell observed another cluster, in the 
Southern hemisphere, but nearer the equator than those seen by 
Dawes, and in a bright belt. [See PI. XL Figs. 81-4.] It was 
much more difficult to catch these than the former ones. 
Luminous spots were observed also in 1858, 1859, and 1860 by 
Sir W. K. Murray , and in 1870 by various observers. 



1 Month. Not. vol. xix. p. 52. Dec. 
1858. One of them (in the drawing at 
least) is precisely like a garden slug ! 

m Month. Not., vol. x. p. 134. April 
1850. 



n Month. Not., vol. xviii. pp. 6 and 49. 
Nov. and Dec. 1857. 

Month. Not., vol. xix. p. 51. Dec. 
18585 Ibid, vol. xx. p. 58. Dec. 1859; 
Ibid., vol. xx. p. 331. June 1860. 



178 The Sun and Planets. [BOOK I. 

The most celebrated spot on Jupiter that has ever been recorded 
is that which was known as " the great red spot," first con- 
spicuously noticed in July 1878, and which occupied a position 
immediately South of the dark belt on the Southern boundary 
of the equator. Its large size and singular boldness of out- 
line aroused the keenest interest amongst astronomers. From 
measures made with the i8-in. refractor at Chicago in the 
years 1879-82, the mean dimensions and position of the spot 
were as follows : Length 11-73", Breadth 3-58", Latitude 7-25' S. 
These figures correspond to a length of about 27,000 miles and 
a breadth of 8000 miles. The intense red colour and permanency 
of the spot called for especial remark. Very little change in 
its shape or appearance occurred until the autumn of 1882, when 
it sensibly began to fade ; and during the ensuing year it became 
extremely faint, though still preserving its integrity of form. 
By the spring of 1884 the spot was to be seen with difficulty, as 
it became involved with the dusky belts, and lost much of its 
definiteness of outline. This object offered an excellent means 
for rediscussing the rotation-period of Jupiter. From some 
observations in 1878, compared with his own up to the end of 
1883, Denning found the period to be 9 h 55 m 36-2", from 4586 
rotations ; but the motion was not uniform, for during the interval 
of more than 5 years embraced by the observations the time 
increased 5 seconds. At the Opposition of 1879 it was 9 h 55 34", 
but in 1883 had increased to 9 h 55 m 39". This extensive drift 
in longitude proves the spot to have been atmospheric, and not 
a fixed object on the actual surface of the planet. The rotation- 
period it has exhibited may not therefore coincide with the true 
period of Jupiter. 

Fig. 88 represents the red spot on Jupiter as seen with a 
lo-inch reflector in the summer of 1887. 

During the last few years a brilliant white spot has been 
visible on the equatorial border of the great Southern belt. A 
curious fact in connection with this spot is, that it moves with a 
velocity of some 260 miles per hour greater than the red spot. 
Denning obtained 169 observations of this bright marking 



CHAP. XI.] 



Jupiter. 



179 



during the years 1880-83, an ^ determined the period as 9 h 50 8.7 s 
(5^ minutes less than that of the red spot), and this period 
increased with the time. In 1 880-81 it was 9 h 50 5-8 8 , but 
during 1883 augmented to 9 h 50 ii4 8 . The swifter motion of 
this object enabled it to complete a revolution of Jupiter relatively 




THE GREAT RED SPOT ON JUPITER, JOLT 16, 1887. (W. P. Denning.} 

to the red spot in 45* i4 h 375 m . During the 1115 days which 
elapsed from Nov. 19, 1880, to Dec. 9, 1883, it performed 25 
rotations more than the red spot. Although the latter is now 
somewhat faint, the bright spot gives promise of remaining 
visible for many years. 

During 1886 a large number of observations of Jupiter were 
made at the Dearborn Observatory, Chicago, U.S., by Prof. G. W. 
Hough, using the 1 8 -inch refractor of the observatory. Inasmuch 
as these observations are not only of high intrinsic interest, but 

N 2 



180 The Sun and Planets. [BOOK I. 

are in conflict to some extent with previous records, a somewhat 
full abstract of them will be useful p : 

" The object of general interest is the great red spot. The outline, shape, and size 
of this remarkable object has remained without material change from the year 1879, 
when it was first observed here, until the present time. According to our observa- 
tions, during the whole of this period it has shown a sharp and well-defined outline, 
and at no time has it coalesced or been joined to any belt in its proximity, as has 
been alleged by some observers. 

"During the year 1885, the middle of the spot was very much paler in colour 
than the margins, causing it to appear as an elliptical ring. The ring-form has 
continued up to the present time. While the outline of the spot has remained 
very constant, the colour has changed materially from year to year. During 
the past three years [1884-6] it has at times been very faint, so as barely to be 
visible. 

"The persistence of this object for so many years leads me to infer that the 
formerly-accepted theory, that the phenomena seen on the surface of the planet are 
atmospheric, is no longer tenable. The statement so often made in text-books, that 
in the course of a few days or months the whole aspect of the planet may be changed, 
is obviously erroneous. 

"The rotation-period of Jupiter from the red spot has not materially changed 
during the past three years. The 'mean' period, 1884-5, was 9 h 55 m 40.4'. 
Marth's ephemeris for the present year is based on a period of 9^ 55 40.6*. The 
mean correction to this ephemeris is now [May 1887] only about minus 7 minute?, 
indicating a slightly less value. 

" The oval white spots on the Southern hemisphere of the planet, 9" S. of the 
equator, have been systematically observed at every Opposition during the past 
8 years. They are generally found in groups of three or more, but are rather 
difficult to observe. The rotation-period deduced from them is nearly the same as 
from the great red spot. 

" These spots usually have a slow drift in longitude of about o - 5 daily in the 
direction of the planet's rotation, when referred to the great red spot ; corresponding 
to a rotation-period of 20 seconds less than the latter." 

It is not known what is the physical nature of either the dark or 
the luminous spots, but observations by Brett indicate (he thinks) 
that the large white patches on the equatorial zone of Jupiter 
cast, shadows : thus showing that these patches project above the 
general surface visible to us. The appearances presented point to 
the conclusion that we do not see the actual body of the planet 
itself either in the dark belts or in the bright ones q . The usual 
form of both kinds of spots is more or less circular. 

It has been already pointed out in Chap. I. (ante) that some 

P Annual Report of Chicago Ast. Soc. 1 Month. Not., vol. xxxiv. p. 359. 

1887, p. 10. May 1874. . 



CHAP. XL] Jupiter. 181 

relationship has been thought to exist between Sun-spots as 
regards their period and the position of Jupiter in its orbit ; but 
Ranyard extends this idea considerably. He points out an 
apparent identity in point of time between the prevalence of 
spots on the Sun and spots on Jupiter, and proceeds to infer that 
spots on Jupiter are indicative of disturbance on Jupiter, and that 
both classes of phenomena are dependent upon some extraneous 
cosmical change, and are in no sense related as cause and effect, 
the supposed cause being Jupiter's attraction, and the supposed 
effect an atmospheric tide on the Sun. The observations of 
Jupiter which are available for the confirmation of the truth of 
this theory are, previous to 1850, too few and too casual to be 
conclusive ; but such as they are they have been tabulated by 
Ranyard, and unquestionably countenance his theory 1 *. Brown- 
ing suggests that evidence exists to show that the red colour of 
Jupiter's belts is a periodical phenomenon coinciding with the 
epoch of Sun-spot maxima 8 . That in a general way the colour 
of Jupiter varies from time to time he is firmly convinced. 

Cassini, by closely watching the spot which he first saw in 
July 1 665, noticed movement, and regarded this as a proof of the 
planet's axial rotation, the period of which he found to be about 
9 h 56. The independent observations of Airy and Madler in 
J ^35 gi ye 9 h 55 m 21 '3 s , an d 9 h 55 m ^'5 S > an d afford another 
illustration of the care bestowed by Cassini on his astronomical 
researches. The later observations of Cassini, those of Sir W. 
Herschel, and those of Schroter indicate results not free from 
anomalies ; Sir William's various determinations fluctuated to an 
extent of nearly 5 m , a discordance far beyond that which is 
assignable to errors of observation ; and the unavoidable conclu- 
sion is, that the spots employed by those 3 astronomers in their 
investigations were affected (as they themselves believed) by a 
proper motion of their own. Schmidt found the period to 
be 9 h 55 m 28'7 8 . 

r Month. Not., vol. xxxi. p. 34, Dec. 5 Month. Not., vol. xxxi. p. 75, Jan. 

1870; p. 201, May 1871; and p. 224, 1871. 
June 1871. 



182 Irie otm and Planets. [BOOK I. 

The axial rotation of Jupiter being so much quicker than that 
of the Earth, combined with its diameter being so much greater, 
results in the rotating velocity of a particle at its equator being 
greater than on any other planet 466 miles per minute, against 
the Earth's 17 miles per minute. It will at once be per- 
ceived that the intensity of the centrifugal force must be very 
great, and the polar compression likewise. Hind calls attention 
to this rapid rotation as offering some compensation, by the heat 
which it must evolve, for the diminished power of the Sun's rays 
at the distance of Jupiter. 

Under favourable circumstances Jupiter, like Mars, rivals 
Venus in brilliancy, and even casts a shadow. G. P. Bond found 
that for photographic purposes its surface reflects light better 
than that of the Moon in the ratio of 14 to i *. Zollner has 
calculated that Jupiter reflects 0*6 a of the light it receives, the 
Moon reflecting but 0-17 of the incident light. Bond computed 
that Jupiter actually emits more light than it receives (!) : but 
whether we accept this problematical result, or the more trust- 
worthy one obtained by Zollner, strong indications of inherent 
luminosity in Jupiter seem to exist ; and this points to the 
conclusion that this planet is itself a miniature Sun. The heat 
derived from the Sun only would leave water on Jupiter's 
surface above 500 below freezing point, so that any clouds must 
arise from internal heat. Moreover, if we conceive the Earth 
and Jupiter to have been simultaneously created, Jupiter would 
retain its heat for ages after the Earth had cooled down. 

Seen from the Earth the apparent motion of Jupiter is some- 
times retrograde. The length of the arc of retrogradation varies 
from 9 51' to 9 59', and the time of its performance from u6 d 
i8 h to i22 d I2 h . The retrograde motion begins or ends, as the 
case may be, when the planet is at a distance from the Sun, 
which varies from 113 35' to 116 42'. u 



e Month. Not., vol. xxi. p. 198. May planet is from the Sun, the less will be 

1 86 1. the extent of its arc of retrogression, but 

11 It may here be noted that, as a the greater will be the time occupied in 

general rule, the farther a superior describing it. 



CHAP. XL] 



Jupiter. 



183 



In by-gone days Jupiter was not without its supposed astro- 
logical influences. It was considered to be the cause of storms 
and tempests, and to have power over the prosperity of the 
vegetable kingdom. Pliny thought that lightning, amongst other 
things, owed its origin to Jupiter. An old MS. Almanac for 
1386 states, that " Jubit es hote and moyste, and doos weel til al 
thynges, and noyes nothing." 

Jupiter is attended by 4 satellites x , 3 of them seen for the first 
time by Galileo, at Padua, on January 7, 1 6io y , but not determined 
to be satellites till the following day, whilst the whole four 
were not seen all together till Jan. 13. They shine with the 
brilliancy of stars of the 6 th or 7 th magnitude ; but, owing to 
their proximity to their primary, are usually invisible to the 

Fig. 89. 




JUPITER AND ITS SATELLITES. 



naked eye, though several instances to the contrary are on 
record. Mr. C. Mason states that on April 15, 1863, finding 
Jupiter conveniently placed for the purpose, he determined to 
make a systematic attempt to solve the problem frequently 
declared to be an impossibility. After a steady gaze of 8 m or 
io m he was able to assure himself that in close proximity to 
Jupiter he could see a little star. Having resorted to various 
precautions to prevent self-deception, he at length turned his 



1 Named by Simon Marius, a frau- 
dulent claimant of their discover}', lo, 
Europa, Ganymede, Callisto. These 
names have never been in use. 



7 Siderius Nuncitis ; Opere di Gali- 
leo, vol. ii. p. 15 et seq. Ed. Padua. 
1 744. Au English Translation by E. S. 
Carlos was published in London in 1880. 



184 



The Sun and Planets. 



[BOOK I. 



refractor of 4^ inches aperture on the planet and found in the 
position corresponding to that indicated by the naked eye (allow- 
ance being made for inversion) all the 4 satellites on the same 
side of the planet. He states that until referring to the Nautical 
Almanac a few minutes before using the telescope he had no idea 

Fig. QO 



JUP1TEB SEEN WITH THE NAKED EYE, APRIL 15, 1863. (Mason.) 

as to their configuration, and is the more convinced that with 
the naked eye he really did see the 4 as one 2 -. It is quite certain 
that satellites II and III were seen on Jan. 15, 1860, by some 
officers of H. M. S. "Ajax" in Kingstown Harbour, near Dublin a . 
Mr. Levander and others at Devizes asserted that on April 

Fig. 91. 




JUPITER SEEN WITH A TELESCOPE, APRIL 15, 1863. (Mason.) 

21, 1859, they saw 2 of these bodies. In 1852 an American 
missionary of the name of Stoddard, at Oroomiah in Persia, re- 
peatedly saw two satellites in the twilight, so long as Jupiter itself 
was devoid of an overpowering glare. Wrangel, the celebrated 
Russian traveller, stated that when in Siberia he once met an 
hunter who said, pointing to Jupiter, " I have just seen that large 



* Month. Not., vol. xxiii. p. 215. May 
1863. 



1860. 



Month. Not., vol. xx. p. 212. March 



CHAP. XI.] 



Jupiter. 



185 



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186 The Sun and Planets. [BOOK I. 

star swallow a small one, and vomit it shortly afterwards." The 
Russian remarks that the sportsman here referred to an immer- 
sion and subsequent emersion of the III rd satellite, on which 
Arago, who makes the citation, says, " It is well known that the 
acuteness of sight of those natives and of the Tartars has become 
proverbial." Other similar observations, including one by him- 
self, are given by Webb b . so that we may now regard the 
question of possibility as decided in the affirmative. 

The satellites of Jupiter are capable of being seen with so 
little optical assistance that it is worth while to enter at some 
length into a consideration of them. 

They are distinguished by ordinal numbers preceding out- 
wards. Thus the "I st " satellite is the one nearest to the 
primary ; the " IV th " the one most distant therefrom. To 
determine which is which, the diagrams given in the Nautical 
Almanac will usually be necessary, but the III rd , as the largest 
and brightest, will generally be identified with least difficulty. 
In small telescopes it is scarcely possible to say that there is 
anything to distinguish the satellites from stars, beyond a 
noticeably greater steadiness of light ; increased power will 
reveal discs, but a very considerable augmentation is requisite 
for detecting physical peculiarities. " The discovery of 4 bodies 
revolving round a primary, exhibited a beautiful illustration 
of the Moon's revolution round the Earth, and furnished a most 
favourable argument in favour of the Copernican theory c . The 
announcement of this fact pointed out also the long vista of 
similar discoveries which have continued from time to time 
down to the present day to enrich the solar system, and to shed 
a lustre on the science of astronomy." 

The eclipses, occultations, and transits of the Jovian satellites 
offer an endless series of interesting, and indeed useful, pheno- 
mena. The I st , II nd , and III rd satellites, in consequence of the 
smallness of the inclinations of their orbits, undergo once in 

b Celest. Objects, p. 144. and Romish ecclesiastics, who assailed 

c The argument, however, failed to Galileo's views respecting these satellites 
command the acceptance of divers Popes with great bitterness for many years. 



CHAP. XI.] Jupiter. 187 

every revolution an eclipse in the shadow cast by the planet 
into space. The IV th , however, frequently escapes this ordeal, 
in consequence of the plane of its orbit being somewhat more 
inclined than is the case with the others, and its distance from 
the primary being so considerable. 

When the satellites enter the shadow the immersion is said to 
take place ; when they come out of it, the emersion terms which 
explain themselves. Closely associated with the eclipses are the 
occultations a word employed to express the concealment of the 
satellites by the direct interposition of the planet itself, indepen- 
dently of the shadow. When the planet has passed its conjunc- 
tion with the Sun, the shadow is projected on the Western side, 
and at this time both the immersions and emersions of the III rd 
and IV th satellites may be observed, but not always those of the 
II nd ; and only the emersions of the I st , in consequence of its 
proximity to the planet causing it (after first undergoing an 
occultation) to enter the shadow behind the planet. When 
Jupiter is near its Opposition to the Sun, the immersions and 
emersions take place very close to the planet's limbs. As the 
planet again approaches Conjunction the shadow is projected on 
the Eastern side, giving rise to phenomena partly comple- 
mentary to those set forth above. In other words, whilst the 
immersions and emersions of III and IV are always visible, 
and those of II frequently visible, the immersions only of I can 
be perceived because it emerges behind Jupiter ; when this one 
does reappear it is on emersion from an occultation. 

The occultations "generally require much more powerful 
instruments for their satisfactory observation than the eclipses. 
With a telescope of adequate power we may trace the gradual 
disappearance of the satellite from the first contact with the limb 
of the planet to its final obscuration behind the disc ; and, as 
viewed with such an instrument, these phenomena are highly 
interesting. The occultations of the IV th satellite are usually 
visible both at disappearance and at reappearance ; those of the 
III rd also are frequently so observable. But it happens much 
more rarely that the complete phenomenon can be observed 



188 The Sun and Planets. [BOOK I. 

in regard to the II nd satellite, while the immersion and emer- 
sion of the I st can only be visible a day or two before or after 
the Opposition of Jupiter, as at all other times either the im- 
mersion or emersion must happen while the satellite is obscured 
in the planet's shadow. Thus it most usually occurs that from 
Conjunction to Opposition the reappearances only of the I st and 
II nd satellite can be observed, and the disappearances only from 
Opposition to Conj unction d ." 

Far more interesting are the transits of the satellites and their 
shadows across the planet phenomena which, it is easy to under- 
stand, are of frequent occurrence when the satellites are in those 
parts of their respective orbits which lie nearest to the Earth. 
The satellites appear on the disc of their primary as round lumi- 
nous spots preceded or followed by their shadows, which show 
themselves as round black or blackish 6 spots. The shadow 
precedes the satellite when Jupiter is passing from Conjunction 
to Opposition, but follows it when the primary is between 
Opposition and Conj unction. When actually in Conjunction the 
shadow is in a right line with the satellite, and the two may be 
superposed. 

Some peculiarities in the appearance of the satellites during 
transit are too well attested to be passed over. Ill in particular 
is nearly always seen almost or quite as dark as its shadow, but 
on rare occasions appears dusky and shaded. IV has been 
often seen dark f , but, according to Dawes, II has never had the 
slightest shading on the disc within his knowledge, and I only 
a grey tinge, inferior by many shades to that usually possessed 
by III. Contrast has evidently a good deal to do with the 
bringing out of these shadings, but the circumstances attending 

d Hind, Sol. Syst., p. 100. (Modified vol. viii. p. 37. Feb. 1870.) 

in one place.) f Roberts (Month. Not., vol. xxxiii. p. 

6 Blackish, because the visible margin 412. April 1873); Firmstone (Ibid., p. 

is not that of the true shadow, but of a 460. May 1873) ; Burton (Ibid., p. 472. 

penumbra which surrounds the shadow, Jnne 1873), &c. On Aug. 21, 1867, 

though it is rare for this penumbra to Prince saw IV as a " round black spot," 

be observable as an actual ring sur- its colour being as nearly as possible that 

rounding the shadow. (See an instance of its own shadow " (Month. Not., vol. 

recorded by T. H. Buffham in Ast. Reg., xxvii p. 318). 



CHAP. XL] 



Jupiter. 



189 



Fig. 92. 



the recorded variations in this intensity are less intelligible. 

J. D. Cassini, Maraldi, Pound 8 , Messier h , Schrb'ter, and Sir W. 

Herschel were amongst the earlier observers of 

these peculiarities, and W. C. Bond, Lassell, and 

Dawes amongst the more modern ones. Bond 

saw III as a well-defined black spot on Jan. 28, 

1848, and again on March n. He stated that, 

on March 1 8, it entered upon the disc as a very 

bright spot, more brilliant than the surrounding 

surface ; that 2o m later it had so decreased in 

brightness as to be hardly perceptible, and that 

in another few minutes a dark spot suddenly 

appeared in its place, which was seen for 2| h . This spot was 

sufficiently conspicuous to be measured with a micrometer, was 




THE IV th SATELLITE 

OF JDPITER, 
MARCH 26, 1873. 

(O-. W. Roberts.) 



Fig- 93- 



Fig. 94. 





THE III rd SATELLITE OF 
JDPITER, JAN. 31, i860. 



THE IV th SATELLITE OF 
JUPITER, FEB. 12, 1849. 

Dawes. 



perfectly black, nearly round, and on the satellite. The con- 
verse of this the satellite dark first and bright afterwards 
was witnessed by Prince and Brodie on Jan. 31, 1860*. 

On June 26, 1828, II, having entered on the disc of Jupiter, 
was seen 12 or 13 afterwards outside the limb, where it re- 
mained visible for at least 4 and then suddenly vanished. 
Three observers of eminence (Sir T. Maclear, Adm. Smyth, and 
Dr. Pearson) record this, so there can scarcely have been any 



e Phil. Trans-, vol. xxx. p. 900. 

h Phil. Trans., vol. lix. p. 459. 1769. 



1 Month. Not., vol. xx. p. 212. 
1860. 



March 



190 The Sun and Planets. [BOOK I. 

individual optical illusion, much less deception. It has been 
suggested that an eclipse of the satellite by another satellite 
would meet the facts of the case, provided we could establish a 
doubt as to whether these observers for a certainty saw the 
satellite previously on the disc of the planet 

Fig. 95. 




JUWTBB WITH SATELLITE IN TRANSIT, JUNE 26, 1828. (Smyth.} 

Lassell has found the shadow of IV very much larger than 
the satellite itself, even to the amount of double the diameter, 
and the same shadow larger than that of III, though the satel- 
lite itself is smaller than III. The shadow of II has been seen, 
it is said, to possess an irregular outline, but the observation is 
not well attested. 

On April 5, 1861, Mr. T. Barneby saw the shadow of III first 
in the shape of a broad dark streak such as the cone of the 
shadow would represent in a slanting direction, but it shortly 
afterwards appeared as a circular spot perfectly dark and much 
larger than the shadow (which was visible at the same time) of 
I. I cite this passage chiefly because of the information about 
the form of the projection of the shadow, which, though very 
reasonable and obvious, is noticeable as the only instance I have 
met with. 

On April 17, 1861, the Rev. R. Main saw II occulted by I, and 
the two appeared as one for some y m or 8 m . 

On Jan. 14, 1872, Mr. F. M. Newton saw I superposed on its 
shadow, so that the satellite appeared to be surrounded by a 
dark ring. This observation seems to be unique k . The nearest 

k Letter in Eng. Mech., vol. xxiii. p. 562. Aug. n, 1876. 



CHAP. XI.] Jupiter. 191 

approach to it is an observation by Mr. G. D. Hirst, on May 13, 
1876, of I in transit partly occulting its own shadow, so that 
the shadow appeared as a narrow black crescent. The satellite 
itself was not seen except when near the edge of the planet's 
disc 1 . 

Fig 96 represents a singular observation made by Trouvelot 
at Cambridge, U.S., on April 24, 1887. 

Fig. 96. 




JUPITER'S IST SATELLITE IN TRANSIT, WITH A DOUBLE SHADOW, 
APRIL 24, 1877. (Trouvelot,} 

" The shadow of the first satellite which had entered on Jupiter 
39 minutes previously had not yet quite gone a quarter of its 
way across the disc. This shadow, black and of a sensibly 
elliptical form, doubtless on account of the fact that it was seen 
projected not far from the edge of a spherical surface, almost 
touched at its most northern point the northern edge of the 
pink equatorial zone. It was preceded on its western side by 
a rather dark spot, which was of exactly the same shape and 
size, and only separated from the shadow by a space equal at 

1 Letter in Entj. Mech., vol. xiv. p. 535. Feb. 9, 1872. 



192 The Sun and Planets. [BOOK I. 

the most to one-third of the equatorial diameter of the shadow. 
This remarkable spot was not exactly on the same horizontal 
line as the shadow of satellite I, but lay about one third of 
the vertical diameter of the shadow towards the South." 

Trouvelot goes on to say that he watched the phenomenon for 
altogether i h 20, or until the primary shadow had accomplished 
about f of its journey across the planet, when it ceased. He 
assured himself that it was neither a planetary spot, properly so 
called, nor a satellite that he had seen, and he regarded it as 
simply a secondary shadow a shadow of the main shadow seen 
projected on a lower stratum of Jupiter's atmosphere (or even it 
might be on the solid body of the planet), which would account 
also for the secondary shadow being much less intense than the 
primary or ordinary one m . 

As to certain irregularities of figures presented by IV when 
seen as a dark spot on the disc of Jupiter, reference may be made 
to a paper by Burton 11 . 

The phenomena exhibited by the satellites in transit have 
been carefully studied by Spitta, and his conclusions in a 
summary form will be useful for reference : IV is fainter than the 
others on approaching the limb of the planet ; bright for first i o 
or 1 5 minutes of transit ; lost for about the same time ; reappears 
as a dark spot, becoming jet black : II always bright during 
transit; brilliancy least affected on approaching limb: III 
sometimes becomes lost, reappearing as a dark spot ; at others, 
remains white throughout : I after becoming lost, usually turns 
one of the shades of grey to nearly black . 

Jupiter's satellites move in orbits nearly circular, and between 
the motions of the first three a singular relation exists: The 
mean sidereal motion of I added to twice that of III, /'* constantly 
equal to three times that of II ; so that the sidereal longitude of I, 
plus twice that of III, minus three times that of II, yields a re- 
mainder always constant, and in fact equal to 180. This relation 

ra L'Astronomie,.vo\. vi. p. 414. Nov. 1887. 
" Month. Not , vol xxxiii. p. 472. June 1873. 
Month. Not., vol. xlviii. p. 34. Nov. 1887. 



CHAP. XL] 



Jupiter. 



193 



will be better understood by an inspection of the following 
table : 

Sidereal motion 
per second of time. 

Satellite I. 8-478706 x i = 8-478706. (a) 
II. 4-223947 x 3 = 12-671841. (/>) 
III. 2-096567 x 2 = 4-193134. (c) 

Fig. 97. 




PLAN OF THE JOVIAN SYSTEM V. 



P The satellite orbits in this and the 
follow.ing chapters are all drawn to the 
same scale. No diagram on a plane 
on the same scale of the orbits of 



the satellites of Mars is given in this 
volume, because on the scale here em- 
ployed, those orbits would be of micro- 
scopic dimensions. 



C) 



194 The Sun and Planets. [BOOK I, 

Adding together a and c, we get 12-671840, which quantity is to 
5 places of decimals the same as b. From this it follows that for 
an enormous period of time the 3 interior satellites cannot 
all be eclipsed at the same time; for in the simultaneous 
eclipses of II and III, I will always be in conjunction with 
Jupiter, and so on q . Making use of his own tables, Wargentin 
has calculated that simultaneous eclipses of the 3 satellites 
cannot take place before the lapse of 1,317,900 years 1 , and an 
altefation of only 0-33" in the annual motion of II would 
suffice to render the phenomenon for ever impossible. 

D' Arrest pointed out the commensurability, within a few hours, 
of 5187 revolutions of I, 2583 of II, 1281 of III, and 548 of 
IV, in 25 y 55 d 5 when the same geometric configuration will 
recur. 

The exact figures are given by him 8 as follow : 

Revolutions. Days 

Satellite I. 5187 = 9 [80-27. 
II. 2583 = 9180-23. 
III. 1281 = 9180-14. 
IV. 548 = 9 l8o> 95- 

Between satellites III and IV the following comparatively 
coarse approximation subsists. Seven times the period of the .former 
(5o d i h 57 m 53 '5 20 s ) exceeds by only 2i m 19* 7 s three times the 
period of the latter (50* i h 36 33'8i3 8 ). Moreover the periods of 
I, II, and III stand in the ratio of i, 2 and 4, as near as may be. 
The following special elements are given by Hind*. " The 
line of apsides of the III rd satellite revolves in about i37 y , and 
that of the IV th in about 5i6 y . The lines of nodes of the 3 
exterior satellites revolve in a retrogade direction, as is the case 
with the nodes of the lunar orbit ; the period for the II nd is 3O y , 
for the III rd i40 y , and for the IV th 52o y ." 

It occasionally, but very rarely, happens that all 4 satellites 
are for a short time invisible, being either directly in front of, or 

i Laplace demonstrated by the theory r Ada Soc. Upsal.,p. 41. 1743. 

of Gravitation that if this relation be s Ast. Nach., vol. Iviii. No. 1377. 

once approximately begun, it will always Aug. 25, 1862. 

last. l Sol. Syst., p. 98. 



CHAP. XI.] Jupiter. 195 

behind, the planet. Such was the case, according to Molyneux u , 
on Nov. 2, 1681 (o. s.) The same thing was noticed by Sir W. 
Herschel on May 23, 1802; by Wallis on April 15, 1826; by 
Dawes and W. Griesbach on Sept. 27, 1843. Dawes published 
in 1862 an account of his observations w . Jupiter's (apparent) 
deprivation of its satellites lasted about 35. A repetition of 
this phenomenon occurred on Aug. 21, 1867, when the planet 
was for if h apparently without satellites projected on the sky. 

The satellites appear to vary in brilliancy in a way wholly 
inexplicable. I have already stated that III is commonly the 
brightest but Maraldi and Bond have seen the contrary. On 
the whole, perhaps, we are justified in saying that the faintest is 
IV ; but the lustre of this is irregular : in 1 7 1 1 Bianchini and 
another, and on June 13, 1849, Lassell, saw it so feeble as to be 
almost invisible, whilst Webb repeatedly saw it surpass III. 
This observer wrote " Spots . . . may easily cause this variable 
light; but a stranger anomaly has been perceived, the discs 
themselves do not always appear of the same size or form. 
W. Herschel noticed the former fact, and inferred the latter ; and 
both have been since confirmed by others. Beer and Madler. 
Lassell, Secchi and Buffham have sometimes seen the disc of 
II larger than I; and Lassell, and Secchi and his assistant, 
and Burton have distinctly seen that of III irregular and 
elliptical; and according to the Roman observers the ellipse 
does not always lie the same way: Mitchell also, with an i i-inch 
achromatic, has observed this disc irregular and hazy. Buffham 
has often found IV the smallest of all, and irregular-looking. 
Phenomena so minute hardly find a suitable place in these pages, 
but they seem too singular to be omitted ; and in some cases, 
possibly small instruments [?] may indicate them ; at least, 
with an inferior fluid achromatic reduced to 3 inches aperture I 
have sometimes noticed differences in the size of the discs which 
I thought were not irnaginary x ." 

Sir W. Herschel, by attentive and prolonged observation, was 

u Opticks, p. 271. w Month. Not., vol. xxii. p. 292. June 1862. 

x Celest. Objects, 4 th ed., p. 162. 

O 2 



19(> The Sun and Planets. [BOOK I. 

led to infer that each of the satellites rotated on its axis in the 
same time that it made a sidereal revolution round its primary 
thus presenting an analogy to the case of our Moon. The imme- 
diate reason which led to this conclusion was a belief that the 
variation in their brilliancy always recurred in nearly the same 
positions of the satellites with respect to Jupiter and the Sun, 
which supposition had previously presented itself to the mind of 
Cassini y . But modern observations do not harmonise with these 
statements ; that is to say, we are not entitled to affirm now 
that peculiarities in the appearances of the satellites correspond 
with definite orbital positions. On the contrary, the peculiar- 
ities observed are not governed by any known law of time or 
place. 

Arago thus summed up Sir W. Herschel's photometric deduc- 
tions. " The I st satellite is at its maximum brightness when it 
attains the point of its orbit which is almost midway between 
the greatest Eastern Elongation and its Conjunction. The bright- 
est side of the II nd satellite is also turned towards the Earth 
when that body is between the greatest Eastern Elongation and 
Conjunction. The brightness of the III rd satellite attains 2 
maxima in the course of a revolution, namely at the 2 Elonga- 
tions. The IV th shines with a bright light only a little before 
and a little after Opposition 2 ." 

Various observers have assigned colours, or rather tinges of 
colour, to the different satellites, but the results are not suffi- 
ciently of accord to be worth citing. 

Eclipses as viewed on Jupiter take place on a grand scale ; for 
in consequence of the small inclinations of the orbits of the 
satellites to the planet's equator and the small inclination of the 
latter to the ecliptic, all the satellites, the IV th excepted, are 
eclipsed some time in every revolution ; so that a spectator on 
Jupiter might witness during the Jovian year 4500 eclipses of 
the Moon (Moons) and about the same number of the Sun. 

Soon after their discovery it suggested itself to the reflecting 

y Mem. Acad. des Sciences, vol. i. p. 266. 
* Pop. Ast., vol. ii. p. 549. Eng. ed. 



CHAP. XL] Jupiter. 197 

mind of Galileo that eclipses of the satellites of Jupiter might be 
made useful for determining the longitude. Regarding eclipses 
as instantaneous phenomena visible at the same moment in every 
place which has the planet above its horizon, it is clear that a 
comparison of observations recorded in 2 local times would afford 
data for determining the difference of time (longitude) between 
the places to which the times belong. Eclipses accurately pre- 
dicted for one meridian when observed under another one would 
afford a still more advanced means of ascertaining the difference of 
longitude between them. These eclipses could be predicted if 
sufficiently accurate tables of the satellites were in existence ; 
but at sea, where the problem has chiefly to be solved, they 
cannot be observed with the most refined accuracy, and on land 
some difficulties present themselves ; so that the method to some 
extent breaks down, and is only available where very rough 
approximations will suffice. 

It was to observations of one of the satellites of Jupiter, and 
Homer's discussion of them in 1675, that we owe the discovery that 
light is not propagated instantaneously through space*. It was 
found that the calculated times of the eclipses did not correspond 
with the observed times, and that the difference was a quantity 
constantly affected by opposite signs of error according as Jupiter 
was in perigee or apogee. In the former case the eclipse always 
occurred before the calculated time ; in the latter, always after 
it. The regularity with which these anomalies showed them- 
selves led Homer to suspect that they had their origin in the 
variations which occurred in the distance of Jupiter from the 
Earth : that as this distance increased or diminished so a longer 
or a shorter period was requisite for light to traverse the space 
between the 2 planets. Assuming from the data in his posses- 
sion that light travelled at the rate of 192,000 miles per second, 
and required i6i m to traverse the diameter of the Earth's orbit, 
and applying this (as yet hypothetical) conclusion to the eclipses 
in the form of a trial-correction, Homer promptly obtained proofs 
of the accuracy of his reasoning ; but it was Bradley 's discovery 
" Opere di Galileo, vol. ii. p. 33. Padua ed., 1 744. 



198 The Sun and Planets. [BOOK I. 

of aberration some half a century later which completely 
demonstrated the soundness of Homer's views and caused their 
general acceptance. The modern experiments of Fizeau have 
given for the velocity of light a result but slightly differing in 
amount from Romer's, namely, 194,000 miles per second b . 

Like most new discoveries Romer's did not, when promulgated, 
find favour in the scientific world, and many years elapsed ere it 
was generally accepted. 

The mass of Jupiter has never been a very doubtful quantity, 
all the values of it being much more in accord with one another 
than is usually the case. Laplace, from Pound's observations of 
the IV th satellite, placed it at r^Vr 5 Bouvard, from the pertur- 
bations of Saturn, at y^Y^ 5 Nicolai, from the perturbations of 
Juno, at TtrsVrs-; Encke, from the perturbations of Vesta, at 
Ttrsff 5 and from perturbations of the Comet bearing his name, 
at xo-Vr ; Santini at T (r 3o - ; Bessel at j^VsT ; Airy, from motions 
of the satellites, at T^tWr ; Kriiger, from observations of Themis, 
at TTT/TTT 5 Jacob, from the motions of the satellites, at j^V-ir 5 
and Moller, from the motions of Faye's Comet, at xmrV-TF 5 Schur, 
from heliometer measures of the satellites, at x7riV^- Any one 
of the 4 last values may be taken to be substantially exact. 

" The most ancient observation of Jupiter which we are ac- 
quainted with is that reported by Ptolemy in Book X. chap. iii. 
of the Almagest, and considered by him free from all doubt. It is 
dated in the 83 rd year after the death of Alexander the Great, on 
the 1 8 th of the Egyptian month Epiphi, in the morning, when the 
planet eclipsed the star now known as 8 Cancri. This observation 
was made on Sept. 3, B.C. 240, about i8 h on the meridian of 
Alexandria." 

This is a convenient place to mention the " Great Inequality " 
in the motion of Jupiter and Saturn, so far as the fact of its 

b In consequence of the increase in before the parallax question came up 

the received value of the Sun's parallax for general discussion pointed to the 

a reduction in the velocity of light by same conclusion. The value for the 

several thousands of miles per second velocity of light now generally accepted 

must be assumed, and singularly enough is about 186,660 miles per second, 
some experiments of Foucault's made 



CHAP. XI.] Jupiter. 199 

existence is concerned, for a particular account of it would be 
altogether foreign to the purposes of this work c . The period of 
each of these planets is subject to a continuous change owing to 
the mutual influence exerted by each on the orbit of the other 
and the time required for this change to go through its various 
stages is the Period of the Great Inequality. It amounts to 
918 years. 

The Tables of Jupiter used till recently were those of A. Bou- 
vard, published in 1821, but the new and far superior Tables of 
Le Verrier have superseded them d . For the satellites, Damoi- 
seau's Tables (published in 1836) are employed. As regards the 
satellites there is room for much improvement in the Tables at 
present employed. They fail to give results characterised by the 
precision which modern science demands. 

c See Sir J. Herschel's Outlines,^. 502. first time in England in the preparation 
ll These tables were employed for the of the Nautical Almanac for 1878. 



200 The Sun and Planets. [BOOK I. 



CHAPTER XII. 



SATURN . Fj 

Period, &c. Figure and Colour of Saturn. Belts and Spots. Observations of 
the Belts by Holden. By Ranyard. Bright spot recorded by Hall. Probable 
atmosphere. Observations of Galileo, and the perplexity they caused. Logo- 
griph sent by him to Kepler. Huygens's discovery of the Ring. His logn- 
ffriph. The bisection of the Ring discovered by the brothers Ball. Sir W. 
HerscheVs Doubts. Historical epitome of the progress of discovery. The 
" Dusky" Ring. Facts relating to the Rings. Appearances presented by them 
under different circumstances. Rotation of the Ring. Secchis inquiries into 
this. The Ring not concentric with the Ball. Measurements by W. Struve. 
Other measurements. Miscellaneous particulars. Theory of the Ring being 
fluid. Now thought to consist of an aggregation of Satellites. The " Beaded " 
appearance of the Ring. O. Struve's surmise about its contraction. Irregu- 
larities in the appearances of the ans<B. Rings not bounded by plane sur- 
faces. Mountains suspected on them. An atmosphere suspected. Physical 
observations between 1872 and 1876 by Trouvelot. Observations^ MM. Henry. 
By Keeler. Brightness of Rings and Ball. Bessets investigations into the 
Mass of the Rings. Saturn attended by 8 Satellites. Table of them. 
Physical data relating to each. Elements by Jacob. Coincidences in the 
Rotation-periods of certain of them. Transits of Titan. Celestial phenomena 
on Saturn. Lockyer^s summary of the appearances presented by the Rings. 
Peculiarity relative to the illumination of lapetus. Mass of Saturn. Ancient 
observations. Saturnian Astronomy. 

TNFERIOR in size to Jupiter only, Saturn may fairly be pro- 
nounced to be the most interesting member of the Solar 
System. It revolves round the Sun in io759 - 2 d or 29'45 y at a 

* For drawings, &c. of Saturn, see (Dawes) ; Ibid.,xv. p. 79(Dawes); Ibid., 

Annals of Harvard Coll. Obs., vol. ii. vol. xvi. p. 120 (one fig. by Jacob); Ibid., 

(1 20 drawings by the Bonds); Ast. Nach., vol.xviii.p. 75 (abstract of Harvard Obs.) ; 

vol. xxviii. No. 650, Nov. 1848 (J. F. J. and vol. xxii. p.. 89 (two figs, by Jacob) ; 

Schmidt) ; Ibid., vol. xxxix. No. 929, Student, vol. ii. p. 240 (Browning). 

Jan. 8, 1855 (Secchi) ; Mem. R.A.S., vol. Month. Not., vol. xliv. p. 407 (Pratt) ; 

iv. p. 383 (Kater) ; Ibid., vol. xxi. p. 151 Ibid., vol. xlv. p. 401 (Green); Ast. Nach., 

(8 figs, by Lassell); Month. Not., vol. xi. vol. cxii. No. 2682 (Lamp) ; Month. 

p. 23 (Dawes and Lassell) ; Ibid., vol. Not., vol. xlvii. p. 514 (Elger); L'Astro- 

xiii. p. 1 6 (Dawes) ; Ibid., vol. xiv. p. 17 nomie, vol. vi. p. 208 (Stuyvaert). 



Fig. 98. 



Plate XII. 




CHAP. XII.] Saturn. 203 

mean distance of 886,065,000 miles, which an orbital eccentricity 
of 0-056 may increase to 931,033,000 or diminish to 841,097,000 
miles. Its apparent diameter varies between i5'i" in conjunc- 
tion, and 20-7" in opposition, and its real (equatorial) diameter 
may be taken at 75,036 miles. Its polar compression is larger 
than that of any other planet, Jupiter not excepted: but it is 
usually less noticeable than that of Jupiter because the ring 
distracts the eye. Sir W. Herschel's value of the compression 
is TirW; Bessel's T ^ T ^ ; the Rev. R. Main's T .^ b ; and Hind's ^ .\^. 

Saturn has no perceptible phases. The maximum defalcation of 
light under extreme circumstances is so small that the maximum 
breadth of the shaded area can hardly be ^ of a second of arc 
a quantity inappreciable. 

The figure of Saturn is now quite understood to be that of an 
oblate spheroid, but at one time considerable doubt existed about 
the matter in consequence of Sir W. Herschel having advanced 
the opinion, from observations made in April 1805, that the 
planet was compressed at the equator as well as at the poles ; 
or, as it is generally phrased, that it resembles a parallelogram 
with the corners rounded off. so as to leave both the equatorial 
and the polar regions flatter than they would be in a regular 
spheroidal figure. This opinion, never received with much 
favour (though not entirely unconfirmed by later observers), is 
now almost universally repudiated, chiefly owing to the micro- 
metrical measurements performed by Bessel in 1 833 and by Main 
in 1848. Some optical illusion was probably at the foundation 
of it, though it is right to say that the notion is believed in to 
this day by some persons, and ascribed to an actual upheaval of 
the planet's surface recurring from time to time and due to 
quasi-volcanic causes. It must also be added, that (as in the 
case of Jupiter) we only see the outline of Saturn's atmosphere and 
not that of the solid (or fluid) body of the planet itself. 

Belts exist on Saturn resembling those of Jupiter, but they 

b See Month. Not., vol. xiii. p. 79, Jan. memoir by the same observer appears in 
1853, for others, and same vol., p. 152, for Mem. M.A.S., vol. xviii.p. 27, 1850. 
a note by the Rev. R. Main: an important 



204 



The Sun and Planets. 



[BOOK I. 



are very much fainter. They are probably of the same physical 
character. 

In November and December 1883 several observers noticed a 
singular configuration of dark and bright belts on Saturn, the 
character of which will be best understood by a careful perusal of 
the following description by Professor E. S. Holden. Under date 
Dec. 2 he writes : " The S. pole is mottled, especially so near of the 
shadows. The bright equatorial belt is bounded on the S. by a 
narrow dark streak some 2" wide ; it is the darkest thing on the ball. 

Fig. 99. 




SATURN, Dec. 2, 1883. (Holden.} 

S. of this is an equally narrow bright streak, then S. of this is the 
nearly uniform S. hemisphere. N. of the equatorial bright belt 
is a narrow dusky belt (i"*5 ?), then a narrow bright belt (i"\5 '?), 
and then a dark band, which is the dusky ring itself (ring C). 
The principal division is seen all around ; the division in ring A 
is seen at both ends. The shadow of the ball on the ring is as 
drawn. It is wider and of a different shape on the preceding side, 
as drawn. I did not specially look for (nor see) the shadow of 
the ball on the ring C " [this being a test of good images]. 

Fig. 114 (p. 224) gives a view of an isolated narrow belt, 
stretching right across the ball, seen by Ranyard on Nov. 4. 
1883, and subsequently. 



CHAP. XII.] Saturn. 205 

It was Lassell's opinion that, taking the planet as a whole, it 
may be said that the South pole is generally darker than the 
North pole and more blue in tinge. The dark belts on the planet 
are often thought to exhibit a greenish hue. The planet's or- 
dinary colour is yellowish white, the belts inclining to grayish 
white. Browning finds that large apertures bring out the 
existence of considerable diversities of colour on Saturn. Any 
first-class telescope of 4 inches aperture will exhibit the marked 
distinction between the yellow tint of Saturn's globe and the 
silvery or bluish white hue of ring B. 

The belts of Saturn differ from those of Jupiter in the respect 
that they exhibit at times a sensible curvature, whilst those of 
Jupiter are rectilinear. Hence we draw the conclusion that if 
the belts of Saturn are parallel to the planet's equator (as 
probably is the case), then the plane of this equator must make a 
rather considerable angle with the ecliptic. A quintuple belt 
furnished Sir W. Herschel with the means of determining the 
period of the planet's axial rotation, which he fixed at 
lo h jgm O-44 8 , from observations extending over 100 rotations 
between Dec. 4, 1793 an d Jan. 16, 1794. He is said to have 
subsequently made the period to be io h 29 i6-8 s . Schroter's 
results exceed this, but contradict one another considerably. 
His highest result was as much as 1 2 h . 

Spots on Saturn are very rare. The instances on record 
hardly number a dozen. On Dec. 7, 1876 A. Hall at Washington 
observed a bright spot 2" or 3" in diameter, round, and well de- 
fined, and brilliantly white. It lasted nearly a month, and was 
seen by several observers' 1 . It yielded for the period of Saturn's 
rotation io h 14 23-8". 

Sir W. Herschel considered that he had obtained decided 
indications of the existence of an atmosphere on Saturn: the 
satellites when undergoing occultation never disappeared instan- 
taneously, but seemed to hang on the planet's limb, in one case 
for as long as 2O m . Such a retardation would imply a horizontal 

c Phil. Trans., vol. Ixxxiv. p. 62. 1 794. 

11 Ast. Nach., vol. xc. No. 2146, Aug. 16, 1^77. 



206 The Sun and Planets. [BOOK I. 

refraction of 2", but no confirmation of this has been obtained by 
any subsequent observer. The same observer found other proofs 
of an atmosphere : an examination of the polar regions on various 
occasions shewed that according as they were turned towards or 
from the Sun a difference of hue was perceptible, which might 
reasonably be supposed to be due to snow in those regions 
melting under the Sun's rays, and accumulating in the absence 
of those rays, as has been explained when speaking of Mars. 

When Saturn was first telescopically examined by Galileo, he 
noticed that it presented a very oval outline, which in his 
opinion gave the notion of a large planet having on each side of 
it one smaller one. He added, that with telescopes of superior 
power, the planet did not appear triple, but exhibited an oblong 
form, somewhat like the shape of an olive 6 . 

Continuing his observations, the illustrious astronomer was not 
long in noticing that the two (supposed) bodies gradually de- 
creased in size, though still in the same position as regards their 
primary f , until they finally disappeared altogether 8 . Galileo's 
amazement at this was unbounded, and his third letter to Welser, 
dated Dec. 4, 1613, in which he expresses his feelings on the 
subject, is still extant. He remarks : 

" What is to be said concerning so strange a metamorphosis ? 
Are the two lesser stars consumed after the manner of the solar 
spots ? Have they vanished or suddenly fled ? Has Saturn, per- 
haps, devoured his own children? Or were the appearances 
indeed illusion or fraud, with which the glasses have so long 
deceived me, as well as many others to whom I have shewn 
them ? Now, perhaps, is the time come to revive the well-nigh 
withered hopes of those who, guided by more profound contem- 
plations, have discovered the fallacy of the new observations, 
and demonstrated the utter impossibility of their existence. I 
do not know what to say in a case so surprising, so unlocked for, 
and so novel. The shortness of the time, the unexpected nature 

Of ere di Galileo, vol. ii. p. 41. Padua 1612, when of course Saturn would in 

ed., 1744. such a telescope as Galileo's appear to 

f Ibid. be destitute of all appendages what- 

* A nodal passage took place in Dec. ever. 



CHAP. XII.] Saturn. 207 

of the event, the weakness of my understanding, and the fear of 
being mistaken, have greatly confounded me h ." Galileo was so 
disgusted that he entirely abandoned observations of Saturn. 

The original discovery was announced to Kepler in the 
following logogriph 1 : 

smaismrmilmepoetalevmibvnenvgttaviras ; 

which, being transposed, becomes 

altissimvm planetam tergeminvm observavi ; 
" I have observed the most distant planet to be tri-form.'' 

As time wore on, more correct ideas were obtained of the phe- 
nomenon, which gradually came to be looked upon as due to the 
existence of two ansse, or handles, to the planet, though the cause 
of their disappearance from time to time was yet unexplained. 
Astronomers are indebted to Mr. C. L. Prince for having called 
attention to an important stage in the development of true ideas 
as to the causes of the changes seemingly undergone by Saturn. 
In 1876 he unearthed and had engraved some curious old 
drawings made by Gassendi between 1633 and 1656, and pub- 
lished in Gassendi's Works k . But it was not till after the lapse 
of nearly 50 years from the time of Galileo's discovery that 
the true cause of the appearance seen by him and others became 
known. C. Huygens was the discoverer, and he intimated his 
discovery in the following logogriph 1 :- 

aaaaaaa ccccc d eeeee g h 

iiiiiii 1111 mm nnnnnnnnn 

oooo pp q rr s ttttt uuuuu ; 

which letters, when placed in their proper order, give 

annulo cingitur, tenui, piano, nusquam cohaerente, ad eclipticam inclinato ; 

" The planet is surrounded by a slender flat ring inclined to the ecliptic, but which 
nowhere touches the body of the planet." 

h Opere di Galileo, vol. ii. p. 152. m T.Maurice (Indian Antiquities, vol. 

Padua ed., 1744. vii. p. 605 ; see also vol. ii. p. 302) gives 

' Opere di Galileo, vol. ii. p. 40. Padua an engraving of Sani, the Saturn of the 

ed., 1744. Hindus, from an image in an ancient 

k Vol. iii. Lyons, 1658. See Month. pagoda. A circle is formed around him 

Not. E.A.S., vol. xxxvi. p. 108, Jan. 1876. by the intertwining of two serpents; 

1 De Batumi Luna Olservatio Nova. whence the writer infers that, by some 

Hagse, 1656. Followed in 1659 ' 3V ^e- means or other, the existence of Saturn's 

tailed particulars in the Sy sterna Satur- ring may have been known in remote 

nium. ages. The same thing is observable in 



208 



Tlic Sim and Planets. 



[BOOK I. 



It must not be supposed that this discovery was the result of 
a chance inspiration. On the contrary, Huygens seems to have 
spent several years in scrutinising Saturn before he finally 
decided that the theory of a ring round the planet was the only 
one which would reconcile the various observed facts. 

With the view of commending his hypothesis to the attention 
of astronomers, Huygens ventured to predict that in the month 
of July or August 1671 the planet would again appear round; 
and in this he was nearly correct, for Cassini, watching the 
disappearance of the ring, found the planet presenting this aspect 
in May 1671, or within 2 months of the time foretold by 
Huygens. 



Fig. 100. 



Fig. 102. 




(Ball, 1665.) (HeveUus, 1675.) (Cattini, 1676.) 

THREE I7TH CENTURY SKETCHES OF SATURN AND ITS RING. 

As advances have been made in the manufacture of telescopes, 
so our knowledge of the Saturnian system has been increased. 
In 1675, within a very few years of Huygens's discovery, Cassini 
discovered that what Huygens saw as one ring was in reality 
a combination of two, lying concentrically, one within the other". 
Sir W. Herschel was for a long time very unwilling to allow 
that this division was actually such in fact ; and he did not 
become convinced until he had executed a very protracted series 
of observations extending over several years. He coupled his 
acceptance of the division with a strong assertion that it was the 
only one that existed. 



Assyrian sculptures ; but it must in can- 
dour be added that this ring-surrounded 
Deity possessed a signification (impossible 
to be alluded to here) in the ancient 
Phallic worship. 

n For some particulars of a controversy 
which raged in 1882 respecting the share 
of credit for this discovery supposed to be 



due to others besides Cassini see Observa- 
tory, vol. v., 1882, passim. It arose out of 
misconceptions as to the meaning of a 
passage which appears in Phil. Trans., 
vol. i. p. 152. Cassini's sketch will be 
found in Lowthorp's abridgement of Phil. 
Tran*., vol. i. p. 288. 



Figs. 103-5. 




1853: Nov. 2. (Dawes.) 




1848. (W. C. Bond.-) 




1856: Jan. 8. (Jacob.) 



SATURN. 






CHAP. XII.] Saturn. 211 

But we have now certain knowledge of the existence of more 
than 2 rings, and the system must be described as a multiple one. 

It is stated by Lalande that Short, the celebrated optician, 
perceived several concentric streaks on the outer ring. It is not 
known that Short left any record of his own relating to this. 

Between June 19 and 26, 1780, Sir W. Herschel p perceived a 
slight dark streak close to the interior edge of the western ansa. 
It had disappeared on June 29, and no corresponding appearance 
at all was seen on the other ansa. 

In Dec. 1823 ^' Quetelet, at Paris, with a Cauchoix achro- 
matic of 10 inches aperture, thought he saw a division in the 
exterior ring q . 

On Dec. 17, 1825, Capt. Kater, with a 6-inch Newtonian re- 
flector, perceived in the exterior ring numerous black streaks 
very close to each other 1 . On Jan. 16, 1826, with another 
telescope, the same observer saw similar markings, but as on 
Jan. 22, 1828, none whatever could be perceived, he concluded 
that they had no permanent existence. 

On April 25, 1837, Encke 8 , at Berlin, assured himself of the 
existence of a division in the exterior ring ; on May 28 following 
he was able to procure measurements which shewed that the old 
ring was unequally divided, the wider portion lying outermost. 

On May 29, 1838, Di Vico, at Rome, perceived not only this 
division, but two similar divisions in the interior ring. 

On Sept. 7, 1843, Lassell and Dawes * saw a decided division 
in the exterior ring at both ends, but placed it near the outermost 
edge, thereby failing to agree with Encke's measurements of 1837. 

This subdivision of the exterior ring is now generally ac- 
cepted u , and De La Rue's beautifully executed engraving (Fig. 98, 
Plate XII) conveys a good idea of it. 

Astronomic, vol. iii. Paragraph 3228. * Month. Not., vol. vi. p. 12. 

2nd ed., Paris, 1771. u Jacob on the contrary expressed in 

P Phil. Trans., vol. Ixxxii. p. 8. 1792. unequivocal terms his conviction that the 

1 Mem. R. A. S., vol. iv. p. 388. 1831. black mark or so-called division in the 
r Mem. E. A. S., vol. iv. p. 384. 1831. exterior ring was merely a depression. 
8 Mathematische Abhandlungen der He was confident that it reflected the 

Konigl. Akad. Wissenschaften Berlin, planet's shadow, shewing an apparent 
1838, p. 5. projection, such as every shadow falling 

P 2 



212 The Sun and Planets. [BOOK I. 

The discovery of another curious and interesting feature has 
now to be dealt with. In 1838 Galle, in examining Saturn, 
noticed a gradual shading off of the interior bright ring towards 
the ball. He published a note of this observation, but little or 
no attention seems to have been paid to it x . On Nov. u, 1850, 
G. P. Bond perceived a luminous appearance between the ring 
and the planet : subsequent observations by himself and his 
father shewed that this luminous appearance was neither more 
nor less than another ring. Neither of these observers could 
satisfactorily determine whether this dusky ring (as it soon came 
to be called) was actually in contact with the interior bright 
ring, but they thought it was not y . Before the arrival of the 
American mail conveying intelligence of this new ring, Dawes had 
found it. On Nov. 29 he entered in his Journal the following 
remark : " After a few seconds of uncommonly sharp vision, I 
involuntarily exclaimed, ' Obvious.' There is a shading, like 
twilight, at the inner portions of the inner ring 2 ." This acute 
observer was not long in ascertaining the annular character of 
the " shading," and moreover he found (as did O. Struve also) 
that the dusky ring was occasionally divided into 2 or more con- 
centric rings. This fact is not indicated in De La Rue's en- 
graving, but the transparent nature of the entire ring is well 
shewn. On Dec. 3, Lassell, while on a visit to Dawes, saw " some- 
thing like a crape veil covering a part of the sky within the inner 
ring :" this observation was made in consequence of a hint given 
by Dawes as to what he himself had seen a . 



on a groove has. {Month. Not., vol. xvi. y Mem. Amer. Acad. of Arts and 

p. 126, March 1856; vol. xvii. p. 174, Sciences, vol. v., (N.S.), p. in. 1855. 

April 1857.) Hippisley and Watson dis- z Month. Not., vol. xi. p. 23. Dec. 1830. 

believed in a division, and adhered to the A passage in Phil. Trans., vol. xxxii. 

opinion that the mark is merely a mark, p. 385, I7 2 3> by Hadley, almost leads one 

and that its breadth varies. Month. Not., to infer that he had seen the dusky ring, 

vol. xiv. p. 163, March 1854; vol. xvi. though without being able to make up his 

p. 152, April 1856.) mind as to what it was. Hind, in Month. 

x Math.AbhanrJl.Konigl. Akad. Wis- Not., vol. xv. p. 32, Nov. 1854, expresses 

senschaften Berlin, 1838, p. 7. See also his belief that a record of Picard's will 

Ast. Nach., vol. xxxii. No. 756. May 2, fairly bear the interpretation that on 

1851; and Month. Not., vol. xi. p. 184. June 15, 1673, he saw the dusky ring, with 

June 1851. the like comprehension as Galle. 



Figs. 106-8. 



Plate XIV. 




1 86 1 : April 7. (De La Eue.) 




1 86 1 : Nov. 13. (Jacob.} 







1 86 1 : Dec. 4. (Jacob.) 



SATURN. 



Figs. 109-11. 



Plate XV. 




1 86 1 : November. (Anon.) 




1861: Dec. 26. (Wray.~) 




1862: Jan. 5. (Wray.) 



SATURTST. 



CHAP. XII.] Saturn. 217 

The transparency of the dusky ring was not ascertained till 
1852 ; Jacob, Dawes, and Lassell share this discovery between 
them b . 

Figs. 1 10-1 1 on Plate XV. relate to a very interesting observa- 
tion made by Wray on Dec. 26, 1861. He saw "A prolongation 
of very faint light stretched on either side from the dark shade on 
the ball, overlapping the fine line of light formed by the edge of 
the ring, to the extent of about one-third its length, and so as to 
give the impression that it was the dusky ring, very much thicker than 
the bright rings, and seen edgewise projected on the sky c ." 

It has been thought that the dusky ring is wider and less faint 
than formerly. On March 26, 1863, Carpenter found it to be 
" nearly as bright as the illuminated ring," so much so that it 
" might easily have been mistaken for a part of it d ." 

On Oct. 29, 1883, Davidson with a 6*4 inch refractor found an 
undoubted difference in the brightness of the dusky ring at the 
2 ansse, the preceding ansa being decidedly brighter than the 
following one ; different eye-pieces yielded the same result, and 
another observer concurred in the opinion e . 

Having said this much on the history of these discoveries, some 
facts connected with the rings must now be set out. Their true 
form is no doubt circular, or nearly so ; but as we always see 
them foreshortened, they appear more or less oval when the 
Earth is above or below the plane of the rings, but when we are 
nearly in the plane they appear as a single straight line, or 
something like it. When we are exactly in the plane they dis- 
appear altogether, except in very large telescopes. Figs. 112 and 
113 will make this sufficiently clear. In the true position 
of the rings during Saturn's revolution round the Sun there is 
no change : they remain continually parallel to each other. 



b Perhaps this sentence requires to be planet of deeper shade than usual, 

qualified, for Galle, in his drawing, re- c Month. Not., vol. xxiii. p. 86. Jan. 

presents the planet seen through the 1863. 

ring; but it must be remarked that he d Month. Not., vol. xxiii. p. 195. April 

did not know he was looking at a ring, 1863. 

and only intended to draw what was (and e Observatory, vol. vii. p. 85. March 

readily might be) taken for a belt on the 1 884. 



218 



The Sun and Planets. 



[BOOK I. 



The plane of the rings is inclined 28 10' to the ecliptic, and 
intersected it in 1860 in longitude 167 43' 10" and 347 43' 10" 
(17! of Virgo and Pisces) ; the former point being the place of 
the ascending node, and the latter that of the descending node. 
According to Bessel the longitude of the node of the ring 
referred to the ecliptic increases at the rate of 46-46 z" per annum, 

Fig. 112. 




GENERAL VIEW OF THE PHASES OF SATUBN S KINGS. 

Whether viewed from the Earth or from the Sun, the pheno- 
mena seen in connexion with the rings of Saturn are much the 
same, but the motion of the Earth in its orbit (the inclination of 
which differs somewhat from that of Saturn) gives rise to certain 
phases in the rings which would not be witnessed by an observer 
placed on the Sun. " Thus it usually happens that there are 2, 
if not 3 disappearances f , about the time of the planet's arrival at 
the nodes. The plane of the ring may not pass through the 
Earth and Sun at the same time, but the ring may be invisible 

' There can really never be more than two disappearances. ^Procter, Saturn. 
p. 90.) 



CHAP. XII.] 



Saturn. 



219 



under both conditions, because its edge only will be directed 
towards us. It is also invisible when the Earth and Sun are on 
opposite sides of its plane a state of things that may continue 
a few weeks : in this case we have the dark surface turned 
towards our globe. In very powerful telescopes it has been found 
that the disappearance of the ring is complete under the latter 



Fig. 113- 



1877 



1885 




1808 



1891 

PHASES OF SATUBN'S RINGS AT THE DATES SPECIFIED. 



condition ; it has, however, been perceived as a faint broken line of 
a dusky colour, not only when the Sun is in its plane, but like- 
wise when its edge is directed to the Earth. Our remarks must 
be considered as applying to observations with telescopes in 
common use." The foregoing quotation is from Hind * ; a fuller 
account is given by Sir John Herschel h . 

Saturn's period being 29-45 8 y , the half of this, or 14-729^ will 
be the average time elapsing between 2 nodal passages. Such a 



Introd. to Ast., p. 107. 



Outlines of Ast., p. 343 et seq. 



220 The Sun and Planets. [BOOK I. 

passage took place in 1877. The Northern surface of the ring 
had then been visible for J4'7 y . 

In June 1885 the planet was in 77.5 of longitude, one of the 
two places at which the greatest opening of the rings occurs. 
The breadth will diminish till 1891, when the motion of the 
planet and of the Earth will again bring the ring edgewise to the 
Earth and cause it to disappear, the Sun being South of the plane, 
and the Earth crossing to the North. 

In 1893 the Sun, passing through the plane of the ring, will 
begin to illuminate its Northern surface, and the Earth being 
also on that side, the ring will reappear. After a few months 
the Earth will go to the South, and the Sun remaining on the 
North, a second disappearance will take place. The ring will 
remain invisible, in consequence of presenting its unilluminated 
side to us, till the Earth once more passing through the plane of 
the ring to the North, will bring the Northern side into view 
a state of things which will last till 1907. 

It will be seen from De La Rue's drawing of 1856, and 
from others taken at the epoch of maximum breadth, that 
the ball is at such times entirely encompassed by the ring, and 
that thus the outline of the whole system is a perfect ellipse : 
this state of things always lasts for several months. The ring of 
Saturn is most open when the planet is in either Gemini or 
Sagittarius. 

By a careful examination of the ring Sir W. Herschel ascer- 
tained that it revolves round the ball in io h 32 15" a period 
not greatly in excess of that of the planet's own axial rotation : 
the direction is the same in both cases. There are, however, great 
difficulties in the way of admitting this rotation '. 

In 1 854-5-6, Secchi executed numerous measures of the rings, 
but they exhibited considerable discordances. He afterwards 
found that whilst those of 2 consecutive days did not harmonise, 
those of 3 and 9 days did ; and the idea then occurred to him 
that the results might be explained by supposing the ring to 

! It is noteworthy that previously to in the text, Laplace calculated that the 
Sir W. Herschel finding the result given rings ought to rotate in io h 33 36". 



CHAP. XII:] Saturn. 

be elliptical, presenting sometimes its longer, sometimes its shorter 
diameter. He failed to reconcile Herschel's period of rotation 
with his own observations, but found that a period which corre- 
sponds with that which a satellite placed on the margin of the ring 
would have (namely, I4 h 23 i8 8 ) would satisfy them k . 

O. Struve introduced a system for conveniently distinguishing 
the rings from each other, in writing and speaking, which is now 
generally adopted. He called the exterior bright ring A, the 
interior bright ring B, and the dusky one C. When reference 
is made to the system as a whole it is very usual to say ' ring,' 
in the singular number, no one ring in particular being thereby 
meant. 

The ring is not concentric with the ball. Gallet of Avignon 
announced this in ] 664, placing the ball nearer to the East ansa. 

In 1827, Schwabe expressed his belief that the ring was 
eccentric, but in the opposite direction to that assigned by Gallet. 
Harding confirming Schwabe's opinion, W. Struve took the 
matter in hand micrometrically, and found that at the mean 
distance of Saturn from the Earth, whilst the diameter of the 
Eastern vacuity was irzSS", that of the Western was only 
u'073", shewing a difference of 0-215" in favour of the former. 
This peculiarity has been shewn to be essential to the stability 
of the system of the rings : without this feature and without 
rotation they would fall upon the planet. 

The following angular measurements, reduced to the mean 
distance of the planet (and calculated on the solar parallax of 
8-80"), are by the same observer : 

English 
// Miles. 

Outer diameter of exterior ring ... ... ... 40-095 = 172,240 

Inner diameter ,, ... ... ... 35-289 = 151,590 

Breadth ... ... ... 2-403 = 10,320 

Outer diameter of interior ring ... ... ... 34-475 = 148,100 

Inner diameter ,, ... ... ... 26-668 = 114,560 

Breadth 3'93 = ^,765 

Interval between the two ... ... ... 0-408 = 1,750 

Distance of ring from ball ... 4-339 = 18,640 

Equatorial diameter of ball ... ... ... 17-60 = 75,600 

k Month. Not., vol. xvi. p. 52. Jan. 1856. 



222 



The Sun and Planets. 



[BOOK I. 



The measures of De La Rue 1 , Main, and Jacob 11 are appended 
for comparison : 





De La Rue. 


Main. 


Jacob. 






n 


n 


Outer diameter of exterior ring 


39-83 


30.75 


sn-qq 


Inner diameter , , 


35-33 




35-82 


Breadth ,, ... 


2-2? 




2-o8 


Outer diameter of interior (middle) ring 


33-45 




34-85 


Inner diameter ,, ,, 


26-91 


27.65 


26-27 


Breadth ,, ,, 


3-27 




4-29 


Interval between the two 


0-94 




0-48 


Distance of ring from ball 


4-62 


5-07 


4-16 


Equatorial diameter of planet 


17-66 


I7-RO 


17-04 











There are some particulars relating to the rings which cannot 
well be classified. Sir J. Herschel estimated their thickness at not 
more than 250 miles ; G. P. Bond cut this down to 40 miles. 
Peirce p thought that there were good grounds for supposing them 
to be fluid rather than solid ; but the opinion which meets with 
most favour now is that they are a dense aggregation of small 
satellites, densest where brightest, widest apart where most faint. 
In fact it may be shewn that if a system of rings of such propor- 
tion was constructed of iron it must become semi-fluid under the 
forces it would experience. Considered as a system, the rings 
are sensibly more luminous than the planet (a fact which Hooke 
pointed out as long ago as 1666), and B is brighter than A. 
B itself is perceptibly less bright at its inner edge than elsewhere. 
At the epoch of the Saturnian equinoxes the ansse do not both 
disappear and reappear at the same time, and at these periods 
they are sometimes of unequal magnitude. 

On Oct. 9, 1714, 6 days before the actual passage of the Earth 
through the plane of the ring, and whilst the ansse were de- 
creasing, Maraldi noticed that the Eastern one appeared a little 



1 Month. Not., vol. xvi. p. 43. Dec. 1855. 

m Hid., p. 30. 

" Ibid., p. 124 (March 1856). 

An important series by Bessel will 



be found in Ast. Nach., vol. xii. Nos. 
274-5. Feb. 18, and March 7, 1835. 

P Gould's Astronomical Journal, vol. 
ii. p. 17. June 16, 1851. 




DURING THE WINTER OF 1883-4. (Ranyard.) 




Feb.-March, 1884. (Henry.) 




Feb. 1887. (Terby.) 



SATURTST. 



CHAP. XII.] Saturn. 225 

broader than the other for 3 or 4 nights, and yet it vanished 
first q . He was induced to suspect that the anste had changed 
places by rotation, and that at any rate the surface of the rings 
was very irregular, the 2 rings lying moreover in different 
planes. 

Heinsius, Varela, Messier, and many others have noticed the 
ansse to be of different lengths, and that one is frequently visible 
without the other. When only one is visible, it is most fre- 
quently that on the Western side a fact for which it is difficult 
to account. 

Fig. 116 represents Saturn as drawn by Terby of Louvain with 
an 8-inch Grubb Refractor. He remarks that the drawing 
brings out especially the following features : Encke's division ; 
Henry's bright streak in ring A opposite Cassini's division ; 
Struve's division between rings B and C especially on the East ; 
the black patches in the dusky ring especially on its West side ; 
the indentation of the shadow of the ball on the Cassini division 
on the West ; the shadow cast by the ball on the dusky ring ; 
and lastly the transparency of the dusky ring which permits the 
ball to be seen through it r . 

When at its nodes the ring frequently appears broken, shewing 
merely luminous elongated beads seemingly detached from one 
another. For a long time astronomers were in doubt as to the 
cause of these appearances, and it was not till so recently as 
1848 that the question was cleared up. In that year the ob- 
servers at Harvard College, U. S., instituted a careful inquiry, 
and their micrometrical observations shewed that these " beads " 
were due to the concurrent effect of light reflected by the edges, 
external and internal, of the rings. The Figures [117-18] are 
copied from Bond's memoir, but ring C is omitted that matters 
may be simplified. What follows I cite from Webb, who 
devoted much time to the elucidation of Saturnian facts. " It 
must be borne in mind that this design is an intentional exaggera- 
tion for clearness' sake, representing the dark surface much 

i Mem. Acad. des Sciences, 1715, p. 12. 
r Observatory, vol. x. p. 163, April 1887. 

Q 



226 



The Sun and Planets. 



[BOOK I. 



more expanded than it ever really is, and the thickness of the 
rings many (they say perhaps 10) times too great. To this 
they add the qualification that the edges should be rounded ; 

Fig. 117. 




DIAGRAM ILLUSTRATING THE PHENOMENON OF SATURN'S RING " BEADED." 

and I should be inclined to suggest another, that A may probably 
be much thinner than B, so that its inner edge would add 

Fig. 1 1 8. 




DIAGRAM ILLUSTRATING THE PHENOMENON OF SATURN'S RING "BEADED." 

little to the effect. Comparing, then, Fig. 117 with Fig. 118. 
we should have, i. A narrow dark band upon the planet. 



CHAP. XII.] Saturn. 227 

slightly curving upwards, and consisting of both the dark side, 
of the ring and its shadow (the latter not inserted in Fig. 114). 
2. The outer edge of A visible throughout, but with extreme diffi- 
culty when alone, as between I and c, and f and ff, and towards 
a and h. 3. Two brighter portions from c to d, and from e to /, 
where the light of A is reinforced by that reflected by the inner 
edge of B. 4. Two bright knots where the same light, strength- 
ened by the concurrent reflection from the inner edge of A and 
the outer of B (the latter, it may be presumed, many times out- 
weighing the former), reaches us through the opening of [Cassini's] 
division. This the Americans considered fully satisfactory, the 
curvature of the black stripe having been noticed, and estimated 
at O'25"; the extremities of the line, and the beads, falling be- 
neath its direction, as from the diagram they ought to do, and 
the accordance of measures fully bearing out the impression 
of Nov. 3, that the ' interruptions in the light of the ring are so 
plainly seen, that no one could for a moment hesitate as to their 
explanation.' " 

O. Stru ve many years ago propounded a theory B that the rings 
were expanding inwards (so that ultimately they would come in 
contact with the ball) ; and also that between the time of J. D. 
Cassini and Sir W. Herschel the breadth of the inner ring had 
increased in a more rapid ratio than that of the outer ring, while 
the exterior diameter of A was unchanged. Struve drew this 
conclusion from the early observations of Huygens and others : 
but it is doubtful if these are to be relied upon ; and Main 
considered that micrometric measures obtained by himself showed 
the theory to be untenable. Kaiser also considered it to be 
destitute of foundation 1 . On the other hand, both Hind u and 
Secchi x favour the idea of change. 

The rings cast a shadow ; and from observing this shadow 

8 Mem. de VAcad. des Sciences de St. Jan. 1856, for an abstract of Kaiser's 

Pttersbourg, 6th ser., Math, et Phys., memoir. 

vol. v. 1852. An abstract of it appears n Month. Not., vol. xv. p. 31. Nov. 

in Month. Not., vol. xiii. p. 22. Nov. 1854. 

1852. * Month. Not., vol. xvi. p. 50. Jan. 

' See Month. Not., vol. xvi. p. 66, 1856. 

Q 2 



228 The Sun and Planets. [BOOK I. 

some persons have been led to think that the surfaces of the rings 
are con vex y , and that they do not lie in precisely the same plane. 
Sir J. Herschel doubted the former being a legitimate conclusion 
from observation, but admitted its theoretical probability 2 . Lassell 
considered that C often changes colour, each end being alter- 
nately bluish-gray and brownish a . This may indicate rotation. 
Hippisley thought that there was evidence that the ring A lies 
in a different plane from the others, and that B is thicker in the 
middle than at either of the edges b . Sir W. Herschel surmised 
that the ring is not flat, but that the inner edge was hemi- 
spherical or hyperbolical . The outer edge of B is commonly 
the brightest portion of the system, but Schwabe and Webb 
believed it to be variable. The inner edge of the same ring 
is usually much the dullest, but occasionally it brightens up. 
G. P. Bond in 1 856 regarded the dark shading visible at the inner 
edge of B as a sharply-defined dark area, elliptical in form and 
concentric with the rings, but of greater eccentricity. Prince 
" is convinced " that C is becoming more and more illuminated d . 
Lassell and De La Rue have suspected the existence of mountains 
on the rings, in consequence of elevations appearing in the shadow 
projected on the ball 6 . [Fig. 106, PL XIV.] Jacob saw the effect, 
but doubted the assigned cause, preferring to think that it is an 
illusion arising from inequalities in the depth or tone of the 
shadow f . In 1848, when the unilluminated side was turned to- 
wards us, Dawes saw traces of the shadow, of a coppery hue, and 
he regarded this as an effect due to a rather dense atmosphere g : 
but more than this, the atmosphere causing a refraction of the 
solar light on each side of the ring would reduce the shadow of 
the ring to a penumbra, and thus account for it being impercep- 
tible when the Sun was in the plane of the ring. Sir W. Herschel 

y De La Rue's drawing forcibly con- d Month. Not., vol. xx. p. 212. March 

veys the impression of this as regards B. 1860. 

* Outlines of Ast., p. 343. * Ibid., vol. xxi. pp. 177 and 236. 

n Month. Not., vol. xiii. p. 147. March April and June 1861. 

1853. ' Ibid., vol. xxi. p. 237. June 1861. 

b Month. Not., vol. xiv. p. 163. March & Month. Not., vol. x. p. 46, De- 

1854. cember 1849, and vol. xxii. p. 298, June 
Phil. Trans., vol. xcvi. p. 463. 1806. 1862. 



CHAP. XIL] Saturn. 229 

had previously believed that an atmosphere surrounding the ring 
alone would explain a distortion which he noticed in 1807, at 
the South pole, in optical proximity to the ring ; the other pole 
being at the same time clear of the ring and free from distortion 11 . 
Between 1872 and 1876, using the 26-inch refractor of the 
Washington observatory and 2 smaller instruments, Trouvelot 
spent much time in carefulty studying the planet Saturn. His 
observations were numerous, and the conclusions he drew, 
important. The following are some of them in a condensed 
form 1 : The inner margin of A 2 , limiting the outer border of 
the principal division, shewed on the ansse some singular dark 
angular forms attributable to an irregular and jagged conform- 
ation of the inner border of A 2 , either permanent or temporary; 
the surface of A 15 A 2 and B frequently exhibited a mottled 
or cloudy appearance on the ansse ; the thickness of the system 
of rings increases from the inner margin of C to the outer margin 
of B, a fact which is shown by the form of the planet's shadow 
thrown upon the rings ; the cloud -forms seen near the outer 
edge of B attain different heights and change their relative 
position either by the rotation of the rings on an axis, or by 
some local cause a fact indicated by the rapid changes in the 
indentation of the shadow of the planet ; the inner portion of C 
disappears in the light of the planet at that part which is pro- 
jected upon its disc ; contrary to the observations hitherto made, 
C is not transparent throughout ; C grows more dense as it 
recedes from the planet, so that at about the middle of its width 
the limb of the planet entirely ceases to be visible through it; 
the matter composing C is agglomerated here and there into 
small masses which almost wholly prevent the light of the planet 
from reaching the observer. 

h Phil. Trans, vol. xcviii. p. 162. of the outer king, and calls C the ring 

1808. which all other astronomers, following 

' American Journal of Science and O. Struve, always indicate by the letter 

Arts, 3rd Ser., vol. xii. p. 447. June B. I have altered Trouvelot's letters to 

1876. Trouvelot has adopted a special accord with the recognised nomenclature, 

nomenclatureofhisownwhichiscalculated indicating the sub-divisions of A by 

to cause great confusion. He designates calling them A, and A 2 respectively, 
by A and B the outer and inner portions 



230 The Sun and Planets. [BOOK I. 

In February and March 18 84 the brothers Henry, using at the 
Paris Observatory a refractor of 15 inches aperture, armed with 
a power of 1000, remarked around the inner edge of A a narrow 
bright ring bounded by a black line. This new ring, not (it 
would seem) previously noted, was about 1*5" wide; in other 
words, was about as wide as Cassini's well-known division. 
But the fact which especially struck these observers was the 
non-visibility of Encke's great division in A. That division, 
so familiar to all who have observed and drawn Saturn during 
the last 50 years, had in the judgment of MM. Henry completely 
disappeared. They stated that nothwithstanding very favourable 
atmospheric conditions it was impossible to detect on A any 
markings but the narrow bright circle mentioned above k . The 
disappearance of Encke's division seems to have been lately 
remarked in America. 

The following observations of Saturn by Keeler with the 
great 36-inch telescope of the Lick observatory present some 
very interesting and novel points : 

*' The object of greatest interest to me was the outer ring. It is usually drawn 
with a division at about one-third of its width from the outer edge, sometimes fine 
and sharp and sometimes broad and indefinite. Many drawings which I have 
examined place this line or shade near the centre of the ring. In a series of drawings 
which I made with the 1 2-inch equatorial of this observatory, from a careful study of 
Saturn during the finest nights of the past summer [1887], the outer ring is repre- 
sented with a faint broad shading in the centre, diminishing gradually toward the 
edges, which are therefore relatively bright. 

" The 36-inch equatorial shewed, at a little less than one-fifth of the width of the 
ring from its outer edge, a fine but distinct dark line, a mere spider's thread, which 
could be traced along the ring nearly to a point opposite the limb of the planet. 
This line marked the beginning of a dark shade which extended inwards, diminishing 
in intensity nearly to the great black division. At its inner edge the ring was of 
nearly the same brightness as outside the fine division. No other markings were visible. 

"It is easy to see how, with insufficient optical power, this system of shading 
could present the appearance of an indistinct line at about one-third the width of the 
ring from its outer edge. The broad band alone would make it appear near the 
centre of the ring, and the effect of the line, itself invisible, would be to displace the 
greatest apparent depth of shade in the direction of the outer edge. Two nights 
after the observations just described I re-examined Saturn very carefully with the 
12-inch equatorial, but could not perceive the narrow line, although I was then aware 
of its existence, and the definition was excellent '." 

k IS Astronomic, vol. iii. p. 230. June 1884. 
1 Sid. Mess., vol. iii. p. 80. Feb. 1888. 



CHAP. XII.] Saturn. 231 

In general the brightness of the ball and of the rings is toler- 
ably uniform, but there are exceptions to this rale. In April 
1862 Lassell noted the rings to be very dull compared with the 
ball, but this might have been due to the small elevation of the 
Sun above the plane of the ring. Probably any peculiarities of 
this nature which may be noticed from time to time are optical 
effects, and do not depend on actual change. Trouvelot however 
found the ball less luminous at its circumference than at its 
centre, a fact which seems indicative of the existence of an 
atmosphere. 

Bessel entered upon some investigations to determine the mass 
of the rings, by ascertaining their perturbing effect on the orbit 
of the 6 th satellite, Titan. He estimated it at y^ of the mass 
of the planet. The thickness of the rings being too minute 
for measurement, no precise determination of their density is 
attainable ; if, however, we assume it as approximately equal to 
that of the planet, as is probably the case, it will follow that the 
thickness is about 138 miles a quantity which is very nearly 
the mean of the 2 estimations of Sir J. Herschel and Bond. 
Supposing this to be correct, at the mean distance of the planet 
the rings would only subtend an angle of about 0-03"; it may 
therefore be readily inferred that the ring will at stated times 
become wholly invisible even in the most powerful telescopes. 

Saturn is attended by 8 satellites, 7 of which move in orbits 
whose planes coincide nearly with that of the planet's equator, 
and therefore with the plane of the rings also : the orbit of the 
remaining and most distant satellite is inclined about 12 14' 
(Lalande) to the aforesaid plane. One consequence of this coin- 
cidence in the planes of the orbits of the first 7 satellites is that 
they are always visible to the inhabitants of both hemispheres 
when not under eclipse in their primary's shadow. 

In dealing with the satellites of Saturn, I continue to follow 
my usual plan of tabulating as much information as possible, 
but when we have proceeded beyond Jupiter, data concerning 

m Conn, des Temps, 1838, p. 29. 



232 The Sun and Planets. [BOOK I. 

satellites become both scarce and contradictory, and it is fre- 
quently necessary to give alternative statements. 

The figures in the column of "Diameter" are, with the ex- 
ception of Titan's, extremely doubtful, and this impairs the value 
of Proctor's calculations given at the foot of the Table opposite. 

Mimas. Beer and Madler's reduction of Sir W. Herschel's ob- 
servations in 1 789 gives for the epoch of Sept. J4 d i3 h 26 m Slough 
M.T., the Saturnicentric A. at 264 16' $6", the longitude of the 
peri-saturnium at 104-42, and the eccentricity at 0-068. 

Fig. 119. 




GENERAL VIEW OF SATURN AND ITS SATELLITES. 

Encetadits. Beer and Madler, also from Sir W. Herschel's ob- 
servations, gave for the epoch of 1789, Sept. I4 d n h 53, the A. 
at 67 56' 26" : they considered the orbit to be circular in the plane 
of the ring. Hind says that Enceladus was seen by Sir W. 
Herschel on Aug. 19, 1787. 

Tetkys. Lament, from his own observations in 1 836, found for 
the epoch of April 23* 8 h 27 Greenwich M.T., the A to be 158 31', 
the longitude of the peri-saturniuin 357 37', the & 184 36', 
the eccentricity 0-0051, and the inclination of the orbit to the 



CHAP. XII.] 



Saturn. 



233 



O 

02 
H 

HH 
H-3 



EH 
<^ 



MH 

H 



- 


1* 


r- 





CO 


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I > l>l 


<j 3 


3 S 




















I " 7 *s 


^8 


23 


O CO 
CO 


o 


S 


VO 

VO 





2, 


t 


oo 


d 

o 


ti?i 

^ fl ^, ^ 


1 


00 


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o 

ITS 


8 


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re 
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~ 


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oo 


1 

o 


o * W J 3 

00 "3 >! "^ 


Q 


M 

II 


6 




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6 


o" 
6 


6 


6 




VO 

6 


_c 

bp 


h^ O * 

1-6 -si 


H 


a 


f 

6 


Uncertain. 


LO 

8 

6 




o 
6 




6 




6 


6 


o 
6 


servers be 


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fl * ^3 3 


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Q te" T3 O 
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cc 
t>. 


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SirW.Herschel. 


s 


!s 

ri 

o 

ft 


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w 

d 


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1 

ft 


kt Dawes claimed 1 
|8. 


best estimates of 
) the space covere 
s that of our Moo 
Moon, they could 
the full Moon." ( 


1 


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- 


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= 8 3 ^ 



234 The Sun and Planets. [BOOK I. 

plane of the ring 1 33'. Sir John Herschel, about the same time, 
found the A to be 313 43', the longitude of the peri-saturnium to 
be 53 40', the eccentricity 0-04217, and the orbit to be precisely 
in the plane of the ring. The serious differences in these two 
results are to be ascribed to errors in the observations arising 
from the difficulty attending them, but such differences naturally 
make us distrust the entire batch of figures. 

Dione. Sir John Herschel in 1836 found the A to be 327 40', 
the longitude of the peri-saturnium 42 30', the eccentricity 0*0206, 
and the orbit to be precisely in the plane of the ring. 

Rhea. Sir John Herschel in 1835-7 found the A to be 353 44', 
the longitude of the peri-saturnium 95, and the eccentricity 
0-02269. The inclination is very small. 

Ti(an n , as the satellite most easily seen, has naturally received 
most attention. Bessel's determination of its orbit is reputed to 
be the most complete. For the epoch of 1 830-0 he gave the A at 
137 21', the longitude of the peri-saturnium at 256 38", and the 
eccentricity 0-029314. The line of apsides has a direct motion on 
the ecliptic of 30' 28" annually, completing a revolution in 7 1 8 
years, the nodes completing a revolution in 3600 years. 

Hyperion has been so recently discovered that its orbit has not 
been very fully investigated. From Washington observations 
made in 1875 Hall found the A to be 120 12', the longitude of the 
peri-saturnium 173, the eccentricity 0-118, and the inclination 
of the orbit 6 12'. Lassell's observations made at Malta in 1852 
and 1853 agree with these conclusions in part, but Hall remarks 
that neither Lassell's observations nor those at Washington " fix 
the position of the satellite in its orbit with much certainty, since 

n When Huygens discovered this sa- says : " 'Tis highly probable that there 

tellite in 1655, he was imprudent enough may be more than 5 moons revolving 

to predict that there were no others, round this remote planet [the number of 

because Titan being the 6th secondary satellites which Saturn was then known 

planet, and there being only 6 primary to possess] ; but their distance is so great 

planets known, Nature's (supposed) laws as that they have hitherto escaped our 

of symmetry were satisfied. The danger eyes, and perhaps may continue to do so 

of prediction in matters of this kind is for ever ; for I do not think that our 

well illustrated in the case of Mr. John telescopes will be much further im- 

Harris, F.R.S. That learned gentleman proved !! " 
published a book in 1729, in which he 






Fig 1 20. 



Plate XVII. 




CHAP. XII.] 



Saturn. 



237 



they were made when the plane of the orbit was nearly edgewise 
to the observer." He adds : " If we examine the elements we 
shall see that Hyperion moves nearly in the plane of the orbit of 
Titan, and considering the values of the eccentricities it will be 
seen that these satellites can approach very near each other ." 
Hyperion was seen by Bond on Sept. 16, 1847, and by Lassell on 
Sept. 1 8, but it was not till the date given in the table that its 
character was determined. 

lapetus. Lalande for the epoch of 1790 gave the A. at 269 37', 
and the S3 at 150 27', reckoned on the orbit. 

The following elements are by Captain Jacob p : 





1857- 
Jan. 0. 
X 


jr 


Q 


t 

To Eclip. 





Semi-axis 
maj. 
a 


Daily 

Sat' centric 
Mot. 
/* 


Mimas 


o / 
2IO + 


O 1 

1 


o / 
1 


1 

2 


2 


// 

2 


o 
^Sl-04.7 


Enceladus 


301 55 


? 


! 


? 


? 


2 


262.732 


Tethys 


281 42 


109 7 


167 37 


28 10 


0.01086 


42-60 


190-697 


Dione 


"5 3 


M5 4 


167 37 


28 10 


0.003IO 


54-85 


I31-534 


Rhea 


288 43 


185 o 


167 19 


28 8 


O-OOO8O 


76-I3 


79-690 


Titan 


299 42 


257 6 


167 58 


27 36 


0-027937 


I 76-90 


22-577 


Hyperion 


? 


? 


? 


2 


2 


2 


? 


lapetus . . . 


78 9 


349 20 


143 i 


18 37 


0.028443 


514-96 


4-538 



A. Hall considers that the orbits of the 5 inner satellites are 
sensibly circular and that they move in the plane of the ring or 
nearly so, but it will be readily understood that the small 
apparent size of most of these satellites, and the consequently 
limited number of telescopes and observers which can be brought 
to bear on them, materially retards the attainment of any more 
perfect acquaintance with their motions, though it is reasonable 
to hope that the multiplication of large instruments and experi- 
enced observers now taking place will ere long lead to a develop- 
ment of our knowledge of the orbits of these satellites. 



Ast. NacTi., vol. xcv. No. 2263. June 
17, 1879. 



p Month. Not., vol. xviii. p. i. 

1857- 



Nov. 



238 The Sun and Planets. [BOOK I. 

Sir J. Herschel pointed out the curious circumstance that the 
period of Mimas is \ that of Tethys, and the period of Enceladus 
\ that of Dione q . Monck puts these facts in the shape that the 
ratio of these 4 periods are 2, 3, 4 and 6, adding that the period 
of lapetus is very nearly 5 times that of Titan. D' Arrest further 
called attention to the commensurability within yV 1 , or 2f h , of 
274 revolutions of Mimas, 170 of Enceladus, and 85 of Dione*. 

Kirkwood has discovered a still more complicated relationship, 
which may be thus enunciated : To 5 times the daily angular 
motion in its orbit of Mimas add the daily motion of Tethys, 
and 4 times the daily motion of Dione, and the sum total will be 
equal to 10 times the daily motion of Enceladus. 

The disappearance of the ring, in 1862, was taken advantage 
of by various observers for watching the rare phenomenon of a 
transit of the shadow of Titan across the planet. The satellite 
itself was not seen on any occasion, but Dawes and others ob- 
tained several good views of the shadow 8 . Again in 1877 the 
shadow of Titan was seen by Common, and others. The only 
observation of this kind prior to 1862 appears to have been 
made by Sir W. Herschel on Nov. 2, 1789. Dawes on May 25, 
1 862, saw an eclipse of this satellite in the shadow of Saturn 
the only instance on record. 

It must not be supposed that Titan is the only satellite of 
which an eclipse, transit, or occultation is possible, for all the 
satellites are occasionally subject to these effects. This is 
especially true of the two innermost ones, but the small apparent 
size of all except Titan hinders observation of them. 

Celestial phenomena on Saturn must possess extreme grandeur 
and magnificence, the rings forming a remarkable series of arches 
stretched across the Saturnian heavens. The nearest satellite, 
Mimas, traverses its orbit at the rate of 16' of arc in a minute of 
time, so that, as viewed from Saturn, it moves in 2 minutes over 
a space equal to the apparent diameter of the Moon. Considering 

"> Month. Not., vol. vii.p. 24. Dec.i845- Month. Not., vol. xxii. pp. 264, 297, 

r At. Nach.,l\i\. No. 1364. June 14. &c. May and June 1862. 
1862. 



CHAP. XII.] Saturn. 239 

the remoteness of Saturn from the Sun its satellites play a 
somewhat important part in the Saturnian sky as reflectors of 
sun-light. Nevertheless the space occupied by all of them, taken 
together, is (as stated on a previous page) only about 6 times 
that covered by the Moon. 

Lockyer thus summarises the phases of Saturn's ring as seen by 
an observer placed on the planet itself*: "As the plane of the 
ring lies in the plane of the planet's equator, an observer at the 
equator will only see its thickness, and the ring therefore will put 
on the appearance of a band of light passing through the East and 
West points and the zenith. As the observer, however, increases 
his latitude either North or South, the surface of the ring-system 
will begin to be seen, and it will gradually widen, as in fact the 
observer will be able to look down upon it ; but as it increases 
in width it will also increase its distance from the zenith, until 
in lat. 63 it is lost below the horizon, and between this latitude 
and the poles it is altogether invisible. Now the plane of the 
rings always remains parallel to itself, and twice in Saturn's 
year that is, in two opposite points of the planet's orbit it 
passes through the Sun. It follows, therefore, that during one- 
half of the revolution of the planet one surface of the rings is lit 
up, and during the remaining period the other surface. At night, 
therefore, in one case, the ring-system will be seen as an illumin- 
ated arch, with the shadow of the planet passing over it, like the 
hour-hand over a dial ; and in the other, if it be not lit up by 
the light reflected from the planet, its position will only be 
indicated by the entire absence of stars. 

" But if the rings eclipse the stars at night, they can also eclipse 
the Sun by day. In latitude 40 we have morning and evening 
eclipses for more than a year, gradually extending until the Sun 
is eclipsed during the whole day that is, when its apparent 
path lies entirely in the region covered by the ring ; and these 
total eclipses continue for nearly 7 years: eclipses of one 
kind or another taking place for 8 years 292 days. This will 
give us an idea how largely the apparent phenomena of the 
4 Elementary Lessons in Astronomy, p. 117. 



240 



The Sun and Planets. 



[BOOK I. 



heavens, and the actual conditions as to climates and seasons, 
are influenced by the presence of the ring." 

The only physical fact which has been discovered in relation 
to the satellites of Saturn concerns lapetus. Cassini lost that 
satellite soon after its discovery, but a larger telescope enabled 
him to find it again, and moreover to ascertain that it was 
subject to considerable variations of brilliancy. Sir W. Herschel, 
with a view of establishing this fact beyond doubt, paid much 

Fig. 121. 




THE APPARENT OBBITS OF THE SEVEN INNER SATELLITES OF SATUBN TO FACILITATE 
THEIR IDENTIFICATION (l888). 

** The date of Titan '* Eastern Elongation being known (= o), it will on subsequent days be 
found in the positions corresponding to the daily intervals marked on the diagram. 

attention to lapetus. He was able to confirm Cassini's opinion, 
and decided that it actually did experience a considerable loss of 
light when traversing the Eastern half of its orbit. He found 
that 7 past Opposition was the place of minimum light. The 
conclusions deducible from this are (as Cassini himself pointed 
out), that the satellite rotates once on its axis in the same time 
that it performs one revolution round its primary ; and that there 
are portions of its surface which are almost entirely incapable of 
reflecting the rays of the Sun. 

The mass of Saturn has been given at 7 ^V T by Newton ; at 
*sW by Laplace ; at ^^ r? by Bouvard ; and at -5-5^-5 by Bessel. 
Jacob thought from his own observations that the mass of the 
whole Saturnian system did not differ much from ^ T Vs- The 
most recent value is A. Hall's, 



CHAP. XII.] Saturn. 241 

" The most ancient observation of Saturn which has descended 
to us was made by the Chaldaeans, probably at Babylon, in the 
year 519 of Nabonassar's period, on the i4th of the month Tybi, 
in the evening ; when the planet was observed to be 2 digits 
below the star in the Southern wing of Virgo, known to us as 
y Virginis. The date given by Ptolemy, who reports this observa- 
tion in his Almagest [Kb. xi.], answers to B.C. 228, March i u ." 

An occultation of this planet by the Moon is recorded to have 
been observed by one Thius, at Athens, on Feb. 2 1 , 503 A.D. 

Cassini observed in 1692 the occultation of a star by Saturn's 
satellite Titan. No other instance of this kind is on record. 

From Saturn the Sun appears only about 3' in diameter, and 
the greatest elongations of the planets are : Mercury, 2 19'; 
Venus, 4 21'; Earth, 6 i' ; Mars, 9 11'; Jupiter, 33 3' so 
that a Saturnian, assuming his visual powers to resemble ours, 
can only see Jupiter, Uranus, and Neptune with the naked eye, 
and Mars perhaps with some optical aid. Saturn, on account of 
its slow dreary pace, was chosen by the alchemists as the symbol 
for lead. 

In computing the places of Saturn, the Tables of A. Bouvard, 
published in 1821, were long used, but new Tables by Le Verrier 
have superseded them. Tables of the satellites have still to be 
formed, and are a great desideratum. 

u Hind, Sol. Syst., p. 117. 



242 The Sun and Planets. [BOOK I. 



CHAPTEK XIII. 

URANUS. $ 

Circumstances connected with its discovery by Sir W. Herschel. Names proposed 
for it. Early observations. Period, &c. Physical appearance. Belts visible 
in large telescopes. Position of its axis. Attended by 4 Satellites. Table of 
them. Miscellaneous information concerning them. Mass of Uranus. Tables 
of Uranus. 

ON March 13, 1781, whilst engaged in examining some small 
stars in the vicinity of H Geminorum, Sir W. Herschel 
noticed one which specially attracted his attention : and desirous 
of knowing more about it, he applied to his telescope higher 
magnifying powers, which (in contrast to their effect on fixed 
stars) he found increased the apparent diameter of the object 
under view considerably; this circumstance clearly proving its 
non-stellar character. Careful observations of position shewing 
it to be in motion at the rate of ?.\" per hour, Herschel con- 
jectured it to be a comet, and made an announcement to that 
effect to the Royal Society on April 26*. Four days after its 
first discovery it was observed by Maskelyne, then Astronomer 
Royal, who seems to have suspected at the time its planetary 
character, and in the course of the following 2 or 3 months it 
received the attention of all the leading observers of Europe. So 
soon as sufficient observations were accumulated, attempts were 
made by various calculators to assign parabolic elements for the 
orbit of the new body ; though but little success attended their 
efforts. It was found that although a parabola might be obtained 
which would represent with tolerable accuracy a limited number 

Phil. Trans., vol. Ixxi. p. 492. 1781. 



CHAP. XIII.] Uranus. 243 

of observations, yet a larger range always revealed discrepancies 
which defied all endeavours to reconcile them with positions 
assigned on any parabolic hypothesis. The final determination 
was only arrived at step by step, and to Lexell must be ascribed 
the credit of first announcing, with any amount of authority, 
that the stranger revolved round the Sun in a nearly circular 
orbit, and that it was a planet and not a comet ; though priority 
for this honour has been contested on behalf of Laplace. 

The question of a name for the new planet was the next 
subject of debate. Herschel himself, in compliment to his 
sovereign and patron King George III, proposed that it should 
be called the Georgium Siclus ; Lalande or, as some say, Laplace 
suggested the personal name of Herschel ; but neither of these 
gave satisfaction to the Continental astronomers, who all declared 
for a mythological name of some kind. Prosperin considered 
Neptune appropriate, on the ground that Saturn would then be 
found between his two sons Jupiter and Neptune. Lichtenberg 
advanced the claims of Astraa, the goddess of justice, who fled to 
the confines of the system. Poinsinet thought that as Saturn 
and Jupiter, the fathers of the gods, were commemorated astro- 
nomically, it would be unpolite longer to exclude the mother, 
Cylele. Ultimately, however, Bode's Uranus prevailed over all 
others. A symbol was manufactured out of the initial of Her- 
schel's surname, though in Germany, at the instigation of Kb'hler, 
one not differing much from that of Mars was adopted. 

It soon became a matter of inquiry whether the new planet 
had ever been seen before, and here may be brought in a note 
of Arago's : " If Herschel had directed his telescope to the con- 
stellation Gemini 1 1 days earlier (that is, on March 2 instead of 
March 13), the proper motion of Uranus would have escaped his 
observation, for on the 2nd the planet was in one of its stationary 
points. It will be seen by this remark on what may depend the 
greatest discoveries in astronomy b ." A careful inspection of the 

b On this remark of Arago's Holden motion. Does any one suppose that ' a 

says: "This is an entire misconception, new and singular star' like this would 

since the new planet was detected by its have been once viewed and then for- 

physical appearance and not by its gotten?" (Life of W. fferschel, p. 49.) 

R 2 



244 The Sun and Planets. [BOOK I. 

labours of former astronomers shewed that Uranus had been ob- 
served and recorded as a fixed star on 20 previous occasions : 
namely, by Flamsteed c in 1690, on Dec. 13; in 1712, on March 
22; in 1715, on Feb. 21, 22, 27, and April 18 (all o.s.) ; by 
Bradley in 1748, on Oct. 21 ; in 1750, on Sept. 13, and in 1753, 
on Dec. 3 ; by Mayer in 1756, on Sept. 25 ; and by Le Monnier 
no less than 12 times in 1750, on Oct. 14 and Dec. 3 ; in 1764, 
on Jan. 15; in 1768, on Dec. 27 and 30 ; in 1769, on Jan. 15, 16, 
20, 21, 22 and 23; and in 1771, on Dec. 18. Had Le Monnier 
been a man of order and method it can scarcely be doubted that 
he would have anticipated Sir W. Herschel. Arago recollected 
to have been shewn by Bouvard one of Le Monnier's observations 
of the planet written on a paper bag, which originally contained 
hair-powder purchased at a perfumer's ! 

It will readily be understood that these early observations 
have been of great service to computers, inasmuch as they have 
been enabled to determine the elements of the planet's orbit with 
greater accuracy than they could otherwise have done simply by 
the aid of modern observations. 

Uranus revolves round the Sun in 30,6867 days, or rather 
more than 84 of our years, at a mean distance of 1,781,944,000 
miles. The eccentricity of its orbit, which amounts to 0-04667 
(rather less than that of Jupiter), may cause this to extend to 
1,865,107,000 miles, or to fall to 1,698,781,000 miles. The 
apparent diameter of Uranus varies but slightly, as seen from the 
Earth ; and its mean value is about 3'4". ( Seeliger, 3^82" : 
Millosevich, 3'96.") The real diameter is about 31,000 miles. 
Sir W. Herschel saw the planet's outline strongly elliptical 
in 1792 and 1794, after having noted it to be round in 1782. 
Madler at-Dorpat in 1842 and 1843 measured the ellipticity to be 
TO- or iV Arago however pointed out that a polar compression 
may exist but not always be visible, because a spheroid, when 
viewed in the direction of its axis, will necessarily present a truly 

c Le Verrier, in his investigation of adopted another dated April 18, 
the theory of Uranus, rejected Flam- (_Grant, Hit>t. Phys. Ant., p. 165.) 
steed's observation of Feb. 22, 1715, and 



CHAP. XIII.] Uranus. 245 

circular outline, and this seems both the proper and a sufficient 
way of reconciling discordances on the subject which have been 
noted. Buff ham on Jan. 25, 1870, thought that the ellipticity 
was " obvious d ." Safarik after many observations between 1877 
and 1883 considered the ellipticity to be "striking" and there- 
fore in fact "considerable 6 ." Prof. C. A. Young in 1883 
measured the planet on several occasions and obtained an ellip- 
ticity of T V- He considers that there can be no " reasonable doubt 
that the planet's disc is considerably flattened, its equator lying 
sensibly in the same plane with the satellite-orbits f ." Schia- 
parelli too in 1884 obtained as he thought clear proofs of an 
ellipticity of T V But the measures of Seeliger at Munich and 
Millosevich at Rome in 1883 negative the idea. 

It has been calculated that the amount of light received by 
Uranus from the Sun is equal to about the quantity which would 
be afforded by 300 Full Moons. The inhabitants of Uranus can 
see Saturn, and perhaps Jupiter, but none of the planets included 
within the orbit of the latter. 

The physical appearance of Uranus may be disposed of in a few 
words. Its disc is commonly considered to be uniformly bright, 
bluish in tinge and without spots or belts. Yet both Lassell and 
Buffham have fancied they have seen traces of an equatorial belt 
and of inequalities of brilliancy on the planet's surface. Writing 
in 1883 Prof. C. A. Young says: " Whenever the seeing was 
good 2 belts were always faintly but unmistakeably visible on 
each side of the equator much like the belts of Saturn. On one 
or two occasions other belts were suspected near the poles g ." 
Schiaparelli too with an 8-inch refractor has detected faint spots 
and differences of colour on the disc of Uranus. The period of 
axial rotation is unknown, but analogy h leads us to suppose that 
it does not differ materially from that of Jupiter or Saturn. 
Buffham has ventured on a conjecture that some indications of 

d Month. Not., vol. xxxiii. p. 164. f Observatory, vol. vi. p. 331. Nov. 

Jan. 1872. 1883. 

8 Ast. Nach., vol. cv. No. 2505. Ap. E Observatory, vol. vi. p. 331. Nov. 

14, 1883. Observatory, vol. vi. p. 183. 1883. 
June, 1883. h See p. 68, ante. 



246 The Sun and Planets. [BOOK I. 

spots seen by him imply a Kotation-period of I2 h . Sir W. 
Herschel once fancied he had seen traces of a ring or rings, but 
the observation was not confirmed by himself, nor has it been by 
others since. Uranus is just within the reach of the naked eye 
when in Opposition, and may be found without a telescope if the 
observer knows its precise place 1 . 

The direction of the axis of Uranus was supposed by Sir W. 
Herschel to be such that if prolonged it would at each end meet 
the planet's orbit. In consequence of this " the Sun turns in a 
spiral form round the whole planet, so that even the two poles 
sometimes have that luminary in their zenith k ." Buff ham very 
roughly makes the inclination of the axis 10. 

Fig. 122. 




URANUS, 1884. (Henry .) 

MM. Henry of Paris, in giving the accompanying sketch of 
Uranus as seen during 1884, say that they were able to detect 
constantly the existence of 2 belts, straight and parallel to one 
another, placed almost symmetrically on each side of the centre 

1 It is a somewhat singular fact that discover the Georgium Sidus, and strip 

the Burmese mention eight planets : the the illustrious Herschel of his recent 

Sun, Moon, Mercury, Venus, Mars, Ju- honours." 

piter, Saturn, and Rahii, which latter is k Sir W. Herschel, quoted in Smyth's 

invisible. "An admirer of Oriental lite- Cycle, vol. i. p. 205. 
rature," says Buchanan, " would here 



CHAP. XIII.] 



Uranus. 



247 



o 

CO 
W 
H 



W 

H 



a c 
3g 


* 2 H? ^ 






VO 





10 Th 06 


H? 


1 

DH 

1 

E 








-00 1^. rj 
B p PI irj 


VO 


03 


PI CO VO 

M M 


M 




O -<f OO 


CO 


8 


o o 


^-* O^ co 


8 


1 


PI J>- OO 


M 

oo 

CO 


.2 

p 






i : 
s 


00 t^ 00 
O P> * 


. 


r 


00 H. OO 

M I-H 


M- 




HI I- 1 
00 H< 






111 


" 


I* 




^" ^i- OO 
00 00 ^ 

HI HI M 


^ 









s 


o 
so 


, 




1 

a S * 
^ -S fe 






g 02 ^ 
e8 . .Jj 

ni o cc 




Si 

og 


CO "*< M P4 




^ 1 1 

5 S 1 


I 




- 



248 The Sun and Planets. [BOOK I. 

of k the planet. Between these 2 belts there was discernible a 
fairly bright zone, which seemingly corresponded to the equatorial 
region of the planet. The 2 poles were darkish ; however, the 
upper pole in the engraving always appeared brighter than the 
lower. They also found as the result of a great number of 
measures that the direction of the belts did not coincide with 
the major axis of the apparent orbit of the satellites, but formed 
with it an angle of 40, so that the position- angles observed 
were 56 for the belts and 16 for the major axis at the same 
epoch. MM. Henry suggest that in supposing, as it seems 
reasonable to do, that the belts of Uranus are parallel to its 
equator, and remembering that the latitude of the Earth above 
the plane of the orbit of the satellites when the observations 
were made was about 9, there follows the result that the angle 
between the plane of the equator of Uranus and the plane of the 
orbits of the satellites is about 41. 

Uranus is attended by at least 4 satellites, 2 of which were 
discovered by Sir W. Herschel, and 2 by recent observers } . 
Such is their extreme minuteness that only the very largest 
telescopes will shew them, and for this reason our knowledge of 
them is very limited. Their chief peculiarity is the inclination of 
their orbits, which for direct motion amounts to + 98 ; in other 
words, their Urani-centric motion is retrograde, the planes of the 
orbits lying nearly perpendicular (180 98 = 82) to the 
planet's ecliptic. The satellites, as Sir W. Herschel remarked, 
describe the Northern halves of their orbits, included between 
the ascending and descending nodes, in virtue of movements 
directed from E. to W. 

Sir J. Herschel pointed out a test by which astronomers can 
ascertain whether their instruments are sufficiently powerful and 
their sight sufficiently delicate to undertake with any reason- 
able hope of success a search for these satellites. Between the 
stars /8 1 and ft 2 Capricorni, about the middle of the interval in 

1 Sir W. Herschel thought that he had that Herschel's conclusions must have 

discovered 6 satellites, which with the 2 been based on some misapprehension : 

discovered by Lassell and Struve would that is to say, that he mistook small stars 

make a total of 8 ; but it is now accepted for satellites. 



CHAP. XIII.] Uranus. 249 

R. A. and slightly to the N., there is a double star whose com- 
ponents are of mags. 16 and 17 [=13 and 13*6 of Argelander's 
magnitudes], and 3" apart. No instrument incapable of shewing 
these two stars is suitable for observing the satellites of Uranus. 
In fact Sir John remarked that in comparison with the Uranian 
satellites these two stars are "splendid objects." 

Under these circumstances I shall be pardoned if I omit the 
details of the observations made by Sir William Herschel", his 
son , Lamont p , O. Struve, and Lassell q , more especially as the 
substance of them has been reproduced by Hind r and Arago 8 . 
Suffice it then to remark that, according to Lassell, Ariel and 
Umbriel are of nearly equal brightness, whilst Titania and 
Oberon are both much brighter than the 2 innermost satellites. 

Under date of Jan. n, 1853, Lassell said lie was fully persuaded 
that either Uranus has no other satellites than these 4, or if it has, 
they remain yet to be discovered but the 

assumption of 8 satellites was accepted 
by Arago and other influential astron- 
omers. Lassell, writing in 1864 from 
Malta, on the occasion of his second 
visit, reiterated his former statement. 

It was found by Sir W. Herschel 
that the satellites disappeared when 
within a short distance (j' or there- 
abouts) of the planet. This occurred 

PLAN OF THE URANIAN SYSTEM. 

whichever was the side of the planet 

on which the satellites happened to be, thus negativing the 
possibility of the phenomenon being due to an atmosphere on 
Uranus ; and Sir William was led to assume that it was merely 
an effect of contrast the comparatively great lustre of the planet 
overpowering the feeble glimmer of the satellites. 

m Cited by Arago in Pop. Ast., vol. ii. P Mem. R.A S., vol. xi. p. 51. 1840. 

p. 628, Eng. ed., and by Smyth, Celest. 1 Month. Not., vol. viii. p. 44, Jan. 

Cycle, vol. ii. p. 475. 1848; vol. xii. p. 152, March 1852; vol. 

a Pkil.T>-ans., vol. Ixxvii. p. 125, 1787; xiii. p. 148, March 1853. Mem. R.A.S., 

vol. Ixxviii. p. 364, 1788 ; vol. Ixxxviii. vol. xxxvi. p. 34, 1867. 

p. 47, 1798; vol. cv. p. 293, 1815. r Sol. Syst., p. 121. 

Mem. R.A.S., vol. viii. p. i. 1835. s Pop. Ast., vol. ii. p. 623. 




250 



TJie Sun and Planets. 



[BOOK I. 



Hind, from Lassell's observations at Malta in 1852, has deduced 
the following elements : 

III. TlTANIA. 

Radius of orbit at the mean distance of Ijl ... 33-88" = 288,000 miles. 

Longitude of ascending node ... ... ... 165 25' 

Inclination of orbit ... ... ... ... 100 34' 

IV. OBERON. 

Radius of orbit at the mean distance of 1$ ... 45-20" = 384,000 miles. 
Longitude of ascending node ... ... ... 165 28' 

Inclination of orbit ... ... ... ... 100 34' 

From the distance of Titania the same computer obtained ^-5^-5 
as the mass of Uranus, Oberon indicating ^r^ ; results fairly 




THE APPARENT OBBITS OF THE SATELLITES OF URANUS. 

** The small circle represents the planet : the arrows, the direction in which the tatellites move : each 
black dot, a day's interval reckoned from O, the epoch of the preceding Northern elongation. 

in accord with those of other observers, when the difficulties in 
obtaining data are considered. Encke's value was -ST^V g-> Lament's 
irrtfT> Madler's ^Tlr^ ^- Hall's s-svsr> Adams's v^w Littrow's 
Ttfinr> an d Bouvard's T7 | T ^. Bouvard's value is now very 
generally rejected as excessive. 

In computing the places of Uranus the Tables of A. Bouvard, 
published in 1821, were used up to quite a recent date. From 
what appears in the following chapter it will be evident that they 



CHAP. XIII.] Uranus. 251 

were susceptible of material improvement, and they have now 
given place to those completed in 1872 by an American astro- 
nomer, Professor S. Newcomb, as to which it may be observed 
that they do not countenance the idea that there exists a trans - 
Neptunian planet. Newcomb has also framed Tables of the 
Satellites of Uranus *. 

i Washington Obs., 1873, Appendix I. 



252 The Sun and Planets. [Boos. I. 



CHAPTER XIV. 



NEPTUNE a . T 

Circumstances which led to its discovery. Summary of the investigations of Adams 
and Le Verrier. Telescopic labours of Challis and Galle, The perturbations 
of Uranus by Neptune. Statement of these perturbations by Adams. Period, 
&c. Attended by I Satellite. Elements of its orbit. Mass of Neptune. 
Observations by Lalande in 1 795. 

MORE than half a century ago an able French astronomer, 
M. Alexis Bouvard, applied himself to the task of making 
a refined investigation of the motion of Uranus, in order to 
prepare Tables of the planet. He had at his disposal the various 
observations by Flamsteed and others, made prior to the direct 
optical discovery of Uranus, and those made by various astro- 
nomers subsequent to that event in 1781. In working these up 
he found himself able to assign an ellipse harmonising with the 
first series, and also one harmonising with the second ; but by no 
possibility could he obtain an orbit reconcileable with both. As 
the less objectionable alternative, Bouvard decided to reject all the 
early observations and to confine his attention solely to those 
more recent b . In this way he produced, in 1821, Tables of the 
planet, fairly representing its motion in the heavens. This 
agreement, however, was not of long duration, and a few years 

a Many French writers deal with the tended to rob a deserving Frenchman of 

discovery of Neptune in a way that is not his share in the honours. Science ought 

fair. Nothing is more common than to to be international, and to rise above 

meet with a narrative of the incident such petty insinuations, 
either without any mention, direct or in- b A memorable illustration of the folly 

direct, of Mr. J. C. Adams, or with some and impolicy of rejecting any observation, 

casual remark more or less implying that merely because it opposes or seems to 

the English version is a trumped-up story oppose a pre-conceived theory, 
due to national jealousy, and only in- 



CHAP. XIV.J Neptune, 253 

only elapsed before discordances appeared of too marked a 
character to be possibly due to any legitimate error in the Tables : 
constructed in the form in which they existed it was evident 
that they were defective in principle. Eouvard himself, who 
died in 1 840, seems to have fancied that an exterior planet was 
alone the cause of the irregularities existing in the motion of 
Uranus, and the Rev. T. Hussey was led to assert this in decided 
terms in a letter to Airy in 1834. This conviction soon forced 
itself on astronomers , and amongst others on Valz, Madler, and 
Bessel. Bessel, it would seem, entertained the intention of mathe- 
matically inquiring into the matter, but was prevented by an 
illness, which eventually proved fatal. 

Mr. J. C. Adams, whilst a student at St. John's College, Cam- 
bridge, resolved to attack the question, and, as he found sub- 
sequently, entered a memorandum to this effect in his diary under 
the date of July 3, 1841, but it was not till January 1843 that he 
found himself with sufficient leisure to commence. He worked 
in retirement at the hypothesis of an exterior planet for i f years, 
and in Oct. 1845 forwarded to Airy some provisional elements 
for one revolving round the Sun at such a distance and of such 
a mass as he thought would account for the observed pertur- 
bations of Uranus. This was virtually the solution of the 
problem in a theoretical point of view, and it is much to bo 
regretted that neither the result nor any of the circumstance.s 
attending it were made public at the time. 

In the summer of J 845, Le Verrier, of Paris, turned his atten- 
tion to the anomalous movements of Uranus, and in the November 
of that year published his first memoir to prove that they did 
not depend solely on Jupiter and Saturn. In June 1846 the 
French astronomer published his second memoir to prove that 
an exterior planet was the cause of the residual disturbance. He 

c As far back as October 25, 1800, La- This statement is reputed to depend on a 

lande and Burckhardt came to the con- note to this effect found amongst Lalande's 

elusion that there existed an unseen papers presented to the Academy of 

planet beyond Uranus, and they occupied Sciences in 1852, but I am not acquainted 

themselves in trying to discover its posi- with any other authority for it. 
tion. (Year Book of Facts, 1852, p. 282.) 



254 The Sun and Planets. [BOOK I. 

assigned elements for it, as Adams had done 8 months previously. 
A copy of the memoir reached Airy on June 23, and finding 
how closely in accord Le Vender's hypothetical elements were 
with those of Adams, which were still in his possession, he was 
so impressed with the value of both, that on July 9 he wrote to 
Professor Challis of Cambridge to suggest the immediate employ- 
ment of the large "Northumberland" telescope in a search for 
the planet. The proposal was agreed to, and on July u a 
systematic search was commenced. Challis, not being in posses- 
sion of the Berlin Star Map of the particular locality in which it 
was supposed that the looked-for planet would be found, was 
forced to make observations for the formation of a map for 
himself ; this was done, but much valuable time was occupied. 
When matters had reached this stage Sir J. Herschel seized an 
opportunity which happened to present itself, and thus addressed 
the British Association at Southampton on Sept. 10, 1846: 
The past year has given us the new planet Astrsea " it has done 
more it has. given us the probable prospect of the discovery of 
another. We see it as Columbus saw America from the shores 
of Spain. Its movements have been felt, trembling along the 
far-reaching line of our analysis, with a certainty hardly inferior 
to that of ocular demonstration d ." The Map was eventually got 
ready, but it was not till Sept. 29 that Professor Challis found an 
object whose appearance attracted his attention, and which was 
subsequently proved to be the new planet so anxiously sought. 
It was likewise ascertained afterwards that the planet had been 
observed for a star on Aug. 4 and 12, and that the supposed star 
of Aug. 12 was wanting in the zone of July 30. The non- 
discovery of its planetary nature on Aug. 1 2 was due to the fact 
of the comparisons not having been carried out quite soon 
enough; a pardonable though regrettable circumstance. It 
should be added that it was not until Oct. i that Challis heard 
of Galle's success on Sept. 23. (See post.) 

In August Le Yerrier published a third memoir, containing re- 
vised elements, in which particular attention was paid to the 

d Athenteum, Oct. 3, 1846, p. 1019. 



CHAP. XIV.] Neptune. 255 

probable position of the planet in the heavens. On Sept. 23 a 
letter from him, containing a summary of the principal points of 
this memoir, was received by Encke of Berlin, whose co-operation 
in searching telescopically for the planet was requested. The 
Berlin observers had the good fortune to have just become pos- 
sessed of Bremiker's Berlin Star Map for Hour XXI. of R.A., 
which embraces that part of the heavens in which both Adams 
and Le Verrier expected that the new planet would be found, 
and resort to this Map was suggested by D'Arrest, then a young 
student at the Berlin Observatory. On turning the telescope 
towards the assumed place, Galle, Encke's assistant, called out 
the visible stars one by one, and D'Arrest checked them by the 
Map. After a while Galle saw what seemed to be a star of the 
8 th magnitude, which was not laid down on the Map. Further 
observations on Sept. 24 placed it beyond a doubt that this 
8 th magnitude star was in reality the trans-Uranian planet ; a 
discovery, the announcement of which, as may be well imagined, 
created the liveliest sensation. The French astronomers, with 
Arago at their head, disputed with unseemly violence the equal 
claims of Adams to participate with Le Verrier in the honours ; but 
Airy, then Astronomer Royal, laid before the Royal Astronomical 
Society, on Nov. 13, such an overwhelming chain of evidence 
in favour of our distinguished countryman's exertions as seems 
to all impartial minds to have finally settled the question 6 . 

The intellectual grandeur of this discovery will be best ap- 
preciated, so far as a non-mathematical reader is concerned, by 
placing in juxtaposition the observed longitude of the new planet 
when telescopically discovered, and the computed longitudes of 
Adams and Le Verrier. 

e The foregoing is a very bare outline case will be found stated in Arago's Pop. 
of the case, which is a most interesting Ast., vol. ii. p. 632 ; the English trans- 
one. Grant (Hist. Phys. Ast., p. 165 et lator's notes to the passage are very 
seq.) gives full particulars ; and reference appropriate. A very full statement of 
may also be made to Month. Not., vol. the facts of the case from a quite recent 
vii. p. 121, Nov. 1846; Mem. R.A.S., stand-point will be found in an obituary 
vol. xvi. p. 385, 1847; Athen&um, Oct. notice of Prof. Challis, in Month. Not., 
3, 1846; Adm. Smyth's Speculum Hart- vol. xliii. p. 160. Feb. 1883. D' Arrest's 
wellianum, p. 405 ; and Sir J. Herschel's share in the work will be found explained 
Outlines of Ast., p. 533. The French in Copernicus, vol. ii. p. 63, 1882. 



256 



The Sun and Planets. 



[BOOK I. 



HELIOCENTRIC POSITIONS. 

Observed by Galle 326 52' 

Computed by Adams ... 3 2 9 T 9' 

Computed by Le Verrier 3^6 o'; 

Adams A C O = + 2 27' 

Le Verrier A C - O - o 52'. 

From this it will be seen that Le Verrier' s computation proved 
to be slightly the more accurate of the two, a fact which in no 
respect militates against the equality of the merits of the two 
great mathematicians. 

After considerable discussion Neptune was the name agreed 

upon for the new planet; 
Galle's suggestion of Janus 
being rejected as too signi- 
ficant. 

" Such," in the words of 
Hind, "is a brief history 
of this most brilliant dis- 
covery, the grandest of 
which astronomy can 
boast, and one that is des- 
tined to a perpetual record 
in the annals of science 
an astonishing proof of the 
power of the human intel- 
lect." 

The accompanying dia- 
gram shews the paths of Uranus and Neptune from 1781 to 1840, 
and will help to illustrate the direction of the perturbing action 
of the latter planet on the former. 

From 1781 to 1822 it will be evident, from the direction of the 
arrows, that Neptune tended to draw Uranus in advance of its 
place as computed independently of exterior perturbation. 

In 1822 the two planets were in heliocentric conjunction, and 
the only effect of Neptune's influence was to draw Uranus 
farther from the Sun, without altering its longitude. 




ILLUSTRATION OF THE PERTURBATION OP 
URANUS BY NEPTUNE. 



CHAP. XIV.] Neptune. 257 

From 1822 to 1830 the effect of Neptune was to destroy the 
excess of longitude accumulated from 1781, and after 1830 the 
error in longitude changed its sign, and for some years subse- 
quently Uranus was retarded by Neptune ; having by 1 846 
fallen 128" behind its place as predicted from Bouvard's tables. 

Prof. Adams has kindly furnished me f with the following ex- 
planatory comment on the above diagram (Fig. 1 25) : 

" The arrows rightly represent the direction of the force with which Neptune acts 
on Uranus taken singly, but the diagram does not represent the direction of the disturb- 
ing force which Neptune exerts on Uranus relatively to the Sun, and this latter force 
is what we must take into account in computing the planetary perturbations. To find 
this disturbing force, we must take the force of Neptune on the Sun, reverse its 
direction, and then compound this with the direct force of Neptune on Uranus. 

"Thus if S denote the Sun, U Uranus, and N Neptune, the force of Neptune on 

Uranus will be in the direction UN and will be proportional to ? and the force 

of Neptune on the Sun will be in the direction SN and will be proportional to 



i 



Hence if we produce NS, if Fig. 1 26. 




necessary, to V and take NV= ^ , the 

reversed force of Neptune on the Sun will 

be represented by NV, provided the direct 

force of Neptune on Uranus be represented 

by UN. Hence the disturbing force of 

Neptune on Uranus relatively to the Sun 

will be represented on the same scale in THE PERTUBBATTO N OF URANUS BY 

magnitude and direction by UV, the direc- NEPTUNE. 

tion being indicated by the arrow in the ,, v 

figure, and the magnitude of the disturbing force being proportional to _, 

" It is not possible to state the effect of Neptune's action on the motion of Uranus 
in such simple terms as you have attempted to do, since it is necessary to take into 
account the action of Neptune in order to find the correct elements of the orbit of 
Uranus, and consequently the corrections of the assumed elements must be taken as 
additional unknown quantities which must be determined simultaneously with the 
perturbations depending on Neptune." 

Neptune revolves round the Sun in 60,126 days, or 164-6 years, 
at a mean distance of 2,791,750,000 miles, which an eccentricity 
of 0-0087 will increase to 2,816,094,000 miles, or diminish to 
2,767,406,000 miles. The apparent diameter of Neptune only 
varies between 2-6" and 2-8". Its true diameter is about 37,200 

f Private letter dated Cambridge, May 8, 1884. 
S 



258 



The Sun and Planets. 



[BOOK I. 



miles a diameter somewhat greater than that of Uranus. No 
polar compression is perceptible. 

Neptune is destitute of visible spots and belts, and at present 
the period of its axial rotation is unknown. But it deserves to 
be stated that on 14 nights in November and December 1883 
Maxwell Hall in Jamaica observed periodical variations in the 
light of Neptune which he thought might have been due to an 
axial rotation occupying 7 h 55 m 1 2 s . He arrived at this result 
after finding that the planet's light seemed to change from a 
maximum star mag. of 1\ to a minimum of 8^ in a period of 
something under 4 hours g . Lassell, Challis, and Bond at various 
times suspected the existence of a ring but nothing certain is 
known on the subject. It would be very desirable to have a large 
reflector like Lord Rosse's, or a large refractor like those at the 
Lick and Vienna Observatories, devoted to a series of observations 
of this planet and Uranus, for it is nearly certain that no other 
existing instruments will add much to our present extremely 
limited knowledge of the physical appearance of these planets. 

Neptune is known to be attended by only one satellite, dis- 
covered by Lassell in 1 846, but both that observer and the late 
W. C. Bond subsequently imagined that they had obtained traces 
of the existence of a second. 

The following table furnishes all the information we at present 
possess about Lassell's confirmed satellite : 

THE SATELLITE OF NEPTUNE. 







Mean Distance. 










Discoverer. 




Sidereal Period. 


Apparent 
Star mag- 
nitude. 


Max. 
Elong. 


Radii of 








T- 1 - 


















<l. b. in. 


d. 




H 


I 


Lassell. 1846, Oct. 10 


I2-OO 


223,OOO 


5 21 3 


5-88 


H 


18 



Changes appear to be in progress in the plane of the orbit of 
this satellite the precise nature of which await further observa- 
tion and explanation b . 

* Month. Not., vol. xliv. p. 257. March 1884. 
h Observatory, vol. xi. p. 446. Dec. 1888. 



CHAP. XIV.] 



Neptune. 



259 



Hind gives the following elements 1 : 

Epoch 1852, Nov. o-o G. M. T. 

o / 

Mean anomaly .................. 243 32 

Peri-neptunium .................. 177 30 

8 ... 175 40 

........................... 151 o 

Eccentricity .................. 6 5=0-1059748 

Period ..................... 



The elements are calculated for direct motion ; accordingly it 
will be noticed that the actual Neptunicentric motion of the 
satellite is retrograde a circumstance which, except in the case of 
the Uranian satellites, is without parallel in the solar system as 
regards either planets or satellites; though there are many 
retrograde comets. 

Fig. 127. 





PLAN OP THE ORBIT OF THE 
SATELLITE OF NEPTUNE. 



ORBIT OF THE SATELLITE OF 
NEPTUNE. 



The mass of Neptune has been variously estimated at a 
by Safford ; at ^\^ by Bond ; at ^^ by A. Hall ; at 
by Littrow; at r g^g^ by Peirce ; at TS^T^ by Holden ; at 1^7 
Hind, from a combination of early measures ; at rrHir by Lassell 
and Marth ; at TTTS^ by Hind from Lassell's Malta measures ; 
a ^ TTT7T by O. Struve ; and at TT l T -g- by Miidler. 

The only known observations of Neptune made previously to 
its discovery in 1846 are two by Lalande, dated May 8 and 10, 
1795, and one by Lament of Oct. 25, 1845. Two by the same 



1 Month. Not., vol. xv. p. 47. Dec. 
1854. For some of Lassell's observations 



see vol. xii. p. 155, March 1852, and vol. 
xiii. p. 37, Dec. 1852. 



S 2 



260 Th,e Sun and Planets. 

astronomer on Sept. 7 and n, 1846, were probably due to Le 
Verrier's announcement made just before, and therefore are not 
entitled to be regarded as casual ones. 

Owing to its immense distance from the Sun, only Saturn and 
Uranus can be seen from Neptune. Though deprived of a view 
of the principal members of the solar system, the Neptunian 
astronomers, if there be any, are well circumstanced for making 
observations on stellar parallax ; seeing that they are in pos- 
session of a base-line of 5,584,000,000 miles, or one more than 
30 times the length of that to which we are restricted. 

Our present knowledge of the movements of Neptune is de- 
rived from the investigations of the late S. C. Walker, of Phila- 
delphia, U.S., and from the Tables of M. Kowalski and Professor 
S. Newcomb. Newcomb has also framed Tables of the satellite 
of Neptune k . 

The question of a possible planet beyond Neptune has received 
some attention, but whether such a planet exists, and whether 
we are ever likely to see it, are problems towards the solution of 
which very little progress has yet been made *. 

The question of the existence of a Trans-Neptunian planet has 
been discussed from a novel stand-point by Flammarion. He 
bases his conclusions that such a planet does exist on considera- 
tions connected with the grouping of the comets whose periodicity 
is open to no doubt. He seeks to show that all the 4 major planets 
beyond Mars have seemingly a group of comets associated with 
them in some way ; and that beyond Neptune there is a group of 
comets to influence which no planet is yet known to exist. Hence 
his conclusion that such a planet does exist but that we have not 
yet seen it. This in brief is Flammarion 's argument, which is 
worked out with considerable ingenuity and care, but with 
materials borrowed, without acknowledgement, from others" 1 . 

k Washington Obs., 1873, Appendix I. 8vo., Washington, U.S., 1880; Ast. Nach., 

1 See Prof. G. Forbes's Comets and vol. cxiii. No. 2698, Dec. 21, 1885. 

Ultra-Neptunian Planets ; also a paper Todd's search extended over 4 months 

by D. P. Todd of the American Nautical during the winter of 1877-8. 

Almanac Office entitled "Preliminary m IS Astronomic, vol. iii. p. 81. March 

account of a speculative and practical 1884. Forbes seems to have been the 

search for a Trans-Neptunian planet," originator of this theory. 



BOOK II 

ECLIPSES* 



CHAPTER I. 

GENERAL OUTLINES. 

Definitions. Position of the Moons orbit in relation to the Earth's orbit. Con- 
sequences resulting from their being inclined to each other. Retrograde motion 
of the nodes of the Moon's orbit. Coincidence of 223 synodical periods with 
19 synodical revolutions of the node. Known as the " Saros." Statement of 
Diogenes Laertius. Illustration of the use of the Saros. Number of Eclipses 
which can occur. Solar Eclipses more frequent than Lunar ones. Duration of 
Annular and Total Eclipses of the Sun. 

THE phenomena which are about to be described are those 
which result from the interposition of some one celestial body 
between a other bodies, the Earth in any case being one of the 3. 
We know well that inasmuch as most of the heavenly bodies are con- 
stantly in motion, the direction of lines drawn from one to another 
must vary from time to time ; and it must occasionally happen that 
3 will come into a right line. " When one of the extremes of 
the series of 3 bodies which thus assume a common direction is 
the Sun, the intermediate body deprives the other extreme body, 

a The portions of this Book which re- edition by my friend Mr. A. C. Kanyard, 
late to Eclipses of the Sun have been facile princeps in this department of 
revised and much extended for this Astronomy. 



262 



Eclipses and Associated Phenomena. [BOOK II. 



either wholly or partially, of the illumination which it habitually 
receives. When one of the extremes is the Earth, the inter- 
mediate body intercepts, wholly or partially, the other extreme 
body from the view of observers situate at places on the Earth 
which are in the common line of direction, and the intermediate 
body is seen to pass over the other extreme body, as it enters upon 
or leaves the common line of direction. The phenomena resulting 
from such contingencies of position and direction are variously 
denominated Eclipses, Transits, and Occultations, according to the 
relative apparent magnitudes of the interposing and obscured 
bodies, and according to the circumstances which attend them." 
We will proceed to consider these several phenomena in detail 
beginning with Eclipses. 

Fig. 129. 




THEORY OF A TOTAL ECLIPSE OF THE 8UN. 



It must be premised that the Moon's orbit does not lie in 
exactly the same plane as the Earth's, but is inclined thereto at 
an angle which varies between 5 20' 6" and 4 57' 22", and for 
which 5 8' 45" may be taken as the mean value. The two points 
where its path intersects the ecliptic are called the Nodes, and the 
imaginary line joining these points is termed the Line of Nodes. 

Fig. 130. 




THEORY OF AN ANNULAR ECLIPSE OF THE SUN. 

When the Moon is crossing the ecliptic from South to North, it 
is passing its Ascending Node ( a ), the opposite point of its orbit 
being its Descending Node ( <3 ). If the Moon should happen to 
pass through either node at or near the time of conjunction, or 
New Moon, it will necessarily come between the Earth and the 



CHAP. I.] 



General Outlines. 



263 



Sun, and the 3 bodies will be in the same straight line ; it will 
therefore follow that to certain parts of the Earth the Sun's disc 
will be obscured, wholly or partially as the case may be : this is 
an Eclipse of the Sun. In the figures above, S represents the Sun, 
E the Earth, and M the Moon. In a total solar eclipse the Moon's 
shadow reaches to and beyond the Earth's surface, the Moon being 
then at or near its minimum distance from the Earth ("perigee "). 
In an annular eclipse the Moon's shadow falls short of the Earth, 
the Moon being then at or near its maximum distance from the 
Earth ("apogee"). 

The Earth and the Moon, being opaque bodies, must cast 
shadows into space ; though of course, owing to the larger size 
of the Earth, its shadow is much the larger of the two. If the 
Moon should happen to pass through either node at or near 
the time of Opposition, or Full Moon, it will be again, as before, 
in the same straight line with the Earth and the Sun ; but the 
Moon will be involved in the shadow of the Earth, and therefore 
will be deprived of the Sun's light ; this causes an Eclipse of the 
Moon. 

Fig. 131. 




THEORY OF AN ECLIPSE OF THE MOON. 



In Fig. 131, S represents the Sun, E the Earth, and m n the 
orbit of the Moon : that the Moon becomes involved in the Earth's 
shadow in passing from m to n is obvious. 

If the orbits of the Earth and the Moon were in the same plane, 
an eclipse would happen at every conjunction and opposition, or 
about 25 times a year ; but as such is not the case, eclipses are of 
less frequent occurrence. According to trustworthy investiga- 
tions, in order that an eclipse of the Sun may take place, the 
greatest possible distance of the Sun or Moon from the true place 



264 Eclipses and Associated Phenomena. [BOOK II. 

of the nodes of the Moon's orbit is 18 36', whilst the latitude of 
the Moon must not exceed i 34' 52". If, however, the distance 
be less than 15 19' 30", and the latitude less than 1 23' 15", an 
eclipse must take place, though between these limits the occurrence 
of the eclipse at any station is doubtful, and depends upon the 
horizontal parallaxes and apparent semi-diameters of the two 
bodies at the moment of conjunction. In order that a lunar 
eclipse may take place, the remark just made will equally 
hold good, except that 12 24', 9 23', 63' 45" and 51' 57", must 
be substituted for the quantities given above. 

The nodes of the Moon's orbit are not stationary, but have a 
daily retrograde motion of 3' 10-64" or an annual one of 1 9 20' 1 9-7", 
so that a complete revolution round the ecliptic is accomplished 
in 18* 2i8 d 2i h 22 m 46 s nearly. The Moon performs a revolu- 
tion with respect to the node in 27 d 5 h 5 m 36" (27-21222222^. 
This is termed a " nodical revolution of the Moon b ," and must not 
be confounded with the " synodical revolution of the Moon." It 
is shorter than the latter, because the retrograde motion of the 
node upon the ecliptic brings the Moon into contact with it 
before she comes again into Conjunction or Opposition as the 
case may be. 

A singular effect produced by the retrocession of the nodes on 
the ecliptic must now be referred to. The Moon's synodical period, 
or the time which she occupies in passing from one conjunction 
or Opposition to another, is 29* I2 h 44 m 2-87" (29-53058872 J5 d ) ; 
223 of these periods amount to 658532J d (i8 y io d 7 h 43); but 
19 revolutions of the Sun with respect to the lunar node" (each 
of 346-620 i d ) are completed in 6585-7 82 d : the near coincidence 
of these two periods produces this obvious result ; that eclipses 

b Sometimes the Draconic Period. instead of remaining stationary. Since 

c If the lunar nodes were immoveable the lunar nodes travel at only 3' 10" per 

the Sun would return to the same poai- day, compared with the Sun's ecliptic 

tions with respect to them every terres- motion of 59' 9", it follows that the nodes 

trial tropical year ; but this luni-nodical require i8-6 d to get over the angular 

revolution of the Sun, if such an expres- distance which the Sun does in i d . De- 

sion maybe used, is less than the tropical ducting then i8-6 d from 365242 d (the 

year for the same reason that the nodical mean solar year), we get 346-(>42 d , as 

lunar month is less than the synodical above, for the period of the Sun's return 

one, the node receding to meet the Sun to the same lunar nodes. 



CHAP. I.] General Outlines. 265 

recur in almost, though not quite, the same regular order after 
the completion of 19 synodical revolutions of the Moon's node. 
The difference between the two periods is 0-46 i d , or io h 49-6 ; 
during which time the Sun describes an arc of 28' 6" relative to 
the lunar node. 

These coincidences will be better brought out by the figures 
being placed in column, thus : 

d. d. 

242 Draconic Periods (27-21222 x 242) = 6585-35777. 

223 Lunations (29-53058 x 223) = 6585-31934. 

19 Returns of Sun ) . 

' AT j [-(346.6201 x 19) =6585-7819. 
to Moon s Node j u 

Some trifling discrepancies in the last column compared with 
the results given above are due to different decimals having been 
employed in the multiplication. 

It was probably a knowledge of these facts which enabled the 
ancient astronomers to predict the occurrence of great eclipses, 
since it is quite certain that they did so in more than one instance 
before the nature of eclipses was fully understood. This cycle 
was known to the Chaldseans as the Saros d . Diogenes Laertius 
records 373 solar and 832 lunar eclipses observed in Egypt; and 
although his testimony is, generally, of no great value, yet it is very 
singular that this is just the proportion of solar and lunar eclipses 
visible above a given horizon within a certain period of time 
(1200-1300 years) a coincidence which cannot be accidental 6 . 

From what I have just said it might be imagined that a 
correct list of eclipses for 18-03 vears would be sufficient for all 
purposes of calculation ; and that the occurrence of an eclipse 
might be ascertained in advance at any distance of time by the 
simple process of adding so many ecliptic periods as were found 

d See Una. Cycl., art. Saros. It has and Colleges, New York, 1879, PP- 
been stated that the Chaldaeans used a 180-4, an< i Newcomb's Popular As- 
triple Saros of 54* 3i d as more correct tronomy, Loud, ed., p. 29. The German 
for purpose of prediction than a single translation of this latter work contains 
one. For a good deal of interesting in- still further and better particulars, 
formation respecting matters incidentally (Trans, by Engelmann, p. 26.) 
connected with the Saros, see Newcomb e Hist, of Ast., L.U.K., p. 15. 
and Holden's Astronomy for Schools 



266 Eclipses and Associated Phenomena. [BOOK II. 

requisite. This would be nearly correct if an eclipse appeared 
under precisely the same circumstances as the one in the pre- 
ceding or following period corresponding to it: but such is not 
the case f . An eclipse of the Moon, which in the year 565 A.D. 
was of 6 digits 8 , was in the year 583 of 7 digits, and in 601 of 
nearly 8. In 908 the eclipse became total, and it remained so for 
about 12 periods, or until the year 1088 : this eclipse continued to 
diminish until the commencement of the i5th century, when it 
totally disappeared in the year 1413. In a similar manner an 
eclipse of the Sun, which appeared at the North Pole in June 
1295, became more southerly at each period. On Aug. 27, 1367, 
it made its first appearance in the north of Europe ; in 1439 it 
was visible all over Europe; at its- 19 th appearance, in 1601, it 
was central in London ; on May 5, 1818, it was visible at London, 
and was again nearly central at that place on May 15, 1836. At 
its 39 th appearance, August 10, 1980, the Moon's shadow will 
have passed the equator, and, as the eclipse will take place near 
midnight, it will be invisible in Europe, Africa, and Asia. At 
every subsequent period the eclipse will go more and more towards 
the south, until, finally, at its 78 th appearance on Sept. 30, 2665, 
it will go off at the South Pole of the Earth, and disappear 
altogether. The time required for a lunar eclipse to go through 
all its Saros changes (so to speak) is 865 years. A similar series 
of solar eclipses will last much longer, or about 1 200 years. 

In the 1 8-year eclipse period, there usually happen 70 eclipses, 
of which 41. are solar and 29 are lunar. In any one year the 

f Halley found that if this period were Moon's contour. The Companion to the 

added to the middle of any eclipse, the Almanac for 1832 contains (p. 8) some 

corresponding one might be predicted to useful memoranda about digits, and a 

within i h 30. According as 4 or 5 leap- description of the path of the central 

years intervene, the period of the Saros line at different periods of the year, 

will be 1 8* io d &c. or i8 y n d &c. The older astronomers treated the digit 

* A digit is the -fa part of the diameter as a measure of surface and indicated by 

of the Sun or Moon ; and of course an its use that -fa of the visible area of the 

eclipse of 6 digits will be understood to Sun, or Moon, was obscured, not -fa of its 

be one in which the disc of the luminary diameter ; but in more recent times the 

is hidden. In the case of a lunar eclipse, word was used as stated at the beginning 

when the magnitude is said to exceed 12 of this note. It is now however quite 

digits, it means that the Earth's shadow obsolete in both senses, and the magnitude 

extends itself so many digits beyond the of every eclipse is expressed decimally. 



CHAP. I.] General Outlines. 267 

greatest number that can occur is 7, and the least 2 : in the 
former case 5 of them may be solar, and 2 lunar ; in the latter 
both must be solar. Under no circumstances can there be more 
than 3 lunar eclipses in i year, and in some years there are 
none at all. Though eclipses of the Sun are more numerous 
than those of the Moon in the proportion of 41 to 29 (say of 
3 to 2), yet at any given place more lunar eclipses are visible 
than solar : because, whilst the former are visible over an entire 
hemisphere, the area of the Earth over which the latter are 
visible is in the case of total or annular eclipses a narrow strip, 
which cannot exceed 180 and is seldom more than 140 miles or 
so in breadth. In the case of partial eclipses of the Sun however 
the range of visibility is, it is true, much wider; for at every 
point of the Earth immersed in the penumbra more or less of the 
eclipse will be seen. 

In a solar eclipse the Moon's shadow traverses the Earth at the 
rate of 1 830 miles an hour, or rather more than half a mile per 
second. This corresponds to 30$ miles per minute; Lalande's 
result is equivalent to 33-1 miles. 

Du Sejour found that, counting from first to last, a solar eclipse 
at the equator may last 4 h 29 44 s , and that at the latitude of 
Paris the maximum period is 3 h 26 22 s , but that the interval 
of time during which the Sun will be centrally eclipsed is very 
small. The duration of the total obscuration is greatest when 
the Moon is in perigee and the Sun in apogee ; for the apparent 
diameter of the Moon being then the greatest possible, while that 
of the Sun is the least possible, the excess of the former over the 
latter, upon which the totality depends, is at a maximum. Now 
the perigean diameter of the Moon =33' 31"; the apogean diameter 
of the Sun=3i'3o". 

A = 33'3i"-3i'3o" = a'i". 

This then is theoretically the arc which has to be described by 
the Moon during the greatest possible continuance of the total 
phase, but in reality the ultimate result is complicated by the 
Sun's apparent motion Eastward and the Earth's axial rotation 
in the same direction. However, taking into consideration the 



268 Eclipses and Associated Phenomena. [BOOK II. 

rapid motion of the Moon, it will be readily understood that, under 
the most favourable circumstances, the Sun cannot remain totally 
eclipsed for more than a few minutes. 

The duration of the obscuration in a total eclipse of the Sun 
varies, catteris paribus, with the latitude of the place of observation, 
and is greatest under the equator. Du Sejour h found that, under 
the most favourable circumstances, the greatest possible duration 
of the total phase at the equator was 7 m 58 s , and that at the 
latitude of Paris it was 6 m 10". 

The duration of an annular eclipse is greatest when the Moon is 
in apogee and the Sun in perigee, for then the apparent diameter 
of the Sun is the greatest, whilst that of the Moon is the least 
possible, and consequently the excess of the former over the latter 
(upon which the annulus depends) is then at a maximum. 

The perigean diameter of the Sun = 32' 35". The apogean 
diameter of the Moon = 29' 22". 

/. A = 32' 35" -29' 22"= 3' 13". 

This then is theoretically the arc which has to be described by 
the Moon during the greatest possible continuance of the annular 
phase, but, as before, some qualification is requisite in dealing with 
the facts which present themselves. Du Sejour found that under 
the most favourable circumstances the greatest possible duration of 
the annular phase at the equator 1 was i2 m 24% and that at the 
latitude of Paris* it was 9 m 56 s . 

It may be desirable briefly to point out the reasons why the 
greatest possible duration of an annular eclipse exceeds that of a 
total one. They are 2 in number : i 8t . Because the excess of the 
perigean diameter of the Sun over the apogean diameter of the 
Moon ( = 3' 13") is greater than the excess of the perigean diameter 
of the Moon over the apogean diameter of the Sun ( = 2' i"). 
2 nd . Because the motion of the Moon in apogee is much slower 
than it is in perigee. 

From the above remarks it will be readily understood that 
though so many solar eclipses happen from time to time, yet 

h Mem. Acad. des Sciences, 1777, p. 318. 
' Ibid., p. 317. k Ibid., p. 316. 



CHAP. I.] General Outlines. 269 

the occurrence of an annular or total one at any particular 
locality is a very rare phenomenon. Thus, according to Halley 1 , 
no total eclipse had been observed at London between March 20, 
1140, and April 22, 1715 (o. s.), though during that interval the 
shadow of the Moon had frequently passed over other parts of 
Great Britain 111 . 

The calculation of eclipses is a matter of considerable com- 
plexity. A paper by Woolhouse, in the supplement to the Nautical 
Almanac for 1836, and the chapters in Loomis's well-known work n , 
may be named as the best guides in our language . Much 
interesting historical matter concerning eclipses will be found in 
the Rev. S. J. Johnson's Eclipses, Past and Present. 

1 Phil. Trans., vol. xxix. p. 245. 1715. derived, ascertained that the eclipse of 

ra It may here be noted that, according 1 140 was not centrally visible in London, 

to recent investigations by Hind, the The line of totality crossed the Midland 

Total solar eclipse of Feb. 3, 1916, will Counties, and did not approach London 

not be visible as such in England, though nearer than Northamptonshire. (See 

a statement to that effect may occasion- letters by Hind inAst. Reg., vol. vii. p. 87, 

ally be met with. On June 30, 1954, April 1869, and vol. ix. p. 209, Sept. 1871; 

occurs the next Total eclipse which will also a paper by the Rev. S. J. J ohnson in 

be visible in Great Britain ; this will be Month. Not., vol. xxxii. p. 332. 1872.) 
seen at the northernmost of the Shetland n Practical Astronomy, pp. 226-90. 

Isles. The eclipse of Aug. n, 1999,18 It is recorded by Rittenhouse that in 

the next that will be visible as a Total his early days he calculated eclipses on 

one in England itself. The line of totality his plough-handle. For a brief sketch 

will pass across Cornwall and Devonshire. of the career of this ' self-made ' man (a 

Hind, in connection with the calcula- pioneer of astronomy in America) see 

tions from which these particulars were Sid. Mess., vol. vii. p. 433, Dec. 1888. 



270 Eclipses and Associated Phenomena. [BOOK II. 



CHAPTER II. 

ECLIPSES OF THE SUN. 

Grandeur of a Total Eclipse of the Sun. How regarded in ancient times. 
Effects of the progress of Science. Indian Customs. Effect on Birds at Berlin 
in 1887. Solar Eclipses may be Partial, Annular, or Total. Chief phenomena 
seen in connexion with Total Eclipses. Change in the colour of the sky. The 
obscurity which prevails. Effect noticed by Piola. Physical explanation. 
Sally's Beads. Extract from Sally's original memoir. Probably due to irra- 
diation. Supposed to have been first noticed by Halley iniji 5 . Sis description. 
The Corona. Hypothesis advanced to explain its origin. Probably caused 
by an atmosphere around the Sun. Remarks by Grant. First alluded to by 
Philostratus. Then by Plutarch. Corona visible during Annular Eclipses. 
The Red Flames. Remarks by Dawes. Physical cause unknown. First 
mentioned by Stannyan. Note by Flamsteed. Observations of Vasseniux. 
Aspect presented by the Moon. Remarks"by Arago. 

A TOTAL eclipse of the Sun is a most imposing spectacle, 
especially when viewed from the summit of a lofty moun- 
tain, and the moon's shadow is seen sweeping upward from the 
horizon towards the observer with a velocity which has been de- 
scribed as perfectly frightful. Professor Forbes, who observed 
the total eclipse of 1 842 from the Observatory of Turin, was so 
confounded by the frightful velocity with which the shadow 
swept over the earth from the distant Alps towards him that he 
felt as if the great building on which he was standing was com- 
mencing to fall over in the direction of the coming darkness. 
Words can but inadequately describe the grandeur and magni- 
ficence of the scene. On all sides indications are afforded that 
something unusual is taking place. At the moment of totality 
the darkness is usually so intense that the brighter planets and 



CHAP. II.] Eclipses of the Sun. 271 

stars of the ist and 2nd magnitude are seen, birds go to 
roost, flowers close, and the face of nature assumes an unearthly 
cadaverous hue ; while not the least striking thing is the sudden 
gust of wind which frequently sweeps over the country with 
some violence at the commencement of totality ; sometimes 
a considerable fall takes place in the temperature of the atmo- 
sphere as the time of the greatest obscuration draws near. 

" During the early history of mankind, a total eclipse of the 
Sun was invariably regarded with a feeling of indescribable 
terror, as an indication of the anger of the offended Deity, or the 
presage of some impending calamity ; and various instances are 
on record of the (supposed) extraordinary effects produced by so 
unusual an event. In a more advanced state of society, when 
Science had begun to diffuse her genial influence over the human 
mind, these vain apprehensions gave place to juster and more 
ennobling views of nature ; and eclipses generally came to 
be looked upon as necessary consequences flowing from the 
uniform operation of fixed laws, and differing from the ordinary 
phenomena of nature only in their less frequent occurrence. To 
astronomers they have in all ages proved valuable in the highest 
degree, as tests of great delicacy for ascertaining the accuracy of 
their calculations relative to the place of the Moon, and hence 
deducing a further improvement of the intricate theory of her 
movements. In modern times, when the physical constitution of 
the celestial bodies has attracted the attention, of many eminent 
astronomers, observations of eclipses have disclosed several in- 
teresting facts, which have thrown considerable light on some 
important points of inquiry respecting the Sun and Moon a ." 

Among the Hindus a singular custom exists b . When during 

a Grant, Hist. Phys. Ast., p. 359. The general holiday, and the natives signified 

truth of the last sentence of this extract the swallowing of the sun by a demon by 

is now more striking than it was in the usual drumming, shrieking, and blow- 

1852. ing of shells, with offerings of rice." 

b In my first edition I wrote " is said Nor is this an isolated incident. The 

to exist," but the following paragraph, cut following account was written of the 

from a newspaper in 1868, and relating eclipse of the Sun of July 29, 1878, by a 

to the great eclipse of Aug. 18, 1868, resident at Fort Sill, Indian Territory, 

will shew that the present reading of the to Mr. Fox, Ex-Mayor of Philadelphia, 

text is preferable : " Tuesday was a U.S., who allowed its publication in the 



272 Eclipses and Associated Phenomena. [BOOK II. 

a solar eclipse the black disc of our satellite is seen advancing 
over the Sun, the natives believe that the jaws of some monster 
are gradually eating it up. They then commence beating gongs, 
and rending the air with the most discordant screams of terror and 
shouts of vengeance. .For a time their efforts are productive of 
no good result the eclipse still progresses. At length, however, 
the terrific uproar has the desired effect on the voracious mon- 
ster ; it appears to pause, and then, like a fish that has nearly 
swallowed a bait and then rejects it, it gradually disgorges the 
fiery mouthful. When the Sun is quite clear of the great 
dragon's mouth, a shout of joy is raised, and the poor natives 
disperse, extremely self-satisfied on account of their having 
(as they suppose) so successfully relieved their deity from his 
late perils. For us times have now happily altered. We do 
not look on a total eclipse of the Sun as a dire calamity, but 
merely as one of the ordinary effects resulting from the due 
working of those laws by which the Supreme Being wills to 
govern the universe. 

The Eclipse of Aug. 19, 1887, deficient though it was in Astrono- 
mical results, yielded some rather interesting observations with 
respect to the effect of the eclipse on birds. In N. E. Germany, 
foresters stated that the birds, which had already begun to sing 
before the eclipse took place, became of a sudden quite silent, and 

Philadelphia Inquirer: "On Monday about his head in a series of extraordinary 

last we were permitted to see the eclipse gesticulations, retreated to his own quar- 

of the sun in a beautiful bright sky. ters. As it happened that very instant 

Not a cloud was visible- We had made was the conclusion of totality. The 

ample preparation, laying in a stock of Indians beheld the glorious orb of day 

smoked glass several days in advance. once more peep forth, and it was unani- 

It was the grandest sight I ever beheld, mously voted that the timely discharge 

but it frightened the Indians badly. of that pistol was the only thing that 

Some of them threw themselves upon their drove away the shadow and saved them 

knees and invoked the Divine blessing ; from the public inconvenience that would 

others flung themselves flat on the ground have certainly resulted from the entire 

face downward ; others cried and yelled extinction of the sun." 

in frantic excitement and terror. Finally See for recent instances of popular 

one old fellow stepped from the door of excitement at eclipses, 2 engravings and 

his lodge, pistol in hand, and fixing his n&rr&tiveBin IS Astronomic, vol. vi.p. 248, 

eyes on the darkened sun mumbled a few July 1887, and relating to the eclipses of 

unintelligible words, and raising his arm, Dec. 16, 1880, and March i, 1877, as 

took direct aim at the luminary, fired off seen at Tashkend and Laos (Indo-China) 

his pistol, and after throwing his arms respectively. 



CHAP. II.] Eclipses of the Sun. 273 

showed signs of disquiet when darkness set in. Herds of deer 
ran about in alarm, as did the small four-footed game. In Berlin 
a scientific man arranged for observations to be made by bird- 
dealers of the conduct of their feathered stock. The results were 
found to vary considerably. In some cases the birds shewed 
sudden sleepiness, even though they had sung before the eclipse 
took place. In other cases great uneasiness and fright were 
observed. Parrots shewed far more susceptibility than canaries, 
becoming totally silent during the eclipse, and only returning 
very slowly to their usual state. 

An eclipse of the Sun may be either partial, annular, or total : 
it is partial when only a portion of the Moon's disc intervenes 
between the Sun and the observer on the Earth ; annular, when 
the Moon's apparent diameter is less than the Sun's, so that when 
the former is projected on the latter it is not sufficiently large 
completely to cover it, an annulus, or ring of the Sun, being left 
unobscured ; and total when the Moon's apparent diameter is 
greater than that of the Sun, which is, therefore, wholly obscured. 
In an annular eclipse, when the centre of the Sun and Moon 
exactly coincide, it is said to be central and annular the Sun ap- 
pearing, for a very short time, as a brilliant ring of light around 
the dark body of the Moon. 

I shall now proceed to describe the principal phenomena which 
are usually witnessed in connexion with solar eclipses. 

Not the least remarkable is the almost invariable change of 
colour which the sky undergoes. Halley, in his account of the 
eclipse of 1715, says: "When the eclipse was about 10 digits 
(that is, when about f of the solar diameter was immersed), the 
face and colour of the sky began to change from a perfect serene 
azure blue to a more dusky livid colour, intermixed with a tinge 
of purple, and grew darker and darker till the total immersion 
of the Sun c ." 

At the moment of totality the suddenly altered conditions of 
illumination give rise to a further change of colour which is so 

c Phil. Trans., vol. xxix. p. 247. 1715. Arago gives an elaborate explanation of this. 
Pop. Ast., vol. ii. p. 358, Eng. ed. 



274 Eclipses and Associated Phenomena. [BOOK II. 

striking that few observers fail to notice it. The lower part of 
the atmosphere within the Moon's shadow is illuminated by light 
from the horizon which has passed through many miles of 
atmosphere near to the earth's surface and has therefore lost 
much from the violet end of the spectrum. The particles floating 
in the lower atmosphere therefore disperse a ruddy light which 
projected upon the deep blue of the upper atmosphere gives rise 
to a combination of colour which may well be described as purple 
or violet. Weeden, who observed the total eclipse of 1 860 near 
Miranda in Spain, says that the heavens during totality seemed 
like a dark purple canopy, hanging low down as it were, in the 
shape of a watch-glass, and covering the earth, excepting 
in a regular belt near the horizon, where the illuminated sky 
beyond the range of the obscurity reflected an orange golden 
light. De La Rue, observing the same eclipse, says that the 
upper part of the sky was of a deep indigo colour, shading 
through a sepia tint into red and orange as it approached the 
horizon. Ranyard, observing the eclipse of 1870 in Sicily, de- 
scribes the colour of the sky as a deep violet, which reminded 
him of the colour of the spectrum near the line H. 

It has also been found that whilst the sky changes colour 
during the progress of an eclipse, similar effects are produced 
upon terrestrial objects. This seems to have been noticed as far 
back as 840 A.D. d Kepler mentions that during the solar eclipse 
which happened in the autumn of 1590, the reapers in Styria 
noticed that everything had a yellow tinge 6 . Similar effects 
have also been described in modern times f . De La Rue, in 
describing the eclipse of 1860, says that the peculiar light cast on 
the spectators impressed him with a feeling of solemnity never 
to be effaced. 

The darkness which prevails during a total eclipse of the Sun 
is not usually so considerable as might be expected. It is, how- 
ever, subject to much variation. Ferrer, speaking of the eclipse 

d Ad Vitellionem Paralipomena, p. f Mem. R.A.S., vol. xv. pp. 12, 14, 

294. and 15, 1846; xxi. passim, 1853; An- 

" Ibid., p. 303. nuaire, 1846, p. 291, &c. 



CHAP. II.] Eclipses of the Sun. 275 

of 1806, says that at the time of total obscuration <: without 
doubt the light was greater than that of the full Moon ." In 
general it has been found that the darkness is sufficiently great to 
prevent persons from reading, though exceptions to this rule have 
been known. The faint illumination which exists at the moment 
of the totality is due to light reflected from those regions of the 
atmosphere which are still exposed to the direct rays of the Sun. 
The corona (which will presently be described) also, no doubt, 
assists in the illumination, but the light received from the corona 
is small compared with that derived from the clouds outside the 
region of totality, for it has frequently been noticed that the 
corona casts no shadow. The degree of obscuration will also 
vary according as the observer is or is not deeply immersed in 
the lunar shadow a fact first pointed out by Halley h . 

Observers inside houses, or so situated amongst buildings that 
the light from the horizon cannot reach them, usually have difficulty 
in distinguishing objects during totality. Mountains or clouds 
upon the horizon and other local causes also seem to affect 
the degree of darkness, so that during the same eclipse the ex- 
perience of observers in different localities may differ considerably. 
Thus during the total eclipse of 1851, Piazzi Smyth could read 
small print, while Professor Adams had only just sufficient light 
to read the face of a box chronometer ; and Sir G. B. Airy says : 
" A candle had been lighted in a lantern about a quarter of an 
hour before the totality. Mr. Hasselgren was unable to read the 
minutes of the chronometer face, without having the lantern held 
close to the chronometer. I had prepared for the occasion 
a circle described upon a card : I desired much to make a 
drawing of the prominences, at least of their positions on the 
limb of the moon, by marking them on this circle, but it was 
impossible for me to see it, and I was obliged to approach 
very closely to the lantern, in order to make the smallest memo- 
randum on the card." 

Mr. Lassell, who observed the same eclipse at Trollhattan, said 

6 Trans. Amer. Phil. Soc., vol. vi. p. 266. 1809. 
h Phil. Trans., vol. xxix. p. 250. 1715. 

T 2 



27(3 Eclipses and Associated Phenomena [BOOK II. 

that the amount of darkness may be appreciated from the fact 
that on withdrawing his eye from the telescope he could neither 
see the seconds-hand of his watch nor the paper sufficiently to 
write the time down '. 

As previously remarked, a solar eclipse of large magnitude and 
still more a Total eclipse is always accompanied by a decided 
decrease in the temperature of the air (in the shade). Mr. G. J. 
Symons from observations in 1858, 1860, and 1870, concludes that 
the air is coldest about hour after the time of the Conjunction 
of the Sun and Moon. 

In the case of the eclipse of 1 842, it was remarked by Piola at 
Lodi, and by O. Struve at Lipesk, that although the obscurity was 
such that stars of the 2 nd and 3 rd magnitudes ought to have been 
visible, yet only those of the i st magnitude were actually seen k . 
M. Belli explained this curious fact by reference to a physiological 
principle. He remarked that during the short interval of total 
obscuration the eye has not sufficient time to recover from the 
dazzling effect of the Sun's rays, and consequently is unable to 
take advantage of the obscurity which actually prevails J . The 
suddenness with which the light succeeds the darkness after 
a total eclipse of the Sun is well known. Halley suggested 
2 explanations of the phenomenon. I st . That previously to the 
total obscuration the pupil of the eye might be very much 
contracted by viewing the Sun, and consequently the organ 
of vision would be less likely to suffer by the effulgence of the 
light than at the instant of emersion, when the pupil has again ex- 
panded. 2 nd . That, as the Eastern margin of the Moon, at which 
the Sun disappeared, had been exposed for a fortnight to the 
direct action of the solar rays, the heat generated during this 
period might cause vapours to ascend in the lunar atmosphere, 
which, by their interposition between the Sun and the Earth, 
would have the effect of tempering the effulgence of the solar 
rays passing through them. On the other hand, the Western 

1 Mem. E.A.S., vol. xxi. p. 47, 1853 ; Biblioth&que Universelle de Geneve, vol. 
and see Mem. R.A.S., vol. xli. p. 185. xliv. p. 368. 

k Giorn. delV lit. Lomb., vol. iv.p. 341 ; l Gioi*n. deW Int. Lomb., vol. iv. p. 341 . 



CHAP. II.] 



Eclipses of the Sun. 



277 



Fig. IS 2 - 



margin of the Moon, at which the Sun re-appeared, had just 
experienced a night of equal length, during which the vapours 
suspended in the lunar atmosphere had been undergoing a course 
of precipitation upon the Moon's surface under a process of 
cooling. In this case, therefore, the solar rays would meet with 
less obstruction in passing through the lunar atmosphere, and, 
consequently, it was reasonable to suppose that they would 
produce a more intense effect m . The second hypothesis requires 
us to suppose the presence of a lunar atmosphere, the existence of 
which modern observation tends to disprove. The first is doubt- 
less the true explanation. 

When the disc of the Moon advancing over that of the Sun has 
reduced the latter to a thin crescent, it is usually noticed that 
immediately before the beginning 
and after the end of complete ob- 
scuration, the crescent appears as 
a band of brilliant points, separated 
by dark spaces so as to give it the 
appearance of a string of beads 
which appear to move and merge 
into one another. While the Moon's 
limb is seen projected upon the 
sun's disc it appears perfectly 
smooth. No lunar mountains can 
be detected projecting beyond the 
general outline. The hypothesis usually advanced to account 
for this smoothness of the Moon's limb is that the irradiation 
from the bright background of the solar surface projects over 
the lunar limb like a fringe, and forms a new even limb inside 
the true rough lunar limb. As the solar crescent becomes thin, 
the irradiation fringe vanishes wherever a lunar projection breaks 
through the thin line of solar light. 

These phenomena are generally known as Bailys Beads, having 
received their name from the late Mr. Francis Baily, who was 




BAILY S BEADS. 



111 Phil. Trans., vol. xxix. p. 248. 1715. 



278 Eclipses and Associated Phenomena. [BOOK II. 

the first to describe them in detail n . His original memoir was 
published in 1836, and from it I make the following quo- 
tation : 

"When [previous to the totality] the cusps of the Sun were about 40 asunder, a 
row of lucid points, like a string of bright beads, irregular in size and distance from 
each other, suddenly formed round that part of the circumference of the Moon that 
was about to enter, or which might be considered as having just entered, on the Sun's 
disc. Its formation indeed was so rapid, that it presented the appearance of having 
been caused by the ignition of a train of gunpowder. This I intended to note as the 
correct time of the formation of the annulus, expecting every moment to see the 
thread of light completed round the Moon, and attributing this serrated appearance 
of the Moon's limb (as others had done before me) to the lunar mountains, although 
the remaining portion of the Moon's circumference was comparatively smooth and 
circular, as seen through the telescope. My surprise, however, was great on finding 
that these luminous points, as well as the dark intervening spaces, increased in 
magnitude, some of the contiguous ones appearing to run into each other like drops 
of water ; for the rapidity of the change was so great, and the singularity of the 
appearance so fascinating and attractive, that the mind was for the moment distracted 
and lost in the contemplation of the scene, so as to be unable to attend to every 
minute occurrence. Finally, as the Moon pursued her course, these dark intervening 
spaces (which, at their origin, had the appearance of lunar mountains in high relief, 
and which still continued attached to the Sun's border) were stretched out into long, 
black, thick parallel lines, forming the limbs of the Sun and the Moon ; when, all at 
once, they suddenly gave way, and left the circumferences of the Sun and Moon in 
those points, as in the rest, comparatively smooth and circular, and the Moon 
perceptibly advanced on the face of the Sun ." 

Mr. Baily then goes on to describe the appearances which he 
saw after the total obscuration ; they were, however, substantially 
the same as those recorded above. 

The most recent full account of " Baily's Beads " is due to 
Mr. Lewis Swift, an American astronomer who observed the 
eclipse of July 29, 1878, at Denver, Colorado. He says : 

"At the eclipse of 1869, I was so captivated with the number, magnitude, and 
unexpected brilliancy of the protuberances, that I failed to give particular attention 
to the beautiful phenomenon of Baily's Beads. On this occasion I observed it very 
carefully, and found it one of the most striking and fascinating features of the 
whole eclipse. Several seconds previous to the formation of the beads, I observed, 
near each end of the solar crescent, a phenomenon which I have never seen described 
in the books. Though reminding me of the ' Black Drop,' which I saw at the late 
transit of Mercury, it was very different from it. At the risk of being considered 
prolix, I will describe it, though to be appreciated it must be seen. Imagine a long 

n They were noticed long before his time. 
Mem. R.A.S., vol. x. p. 5. 1838. 



CHAP. II.] Eclipses of the Sun. 279 

and very narrow crescent cut in a door between two rooms, one brilliantly lighted, 
the other dark, the observer being in the farther end of the latter (imagined to be a 
very long one) looking at the crescent with his telescope. The appearance was as if 
two concealed persons in the lighted room, one each side of the crescent, were busily 
engaged in rapidly protruding and withdrawing a series of long slim black objects 
like slate pencils. They were not broad at their bases as is the ' Black Drop,' and, 
unlike the latter, were not, except in two instances, opposite each other. They were 
seen only near each end of the crescent. This phenomenon was as unique as it was 
unexpected, and lasted for but two or three seconds, and then entirely ceased at 
each end simultaneously, but recommenced in one or two seconds, but farther from 
the end of the lune, and the images were more blunt and less symmetrical, though 
their motions were as before. This lasted but a short time, when all motion ceased, 
as if preparing for a grand denouement, and from each end of the crescent, now 
reduced to a narrow curved line of light, the beads (which are luminous, and thus 
unlike the ' Black Drop ') began to form from each end simultaneously, and in less 
than a half second were completed. They were nearly square, and increased in size 
from each end of the crescent to the centre, which was the largest in exact mathe- 
matical ratio. So symmetrical were they that if half of them had been superimposed 
on the other half they would have agreed in number, curvature, shape, and distance. 
They were visible but a short time say two or three seconds when, giving a few 
pulsating tremors, they vanished altogether. When I take into consideration the 
exact uniformity of their formation as to size, shape, etc., I cannot subscribe to the 
dogma that they are only the sun's light shining through the interstices of the lunar 
mountains. In this case part of the moon's contour, where they were formed, was 
smooth, while the other was exceedingly rough, yet the beads were the same in both 
localities. And those formed at the beginning are precisely similar to those at the 
close of totality, and those of one eclipse just like those of all total and annular 
that have occurred since they were first described by Baily. The assertion here 
seems justifiable that the cause of Baily's Beads is still enshrouded in darkness P." 

The earliest account of the phenomenon of the beads is con- 
tained in Halley's memoir on the total eclipse of 1715. He says : 
" About 2 minutes before the total immersion, the remaining part 
of the Sun was reduced to a very fine horn, whose extremities 
seemed to lose their acuteness, and fo become round like stars ; and, 
for the space of about a quarter of a minute, a small piece of the 
Southern horn of the eclipse seemed to be cut off from the rent by 
a good interval, and appeared like an oblong star rounded at 
both ends q ." The first annular eclipse in which it appears that 
any beads were seen was that of Feb. 18, 1736-7, observed by 
Maclaurin r . 

One of the most interesting appearances seen during a total 

i> Washington Observations, 1876, Ap- 1 Phil. Trans., vol. xxix. p. 248. 1715. 

pendix III. p. 227. r Phil. Trans., vol. xl. p. 177. i?37- 



280 Eclipses and Associated Phenomena. [BOOK II. 

eclipse of the Sun is the corona, or halo of light which surrounds 
the Moon. It usually appears only a few seconds previous to the 
total extinction of the Sun's light, and continues visible for 
about the same interval of time after its reappearance. In 
general, it may be compared to the nimbus commonly painted 
around the heads of the Virgin Mary, the Apostles, &c. Different 
explanations have been advanced to account for this phenomenon : 
Kepler thought it due to the presence of an atmosphere round 
the Moon s : La Hire suggested that it might be produced by the 
reflection of the solar rays from the inequalities of the Moon's 
surface, contiguous to the edge of her disc, combined with their 
subsequent passage through the Earth's atmosphere*; the late 
Professor Baden Powell once conducted a series of experiments 
which tended strongly to support the idea that refraction was 
the cause of it u : on the whole, however, it is now tolerably clear 
that it is due to something in the nature of an atmosphere about the 
Sun. Its figure, the nebulous structures which are seen in it 
all gradually diminishing in density onwards, point to the sup- 
position of its being due to matter encompassing the solar orb, 
and gravitating everywhere towards its centre. Delisle con- 
jectured that the luminous ring might be occasioned by the 
diffraction of the solar rays which pass near the Moon's edge x , 
but Sir David Brewster shewed that this theory, though ingenious, 
is quite untenable y . 

Judged by photographic results, the solar corona is very much 
fainter than the Moon, for whilst its outer portion has been found 
to fail utterly to make any impression on a plate after an exposure 
of 5 seconds, the Moon has been photographed perfectly in O' i to 
O'2 seconds. Moreover Federow in 1842 ; Swan and Chevallier 
in 1851 ; and Lespiault, Burat, and Cuillier in 1860, all observed, 
and specially recorded, that no shadow was cast by the corona. 

The earliest historical allusion to the corona is made by Philo- 
stratus. He mentions that the death of the Emperor Domitian 

8 Ad Vitell. Paralipom., p. 302; Spit. u Mem. R.A.S., vol. xvi. p. 301. 1847. 

Astron., p. 893. x Mem. Acad. des Sciences, 1715, p. 166 

* Mem. Acad. des Sciences, 1 715, p. 161 el seq. 

et seq. 1 Edin. Encyc., art. Astronomy. 



CHAP. II.] Eclipses of the Sun. 281 

had been ' announced ' previously by a total eclipse of the sun. 
"In the heavens there appeared a prodigy of this nature. A 
certain corona, resembling the Iris, surrounded the orb of the 
Sun and obscured his light z ; " (i. e., it appeared coincidently 
with the total obscuration of his light). Plutarch is still more 
precise in his allusion. Speaking of a total eclipse of the Sun 
which had recently happened, he endeavours to shew why the 
darkness arising from such phenomena is not so profound as 
that of night. He begins by assuming, as the basis of his 
reasoning, that the Earth greatly exceeds the Moon in size, and 
after citing some authorities, he goes on to say : " Whence it 
happens that the Earth, on account of its magnitude, entirely 
conceals the Sun from our sight. . . . But even although the 
Moon should at any time hide the whole of the Sun, still the eclipse 
is deficient in duration, as well as amplitude, for a peculiar 
effulgence is seen around the circumference, which does not 
allow the obscurity to become very intense or complete." ('AAAa 
u>erai TLS avyrj Tiepl rrjv trvv, OVK ewcra fiaOelav ylvta-Qai rr]v 
KCU anparov*.) The luminous ring seems to have been 
noticed by Clavius during the eclipse of April 9, 1567 : he 
thought that it was merely the uncovered margin of the Sun's 
disc ; but Kepler shewed that this was impossible. 

There are one or two well-authenticated instances of the 
corona being visible during partial eclipses of the Sun. In 1 842, 
M. D'Hombre Firmas, at Alais, which was contiguous to, though 
not actually in the path of the shadow, states that, " every one 
remarked the circle of pale light which encompassed the Moon 
when she almost covered the Sun b ." Several observers of this 
eclipse noticed that the ring at first appeared to be brightest on 
the side of the solar disc which was first covered by the Moon, 

*LifeofApolloniusofTyana,'by'P}iil- have given in the text. But I am not 

ostratus, Bk. viii. cap. 23. The passage satisfied that he has done so on sufficient 

will be found quoted in Ast. Nach., vol. grounds. 

xxvii. No. 1838, March 31, 1871; and in Plut., Opera Mor. et Phil. vol. ix. p. 

Observatory, vol. ix. p. 129, March 1886, 682. Ed. Lipsiae, 1778. 

where Lynn calls in question both the b Annuaire, 1846, p. 339. 
statements and the deductions which I 



282 Eclipses and Associated Phenomena. [BOOK II. 

but that previously to the close of the total phase, it was 
brightest at the part where the Sun was about to reappear . 

Not the least beautiful phenomena seen during a total solar 
eclipse are the "Red Flames," which become visible around the 
margin of the Moon's disc immediately after the commencement 
of the total phase. Mr. Dawes so minutely described them, 
as they appeared to him in July 1851, that I cannot do better 
than quote his words. He says : 

" Throughout the whole of the quadrant from north to east there was no visible 
protuberance, the corona being uniform and uninterrupted. Between the east and 
south points, and at an angle of about 115 from the north point, appeared a large 
red prominence of a very regular conical form. When first seen it might be about 
i ' in altitude from the edge of the Moon, but its length diminished as the Moon 
advanced. 

" The position of this protuberance may be inaccurate to a few degrees, being 
more hastily noticed than the others. It was of a deep rose colour, and rather paler 
near the middle than at the edges. 

"Proceeding southward, at about 145 from the north point, commenced a low 
ridge of red prominences, resembling in outline, the tops of a very irregular range of 
hills. The highest of these probably did not exceed 40". This ridge extended 
through 50 or 55, and reached, therefore, to about 197 from the north point, its 
base being throughout formed by the sharply-defined edge of the Moon. The 
irregularities at the top of the ridge seemed to be permanent, but they certainly 
appeared to undulate from the west towards the east ; probably an atmospheric 
phenomenon, as the wind was in the west. 

" At about 220 commenced another low ridge of the same character, and extended 
to about 250, less elevated than the other, and also less irregular in outline, except 
that at about 225 a very remarkable protuberance rose from it to an altitude of !', 
or more. The tint of the low ridge was a rather pale pink ; the colour of the more 
elevated prominence was decidedly deeper, and its brightness much more vivid. In 
form it resembled a dog's tusk, the convex side being northwards, and the concave to 
the south. The apex was somewhat acute. This protuberance, and the low ridge 
connected with it, were observed and estimated in height towards the end of the 
totality. 

"A small double-pointed prominence was noticed at about 255, and another low 
one with a broad base at about 263. These were also of the rose-coloured tint, but 
rather paler than the large one at 225. 4 

" Almost directly preceding, or at 270, appeared a bluntly triangular pink body, 
suspended, as it were, in the corona. This was separated from the Moon's edge when 
first seen, and the separation increased as the Moon advanced. It had the appearance 
of a large conical protuberance, whose base was hidden by some intervening soft and 
ill-defined substance, like the upper part of a conical mountain, the lower portion of 
which was obscured by clouds or thick mist. I think the apex of this object must 
have been at least i' in altitude from the Moon's limb when first seen, and more than 

c Mem. R.A.S., vol. xv. p. 1.6. 1846. 



CHAP. II.] Eclipses of the Sun. 283 

i ' towards the end of total obscuration. Its colour was pink, and I thought it paler 
in the middle. 

" To the north of this, at about 280 or 285, appeared the most wonderful 
phenomenon of the whole, A red protuberance, of vivid brightness and very deep 
tint, arose to a height of, perhaps, i-jj-' when first seen, and increased in length to 2', 
or more, as the Moon's progress revealed it more completely. In shape it somewhat 
resembled a Turkish cimeter, the northern edge being convex, and the southern 
concave. Towards the apex it bent suddenly to the south, or upwards, as seen in the 
telescope. Its northern edge was well defined, and of a deeper colour than the rest, 
especially towards its base. I should call it a rich carmine. The southern edge was 
less distinctly defined, and decidedly paler. It gave me the impression of a somewhat 
conical protuberance, partly hidden on its southern side by some intervening substance 
of a soft or flocculent character. The apex of this protuberance was paler than the 
base, and of a purplish tinge, and it certainly had a flickering motion. Its base was, 
from first to last, sharply bounded by the edge of the Moon. To my great astonish- 
ment, this marvellous object continued visible for about 5 seconds, as nearly as I could 
judge, after the Sun began to reappear, which took place many degrees to the south of 
the situation it occupied on the Moon's circumference. It then rapidly faded away, 
but it did not vanish instantaneously. From its extraordinary size, curious form, deep 
colour, and vivid brightness, this protuberance absorbed much of my attention ; and I 
am, therefore, unable to state precisely what changes occurred in the other phenomena 
towards the end of the total obscuration. 

" The arc from about 283 to the north point was entirely free from prominences, 
and also from any roseate tint." 

Astronomers were long unable to determine the nature of 
these rose-coloured emanations ; but it is now accepted that they 
belong to the Sun and consist of gaseous matter (chiefly hydro- 
gen) in an incandescent state rushing upwards with inconceiv- 
able velocity. 

One of these prominences, measured by De La Rue in 1860, 
was 44,000 miles in vertical height above the Sun's surface. 

Julius Firmicus, speaking of the eclipse of July 17, 334, makes 
a remark which may apply to this phenomenon ; otherwise the 
earliest recorded account of the Red Flames is by Captain 
Stannyan, who observed them at Berne during the total eclipse 
of 1 706. He writes to Flamsteed : 

"That the Sun was totally darkened there for 4^ minutes of time; that a fixed 
star and a planet appeared very bright ; and that his getting out of his eclipse was 
preceded by a blood-red streak of light from its left limb, which continued not longer 
than 6 or 7 seconds of time ; then part of the Sun's disc appeared all of a sudden, as 
bright as Venus was ever seen in the night ; nay, brighter ; and in that very instant 
gave a light and shadow to things as strong as the Moon uses to do d ." 

d Phil. Tran*., vol. xxv. p. 2240. 1706. 



284 Eclipses and Associated Phenomena. [BOOK II. 
On this communication Flamsteed remarks ; 

" The Captain is the first man I ever heard of that took notice of a red streak 
preceding the emersion of the Sun's body from a total eclipse. And I take notice 
of it to you [the Royal Society], because it infers that the Moon has an atmosphere ; 
and it3 short continuance, if only 6 or 7 seconds' time, tells us that its height was not 
more than 5 or 6 hundredths part of her diameter 6 ." 

The Red Flames were seen by Halley, Louville f , and C. Hayes 
in 1715, and afterwards by Vassenius, at Gottenberg, who 

says : 

"But what seemed in the highest degree worthy, not merely of observation, but 
also of the attention of the illustrious Royal Society, were some reddish spots which 
appeared in the lunar atmosphere without the periphery of the Moon's disc, amounting 
to 3 or 4 in number, one of which was larger than the other, and occupied a situation 
about midway between the south and west. These spots seemed in each instance to 
be composed of 3 smaller parts or cloudy patches of unequal length, having a certain 
degree of obliquity to the periphery of the Moon. Having directed the attention of 
my companion to the phenomenon, who had the eyes of a lynx, I drew a sketch of its 
aspect ; but while he, not being accustomed to the use of the telescope, was unable 
to find the Moon, I, again with great delight, perceived the same spot, or, if you 
choose, rather the invariable cloud occupying its former situation in the atmosphere 
near the Moon's periphery "." 

A " Red-Flame " of a greenish-blue tinge has been noticed. 
This Arago considered to be an effect of contrast. 

The Red Flames have also been noticed in annular eclipses, 
as in that of 1737, observed by Maclaurin, which appears to be 
the earliest in which the phenomenon was seen h ; and in partial 
eclipses, of which that of 1605, observed by Kepler, is probably 
the first '. 

The aspect presented by the Moon during eclipses of the Sun 
is frequently very singular. Kepler stated that the Moon's surface 
is occasionally distinguishable by a ruddy hue k . Baily, in his 
account of the annular eclipse of 1 836, states, that " previous to 
the formation of the ring, the face of the Moon was perfectly 
black ; but on looking at it through the telescope, during the 
annulus, the circumference was tinged with a reddish purjjle colour, 
which extended over the whole disc, but increased in density of 

" Phil. Trans., vol. xx. p. 2241. 1706. h Phil. Trans., vol. xl. p. 181. 1737. 

f Mem. R.A.S., vol. xxi. p. 90. 1853. ' De Stelld Novd, p. 116. 

8 Phil. Trans., vol. xxviii. p. 135. 1733. k Epit. Astron., p. 895. 



CHAP. II.] Eclipses of the Sun. 285 

colour, according to the proximity to the centre, so as to be in 
that part nearly black 1 ." Vassenius in 1733 and Ferrer in 1806 
are the only observers who state that they have seen the irregu- 
larities in the Moon's surface during a central eclipse, whether 
total or annular m . Arago and others tried to do so in 1842, but 
failed. The fact that the lunar inequalities sometimes are seen 
and at other times are not seen is doubtless owing to meteoro- 
logical causes. 

In 1842 Arago saw the dark contour of the Moon projected 
upon the bright sky 40 after the commencement of the eclipse. 
He ascribes the phenomenon to the projection of the Moon upon 
the solar atmosphere, the brightness of which, by an effect of 
contrast, rendered the outline of the Moon's dark limb discern- 
ible n . The phenomenon appears to be a rare one : at least it is 
recorded by only 3 recent observers . 

On several occasions attempts have been made to detect the 
Moon's shadow in the course of its passage over the surface of 
the Earth. Airy in 1851 succeeded in observing it, but he failed 
in 1842, in which year, however, Plana and Forbes were more 
fortunate. The difficulty arises from the immense velocity of the 
shadow about 30^ miles per minute. The earliest historical 
record of the eclipse-shadow being seen occurs in Duillier's 
account of the eclipse of May 12, 1706 p . 

According to M. Laussedat, one of the horns of the solar 
crescent in 1 860 appeared for a short time rounded and truncated. 
The other horn was contracted nearly to a point, and a small 
patch of light wholly detached was visible beyond the extremity 
of this cusp. 

1 Mem. R.A.S., vol. x. p. 17. 1838. Not., vols. xxvii. p. 185, March 1867, and 

m Phil. Trans., vol. xxxviii. p. 135, xxxiii. pp. 468 and 577, June, &c. 1873; 

J733; Trans. Amer. Phil. Soc., vol. vi. Ast. Reg., vol. xiii. p. 9, Jan. 1875. 

p. 267, 1809. P Mem. Acad. des Sciences, 1706, p. 113 

a Annuaire, 1846, p. 372. (Hist.); Phil. Trans., vol. xxv. p. 2243, 

Noble, Pratt, and Neison, Month. 1706. 



286 Eclipses and Associated Phenomena. [BOOK II. 



CHAPTER III. 

THE TOTAL ECLIPSE OF THE SUN 
OF JULY 28, 1851. 

Observations by Airy. By Hind. By Lassell. 

NOT the least interesting of the total eclipses of the Sun that 
have occurred within the last half-century was that of July 
28, 1851. Though not visible in England, it was seen to great 
advantage in Sweden, to which country many astronomers went 
at the time for the purpose of observing the eclipse. The follow- 
ing remarks are from the pen of Sir G. B. Airy, the then Astro- 
nomer Koyal, who observed the eclipse at Gottenberg : 

" The approach of the totality was accompanied with that indescribably mysterious 
and gloomy appearance of the whole surrounding prospect, which I have seen on a 
former occasion. A patch of clear blue sky in the zenith became purple-black while 
I was gazing on it. I took off the higher power with which I had scrutinized the 
Sun, and put on the lowest power (magnifying about 34 times). With this I saw 
the mountains on the Moon perfectly well. I watched carefully the approach of the 
Moon's limb to that of the Sun, which my graduated dark glass enabled me to see in 
great perfection : I saw both limbs perfectly well defined to the last, and saw the line 
becoming narrower, and the curves becoming sharper, without any distortion or 
prolongation of the limbs. I saw the Moon's serrated limb advance up to the Sun's, 
and the light of the Sun glimmering through the hollows between the mountain 
peaks, and saw these glimmering spots extinguished one after another in extremely 
rapid succession, but without any of the appearances which Mr. Baily has described. 
. . . . I have no means of ascertaining whether the darkness really was greater 
in the eclipse of 1842. I am inclined to think, that in the wonderful, and, I may say, 
appalling obscurity, I saw the grey granite hills, within sight of Hvalas, more dis- 
tinctly than the darker country surrounding the Superga. But whether, because in 
1851 the sky was much less clouded than in 1842 (so that the transition was from 
a more luminous state of sky, to a darkness nearly equal in both cases), or from 
whatever cause, the suddenness of the darkness in 1851 appeared to be much more 
striking than in 1842. My friends, who were on the upper rock, to which the path 



Figs. 133-8. 



Plate XVIII. 




(Airy.} 




(Carrington.) 




(Dawes.} 




(Hind) 




(R. Stephenson, M.P.) 







(&. Williams.') 



THE TOTAL ECLIPSE OP THE SUN OP JULY 28, 1851. 

VIEW8 OF THE BED FLAMES. 



CHAP. III.] Total Eclipse of the Sun, July 28, 1851. 287 

was very good, had great difficulty in descending. A candle had been lighted in a 
lantern, about a quarter of an hour before the totality ; Mr. Hasselgren was unable 
to read the minutes of the chronometer's face without having the lantern held close 
to the chronometer. 

"The corona was far broader than that which I saw in 1842 ; roughly speaking, 
its breadth was a little less than the Moon's diameter, but its outline was very 
irregular. I did not remark any beams projecting from it which deserved notice as 
much more conspicuous than the others; but the whole was beamy, radiated in 
structure, and terminated (though very indefinitely) in a way which reminded me of 
the ornament frequently placed round a mariner's compass. Its colour was white, 
and resembling that of Venus. I saw no flickering or unsteadiness of light. It was 
not separated from the Moon by any dark ring, nor had it any annular structure : it 
looked like a radiating luminous cloud behind the Moon. . . . The form of the 
prominences was most remarkable. One reminded me of a boomerang. Its colour, 
for at least two-thirds of its width, from the convexity to the concavity, was full lake 
red ; the remainder was nearly white. The most brilliant part of it was the swell 
farthest from the Moon's limb ; this was distinctly seen by my friends and myself 
with the naked eye. I did not measure its height; but judging generally by its 
proportion to the Moon's diameter, it must have been 3'. This estimation, perhaps, 
belongs to a later period of the eclipse. ... It was impossible to see the changes 
that took place in the prominences, without feeling the conviction that they belonged 
to the Sun, and not to the Moon. 

" I again looked round, when I saw a scene of unexpected beauty. The southern 
part of the sky, as I have said, was covered with uniform white cloud ; but in the 
northern part were detached clouds, upon a ground of clear sky. This clear sky was 
now strongly illuminated to the height of 30 or 35, and through almost 90 of 
azimuth, with rosy red light shining through the intervals between the clouds. 
I went to the telescope, with the hope that I might be able to make the polariza- 
tion-observation (which, as my apparatus was ready to my grasp, might have been 
done in 3 or 4 seconds), when I saw the sierra, or rugged line of projections, had 
arisen. This sierra was more brilliant than the other prominences, and its colour 
was nearly scarlet. The other prominences had perhaps increased in height, but no 
additional new ones had arisen. The appearance of this sierra, nearly in the place 
where I expected the appearance of the Sun, warned me that I ought not now to 
attempt any other physical observation. In a short time the white Sun burst forth, 
and the corona, and every other prominence, vanished. 

" I withdrew from the telescope, and looked round. The country seemed, though 
rapidly, yet half unwillingly, to be recovering its usual cheerfulness. My eye, how- 
ever, was caught by a duskiness in the south-east, and I immediately perceived that 
it was the eclipse-shadow in the air, travelling away in the direction of the shadow's 
path. For at least 6 seconds this shadow remained in sight, far more conspicuous to 
the eye than I had anticipated a ." 

Mr. J. R. Hind watched the eclipse at Rsevelsberg, near Engel- 
holm. He says: 

".The moment the Sun went out the corona appeared ; it was not very bright, but 
this might arise from the interference of an extremely light cloud of the cirrus class 

a Mem. E.A.S., vol. xxi. p. 5. 1853. 



288 Eclipses and Associated Phenomena. [BOOK II. 

which overspread the Sun at the time. The corona was of the colour of tarnished 
silver, and its light seemed to fluctuate considerably, though without any appearance 
of revolving. Bays of light, the aigrettes, diverged from the Moon's limb in every 
direction, and appeared to be shining through the light of the corona. In the tele- 
scope many rose-coloured flames were noticed ; one, far more remarkable than the 
rest, on the western limb, could be distinguished without any telescopic aid ; it was 
curved near its extremity, and continued in view 4 seconds after the Sun had dis- 
appeared, i. e., after the extinction of ' Baily's beads,' which phenomena were very 
conspicuous in this eclipse, particularly before the commencement of the totality. 
In this case they were clearly to be attributed to the existence of many mountains 
and valleys along the Moon's edge, the Sun's light shining through the valleys and 
between the mountain ridges, so as to produce the appearance of luminous drops or 
beads, which continued visible some seconds. The colour of the ' flames ' was a full 
rose red at the borders, gradually fading off, towards the centres, to a very pale 
pink. Along the southern limb of the Moon, for 40 or upwards, there was a 
constant succession of very minute rose-coloured prominences, which appeared to 
be in a state of undulation, though without undergoing any material change of 
form. An extremely fine line, of a violet colour, separated these prominences from 
the dark limb of the Moon. The surface of our satellite, during the total eclipse, 
was purplish in the telescope ; to the naked eye it was by no means very dark, but 
seemed to be faintly illuminated by a purplish grey light of uniform intensity, on 
every part of the surface. 

" The aspect of nature during the total eclipse was grand beyond description. A 
diminution of light over the Earth was perceptible a quarter of an hour after the 
beginning of the eclipse ; and about ten minutes before the extinction of the Sun, the 
gloom increased very perceptibly. The distant hills looked dull and misty, and the 
sea assumed a dusky appearance, like that it presents during rain ; the daylight that 
remained had a yellowish tinge, and the azure blue of the sky deepened to a purplish 
violet hue, particularly towards the north. But notwithstanding these gradual 
changes, the observer could hardly be prepared for the wonderful spectacle that 
presented itself, when he withdrew his eye from the telescope, after the totality had 
come on, to gaze around him for a few seconds. The southern heavens were then of 
a uniform purple-grey colour, the only indications of the Sun's position being the 
luminous corona, the light of which contrasted strikingly with that of the surrounding 
sky. In the zenith and north of it, the heavens were of a purplish-violet, and 
appeared very near ; while in the north-west and north-east, broad bands of yellowish 
crimson light, intensely bright, produced an effect which no person who witnessed it 
can ever forget. The crimson appeared to run over large portions of the sky in these 
directions, irrespective of the clouds. At higher altitudes the predominant colour 
was purple. All nature seemed to be overshadowed by an unnatural gloom. The 
distant hills were hardly visible, the sea turned lurid red, and persons standing near 
the observer had a pale livid look, calculated to produce the most painful sensations. 
The darkness, if it can be so termed, had no resemblance to that of night. At 
various places within the shadow, the planets Venus, Mercury, and Mars, and the 
brighter stars of the first magnitude, were plainly seen during the total eclipse. 
Venus was distinctly seen at Copenhagen, though the eclipse was only partial in that 
city ; and at Dantzic she continued in view 10 minutes after the Sun had reappeared. 
Animals were frequently much affected. At Engelholm, a calf which commenced 
lowing violently as the gloom deepened, and lay down before the totality had 



CHAP. III.] Total Eclipse of the Sun, July 28, 1851. 289 

commenced, went on feeding quietly enough very soon after the return of daylight. 
Cocks crowed at Elsinborg, though the Sun was only hidden there 30 seconds, and 
the birds sought their resting-places, as if night had come on V 

Mr. W. Lassell, who stationed himself near the Trollhatten 
Falls, thus describes the total obscuration : 

" I may attempt, but I cannot accomplish, an adequate description of the marvellous 
appearances, and their effect upon the mind, which were crowded into this small 
space of 3^ minutes, an interval which seemed to fly as if it were composed of 
seconds and not of minutes ! Perhaps a naked-eye observer would more fully grasp 
the awful effect of the sudden extinguishment of light, the most overpowering of 
these appearances, for, my eye being directed through the telescope, I must have 
been less, though sufficiently, struck with the unprecedented sensation of such 
instantaneous gloom. The amount of darkness may be appreciated from the fact 
that, on withdrawing my eye from the telescope, I could neither see the second-hands 
of my watch, nor the paper sufficiently to write the time down ; and was only able 
to do so by going to the candle, which T had by me burning on the table. Probably 
the suddenness of the gloom, not giving time for the expansion of the pupil of the 
eye, increased the sensation of apparent darkness ; as I was obliged to repair close to 
the candle for the requisite light. After registering the time, I looked out for a few 
minutes with the naked eye over the landscape, north and south. The north was 
clear, and the line of horizon could be distinctly seen. The Sun, covered by the 
Moon, looking like a blue patch in the sky, had now the corona very symmetrically 
formed around it ; but the Moon appeared to my unassisted eye to be not very round 
or smooth at its edge, more as if one had rudely cut out with a knife on a board a 
circular disc of card, the edges somewhat jagged and irregular in outline. 

" The corona itself was perfectly concentric and radiating, some of the rays 
appearing in some parts of the circumference a little longer than in others ; but the 
inequality was not great. I am unable to say whether the corona when first found 
was at all eccentric, for, as it is evident that any one observing with a telescope up to 
the moment of obscuration must have time to take off the dark glass before the 
corona can be seen, and as I had also to note the time, the centres of the Sun and 
Moon must have been pretty closely approximating before I again applied the eye to 
the telescope. It was indeed a great exercise of self-denial to spare the time from 
the exciting phenomena, which was necessary for accurately recording the duration 
of total darkness ; but being inclined to think such record would be disregarded by 
many observers, I took my resolution to secure it." 

The writer then proceeds to say that Venus was the only object 
visible to the naked eye. The corona he describes as " brilliant," 
and he considers that it afforded, speaking roughly, as much 
light as the Moon usually does when at its full. 

" I had intended to direct my attention pointedly to the detection of the ' Red 
Flames,' which I had heard described as but faint phenomena. My surprise and 
astonishment may therefore be well imagined when the view presented itself 

b Sol. Syst., p. 71. 
U 



290 Eclipses and Associated Phenomena. [BOOK II. 

instantly to my eye as I am about to describe, or rather to attempt to give a 
notion of. 

" In the middle of the field was the body of the Moon, rendered visible enough by 
the light of the corona around, attended by the apparent projections from behind the 
Moon of which I have attempted to sketch the positions. The effect upon my own 
mind of the awful grandeur of the spectacle I feel I cannot fully communicate. The 
prominences were of the most brilliant lake colour, a splendid pink, quite defined 
and hard. They appeared to me to be not quiescent ; but the Moon passing over 
them, and therefore exhibiting them in different phase, might convey an idea of 
motion. They are evidently to my senses belonging to the Sun and not at all to the 
Moon ; for, especially on the western side of the Sun, I observed that the Moon 
passed over them, revealing successive portions of them as it advanced. In conformity 
with this observation also, I observed only the summit of one, on the eastern side, 
though my friends observing in adjoining rooms had seen at least two : the time 
occupied by my noticing the time and observing with the naked eye not having 
allowed me to repair again to the telescope until the Moon had covered one, and 
three-fourths of the other. The point of the Sun's limit where the principal ' flame ' 
appeared was (I judged) a few degrees south of the place where the cluster of spots 
was situated, and the flame which I observed on the eastern limb was almost exactly 
where the eastern spot was situated. As, however, some prominences appeared 
adjacent to parts of the Sun's limit not usually traversed by spots, the attempt to 
trace a connexion fails. The first burst of light from the emergent Sun was exactly 
in the place of the chief western flame, which it instantly extinguished. . . . . . 

From the varying lengths of the red flames it is difficult to give an accurate estima- 
tion of their magnitude ; but the extreme length of the largest, on the western 
limb, may have been about 2\'. This estimation is rather rude, as I was so 
absorbed in contemplating their general phenomena that I had not time for exact 
measurement c ." 

c Mem. R.A.S., vol. xxi. p. 47. 1853. 



CHAP. IV.] Annular Eclipse of the Sun, March 1858. 291 



CHAPTER IV. 



THE ANNULAR ECLIPSE OF THE SUN 
OF MARCH 14-15, 1858. 

Summary of observations in England. 

OF the different eclipses which have from time to time been 
visible in England, few have attracted such interest and at- 
tention among all classes of society as that of March 14-15, 1858. 
Though bad weather in most 
cases interrupted or altogether 
prevented observations, yet many 
instructive features were noticed. 
The line of central and annular 
eclipse passed across England 
from Lyme Regis, in Dorsetshire, 
to the Wash, between Lincoln- 
shire and Norfolk, traversing 
portions of Somersetshire, Wilt- 
shire, Berkshire, Oxfordshire, 
and Northamptonshire. The 
following summary of the obser- 




ECLIPSE OP THE SUN, March 14-15, 

1858 ; THE ANNULUS. 



vations made, drawn up by Mr. Glaisher, will be read with 
interest : 

" From returns received between Braemar and the Channel Islands, from 30 to 40 
in number, it is shewn that the depression of temperature during the eclipse was 
about 2-5 at stations north of the line, and nearly 3 at stations on and south of 
the line of central eclipse ; that at places where the usual diurnal increase had taken 
place in the morning the depression of temperature during the eclipse was greater : 
and that at places where such increase had not taken place it was less than the 

U 2 



292 Eclipses and Associated Phenomena. [BOOK II. 

above numbers. Also that at places where the sky was uniformly cloudy during 
the day the decrease in the readings of a black bulb thermometer was less than 1 2, 
while at places where the sky was partially clear the depression was from 17 to 
19, and that, what temperature soever the black bulb thermometer indicated in 
the morning, it fell during the eclipse to that of the temperature of the air at all 
places. 

" The humidity of the air was such that at places north of the line the wet bulb 
thermometer read 2 '6 less ; and on and near the line the depression was 3' 2, and 
south of it was 3*7 below the adjacent dry bulb thermometer. 

" At some places the humidity of the air increased at the time of the greatest 
eclipse, but this was far from being universal. 

" The sky was partially clear at some places on the east and south coasts, in the 
Channel Islands and north of Scotland, and it was for the most part overcast else- 
where. Near the southern extremity of the central line the sky was partially clear, 
and at its northern extremity near Peterborough the clouds were broken ; at most 
intermediate places the sky was wholly overcast. The complete ring was seen at 
Charmouth, and neighbourhood near Lyme Regis, and at Peterborough, but, so far as 
I can learn, at no other places. My own station was on the calculated line of central 
eclipse, near Oundle, in Northamptonshire, and here I saw the Moon and Sun's 
apparent upper limb coincident, or very nearly so, and therefore that I was situated 
on or very near the northern limit of annularity, but distant from the centre line by 
3 or 4 miles. 

" It is very much to be regretted that the unfavourable weather precluded the 
witnessing the very beautiful attendant phenomena upon large solar eclipses. The 
time of year was unfavourable to all optical effects whether of light and shade or 
colour, independently of the particular character of the day, which was more fatal 
still to their exhibition, for even where the Sun was visible their presence was only 
feebly indicated at a few parts of the country. 

" At Oundle the weather for some time previous to the commencement of the 
eclipse was raw and ungenial for the time of year. The wind was gusty and the sky 
overcast, chiefly with cirro-stratus, and dark scud hurrying past the Sun's place from 
the north-west, the clouds occasionally giving way and allowing the Sun to be visible 
by glimpses. Shortly after i o'clock the sky became uniformly overcast, and a small 
steady rain set in for a considerable time. 

" It was long before any sensible diminution of light took place. At 1 2 h 39 a 
gloom was for the first time perceptible to the north, and the crescent of the Sun 
shone out with a bright white light between breaks. At o h 43 ra the gloom was 
general, excepting around the Sun, which appeared the centre of a circle of light, and 
illuminated with fine effect some bold irregular masses of cumulus in its vicinity. At 
o h 45 m the gloom increased, slight rain fell, and the wind rose, birds were heard 
chirping and calling. At o h 53 a severe storm might have been supposed impending, 
and numerous birds were flying homewards. The deepening of the gloom was gradual 
but very slow, and between i h and i h t m was at its greatest intensity ; but even at 
this time the obscurity was not sufficient to require that any employment should be 
suspended. Messrs. Adams and Symons, situated five feet from a shed in an 
adjoining brickfield, spoke of the gloom as very intense for a period of 10 seconds, 
and sufficient to render it difficult to take the readings of the thermometer. A body 
of rooks rose from the ground at this moment and flew homewards ; a flock of 
starlings rose together, and various small birds flew wildly about ; a hare was seen 



CHAP. IV.] Annular Eclipse of the Sun, March 1858. 293 

to run across a neighbouring field, as though it were daybreak ; straw rustled, and the 
silence was peculiar and intense. The darkness and lull was that of an approaching 
thunder-storm. Directly after the greatest intensity the gloom was sensibly and 
instantaneously diminished, and the day was speedily restored to its ordinary 
appearance. 

"After o h 50 the lark ceased to rise, and did not sing; at i h io m it rose again. 
The collected information tends to shew that birds and animals, but particularly 
the former, were affected in some degree in most places ; and that it is probable to 
suppose the gloom was referred by them to the approach of evening, and this not so 
much from the fact of the gloom as from the manner of its approach, without the 
accompanying signs of atmospheric disturbance which usher in a storm, and to which 
birds and animals are keenly sensitive. 

" All over the country rooks seem to have returned to their rookeries during the 
greatest obscuration ; starlings were seen in many places taking flight, whole flocks 
of them together. At Oxford Dr. Collingwood remarked that a thrush commenced 
its evening song. At Grantham pigeons returned to their cote. At Ventnor Dr. 
Martin notes the fact that a fish confined in an aquarium, and ordinarily visible at 
evening only, was in full activity about the time of the greatest gloom. In Greenwich 
Park the birds were hushed and flew low from bush to bush, and at nearly all places 
the song of many birds was suspended during the darkness. At Campden Hill it was 
observed that the crocus closed about the same time, and at Teignmouth that its 
colour changed to that of the pink hepatica. 

" The darkness was not sufficient at any place to prevent moderate-sized print 
being read at any convenient distance from the eye out of doors, but a difficulty was 
sometimes experienced in reading the instruments. At Grantham the darkness is 
described to have been about equal to the usual amount of light an hour before 
sunrise ; near Oxford as about equal to that just after sunset on a cloudy day. The 
general impression communicated was that of an approaching thunder-storm. The 
sudden clearing up of the gloom after the greatest phase was likened by more than 
one observer to the gradual, but somewhat rapid withdrawal of a curtain from the 
window of a darkened room. The darkness is described to have been generally 
attended by a sensation of chilliness and moisture in the air. At Oxford the clouds 
surrounding the Sun were beautifully tinted with red, which merged into purple as 
the obscuration increased. At Grantham as the eclipse progressed the light became 
of a decided grey cast, similar to that of early morning, but at the time of the 
greatest gloom it had a strong yellow tinge. At Teignmouth the diminution of light 
was very great ; the sombre tints of the clouds became much deepened, and the 
remaining light thrown over the landscape was lurid and unnatural. At Green- 
wich the appearance of the landscape changed from a dull white to a leaden, and then 
to a slate-coloured hue ; and as the darkness increased it had much the appearance 
of a November fog closing in on all sides. At Wakefield the tints of the clouds 
changed from the grey slate colour of clouds in a storm, and became of a purple hue. 
At Oundle, my own station, the clouds were one uniform leaden grey or slate-colour, 
and quite in accordance with the general character of the day, nor could I perceive 
that the clouds appeared lower, or, in fact, that there was any very noticeable de- 
parture from the gloom we constantly experience during dull winter weather. 
Throughout the eclipse it occurred to me that the illuminating power of the Sun was 
much more than might have been supposed commensurate with the unobscured 
portion of the disc. When casual breaks permitted it to be visible the illuminated 



294: Eclipses and Associated Phenomena- [BOOK II. 

crescent up to the time of the greatest phase emitted beams of considerable brilliancy, 
which marked out a luminous track in the gloom, and were clearly and well defined 
in extent and figure. As the eclipse proceeded a decided change was to be observed 
in the colour of the Sun itself, which became of a pure silvery brightness, like that 
of Venus after inferior conjunction with the Sun. The absence of all colour in the 
light was remarkable, and at the time when the annulus was nearly formed it 
appeared like a line of silver wire. The departure from the usual amount of light we 
are accustomed to receive on an ordinarily dull day during the greater part of the 
eclipse was so inconsiderable, that we might infer approximately the real amount of 
Sun our average daylight under a cloudy sky is equivalent to. 

"As a photometric test during the eclipse, strips of photographic paper were 
exposed for equal intervals of time every 5 minutes. The result was a scale of tints 
which exhibited clearly the diminishing intensity of the light up to the period of 
greatest obscuration, and the rapid increase beyond. The range of tints is low, owing 
to the cloudy state of the sky, but this does not interfere with the proportionate 
depths of tint ; the time of greatest darkness is distinctly shewn by the very feeble 
discoloration of the paper. The instruments used at Oundle were made specially 
for those observations, and were of a very delicate and accurate construction j the 
meteorological observations were made by Messrs. Adams and Symons. 

" In conclusion, I beg sincerely to thank those gentlemen whose returns have sup- 
plied the data for this investigation, of which we may say, literally as well as 
figuratively, that it exhibits only the faint outline of facts dimly visible through a 
screen of clouds. I think, however, it is reasonable to infer that the great paucity of 
effects and general phenomena witnessed even in places where the Sun was visible, is 
due to the conditions of the atmosphere, attributable alike to climate, time of year, 
and unfavourable weather, and should by no means lessen our confidence in previous 
accounts of the grandeur and beauty of the attendant phenomenon upon solar eclipses. 
Optical phenomena, we all know, are dependent upon the medium through which we 
view them for the nature and power of the effects produced." 

Defective as this record is, from a scientific point of view, 
owing to the unfavourable weather having so generally inter- 
fered with observations, yet it has some interest to Englishmen 
by reason of the fact that phenomena of this character are so 
rarely visible in England. 



CHAP. V.] Total Eclipse of the Sun, July 18, 1860. 295 



CHAPTER V. 

THE TOTAL ECLIPSE OF THE SUN 
OF JULY 18, 1860. 



Extracts from the observations of Sir G. B. Airy. Observations of the Red Flames 
by Bruhns Meteorological observations ly Lowe. 

THE total eclipse of July 18, 1860, presented some noticeable 
features : it owed its interest to the agreeable circumstances 
connected with it a , and its importance to the very extensive 
observations which were made by many astronomers in Europe, 
Africa, and America. 

Sir G. B. Airy stationed himself at the village of Pobes 
in the North of Spain. From his memoir b I make the following 
extracts : 

" On the progress of the eclipse I have nothing to remark, except that I thought 
the singular darkening of the landscape, whose character is peculiar to an eclipse, 
to be sadder than usual. The cause of this peculiar character I conceive to be the 
diminution of light in the higher strata of the air. When the Sun is heavily clouded, 
still the upper atmosphere is brilliantly illuminated, and the diffused light which 
comes from it is agreeable to the eye. But when the Sun is partially eclipsed, the 
illumination of the atmosphere for many miles round is also diminished, and the eye 
is oppressed by the absence of the light which usually conies from it. 

" I had a wax candle lighted in a lantern, as I have had at preceding total eclipses. 
Correcting the appreciations of my eye by reference to this, I found that the dark- 
ness of the approaching totality was much less striking than in the eclipses of 1842 
and 1851. In my anxiety to lose nothing at the telescope I did not see the approach 
of the dark shadow through the air ; but, from what I afterwards saw of its retreat, 
I am sure it must have been very awful." 

a It is to the Himalaya expedition to Spain that allusion is here made. 
b Month. Not., vol. xxi. p. 9. Nov. 1860. 



296 Eclipses and Associated Phenomena. [BOOK II. 
After describing the Red Flames he says : 

" I may take this opportunity of stating, that the colour of these appearances was 
not identical with that which I saw in 1842 and 1851. The quality of the colour 
was precisely the same (full blush-red, or nearly lake), but it was diluted with white, 
and more diluted at the roots of the prominences close to the Moon's limb than 
in the most elevated points. 

" About the middle of the totality I ceased for awhile my measures, in order to 
view the prospect with the naked eye. The general light appeared to me much 
greater than in the eclipses of 1842 and 1851 (one cloudy, the other hazy), perhaps 
10 times as great; I believe I could have read a chronometer at the distance of 
12 inches ; nevertheless, it was not easy to walk where the ground was in the least 
uneven, and much attention to the footing was necessary. The outlines of the moun- 
tains were clear, but all distances were totally lost ; they were in fact an undivided 
mass of black to within a small distance of the spectator. Above these, to the height 
perhaps of 6 or 8, and especially remarkable on the north side, was a brilliant 
yellow or orange sky, without any trace of the lovely blush which I saw in 1851. 
Higher still, the sky was moderately dark, but not so dark as in former eclipses. 
The corona gave a considerable body, but I did not remark either by eye-view or 
by telescope-view anything annular in its structure ; it appeared to me to resemble, 
with some irregularities (as I stated in 1851), the ornament round a compass card. 
But the thing which struck me most was the great brilliancy of Jupiter and Procyon 
so near the Sun. It was impossible that they could have been seen at all, except 
under the circumstance of total absence of illumination on that part of the atmosphere 
through which the light passed. I returned to my measures, but I was soon sur- 
prised by the appearance of the scarlet sierra, announcing the approach of the Sun's 
limb. It disappointed me, for I had reckoned on a much longer time. All our 
party who were aware of the predicted duration fully believed that it must have been 
very erroneous. How the time passed I cannot tell. The Sun at length appeared, 
extinguishing the sierra, but the prominence and cloud remained visible, and my last 
measures were taken after reappearance. The prominences, &c. were then rapidly 
fading, and I quitted the telescope, not without the feeling that I had not done all 
that I had intended or hoped to do." 

The Red Flames were seen, and described by many of the ob- 
servers ; the account given by M. Bruhns is the most complete c . 
He says: 

"Just before the totality, there was visible, on the western border of the Moon, 
only one protuberance and the corona ; but as the last rays of the Sun disappeared, 
more protuberances started out on the eastern side, and the corona shone forth with 
an intense white light, so lustrous in fact as to dim the protuberances. I remarked 
that I saw them better when a clear red glass was held before my eye. 

" During the totality I sketched 4 drawings, and also measured off the position- 
angles of the different protuberances, counting round the circle from the north point 
through the east, &c. 

"The figure marked [Fig. 141. PI. XIX] was drawn during the first minute of 
the totality. The first protuberance is the one already mentioned ; its position-angle 

* Ast. Nach., vol. liv. No. 1292. Jan. 22, 1861. 



Figs. 140-2. 



Plate XIX. 




(Feilitzsch.) 




THE TOTAL ECLIPSE OP THE SUN OP JULY 18, I860. 

TELESCOPIC VIEWS OF THE COBONA AKD BID FLAMES. 



CHAP. V.] Total Eclipse of the Sun, July 18, 1860. 297 

was 35, the length of its base i|' or 2', and its height about the same. The summit 
was somewhat curved, of an intense rose colour, but a little paler at the apex. 

"The second protuberance, situated at 60, was completely separated from the 
Moon, there being between them an interval of %'. For part of its extent it was 
parallel to the Moon's border, it then deviated from it, and ended in a point. Its 
length was i^' or a', its height about |', and of a rose-colour. 

"The third protuberance, having a position-angle of 75, resembled a mountain, 
and had a base of i', and a height of fully 3'. Extending onwards for 50 from this 
protuberance was a narrow fringe, first of a pale red, but a few seconds afterwards it 
came over a splendid rose colour, and of a height of about ', which soon narrowed 
as the Moon passed over it, until at length it was quite covered. 

"A fourth protuberance existed at 155; its base was not more than |', but the 
height was as much as 1 1'. It had a hooked form with the curve trending northwards, 
and likewise of a rose colour. 

" During no part of the totality were there any protuberances visible in the 
southern part of the Sun's disc. 

"In the second minute the above-described protuberances became gradually 
smaller; with the exception of the first, which retained its magnitude and figure 
almost unchanged. The above-described unattached protuberance [No. a] was 
reached by the Moon, and became gradually covered. By the end of the second 
minute the fringe was entirely covered, and at this juncture, on turning to examine 
the western border of the Moon, I perceived several protuberances, not previously 
visible. 

" The protuberance situated at 260, which I will call No. 5, had, at the beginning 
of the second [third?] minute, only a base of %', and about the same height, the 
colour being rose. 

" Between 270 and 300 extended a second streak about \' in height. 

"A sixth protuberance was visible at 310, having a base of 2', and a height of '. 

" Lastly, I found at 340 a seventh protuberance, having a base of i', and a height 
of I', and of a rose colour, like all the preceding. 

"On directing my attention to the first protuberance (the one at 35), I fancied it 
had grown considerably larger. The sharp edge, first seen, had disappeared, and for 
a height of 3' or 4' flaming rays could be discerned, the colour (at the base a bright 
rose) was, at the top, hardly perceptible, but seemed to fade off and become merged 
in the corona. 

" After I had observed these for about half a minute, without perceiving any 
alteration, I quitted the telescope to observe the corona and the sky for a short time 
with the naked eye. The black-looking Moon was surrounded by a crown of clear 
light of unequal breadth. Below [S.] it was considerably greater than above [N.]. 
I estimated that in the former case it was f, in the latter about |, and the general 
appearance of the thing gave me the idea that the Moon was eccentrically placed in 
the corona. 

" The general form of the corona appeared circular, but on the eastern side a long 
ray shot out to a distance of about i ; the breadth of its base was 3', but it tapered 
down to about i'. During the 10 seconds that my attention was directed to it, 
neither the direction nor the length of the ray altered ; its light was considerably 
feebler than that of the corona, which was of a glowing white, and seemed to 
coruscate or twinkle. 



298 Eclipses and Associated Phenomena. [BOOK II. 

" With the naked eye I easily saw Venus and Jupiter, the former being much 
brighter than the latter. Although I knew whereabouts Procyon, Castor, Pollux, 
Mercury, and Saturn were, yet in the few seconds available for seeking for them I 
failed to find them. 

" My assistant, M. Auerbach, who observed the corona, and searched for the stars 
during a longer period than I did, noticed in the south-western quadrant a curved ray 
about 3^ in length, which I in my hurry probably overlooked. He also saw Pollux, 
and another person saw Castor ; but, as far as I am aware, no more than the above 4 
objects were seen by any person in Tarragona. 

"Towards the end of the 3 rd minute of the totality, I again looked through the 
telescope, and made the drawing [Fig. 142, PI. XIX]. The western protuberances 
had altered considerably since the 2 nd minute ; the one at 35 had regained its original 
form and size, the flaming rays, previously spoken of, having disappeared. The pro- 
tuberance in 340 had become much larger, the length of its base being now about 2', 
and the height i '. The red streak extending from 270 to 300 had prolonged itself 
so as to take in the protuberance at 310 [No. 6], and had altogether now a length of 
50, its height having also become augmented from \' to i', and its colour being au 
intense rose. The protuberance at 260 [No. 5] was now separated by about \' 
from the Moon, its breadth being nearly i', and its height |'. Finally, at 240 a new 
and small protuberance had started into view, its base and height were both about j', 
and rose-coloured. 

"As the end of the totality advanced so the protuberances became less distinct, 
the colour became brighter, and immediately after the 3 rd minute of totality the pro- 
tuberances at 240 and 260 disappeared ; the fringe extending itself to a length of 
more than 90, its height being i|', and embraced all the protuberances up to an 
angle of 35. On the first appearance of the solar rays all suddenly vanished, with 
the exception of the first protuberance, which for some time afterwards remained 
visible in the thin red glass." 

Meteorology was not unrepresented in Spain, for Mr. E. J. 
Lowe, at Fuente del Mar, near Santander, with 2 assistants, 
during a period of 5 hours, made upwards of 4000 observations. 
The following is an abstract of Mr. Lowe's results, in his own 
words : 

"Commencing with underground temperature, a thermometer placed 6 inches below 
the surface of the ground ranged between 67-9 and 70-7, i.e. 2-8; at this depth 
the eclipse was not sensibly felt, whereas other thermometers, placed 4 inches, 
2 inches, I inch, and \ an inch below the surface, all exhibited in a very marked 
manner the effect of the eclipse. On the grass the temperature fell to 64 at 3 h 5 ; 
at | inch below the surface, to 69 at 3 h 15 ; at I inch deep, to 69-5 at 3 h 25 ; at 
2 inches, to 71 at 3 h 55 m ; at 4 inches, to 70-7 at 4 h 30 P.M. 

"The temperature on the grass was 77-5 at noon, rising to 91-7 at i h 50, and 
then falling till 3 h 5, and again rising to 85 at 4* io m , giving a range of 27-7. At 
half an inch below the surface of the ground the temperature rose till i h 55 P.M., 
when it was 78-5, and then gradually fell to 69, rising again to 74-7 at 4"' 30 P.M., 
the range being 9-5. At I inch below the surface the temperature rose till i h 55 to 
76-2, fell till 3 h 25 m to 69-5, and rose till 4 h 55 to 74-7, the range being 6-7. At 



Fig. 143. 



Plate XX. 







THE TOTAL ECLIPSE OF THE SUN OP JULY 18, I860. (Tempel.) 



CHAP. V.] Total Eclipse of the Sun, July 18, 1860. 301 

2 inches below the surface the temperature rose till 2 h 5 to 74-4, then fell till 
3 h 55 ra to 7 IlO an d afterwards rose till 4 h 55 to 73-7, the range being 3-4; and at 
4 inches below the surface the temperature rose till 2 h 50 to 73, then fell till 4 h 30 
to 7'7> au( i again rose till 6 h P.M. to 73-2, the range being 2-5. 

" The greatest cold on the ground occurred between 3 h and 3 h 5 P.M. ; ditto, | an 
inch below surface, 3 h io m and 3 h 15 P.M. ; ditto, i inch, 3 h 2o m and 3 h 25 P.M. ; 
ditto, 2 inches, 3 h 5o m and 3 h 55 P.M. ; ditto, 4 inches, 4 h 25 m and 4 h 3o ra P.M. 



TABLE OF TEMPERATUKES. 





Com- 
mence- 
ment of 
Eclipse. 


Middle 
of 
Eclipse. 


End 
of 
Eclipse. 


Range 
during 
Eclipse. 


Of a blackened ball on grass 




104-0 


o 
6.r5 


.0 

94-o 


o 
38-5 


Of a blackened ball in vacuo 


131-0 


66-0 


104-0 


65-0 


In sunshine at 2 feet above ground 


75-5 


63-6 


70-0 


n-9 


In sunshine 2 feet (wet bulb) 


69- > 


59-3 


65-5 


10-2 


Diff. between dry and wet bulb at 2 feet 
In shade at 4 feet 


6-0 
70-0 


4-4 
64-7 


4-5 
71-0 


1-6 
6-3 


In shade at 4 feet (wet bulb) 
In shade at 3 feet .... 


62-5 
70-2 


59-7 
64-2 


63-5 
70-7 


3-8 
6-5 


In shade at 2 feet 


68-5 


62-5 


68-5 


6-0 


In shade at i foot ... . 


70-7 


64- 1; 


70-2 


*>7 













"The barometer rose from i h 4O m till 2 h io ra 0-002 inch, then fell till 3 h 5 0-0017 
inch, and rose till end of eclipse, 0-009 i ncn - 

"Intensity of photographic light, from salted papers conveyed, sensitised, in 
Marion's dark box, exposed for 10 seconds (with a scale of from o to 5), at the 
commencement of the eclipse, 4^ becoming 4 at 2 h 5 m , 3 at 2 h 15, 2 at 2 h 25, 
i at 2 h 40, | at 2 h 5o m , i at 2 h 55 (clear about Sun), } at 3", i at 3 h 5, 
2 at 3 h 25, 2^ at 3 h 40, 3 at 3 h 50, and 4 at 4 h . During totality a paper exposed 
for i minute gave . 

" The wind was N.W. and N.N.W. till 4 h 2o m , then W.S.W., being S.W. at 4 h 25, 
and South at 4 h 45. The wind was brisk at the commencement of the eclipse, quite 
a calm during totality, and a gentle breeze afterwards. The distant prospect was 
very clear, except during totality, when the mountains disappeared, and only near 
objects were visible. 

"The clouds, which were chiefly cumuli, diminished in amount till i h 50, when 
only -$ of the sky was overcast, then increased till 2 h 35 with much cloud till 
3 h 55 m > then again diminished to T % at the termination of the eclipse, the range 
being \$ of the whole sky. Towards totality some of the cumuli became scud, which 
lasted from 2 h 5"" to 3 h io m , giving the strongest impression that the change was due 
to the eclipse. 

"The morning was fine, and from 12'' 45 ' r P.M. sunshine; at i h 25 much open sky 



302 Eclipses and Associated Phenomena. [BOOK II. 

about the zenith; at 2 h 15 a blackness about W. horizon, and slightly so in N. 
and S. ; at 2 h 30 the hills dark, and the blue sky in N. and E. very pale in colour; 
at 2 b 35 m > hills dark, with a blue haze among the more distant mountains ; at 
2 h 40, horizon due W. pink ; at 2 h 45, clear sky, in N. pink; at 2 h 52, splendid 
pink in W. horizon, warm purple in summits of mountains in S., clear sky, in N. 
deep lilac, and in E. very pale blue; at 2 h 57 ra , rapid change, the clear sky in N. 
deep marine blue with a red line. 

" Before totality commenced, the colours in the sky and in the hills were magnificent 
beyond all description ; the clear sky in N. assumed a deep indigo colour, while in 
the W. the horizon was pitch black (like night). In the E. the clear sky was very 
pale blue, with orange and red, like sunrise, and the hills in S. were very red ; on the 
shadow sweeping across, the deep blue in N. changed like magic to pale sunrise tints 
of orange and red, while the sunrise appearance in E. had changed to indigo. The 
colours increased in brilliancy near the horizon, overhead the sky was [of a] leaden 
[hue]. Some white houses at a little distance were brought nearer, and assumed a 
warm yellow tint ; the darkness was great ; thermometers could not be read. The 
countenances of men were of a livid pink. The Spaniards lay down, and their 
children screamed with fear ; fowls hastened to roost, ducks clustered together, 
pigeons dashed against the sides of the houses, flowers closed (Hibiscus Africanus as 
early as 2 h 5 m ); at 2 h 52 m cocks began to crow (ceasing at 2 h 57 m , and recommencing 
at 3 h 5). As darkness came on, many butterflies, which were seen about, flew as if. 
drunk, and at last disappeared ; the air became very humid, so much so that the grass 
felt to one of the observers as if recently rained upon. So many facts have been 
noted and recorded that it is impossible to do more than give a brief statement of the 
leading features." 

The general result of the observations of the eclipse of 1860 
was to shew conclusively that the Red Flames in solar eclipses 
belong not to the Moon but to the Sun. 

An interesting and valuable memoir on this eclipse was pre- 
sented to the Royal Society by Mr. Warren De La Rue d . 

d Phil. Trans., vol. clii., 1862. 



CHAP. VI.] Recent Total Eclipses of the Sun. 303 



CHAPTER VI. 

RECENT TOTAL ECLIPSES OF THE SUN. 



Eclipse of August 18, 1868. Observations by Col. Tennant and M. Janssen at 
Gunloor. Summary of results. Observations of Governor J. P. Hennessy and 
Capt. Reed, R.N. Eclipse of Avgmt 7, 1869. Observations in America by 
Prof. Morton and others. Summary of results. Eclipse of December 22, 1870. 
English expedition in H. M.. S. Urgent to Spain. Observations in Spain 
and Sicily. Eclipse of December II, 1871. Observed in India. Eclipse of 
April 1 6, 1874. Summary by Mr. W. H. Wesley of the recent observations as 
to the Physical Constitution of the Corona. 



eclipse of the Sun of July 18, 1860, described in the last 
chapter, may be said to mark a turning-point in the history 
of eclipse phenomena. It was the first in which photography 
played a conspicuous part, and the experience acquired by the 
numerous observers who went to Spain, paved the way for the 
great photographic and other successes which marked subsequent 
eclipse expeditions. 

The reader who has studied what has been stated in the earlier 
chapters of this Book, respecting the usual accompaniments of 
eclipses of the Sun, will already have acquired a sufficiently com- 
plete general insight into the subject, and therefore in the present 
chapter his attention will be mainly invited to new points. 

The eclipses which will be grouped together here are the fol- 
lowing a : Aug. 18, 1868; Aug. 7, 1869; Dec. 22, 1870; Dec. 

* A very good general summary of the analyse. The information relating to 

eclipse observations made in 1868, 1869, the 1870 eclipse is exclusively from 

and 1870 (accompanied by numerous English sources drawn upon by the 

illustrations) will be found in the Eng- translators. But the most exhaustive 

lish edition of Schellen's Die Spectral- account by far is that furnished in Mem. 



304 Eclipses and Associated Phenomena. [BOOK II. 

u, 1871; April 16, 1874; April 5, 1875; July 29, 1878; May 
17, 1882 ; May 6, 1883 ; Sept. 8, 1885 ; Aug. 29, 1886 ; Aug. 19, 
1887. 

To observe the eclipse of 1868, several expeditions were dis- 
patched from Europe to the East Indies. The most important 
of these was that which under the command of Major Tennant, 
RE., went to Guntoor (Lat. 60 17' 27" N., Long. 5 h 2i m 48 s 
E.) ; but important service was rendered to Science by a French 
observer, M. Janssen, who, accompanied by his wife, stationed 
himself at Guntoor. Another French party, under M. Stephan, 
went to Siam, and a German party to Aden. This last-named 
contingent included MM. Weiss, Oppolzer, and Thiele, all ex- 
perienced astronomers. 

Major Tennant' s arrangements were framed with the object of 
( i ) investigating by the aid of a spectroscope the corona and red 
names (the latter now very generally called the " Solar promin- 
ences"), as regards the source of their light ; (2) examining the 
light of the corona and prominences as regards the polarisation 
thereof, and (3) obtaining photographs during the totality. By 
a due subdivision of labour amongst the different members of 
the expedition this programme was carried to a successful con- 
clusion. Neglecting certain optical effects, common to every 
total eclipse of the Sun, and sufficiently described already in 
connection with previous eclipses, I proceed to note briefly, in 
something like Major Tennant's own words, his deductions as 
to the new results flowing from the labours of himself and his 
colleagues b . 

The corona is to be deemed an atmosphere of the Sun, not 
self-luminous but shining by reflected light. This was proved 
both by the spectroscope and the polariscope. 

During the continuance of the totality, there was seen on the 
North side of the Sun, an enormous horn of light, the apex of 
which was calculated to be about 90,000 miles distant from the 

K.A. S., vol. xli. 1876. This volume is to Mr. A. C. Ranyard's industry. 

a magnificent compilation of Eclipse b Memoirs R.A. S., vol. xxxvii. p. i. 

facts. For it science is mainly indebted 1869. 



CHAP. VI.] Recent Total Eclipses of the Sun. 



305 



Fig. 144 



Sun's limb. This object presented in a striking degree indica- 
tions of a spiral structure, and was presumed to consist of 
incandescent vapours of hydrogen, sodium, and magnesium. 

Capt. Brannll observed that the corona was strongly polarised 
everywhere in a plane passing through the Sun's centre. 

The general phenomena of the total phase are thus described 
by Mr. (now Sir J. P.) Hennessy c : 

"Ten minutes before the total eclipse there seemed to be a luminous crescent 
reflected upon the dark body of the Moon. In another minute a long beam of light, 
pale and quite straight, the rays diverging at a small angle, shot out from the 

Westerly corner of the Sun's crescent. 
At the same time Mr. Ellis noticed a 
corresponding dark band, or shadow, 
shooting down from the East corner of 
the crescent. At this time the sea 
assumed a darker aspect, and a well- 
defined green band was seen distinctly 
around the horizon. The temperature 
had fallen, and the wind had slightly 
freshened. The darkness then came on 
with great rapidity. The sensation was 
as if a thunderstorm was about to break, 
and one was startled on looking up to 
see not a single cloud overhead. The 
birds, after flying very low, disappeared 
altogether. The dragon-flies and butter- 
flies disappeared, and the large drone-like 
flies all collected on the ceiling of the 
tent, and remained at rest. The crickets 
and Cicadse in the jungle began to 
sound, and some birds, not visible, also began to twitter in the jungle. The sea 
grew darker, and immediately before the total obscuration the horizon could not 
be seen. The line of round white clouds that lay near the horizon changed their 
colour and aspect with great rapidity. As the obscuration took place, they all be- 
came of a dark purple, heavy looking, and with sharply defined edges ; they then 
presented the appearance of clouds close to the horizon after sunset. It seemed as if 
the Sun had set at the four points of the horizon. The sky was of a dark leaden blue, 
and the trees looked almost black. The faces of the observers looked dark, but not 
pallid or unnatural. The moment of maximum darkness seemed to be immediately 
before the total obscuration ; for a few seconds nothing could be seen except objects 
quite close to the observers. Suddenly there burst forth a luminous ring around the 
Moon. The ring was composed of a multitude of rays quite irregular in length and 
in direction ; from the upper and lower parts they extended in bands to a distance 




DIAGRAM REPRESENTING THE RAYS 
OF THE CORONA. 

Aug. 1 8, 1868. (Hennery.} 



c Proc. Roy. Soc., vol. xvii. p. 84. 1868. 



306 Eclipses and Associated Phenomena. [BOOK II. 

more than twice the diameter of the Sun. Other bands appeared to fall towards one 
side, but in this there was no regularity, for bands near them fell away apparently 
towards the other side. When I called attention to this, Lieut. Ray said, ' Yes, I see 
them ; they are like horses' tails ; ' and they certainly resembled masses of luminous 
hair in complete disorder. I have said these bands appeared to fall to one side; 
but I do not mean that they actually fell, or moved in any way, during the observa- 
tion. If the atmosphere had not been perfectly clear, it is possible that the appear- 
ance they presented would lead to the supposition that they moved ; but no optical 
delusion of the kind was possible under the circumstances. During the second when 
the Sun was disappearing, the edge of the luminous crescent became broken up into 
numerous points of light. The moment these were gone, the rays I have just men- 
tioned shot forth, and, at the same time, we noticed the sudden appearance of the 
rose-coloured protuberances. The first of these was about of the Sun's diameter 
in length, and about % of the Sun's diameter in breadth. It all appeared at the same 
instant, as if a veil had suddenly melted away from before it. It seemed to be a 
tower of rose-coloured clouds. The colour was most beautiful more beautiful than 
any rose-colour I ever saw ; indeed, I know of no natural object or colour to which 
it can be with justice compared. Though one has to describe it as rose-coloured, yet 
in truth it was very different from any colour or tint I ever saw before. This protu- 
berance extended from the right of the upper limb, and was visible for 6 minutes. 
In 5 seconds after this was visible, a much broader and shorter protuberance 
appeared at the left side of the upper limb. This seemed to be composed of two 
united together. In colour and aspect it exactly resembled the long one. This 
second protuberance gradually sank down as the Sun continued to fall behind the 
Moon, and in 3 minutes it had disappeared altogether. A few seconds after it 
had sunk down there appeared at the lower corresponding limb (the right interior 
corner) a similar protuberance which grew out as the eclipse proceeded. This also 
seemed to be a double protuberance, and in size and shape very much resembled the 
second one ; that is, its breadth very much exceeded its height. In colour, however, 
this differed from either of the former ones. Its left edge was a bright blue, like a 
brilliant sapphire with light thrown upon it. Next to that was the so-called rose- 
colour, and, at the right corner, a sparkling ruby tint. This beautiful protuberance 
advanced at the same rate that the Sun had moved all along, when suddenly it seemed 
to spread towards the left until it ran around J of the circle, making a long ridge of 
the rose-coloured masses. As this happened, the blue shade disappeared. In about 
12 seconds the whole of this ridge vanished, and gave place to a rough edge of 
brilliant white light, and in another second the Sun had burst forth again. In the 
meantime the long rose-coloured protuberance on the upper right limb had remained 
visible ; and though it seemed to be sinking into the Moon, it did not disappear 
altogether until the lower ridge had been formed, and had been visible for 2 
seconds. This long protuberance was quite visible to the naked eye, but its colour 
could not be detected except through the telescope. To the naked eye it simply 
appeared as a little tower of white light, standing on the dark edge of the Moon. 
The lower protuberance appeared to the naked eye to be a notch of light in the dark 
edge of the Moon not a protuberance, but an indentation. In shape the long pro- 
tuberance resembled a goat's horn. . . . Though the darkness was by no means so great 
as I had expected, I was unable to mark the protuberances in my note-book without 
the aid of a lantern, which the sailors lit when the eclipse became total. Those who 
were looking out for stars counted 9 visible to the naked eye ; one planet, Venus, 



CHAP. VI.] Recent Total Eclipses of the Sun. 307 

was very brilliant. . . . On board the Rifleman the fowls and pigeons went to roost, 
but the cattle showed no signs of uneasiness ; they were lying down at the time." 

Captain Reed, R.N., remarked as follows respecting the 
corona : 

" The corona I should not describe as a ring, except in so far as concerned that 
portion of it immediately surrounding the Moon's limb. From this edge it burst 
forth in sharp, irregular-shaped masses, of exceedingly bright light, decreasing in 
brightness as the distance from the Moon increased, and finally resolving into 
numberless bright rays, the visible extremes of which were distant from two or three 
diameters of the Moon. The general appearance of the corona, as seen through my 
glass, struck me forcibly as resembling in form a Brunswick star ; the bright light 
near the Moon resembling the prominent portions immediately surrounding the 
centre, and the rays the more remote portions. I have heard the appearance 
described as representing the glory one sees around the heads of saints in old 
Italian pictures, and to my mind the general appearance could hardly be better 
described." 

The total eclipse of August 7, 1869, was observed by several 
well-equipped parties in the United States. The American ob- 
servations were carried out with great skill, and regardless of 
labour or expense, and resulted in a very complete series of 
excellent photographs d . One of these taken at Ottumwa repre- 
sents the phenomenon of " Baily's Beads," and is, I believe, the 
only photographic record of this phenomenon extant. Professor 
Morton speaks of this as " simply the last glimpse of the Sun's 
edge cut by the peaks of the Lunar Mountains into irregular 
spots." The pictures taken during the partial phase all shew an 
increase of light on the Sun's surface, in contiguity with the 
Moon's limb, as was observed by De La Rue in 1860. Professor 
Morton was at first inclined to attribute this to the existence of 
a Lunar atmosphere ; but subsequent experiments have led him 
to regard the cause as entirely chemical, and not corresponding 
to any celestial phenomenon. An analogous appearance is 
frequently to be seen in terrestrial photographs, and it is now 
generally agreed that the effect is a mere photographic one. 
Professor Pickering at Mount Pleasant noticed that while "the 
sky was strongly polarised all round close up to the corona, that 

4 Eeport on Observations of the Total p. 4, Nov. 1869; p. 173, May 1870; 

EclipseofiheSun,Aug. 7, 1869. Edited Journal of the Franklin Institute, 3rd 

by Commodore B. F. Sands. 4to. Wash- Ser., vol. Iviii. pp. 200, 249, and 354, 

ington, 1869. Month. Not., vol. xxx. Sept.-Nov. 1869. 

X 2 



308 Eclipses and Associated Phenomena. [BOOK II. 

object itself was not a source of polarised light." This obser- 
vation is not in accord with the observations of other eclipses 
(especially 1842, 1851, 1860. and 1868), for it has always been 
found that the light of the corona was strongly polarised. Nor 
indeed do Pickering's observations in 1869 tally with his own 
conclusions arrived at in 1870 in Spain with superior instruments. 
His observations in 1869 were made on an unmagnified image 
of the corona, and his attention was chiefly directed to the polar- 
ized condition of the atmosphere. Prof. Pickering is of opinion 
that his more deliberate observations of the coronal polarization 
made in 1870 are to be preferred, and that the small apparent 
size of the corona and its dazzling brightness as seen with the 
instrument used in 1869 prevented his noticing the polarization 
colours in the coronal light. 

Much more important in every sense than either of the fore- 
going eclipses, wag the eclipse of December 22, 1870. Being 
visible at some very accessible places in Spain, Sicily, and North 
Africa, several expeditions were dispatched to observe it, and 
eventually Her Britannic Majesty's Government placed at the 
disposal of English astronomers, .^2000 and a ship, the Urgent, 
for the conveyance of observers going to Spain and Africa ; and 
the expenses of the party which travelled overland to Sicily 
were defrayed out of this grant. Besides the observing parties 
connected with the expeditions just named, a strong detachment 
of American astronomers, nearly all of them Professors, came 
to Europe. France was only represented by M. Janssen, for the 
eclipse occurring towards the end of the Franco-German War, 
the French had other things to think about. It deserves notice 
that so great was M. Janssen's anxiety to observe the pheno- 
menon, that he determined upon trying to escape from Paris in 
a balloon, and succeeded, carrying with him his instruments. 

Unfortunately the weather was very unsatisfactory, especially 
in the North of Africa, where a cloudless sky had been confi- 
dently anticipated, and accordingly the successful photographs 
of Lord Lindsay's party at Cadiz and of the English party at 
Syracuse, constitute the chief direct results of the efforts made. 



CHAP. VI.] Recent Total Eclipses of the Sun. 309 

The partial failure of the weather is the more to be regretted 
because the preparations made to observe the eclipse were un- 
usually elaborate and costly, and the services of a particularly 
strong body of experienced observers had been secured. The 
general results, though less than had been expected, were un- 
doubtedly of great importance, and constituted a clear advance 
in our knowledge of Solar physics. 

Though attention was paid to other accompaniments of total 
eclipses of the Sun, and useful confirmatory evidence as to other 
matters was accumulated, yet the Sun's corona was in 1870 the 
one main object of attack, and photography, polariscopes, spec- 
troscopes, and ordinary telescopes were all brought to bear on 
the elucidation of the question " What is the corona ? " and im- 
portant information available for answering the question was 
obtained. 

The next eclipse that was widely observed was that of Decem- 
ber 12, 1871, which was visible over a large and accessible tract 
of country in Southern India, Ceylon, and Australia, though in 
the last-named part of the world the weather failed. The 
observations made were as before photographic, spectroscopic, 
and polariscopic. 

It was very generally noticed that the structure of the corona 
was radiated, and several rifts were seen therein. A comparison 
of photographs at different stations, indicates a fixity in these 
rifts which renders it certain that they existed at an immense 
distance from the observers ; in other words, that they were 
neither terrestrial, nor lunar, but solar. 

Fine photographs of the corona in which the definition is very 
sharp were taken at Baikulby Mr. Davis, Lord Lindsay's photo- 
graphic assistant, and six photographs on the same scale were 
taken by Col. Tennant at Dodabetta; and although the dark 
moon is represented by a circle only ^ of an inch in diameter 
and the whole extent of the corona could be covered by a six- 
pence, the definition is so good that on examination under suit- 
able illumination some hundreds of details can be made out and 
measured, and the two series of photographs are found completely 



310 Eclipses and Associated Phenomena. [BOOK II. 

to confirm one another as far as the smallest detail observable. 
In addition to the corona photographs taken at Baikul and 
Dodabetta in Central India, two photographs of the corona were 
secured during this eclipse with an ordinary photographic camera 
at a station near Tjebatjap in Java ; and though these are on a 
very small scale and the definition does not compare with the 
Indian photographs, the rifts and some of the larger structures 
visible in the Indian photographs can be recognized upon them, 
and as far as they go they show that the corona visible in India 
was also visible in Java. 

The line joining the two most marked rifts which are situated 
near to the Sun's poles divides the corona into two halves which 
are roughly symmetrical. The line of symmetry does not ac- 
curately coincide with the Sun's axis, but is inclined to it 
some 10 or 15. On each side of these polar rifts are groups 
of incurving structure which occupy an arc of some 40 on 
the moon's circumference. The curved rays in these groups 
are all bent inwards, and the straighter rays appear to be 
inclined from the radial towards parallelism with the axes of 
the groups. 

Within the polar rifts are several narrow straight or but 
slightly curved rays, none of which are quite radial to the Sun's 
limb. It is worthy of remark that this inclination to the radial 
cannot be a mere effect of perspective. For a line passing 
through the Sun's centre could not be projected so as not to be 
radial to the Sun's limb. There is abundant evidence that many 
of the structures visible in other coronas, as well as that observed 
during the eclipse of 1871, were inclined at considerable angles 
to the normal to the surface of the photosphere. It is difficult 
to conceive how explosions within a gaseous body like the sun 
can give rise to oblique rays, but the evidence for the existence of 
such rays is overpowering. Some of the oblique rays are straight, 
or nearly straight, while others shew considerable curvature, 
and others bend over in one direction in their lower parts, and 
are again curved slightly in a contrary direction above. Such 
double curvature, or contrary flexure, is also to be found in some 



CHAP. VI.] Recent Total Eclipses of the Sun. 311 

of the tree-like forms of structure which on a gigantic scale 
remind the observer of a common type of prominence to be 
seen in the chromosphere. 

The existence of these curving forms is a matter of considerable 
importance, as they appear to indicate the existence of an atmo- 
sphere with currents carrying the matter of which the struc- 
tures are composed, with different velocities at different altitudes. 
The tree-like structures also seem to indicate the spreading out 
within a resisting medium of matter rising from below. None 
of these tree-like structures are to be found in the upper part of 
the corona, though there are several forked and curving rays 
whose form it seems difficult to account for by the action of ex- 
plosive forces and gravity alone. As we proceed towards the 
outer parts of the corona there are more straight rays, and fewer 
contorted structures, indicating that the resisting atmosphere in 
the upper part of the corona is less dense than in the lower. 
The forms of the structures do not seem to afford evidence of a 
repulsive force similar to that which drives the matter of a 
comet's tail away from the Sun, but there are some of them 
in which the bright coronal matter, after having been driven 
upwards in an oblique direction, seems to fall again as if by 
gravity towards the Sun. In most instances however the rays 
which extend to the outer part of the corona grow gradually 
fainter in their upper parts without exhibiting any change of 
direction. 

Mr. W. H. Wesley, the Assistant Secretary of the Royal As- 
tronomical Society, who has given great attention to the numerous 
drawings and photographs of the corona which have been ob- 
tained, says e : 

" One of the most striking features in the corona of almost all the years under 
examination is the existence of a more or less well-marked polar rift, roughly, but 
perhaps never exactly, corresponding with the Sun's axis of rotation, to which it 
appears sometimes inclined as much as 30. In most cases this rift is shewn at both 
poles, but sometimes at one only; in 1882 it does not appear at all. The northern 
and southern rifts are seldom strictly opposite to one another, so that a line drawn 
through them does not pass through the centre of the Sun. The polar rifts are 

e Month. Not., vol. xlvii. p. 500. June 1887. 



312 Eclipses and Associated Phenomena. [BOOK II. 

generally filled with shorter, straighter, and more radial rays, with a background of 
less density than in other parts of the corona. 

" On either side of the polar rift there usually appears a somewhat conical mass, 
composed of rays curving towards each other, forming groups of what Mr. Ranyard 
has appropriately called ' synclinal structure,' which give the quadrilateral or 
cruciform appearance frequently shewn in corona drawings. They mostly seem to be 
situated over the zones of maximum sun-spot activity, and have frequently greater 
extension than other parts of the corona." 



ECLIPSE OP 1851, JOLY 28. 

" Dr. Busch's daguerreotype is remarkable as the first instance of a successful 
photograph of the corona. It shews the general form to a height nowhere much 
exceeding | of a solar diameter. The corona is symmetrical and of hexagonal form, 

Fig- 145- 




OUTLINE OF THE CORONA. 

with a well-marked rift not far from the north and south poles, the southern rift 
being much the broader. On either side of these rifts are indications of synclinal 
masses ; there are also similar masses in the equatorial regions fairly corresponding 
on each side. The orientation of the plate is rather uncertain. Wolf gives 64-2 as 
the relative number of sun-spots for July 1851." 

ECLIPSE OF 1860, JULY 18. 

" In the photographs taken at Desierto de las Palmas, of which I have only seen 
positive copies, there is shewn a very broad rift towards the south pole, and a less 
marked one on the north. The character of the synclinal groups is not clearly 
marked. The corona is fairly symmetrical about a line not much inclined from the 
Sun's axis. Wolfs relative number of sun-spots is 94-9." 

ECLIPSE OF 1869, AUGUST 7. 

" I have not seen the original negatives of the photographs taken at Shelbyville, 
which are the only ones which shew any considerable extent of corona. The 
northern and southern polar rifts are clearly marked and very broad. The bases of 



CHAP. VI.] Recent Total Eclipses of the Sun. 



313 



the four synclinal groups can also be clearly made out, especially that in the north- 
west quadrant. The general axis of symmetry is slightly inclined to the north-west 
and south-east of the Sun's axis. Wolfs relative number of sun-spots is 77-6." 



ECLIPSE OF 1870, DEC. 22. 

" Mr. Brothers's negative, taken at Syracuse, shews a great extent of corona, 
reaching in some parts quite 40' from the limb. The general outline is somewhat 
circular, with a quadrilateral area 

of greater brightness, brighter on Fig. 146. 

the western side. The northern 
polar rift is broad and ill-defined ; 
to the east of the south pole is a 
much narrower and more sharply 
defined rift, easily traceable to the 
limb. To the east and west of this 
are other rifts, and there is struc- 
ture evidently synclinal to the 
north-west; otherwise the photo- 
graph shews but little detail. The 
general axis of symmetry appears 
inclined to the north-west and 
south-east of the Sun's axis as 
much as 20, but the orientation 
is not very certain. The eclipse 
occurred at a period of great solar 
activity, Wolf's relative monthly 




OUTLINE OF THE COBONA, 1870. 



number being 135-4." 



ECLIPSE OF 1871, DEC. 12. 

"Lord Lindsay's and Col. Tennant's excellent series of negatives shew a corona 
remarkably symmetrical, about a line inclined about 10 to the north-west and south- 
east of the Sun's axis. The northern and southern polar rifts are well defined, 
nearly opposite to one another, and very similar in character. The four synclinal 
groups are well marked, appearing to indicate zones of synclinal structure extending 
nearly from the pole to about 40 north and south latitude. These groups are 
generally separated from the equatorial portions by narrow definite rifts. The 
western margin of the south-east synclinal group shews a distinct tendency to double 
curvature a form which reappears in 1883 and 1885. The extension is greatest in 
the equatorial regions, giving a somewhat hexagonal form to the corona. The great 
polar rifts are filled with short straight rays. 

"The greatest extent of the photographic corona does not exceed 27'} but the 
minuteness of the detail near the limb, which with a strong transmitted light can be 
seen through the densest part of film, has never been equalled in any subsequent 
eclipse photograph. The remarkable feature of the lower structure is the prevalence 
of rays completely curving over, and of branching rays, somewhat resembling a 



314 Eclipses and Associated Phenomena. [BOOK II. 

frequent form of solar prominence. Few of these reach a height of more than 5' 
from the limb; above this height the rays are generally straight or more slightly 
curved. 

" It is impossible to be certain whether these lower details are really near the limb, 
or whether they are rays on the nearer or further parts of the corona, seen fore- 
shortened. In the latter case, they could hardly be the extreme ends of coronal rays, 

Fig. 147. 




OUTLINE OF THE CORONA, 1871. 

as these invariably fade away so much towards their extremities that they would 
certainly be lost on the dense background. On the whole, the difference of character 
between the higher and the lower details lends great probability to the view that the 
latter are really near the limb. Mr. Ranyard considers that the more contorted 
character of these lower structures indicates the existence of a resisting atmosphere 
in the lower part of the corona. It seems evident, at least, that many of the 
curvatures of the coronal rays could not be caused by gravity alone. Still when we 
consider what an intricate mass of crossing and interlacing rays must be produced by 
perspective as we approach the limb, we must feel that the question cannot be 
decided with certainty. 

" The eclipse occurred at a time of somewhat less solar activity than that of the 
previous year, Wolf's relative monthly number being 98 "O." 

No photographs having been taken of the eclipse of 1874, no 
annotations on the corona of that eclipse appear in Mr. Wesley's 
paper. I have however thought it would be well to annex a 
hand-drawing thereof. 



CHAP. VI.] Recent Total Eclipses of the Sun. 315 

Fig. 148. 




THE TOTAL ECLIPSE OF THE SUN OF APRIL l6, 1874. 

Naked-eye view of the outer Corona. (H. 2?. P. Bright.) 

Mr. Wesley then proceeds to deal with the eclipses subsequent 
to 1874: 

ECLIPSE OF 1875, APRIL 6. 

" The small size of the photographs taken by Dr. Schuster renders it impossible to 
make out more than the general character of the corona, and from the same cause the 
orientation is not very accurately determined. The corona is somewhat symmetrical 
about a line nearly coinciding with the Sun's axis, the northern and southern polar 
rifts being very broad and well marked. Four synclinal groups are plainly seen, 
their axes making angles of more than 45 with the Sun's axis. The polar rifts are 
filled up, but not to a great height, the polar extension of the corona being only about 



316 Eclipses and Associated Phenomena. [BOOK II. 



Fig. 149. 



half the equatorial, where the greatest height is nearly a solar diameter. The half of 
the corona lying to the east of the axis is decidedly larger than that to the west, so 
that the nearly straight lines which bound the corona north and south converge towards 

the west. Dr. Schuster draws attention to the 
remarkable similarity between this corona and 
that of 1874, of which no photographs were 
taken. He thinks this similarity extends to 
the irregularity in the symmetry just men- 
tioned ; but the want of accordance between 
the drawings made in 1874 renders this un- 
certain. 

" Notwithstanding this general resemblance, 
the solar activity, as indicated by the sun- 
spots, was less than half as great as in the 
previous year, Wolf's relative number for 
April 1874 being 49*1, and for April 1875 

OUTLINE OF THE CORONA, 1875. 2O'S." 




ECLIPSE OF 1878, JULY 29. 

"The photographs which I have examined are two negatives by Mr. Ranyard, made 
at Denver, and a series of 9 positive copies on glass of the photographs taken by 



Fig. 150. 




OUTLINE OF THE CORONA, 



Professor Harkness and Mr. Rogers at Creston and 
La Junta. The exposures of Mr. Ranyard's plates 
were so short that they show but a small extent of 
corona. A drawing combining the detail o^ the 
Creston and La Junta negatives, and shewing a 
further extension of the equatorial rays, from a 
smaller photograph by Mr. Peers, is given in the 
Appendix to the Washington Observations for 
1876. On comparing this drawing with the positives, 
it does not seem very satisfactory. I can make out 
as much or more detail on the positives as on the 
drawing (except the equatorial extension), and no 
doubt much more would be seen on the original 
1 ^7^- negatives. 

"The corona belongs to the same type as those of 1874 and 1875. The equatorial 
extension greatly exceeds the polar, and both the northern and southern rifts are 
widely opened, so that their eastern and western boundaries form nearly straight 
lines tangential to the limb. The northern and southern synclinal groups are so 
much depressed towards the equator that they appear to coalesce into one great mass, 
occupying the whole equatorial region. The rifts are filled with fine rays, straight, 
and nearly radial in the centre of the rift, and becoming more and more curved 
towards its boundaries. In one rift there are as many as 20 separate rays, re- 
markably uniform in length and distance apart, never branching or crossing. The 
two rifts are almost identical in character, but are not opposite each other; the 
northern rift having its general axis inclined about 15 towards the east from the 
Sun's axis, and the southern being more symmetrical with it. 

"The great equatorial extensions, of which the bases only are visible in the 



CHAP. VI.] Recent Total Eclipses of the Sun, 



317 



positives, are very symmetrical in detail, but the western mass is the broader, 
reaching further both to the north and south. These great masses are broadest near 
the limb, and gradually become narrower, so that their northern and southern 
boundaries would meet in a point about 2 diameters from the limb on the western 
side, and rather less on the eastern. These equatorial extensions were, however, 
observed by Newcomb, Langley, and others, to reach to a distance of at least 1 2 
diameters. They must have been very faint, as in the American drawing, combined 
from various negatives, they do not extend more than a diameter. 

" It is a remarkable peculiarity, which I have observed in no other corona, that while 
at the poles it is split up into a great number of fine rays, the equatorial extensions 
are broad smooth masses, shewing scarcely any detail, even at their extreme edges. 

" The eclipse occurred at a time of decidedly low solar activity, Wolf's relative 
number being only 3*3." 

ECLIPSE OF 1882, MAY 17. 

" The negatives taken by Dr. Schuster shew a large extent of corona, reaching in 
several places a height of a solar diameter, one straight ray in the south-west ex- 
tending as far as ij diameter. The 
corona presents none of the features 
which characterised those of 1874, 1875, 
and 1878. Although very irregular in 
detail, it is approximately circular in 
form, and is entirely without that great 
difference between the polar and equa- 
torial extensions which had been so 
striking in the three last eclipses. At 
the same time it shews none of that 
symmetry about a line not very far 
from the Sun's axis that had been more 
or less apparent in most previously 
photographed coronas, and especially in 
that of 1871. This absence of an axis 
of symmetry and of polar rifts is its 
most striking feature. There are groups 
of synclinal structure, but they are not 
of a very definite character, and are 

quite irregularly placed. The solar axis does not pass through the line of least 
extension, as is almost always the case. The only approach to an axis of symmetry 
seems to be about a line nearly at right angles with the Sun's axis. The orientation 
was, however, very carefully made, and in Dr. Schuster's opinion is not more than 
half a degree in error : it nearly agrees with that adopted by Professor Tacchini. 

"The rays are rather more frequently straight than curved, and there is only one 
instance of a ray completely curving over : this is in the south-east ; it reaches a 
height of about 1 2' from the limb. Beneath it are two rays the only ones shewing 
any traces of a branching structure. There are distinct rifts on the western side, 
reaching to the limb ; but they are more filled up with coronal matter than those of 
1871. The rays are in all directions, from radial to tangential, and there are several 
cases of rays crossing each other, but no clear case of a ray of double curvature. 
The lower details of the corona are less distinct than in 1871 ; but this may be due 




OUTLINE OF THE CORONA, 1882. 



318 Eclipses and Associated Phenomena. [BOOK II. 

to the great density of the film near the limb, which is common to all dry-plate 
negatives. The definition of the outer portions is extremely fine. I cannot see any 
evidence of the distinction between an outer and inner corona, which Dr. Schuster 
thinks the photographs shew. Wolf's relative monthly number of sun-spots is 64-5 ; 
a remarkable outburst had occurred during the preceding month, for which the 
number was 95'8." 



ECLIPSE OF 1883, MAY 6. 



Fig. 152. 



" Successful photographs were taken by M. Janssen, and also by Messrs. Lawrance 
and Woods. The most prominent feature is an unusually well-marked rift, partly 

filled with short straight rays, 
near the north pole of the 
Sun's axis, from which the 
general axis of the rift is in- 
clined at an angle of about 
30 to the east. On each side 
of this rift are most charac- 
teristic groups of synclinal 
structure, whose bases meet 
at the limb : the easternmost 
shews a double curvature on 
both sides, but on the western 
edge this appearance seems 
caused by the superposition 
of different rays. There seems 
no regularity in the arrange- 
ment of the rays in the rest 
of the corona, nor any rift 
in the south, corresponding 
to that in the north. The 
general outline of the corona 
is somewhat circular, but the two synclinal groups extend farther than any other 
part. In M. Janssen's long-exposed plate, one of these groups extends nearly as 
far as two solar diameters, which is the greatest extension shewn by any corona 
photograph. Indeed, M. Janssen says that it is much greater than it appeared to 
the eye in his telescope. 

" The solar activity was rapidly decreasing, Wolf's relative monthly number of 
sun-spots being 32-1." 




OUTLINE OF THE CORONA, 1883. 



ECLIPSE OF 1885, SEPTEMBER 8. 

" Several photographs were taken of this eclipse, but the weather was generally 
unfavourable, and few shew much detail. The most marked feature is the southern 
rift, which is broad and well marked, with clear indications of straight rays filling it. 
The only distinctly synclinal group is to the south-east ; its axis makes an angle of 
about 45 with the Sun's axis, and its extension is greater than any other part of the 
corona. The western edge of this group presents a double curvature. The other 
parts of the corona are very irregular, and there does not appear to be any distinct 




CHAP. VI.] Recent Total Eclipses of the Sun. 319 

rift on the north corresponding with the southern rift. There is a marked broad 
depression in the corona, about 35 to the east of the north point of the axis. This 
depression, and the southern rift, appear to divide the corona into two very unequal 
parts, the western one being much the greater. 

"The solar activity, as shewn by the sun-spots, was diminishing ; Wolf's relative 
monthly number being 83-7 for the month of June, and 39-6 for September. 

" The only generalisation with regard to the form of the corona which has seemed 
well supported by the photographic evidence is 
that of Mr. Ranyard, that there is a connection 
between the general form of the corona and the 
solar activity as shewn by the number of sun-spots. 
The corona of a sun-spot maximum has generally 
been somewhat symmetrical, with synclinal groups 
making angles of 45 or less with its general axis. 
The sun-spot minimum coronas shew polar rifts 
much more widely open, synclinal zones making 
larger angles with the axis, and being therefore 
more depressed toward the equatorial regions, in 
which there is usually greater extension. This 

generalisation is well borne out by the maximum 

, ., . . OUTLINE OF THE CORONA, 1885. 

coronas of 1870 and 1871 and the minimum coronas 

of 1867, 1874, 1875, 1878, and apparently 1887. On the other hand, the eclipses of 
1883, 1885, and 1886, do not strikingly confirm the theory. The eclipse of 1883, at 
a time of rapidly decreasing solar activity, shews all the characters of a sun-spot 
maximum corona ; the same in a somewhat less degree may be said of 1885 and 1886, 
at both of which times the solar activity was decreasing. Although the polar rifts 
were wide in 1886, there was no very marked depression of the synclinal groups 
towards the equator, nor any great equatorial extension, although the relative 
number of sun-spots for August 1886 was only 19*0. Striking, therefore, as the 
evidence in favour of the generalisation has been in many years, it still seems probable 
that the form of the corona is modified by other causes at present unknown to us." 



ECLIPSE OF 1886, AUGUST 29. 

" Good photographs were taken at Grenada by Mr. Maunder, Dr. Schuster, and 
Prof. W. H. Pickering. The northern and southern rifts are fairly symmetrical about 
the Sun's axis, and are very wide. The synclinal groups bounding the rifts are well- 
marked, but very unsymmetrical, being depressed towards the equator on the eastern 
side, while the corresponding groups on the west are nearly radial. The south-west 
synclinal group is narrow and conical, extending to a greater height than any other 
part of the corona. On the eastern side the coronal extension is generally less than 
on the western, and the mass of equatorial rays on the east is of much less breadth, 
and is synclinal in character. The separation between the southern synclinal groups 
and the equatorial rays is unusually well-marked. Both polar rifts are filled with 
fine rays of the same character as the polar rays in 1878, but somewhat less regular. 

" One of Pickering's negatives shews very remarkable rays on the western side, 
extending to a height of 60' from the limb, and curving completely over. These are 
by far the highest rays of this character that have ever been photographed. On this 



320 Eclipses and Associated Phenomena. [BOOK II. 

Fig. 154. 




OUTLINE OF THE CORONA, 1 886. 

account they are of great interest if they are genuine coronal features, but Prof. 
Pickering can only detect them on one of his plates, and this was taken on a very 
small scale. Wolfs relative monthly number of sun-spots was 19-0." 

ECLIPSE OF 1887, AUGUST 19. 

"The extremely unfavourable weather which prevailed over Europe greatly interfered 
with the observations, and seems to have prevented successful photographs being 
taken at any of the Russian Stations. A hand-drawing of the corona, made in Siberia 

by Dr. Khandrikoff, is given on Plate XXI. 
Successful photographs, of which positive copies 
have been sent to England, were made by M. 
Sugiyama in Japan. Judgingfrom these copies, 
the corona somewhat resembles that of 1878, 
but the peculiar characters of that year are 
less strongly marked in 1887. The rifts are 
more widely open than in 1886, and the 
masses of rays bounding the rifts are more 
depressed towards the equator. The northern 
rift is filled with regular rays like the polar 
rays of 1878, but in the southern rift are 
broader, denser, and nearly radial masses, 
OUTLINE OF THE CORONA, 1887. gi vin g q uite a different character to this part 

of the corona. Synclinal groups, separated 

from the general mass of equatorial rays, bound the southern rift, but cannot be 
clearly made out in the north. Wolfs relative number of sun-spots for August 
was 21-1, but the mean number for the year was less than that for 1886." 




Fig. 156. 



Plate XXI. 




THE TOTAL ECLIPSE OP THE SUN OP AUQ. 19, 1887. 

(Khandrikoff.*) 



CHAP. VII ] Historical Notices. 321 



CHAPTEE VII. 



HISTORICAL NOTICES . 

Eclipses recorded in Ancient History. Eclipse of 584 B.C. Eclipse of 556 B.C. 
Eclipse 0/479 B.C. Eclipse of 430 B.C. Eclipse of 309 B.C. A llusions in old 
English Chronicles to Eclipses of the Sun. 

THE earliest eclipse on record is one given in the Chinese 
history named the Chou-king ; it has been supposed that a 
solar eclipse happened on Oct. 13, 2128 B.c. b , and that that is 
the one there alluded to. What happened in connection with it 
was this, though I cannot vouch for the details. Ho and Hi the 
Astronomers Royal of the period failed to give timely warning of 
the eclipse, but got drunk instead. The eclipse happened there- 
fore without the proper religious preparations having been made, 
and the land was exposed to the anger of the gods. To appease 
them the officials in question were forthwith executed. If this 
is fact and not romance, the record is a very interesting one, 
contemporaneous as it is with the Patriarchs of the Bible. 

One of the most celebrated eclipses of the Sun recorded in his- 
tory is that which occurred in the year 585 B.C. It is notable, 
not only on account of its having been predicted by Thales, who 
was the first ancient astronomer who gave the true explanation 
of the phenomena of eclipses, but because it seems to fix the 
precise date of an important event in ancient history. Herodotus 

See the Rev. S. J. Johnson's Eclipses of the Moon, Part I, "Observations on 

past and future. The fullest general the Moon before 1750," pp. 27-54 

account of all the early eclipses of im- (Washington, 1878). 
portance is that which will be found in b Mem. E. A.S., vol. xi. p. 47. 1840. 

S. Newcomb's Researches on the Motion 



322 Eclipses and Associated Phenomena. [BOOK II. 

describes a war that had been carried on for some years be- 
tween the Lydians and the Medes ; and gives an account of 
the following circumstances which led to its premature termina- 
tion : 

" As the balance had not inclined in favour of either nation, another engagement 
took place in the 6th year of the war, in the course of which, just as the battle was 
growing warm, day was suddenly turned into night (ffvvrjveiKe uffrt TJJJ no-xn* ow ( - 
aTfdjffrjs rrjv ^i^fprjv f(airivr)s VVKTO. ytvtaOai). This event had been foretold to the 
lonians by Thales of Miletus, who predicted for it the very year in which it actually 
took place. When the Lydians and Medes observed the change they ceased fighting, 
and were alike anxious to conclude peace." Peace was accordingly agreed upon and 
cemented by a twofold marriage. "For without some strong bond, there is little 
security to be found in men's covenants." 

So adds the historian . The exact date of this interesting 
event was long disputed, and the solar eclipses of 610, 593, and 
particularly 585 B.C., were each fixed upon as the one mentioned 
by Herodotus ; and it is only within the last few years that the 
point has been finally settled in favour of the last-mentioned 
eclipse, and that chiefly through the researches of Sir G. B. Airy, 
who gives, as the date of the eclipse in question, May 28, ,585 
B.C. d This is reconcileable with the statements of Cicero and 
Pliny. 

Another important ancient eclipse is that mentioned by Xeno- 
phon, in the Anabasis, as having led to the capture by the Persians 
of the Median city Larissa. In the retreat of the Greeks on the 
eastern side of the Tigris, not long after the seizure of their 
commanders, they crossed the river Zapetes, and also a ravine, 
and then came to the Tigris. At this place, according to Xeno- 
phon, there stood 

" A large deserted city called Larissa, formerly inhabited by the Medes ; its wall 
was 25 feet thick, and 100 feet high ; its circumference 2 parasangs ; it was built of 
burnt brick on an understructure of stone 20 feet in height. When the Persians 
obtained the empire from the Medes, the king of the Persians besieged the city, but 
was unable by any means to take it till a cloud having covered the Sun and caused 
it to disappear completely, the inhabitants withdrew in alarm, and thus the city 
was captured e ." 

c Herod., lib. i. cap. 74. 

d PAt7.2Vo.,vol.cxliii. pp. 191-197. 1853. Month.Not.,vol.xrii\.p. 143. Mar. 1858. 

6 Anal., lib. iii. cap. 4. 7. 



CHAP. VII.] Historical Notices. 323 

The historian then goes on to say that the Greeks in continuing 
their march, passed by another ruined city named Mespila. The 
minute description given by Xenophon enabled Layard, Felix 
Jones, and others, to identify Larissa with the modern Nimrud, 
and Mespila with Mosul. It has been thought that the phenomenon 
to which the Greek author refers as having led to the capture of the 
above-mentioned city, was no other than a total eclipse of the 
Sun, and Airy arrived at the conclusion that the eclipse referred to 
is that which occurred on May 19, 557 B.c. f 

In the same year as that in which, according to the common 
account, the battle of Salamis was fought (480 B.C.), there oc- 
curred a phenomenon which is thus adverted to : 

" At the first approach of spring the army quitted Sardis, and marched towards 
Abydos ; at the moment of its departure the Sun suddenly quitted its place in the 
heavens and disappeared (6 ij\io$ tKXnruv T^V kit rov ovpavov tSprjv d^av^s TJP), though 
there were no clouds in sight, and the sky was quite clear ; day was thus turned into 
night (avrl T)fj.(pr]s rt vii (ytvero) B " 

This account, interpreted as a record of a total solar eclipse, 
has given great trouble to chronologers, and it is still uncertain 
to what eclipse reference is made. If Hind's theory that the 
eclipse of Feb. 17, 478 B.C. is the one referred to, is sound, we 
must consider that the battle of Salamis is an event less remote 
by 2 years than has usually been supposed. Airy " thinks it ex- 
tremely probable " that the narrative relates to the total eclipse 
of the Moon, which happened 478 B.C., March 13* I5 h G.M.T. h 

A total eclipse of the Sun, supposed to have been that of 
August 3, 431 B.C., nearly prevented the Athenian expedition 
against the Lacedaemonians, but a happy thought occurring to 
Pericles, commander of the forces belonging to the former nation, 
the difficulty was got over. 

"The whole fleet was in readiness, and Pericles on board his own galley, when 
there happened an eclipse of the Sun. The sudden darkness was looked upon as an 

f Month. Not., vol. xvii. p. 234. June Pelopidas, 31. Diod. Sic., lib. xv. cap. 

1857. Newcomb doubts this being an 80. Grote, Hist, of Greece, vol. x. p. 424. 

eclipse at all. And see a letter by Lynn h Phil. Trans., vol. cxliii. p. 197. 1853. 

in Observatory, vol. vii. p. 380. Dec. See also Blakesley's Herod., in loco, and 

1884. some criticisms by Lynn in Observatory, 

^ Herod., lib. vii. cap. 37. Plutarch, vol. vii. p. 138, May 1884. 

Y 2 



324 EcMpses and Associated Phenomena. [BOOK II. 

unfavourable omen, and threw the sailors into the greatest consternation. Pericles 
observing that the pilot was much astonished and perplexed, took his cloak, and 
having covered his eyes with it, asked him if he found anything terrible in that, or 
considered it as a bad presage ? Upon his answering in the negative, he said, ' Where 
is the difference, then, between this and the other, except that something bigger than 
my cloak causes the eclipse ' ?' " 

Thucydides says : 

" In the same summer, at the beginning of a new lunar month (at which time 
alone the phenomenon seems possible), soon after noon the Sun suffered an eclipses ; 
it assumed a crescent form, and certain of the stars appeared : after a while the Sun 
resumed its ordinary aspect k ." 

An ancient eclipse, known as that of Agathocles, has also been 
investigated by Sir G. B. Airy, and previously by Baily. It 
took place on August 14, 310 B.C. This eclipse is, according to 
ancient writers, associated with an interesting historical event. 
Agathocles, having been closely blockaded in the harbour of 
Syracuse by a Carthaginian fleet, took advantage of a temporary 
relaxation in the blockade, occasioned by the absence of the 
enemy in quest of a relieving fleet, and quitting the harbour of 
Syracuse, he landed on the neighbouring coast of Africa, at a 
point near the modern Cape Bon, and devastated the Cartha- 
ginian territories. It is stated that the voyage to the African 
coast occupied 6 days, and that an eclipse (which from the 
description was manifestly total) occurred on the 2nd day. Dio- 
dorus Siculus says that the stars were seen 1 , so that no doubt 
can exist as to the totality of the eclipse. Baily, however, found 
that there existed an irreconcileable difference between the cal- 
culated path of the shadow and the historical statement, a space 
of about 1 80 geographical miles appearing between the most 
Southerly position that can be assigned to the fleet of Agathocles 
and the Northerly limit of the phase. " To obviate this discord- 
ance, it is only necessary to suppose an error of about 3' in the 
computed distances of the Sun and Moon at conjunction, a very 
inconsiderable correction for a date anterior to the epoch of the 
Tables by more than 21 centuries 01 ." 

' Plutarch, Vita Peridls. lib. xxii. cap. 6. 

k Thucyd., lib. ii. cap. 28. m Phil. Trans., vol. cxliii. pp. 187-191. 

1 Diodor. Sic., lib. xx. cap. i. Justin.. 1853. 



CHAP. VII.] Historical Notices. 325 

In the work mentioned in the note below n there will be found 
an extremely interesting epitome of all the discussions which 
have taken place respecting the Eclipses of the Sun of 610, 603, 
585, 557, and 310 B.C., together with charts of the tracks of the 
shadow on each occasion. The writer, the late Mr. J. W. 
Bosanquet, F.R.A.S., also brings out very clearly the way in 
which these eclipses are available for settling points of chro- 
nology. 

In the writings of the early English chroniclers are to be found 
numerous passages relating to total eclipses of the Sun. The 
eclipse of August z, 1133, was considered a presage of misfortune 
to Henry I. : it is thus referred to by William of Malmesbury : 

" The elements manifested their sorrow at this great man's last departure. For 
the Sun on that day at the 6 th hour shrouded his glorious face, as the poets say, 
in hideous darkness, agitating the hearts of men by an eclipse ; and on the 6 th day of 
the week, early in the morning, there was so great an earthquake that the ground 
appeared absolutely to sink down ; an horrid noise being first heard beneath the 
surface ." 

The same writer, speaking of the total eclipse of March 20, 
1140, says: 

" During this year, in Lent, on the 13 th of the calends of April, at the 9 th hour of 
the 4 th day of the week, there was an eclipse, throughout England, as I have heard. 
With us, indeed, and with all our neighbours, the obscuration of the Sun also was so 
remarkable, that persons sitting at table, as it then happened almost everywhere, for 
it was Lent, at first feared that Chaos was come again : afterwards learning the 
cause, they went out and beheld the stars around the Sun. It was thought and said 
by many, not untruly, that the king [Stephen] would not continue a year in the 
government p ." 

n Messiah the Prince, or the Inspira- P Hist. Nov., lib. ii. See also Sax. 

tionof the Prophecies of Daniel. 2nded., Chrnn., Thorpe's Trans., p. 233. 8vo. 

8vo. Lond. 1869. London, 1861. 

Hist. Nov., lib. i. 



326 Eclipses and Associated Phenomena. [BOOK II. 



CHAPTER VIII. 
ECLIPSES OF THE MOON. 

Lunar Eclipses of less interest than Solar one*. Summary of facts connected with 
them. Peculiar circumstances noticed duriny the Eclipse of March 19, 1848. 
Observations of Forster. Wargentin's remarks on the Eclipse of May 18, 
1761. Kepler's explanation of these peculiarities beiny due to Meteorological 
causes. Admiral Smyth's account of the successive stages of the Eclipse of Oct. 
13, 1837. The Eclipse of Jan. 28, 1888. The Eclipse of Sept. 2, 1830, a* 
witnessed in Africa by S. and J. Lander. Chaldeean observations of Eclipses. 
Other ancient Eclipses. Anecdote of Columbus. 

A N eclipse of the Moon, though inferior in importance in all 
-^- senses to one of the Sun, is nevertheless by no means 
devoid of interest ; it is either partial or total a , according to the 
extent to which our satellite is immersed in the Earth's shadow. 
In a total eclipse the Moon may be deprived of the Sun's light 
for i h 50, and reckoning from the first to the last contact of the 
penumbra, the phenomenon in its various stages may last 5 h 30, 
but this is the outside limit. The obscuration is found to last 
longer than calculation assigns to it. This is due to the fact 
that no account is taken in the calculations of the denser strata 
of the atmosphere through which the rays have to pass, which 
cause an obstructive effect analogous to that of the solid matter 
of the Earth. From numerous observations made during the 
eclipse of Dec. 26, 1 833, Beer and Madler found that the apparent 
breadth of the shadow was increased by -V on account of the 
terrestrial atmosphere. " Owing to the ecliptic limits of the Sun 

a But never annular, because the from the Earth, is always in excess of 
diameter of the Earth's shadow, at the the diameter of the lunar disc, 
greatest possible distance of the Moon 



CHAP. VIII.] Eclipses of the Moon. 327 

exceeding those of the Moon, there are more eclipses of the 
former luminary than of the latter ; but on account of the com- 
paratively small extent of the Earth's surface to which a solar 
eclipse is visible, the eclipses of the Moon are more frequently 
seen at any particular place than those of the Sun." 

Fig. 157 is designed to illustrate roughly the different conditions 
of eclipses of the Moon. A B is the ecliptic, C D the Moon's 
path. The 3 black circles are imaginary sections of the Earth's 
shadow, when in 3 successive positions in the ecliptic. If the 



. 157- 




CONDITIONS OF ECLIPSES OF THE MOON. 



conjunction in longitude of the Earth and Moon occurs when 
the Moon is at E, it escapes eclipse ; if the Moon is at F, it suffers 
a partial obscuration, but if the Moon is at or very near its node, 
indicated by G, it will be wholly involved in the Earth's shadow 
and a total eclipse will be the result. 

Whereas solar eclipses always begin on the Western side and 
go off on the Eastern, lunar eclipses on the contrary commence 
on the Eastern side and go off on the Western. 

Even when most deeply immersed in the Earth's shadow, our 
satellite does not, except on rare occasions, wholly disappear, but 
may be generally detected with a telescope (and frequently with 
the naked eye), exhibiting a dull red or coppery colour. This 
was exemplified in a very remarkable manner in the case of the 
eclipse of March 19, 1848, on which occasion the Moon was 
seen so clearly that many persons doubted the reality of the 
eclipse. 



328 Eclipses and Associated Phenomena. [BOOK II. 

Mr. Forster, who observed the eclipse at Bruges, writes as 
follows : 

" I wish to call your attention to the fact which I have clearly ascertained, that 
during the whole of the late eclipse of March 19, the shaded surface presented 
a luminosity quite unusual, probably about three times the intensity of the mean 
illumination of the eclipsed lunar disc. The light was of a deep red colour. During 
the totality of the eclipse, the light and dark places on the face of the Moon could be 
almost as well made out as on an ordinary dull moonlight night, and the deep red 
colour where the sky was clearer was very remarkable from the contrasted whiteness 
of the stars. My observations were made with different telescopes; but all presented 
the same appearance, and the remarkable luminosity struck every one. The British 
Consul at Ghent, who did not know there was an eclipse, wrote to me for an explana- 
tion of the blood-red colour of the moon at 9 o'clock V 

As a complement to this observation, I may quote one by 
Wargentin of the total eclipse of May 18, 1761. He says that 
i i m after the commencement of the phase 

'* The Moon's body had disappeared so completely, that not the slightest trace of any 
portion of the lunar disc could be discerned either with the naked eye or with the 
telescope, although the sky was clear, and the stars in the vicinity of the Moon were 
distinctly visible in the telescope c ." 

The red hue was long a phenomenon for which no explanation 
could be found ; by some it was considered to be due to a light 
naturally inherent to the Moon's surface, but Kepler was the first 
to offer a more scientific explanation. He shewed that the phe- 
nomenon was a direct result of the refraction of the Earth's 
atmosphere, which had the effect of turning the course of the 
solar rays passing through it, causing them to fall upon the Moon 
even when the Earth was actually interposed between them and 
the Sun. That the colour of the Moon's surface is red is due to 
the fact that the blue rays of light are absorbed in passing 
through the terrestrial atmosphere, in the same manner as the 
Western sky is frequently seen to assume a ruddy hue when 
illuminated in the evening by the solar rays. On account of the 
variable meteorological condition of our atmosphere the quantity 
of light actually transmitted is liable to considerable fluctuations, 

b Month. Not. ,vol.vui.p.i$2. Mar.i848. stellis vicinis in tubo conspicuis." Other 

c Phil. Trans., vol. li. p. 210. 1761. eclipses, where the same thing occurred, 

The original runs thus: "Tota luna took place on June 15, 1620 (Kepler, 

ita prorsus disparuerat, ut nullum ejus Epist. Ast., p. 825); April 25, 1642 

vestigium, vel nudis, vel armatis oculis, (Hevelius, Selenog., p. 117); an d June 

sensibile restaret. coalo licet sereno, et TO, 1816 ^Beer and Madler). 



CHAP. VIII.] Eclipses of the Moon. 329 

and hence arises a corresponding variation in the appearances 
presented by the Moon's surface during her immersion in the 
Earth's shadow. If the portion of the atmosphere through which 
the solar rays have to pass is everywhere tolerably free from 
vapour, the red rays will be almost wholly absorbed, but not so 
the blue, and the illumination will be too feeble to render the 
Moon's surface visible : as in the instances cited in note c , p. 328. 
If, on the other hand, the region of the atmosphere through which 
the solar rays pass be everywhere highly saturated, the red rays 
will be transmitted to the Moon in great abundance, and its 
surface will consequently be highly illuminated d . Such was the 
case in the eclipse of March 1 848 already referred to. If, more- 
over, the region of the atmosphere through which the rays pass 
be saturated only in some parts and not in others, it follows that 
some portions of the Moon's disc will be invisible whilst others 
will be more or less illuminated. Such an occurrence was seen 
by Kepler 6 on Aug. 16, 1598, and by Sir J. Herschel and Smyth 
on Oct. 13, 1837. 

Smyth has recorded what he saw at each stage of this eclipse 
and it is worth while to give his account f , with the sketch which 
accompanies it, for the two together will serve as a model for 
observers desirous of knowing how to record the progress of an 
eclipse of the Moon. 

" 22 h 55 m o s . A light grey penumbra appearing. 

" 22 h 55 40". The Moon suffused with a copper tint. 

22 h 57 m i2 s . The dark shadow impinged on the lunar limb, and gradually 
marched over Grimaldus (a). 

23!* im j^s. Touched the crater of Aristarchus, the shadow filling the valleys as 
it advanced, then ascending the hills, and extinguishing their bright summits (i). 

jjh j^m 2 s_ Reached the fine regions of Copernicus, part of the cloud to the 
South crossing Gassendus. The stars gradually increasing in brightness (c). 

" 23 h 32 38*. Across the lunar disc, and through the streaky range of Tycho. 
Darkness increased so as to show the Milky Way (d].. 

" 23 h 44 47 s . The umbra passed the rugged mountains of Theophilus, soon after 
which sea-green tints were observable (e). 

d Johnson does not consider that these doubt that the whole question needs more 

explanations accord with the observed investigation and discussion, 

meteorological facts. (Month. Not., vol. e Ad Vitell. Paralipom. 

xlv. p. 44. Nov. 1884.) Monck takes ' Cycle ofCelest. Obj., vol. i. p. 144. 
the same view, and it is- not open to 



330 Eclipses and Associated Phenomena. [BOOK II. 

" 2 3 h 54 m io s . The shadow became more transparent, and the whole orb visible, 
so that the spots and other particulars of the selenography were revealed (/"). 

" o h 8 m 8 s . The sea-green tint spread all over the Moon. A star nearly in a line 
with Aristarchus and Copernicus, close to the moon's limb, was occulted 25 s 
afterwards. 

"o h 22 40'. The moon became lighter all over. Perhaps the retina of the eye 
had been fatigued by the lunar brightness at first, and was now awakening to delicate 
impressions. 

" o h 58 40*. The shadows seemed to be of a dark neutral tint, diluted in its 
intensity by refracted light ; a streak of sea-green towards Aristarchus. Turned the 




ECLIPSE OF THE MOON, OCT. 13, 1837. 

telescope upon the nebula 76 Messier, as a gauge, and saw it beautifully ; but it 
gradually faded as the Moon emerged. 

" i h 28 2i s . While the experiments were being made on nebula, during the total 
obscuration, the green tints were displaced by the copper ones, and a silvery light 
appeared over Grimaldus (g). 

" i h 40 29'. Aristarchus became uncovered, and its brightness rendered the 
obscured part more opaque (A). 

][h g 2 m I2 s. Copernicus and Tycho uncovered. The smaller stars retiring and 
all of them dimming (i). 

" 2 h 20 58'. Theophilus re-appeared almost in full splendour. The nebula 76 
Messier only perceptible from a knowledge of its form and place (&). 



CHAP. VIII.] Eclipses of the Moon. 331 

" 2 h 29 3o s . The small obscured segment of a curious dark tint, lessening with 
a smooth motion (Z). 

" 2 h 31 4 s . The shadow entirely left the moon, and the eclipse terminated. The 
smaller stars vanished, and none but the more brilliant visible. The moon as splendid 
as ever." 

The Rev. Canon Beechey writing of the eclipse of Oct. 4, 1884, 
mentions that during totality the Moon presented " one equal 
flat tint of cold grey, through which every feature of the lunar 
surface was distinctly visible ; " and that the eclipse generally was 
" remarkably similar to the one described by Smyth " as having 
happened on Oct. 13, 1837. 

The following account of the eclipse of Jan. 28, 1888, will be 
found to present several points of interest g : 

"The phase of total eclipse began nominally at 10.30 G. M. T., but it was not 
until fully 20 minutes after this that the last remains of the silvery shading along 
the west limb of the Moon had entirely disappeared. Up till the time that it did 
disappear the familiar coppery hue often seen in total eclipses of the Moon was not 
at all uniformly spread over the Moon's disc ; indeed there was no more than a 
coppery patch somewhat to the east of the centre of the disc for a long time, and I 
doubted whether this usual concomitant of a lunar total eclipse was going to be at all 
a conspicuous feature. However, as time wore on and the middle of the eclipse drew 
near, the whole disc (at 11.20) became overspread with the coppery hue. I speak of 
it under this name because it is the term usually employed, but in reality the tinge 
was more pink than coppery in the usual sense of the word, and it was much paler 
than usual; so much so indeed that in the middle of the totality (at 11.30) it was 
easy enough not only to see the whole disc of the Moon but also to identify some of 
the more conspicuous craters, such as Tycho, Copernicus, and Kepler, as well as 
several of the larger ' seas.' 

" By 11.37 a further change of aspect had manifested itself, and a silvery hue had 
begun to appear on the east limb much sooner than one would have expected in the 
ordinary course of things. 

"During the next 10 minutes a further enfeeblement of the pink hue took place 
more or less all round the margin of the disc, with the result that the Moon (looked 
at with the naked eye) presented an appearance scarcely different from that which 
she oftens presents during a common London fog. 

" At 11.55, a small star which had been occulted by the Moon reappeared, and its 
pure white light offered a curious contrast to the muddy pink of the Moon. 

"Soon after this the atmosphere began to get hazy all round, and before the total 
phase ended (at 12.10) the pink hue had become greatly enfeebled, though it did not 
finally disappear for a considerable time half an hour or more. 

" The haze varied much from minute to minute, and every now and then, when a 
little denser, its effect on the Moon was to make her look like a perfect snowy sphere, 
and her globular form was brought out with intense reality, constituting a sight of 
remarkable beauty." 

* Letter in the Times, Jan. 31, 1888. (G.F.C.) 



332 Eclipses and Associated Phenomena. [BOOK II. 

The celebrated African explorers, the Landers, graphically de- 
scribe what took place on the occasion of the eclipse of the Moon 
of Sept. 2, 1 830. They say : 

" The earlier part of the evening had been mild, serene, and remarkably pleasant. 
The Moon had arisen with uncommon lustre, and being at the full, her appearance 
was extremely delightful. It was the conclusion of the holidays, and many of the 
people were enjoying the delicious coolness of a serene night, and resting from 
the laborious exertions of the day ; but when the Moon became gradually obscured, 
fear overcame every one. As the eclipse increased they became more terrified. All 
ran in great distress to inform their sovereign of the circumstance, for there was not 
a single cloud to cause so deep a shadow, and they could not comprehend the nature 
or meaning of an eclipse. . . .Groups of men were blowing on trumpets, which produced 
a harsh and discordant sound; some were employed in beating old drums; others 

again were blowing on bullocks' horns The diminished light, when the eclipse was 

complete, was just sufficient for us to distinguish the various groups of people, and 
contributed in no small degree to render the scene more imposing. If a European, 
a stranger to Africa, had been placed on a sudden in the midst of the terror-struck 
people, he would have imagined himself to be among a legion of demons, holding 
a revel over a fallen spirit 11 ." 

It is to the Chaldaeans that we owe the earliest recorded obser- 
vations of lunar eclipses, as mentioned by tolemy. The first of 
these took place in the 27 th year of the era of Nabonassar, the 
first of the reign of Mardokempadius, on the 29 th day of the 
Egyptian month Thotk, answering to March 19, 720 B.C., according 
to our mode of reckoning. It appears to have been total at 
Babylon, the greatest phase occurring at about 9 h 30 P.M. The 
second was a partial eclipse only; it happened at midnight on 
the 1 8 th of the month T/wt/t, or on March 8, 719 B.C. The third 
took place in the same year, on the 15 th of the month Phammuth, 
or Sept. i, 719 B.C. The magnitude of the eclipse, according to 
Ptolemy, was 6 digits on the southern limb, and it lasted 3 hours, 
having commenced soon after the Moon rose at Babylon. 

Three eclipses recorded by Ptolemy and which happened in 
523, 502, and 491 B.C., assisted Sir I. Newton in ascertaining the 
terminus a quo from which the "70 weeks " of years were to be 
calculated which the prophet Daniel (ix 24) predicted were to 
precede the death of Christ. And this terminus a quo is on good 

h K. and J. Lander, Journal of an Expedition to explore the Niger, vol. i. p. 366, 
New York, 1844. 



CHAP. VIII.] Eclipses of the Moon. 333 

grounds considered to have been the restoration of the Jews 
under Artaxerxes in his 7 th 'year (457 B.C.) 1 . 

An eclipse occurred in the 4 th year of the 91"* Olympiad, the 
19 th of the Peloponnesian war, answering to Aug. 27, 412 B.C., 
which produced very disastrous consequences to the Athenian 
army, owing to the obstinacy of their general Nicias J '. Modern 
calculations shew that it was total at Syracuse. 

The eclipse of the Moon which happened on March 13, 4 B;C., 
serves to determine the date of our SAVIOUR'S birth. This event 
preceded, by a few weeks, the death of Herod, and, according to 
Josephus k , that occurrence took place soon after a lunar eclipse 
which has been identified as stated 1 . The Nativity took place 
in the Autumn or Winter of 5 B.C. 

An eclipse of the Moon, which happened on March i, 1504, 
proved of much service to Columbus m . His fleet was in great 
straits, owing to the want of supplies, which the inhabitants of 
Jamaica refused to give. He accordingly threatened to deprive 
them of the Moon's light, as a punishment. His threat was 
treated at first with indifference, but when the eclipse actually 
commenced, the natives, struck with terror, instantly commenced 
to collect provisions for the Spanish fleet, and thenceforward 
treated their visitors with profound respect. 

1 H. G. Guinness, Approaching end of 1 See Wieseler, Chronological Synopsis 

the Age, 5th ed., p. 516 : J. B. Lindsay, of the 4 Gospels, p. 51. I cannot see the 

Chrono- Astrolabe, Lond., Bohn, pp. 75 et force of the Rev. S. J. Johnson's reasoning 

seq. in favour of the eclipse of Jan. 9, o B.C. 

J Plutarch, Vita Nicias. Thucyd., lib. (Eclipses, past and present, p. 21.) 

vii. cap. 50. m W. Robertson, Hist, of America, 

k Antiq., xvii. 4. loth ed., vol. i. book ii. p. 240. 



334 Eclipses and Associated Phenomena. [BOOK II. 



CHAPTER IX. 



A CATALOGUE OF ECLIPSES 

rilHE eclipses visible in England have received much attention 
-L from the Rev. S. J. Johnson, and papers of his cited below 
will be interesting to many English readers b . 

The following Catalogue contains all the eclipses which occur 
during the remainder of the 19 th century, excepting solar 
eclipses hardly visible to any inhabited portion of the Earth, and 
lunar eclipses in which less than T V of the Moon's diameter is 
obscured. The time is approximately that of Greenwich, M. 
standing for moming, and A. for afternoon. Under the head of 
"Locality" the letter C points to the path followed by the 
central line ; in cases where this passes very near the North or 
South Pole, it is not traced, but those places only are named 
where the eclipse will be visible (V). The letters N.E. or S.E. 
following the name of a place, indicate the direction taken by the 
shadow after passing the parts in question. 

ft For Catalogues of Eclipses extending called to a very interesting memoir by S. 

over long periods of time see Oppolzer's Newcomb, On the recurrence of Solar 

Canon der Finsternisse in DenTcschriften Eclipses, with Tables of Eclipses from B.C. 

der Kaiserlichen Akad. der Wissen- 7ootoA.D. 2300; in Astronomical Papers 

schaften, vol. lii. Vienna 1887; and IS Art for the use of the American Ephemeris 

de verifier lex dates, Paris 1818, vol. i. and Nautical Almanac, vol. i. Washing- 

p. 269. ton, U.S., 1879. 

In connection with the calculation of b Month. Not., vol. xxxiii. p. 402, Ap. 

Solar eclipses attention may here be 1873: Ib. vol. xl. p. 436, May 1880. 



CHAP. IX.] 



A Catalogue of Eclipses. 



335 



Year. 




Vlonth and 
Day. 


Hour. 


Magni- 
tude. 


Locality. 


1889 





Jan. I 


9 A. 




C Behring's Straights ; Nootka ; Hud- 












son's Bay. 





( 


Jan. 17 


5 iM. 


0.68 


United States. 








June 28 


9 M. 




C S. Africa; Magagascar, S.E. 





( 


July 12 


9 A. 


0-46 


Armenia. 








Dec. 22 


i A. 




C Carthagena ; St. Helena ; Abyssinia. 


1890 





June 17 


10 M. 




C Cape Verde Islands; Smyrna; Pegu. 








Dec. 12 


3 M. 




C Mauritius ; New Zealand ; Tahiti. 


1891 


( 


May 23 


7 A. 


I-3 1 


India. 








June 6 


4JA. 




CN.W. America; N.Pole; Kussia. 





( 


Nov. 1 6 


oJM. 


1.44 


Ireland. 


1892 





April 26 


10 A. 




C S. Pacific. 





( 


May ii 


nJA. 


094 


France. 








Oct. 20 


7 A. 




V N. America. 





( 


Nov. 4 


4| A. 


1-04 


China. 


1893 





April 1 6 


3 A. 




C Easter Island; Guiana; N.E.Africa. 








Oct. 9 


9 A. 




Sandwich Islands ; Peru. 


1894 


( 


Mar. 21 


2|A. 


0.25 


New Guinea. 








April 6 


4 iM. 




C Egypt ; China ; Pacific. 





( 


Sept. 15 


4 IM. 


O-2I 


Canada. 








Sept. 29 


5|M- 




C Madagascar ; New South Wales ; 












New Zealand. 


1895 


( 


Mar. ii 


4 M. 


I. 5 6 


Barbados. 








Mar. 26 


10 M. 




V Atlantic ; Europe ; N. Asia. 








Aug. 20 


of A. 




V N. Asia. 





( 


Sept. 4 


6 M. 


i-54 


Mississippi. 


1896 


( 


Feb. 28 


8 A. 


0-83 


E. Persia. 








Aug. 9 


4|M. 




C Prussia ; E. Siberia ; Pacific. 





( 


Aug. 23 


7 M. 


0-66 


New Mexico. 


1897 





Feb. i 


8 A. 




C New Caledonia ; Easter Is.; Guiana. 








July 29 


4 A. 




C Gallipagos ; Barbados ; Guiana. 


1898 


( 


Jan. 7 


Midnt. 


0-12 


London. 








Jan. 22 


8 M. 




C Fezzan ; Socotra ; N.China. 





( 


Julys 


9 |A. 


O-92 


Russia. 








July 1 8 


7 A. 




V S. America. 





< 


Dec. 27 


Midnt. 


i-33 


London. 


1899 





Jan. ii 


ii A. 




V E. Asia ; N. America. 








June 8 


7 M. 




V N. Europe ; N. Asia. 



336 Eclipses and Associated Phenomena. [BOOK II. 



Year. 




Month and 
Day. 


Hour. 


Magni- 
tude. 


Locality. 


1899 


( 


June 23 


2|A. 


I. 5 


New Guinea. 





( 


Dec. 17 


i|M. 


0-96 


Cape Verde Islands. 


1900 





May 28 


3 A. 




C Mexico ; Azores ; Egypt. 








Nov. 22 


8 M. 




C Benin ; Madagascar ; New South 












Wales. 



According to Hind c the following are the important total 
eclipses of the Sun for the remainder of the present century, 
which are likely to be available for increasing our knowledge of 
solar physics: Dec. 22, 1889, the totality of which lasts for 
3 34 s , and April 19, 1893, lasting 4 m 44 s . 



c Month. Not., vol. xxxii. p. 178 (Feb. 1872). 



CHAP. X.] Transits of the Inferior Planets. 337 



CHAPTEK X. 



TRANSITS OF THE INFERIOR PLANETS. 

Cause of the phenomena. Lord Grimthorpe's statement of the case. Long intervals 
between each recurrence. Useful for the determination of the Sun's parallax. 
List of transits of Mercury. Of Venus. Transit of Mercury of Nov. 7, 1631. 
Predicted by Kepler. Observed by Gassendi. His remarks. Transit of Nov. 3, 
1651. Observed by Shakerley. Transit of May 3, 1661. Transit of Nov. 7, 
1677. Others observed since that date. Transit of Nov. 9, 1848. Observations 
of Dawes. Of Forster. Transit of Nov. n, 1861. Observations of Baxen- 

dell Transit of Nov. 5, 1868 Transit of May 6, 1878. Transit of Nov. 7, 

1881. Summary by Jenkins of the main features of a Transit. Observations 
by Prince. By Langley. Transit of Venus of Nov. 24, 1639. Observed by 
Horrox and Crabtree. Transit of June 5, 1761. Transit of June 3, 1769. 
Where observed. Singular phenomenon seen on both, occasions. Explanatory 
hypothesis. Other phenomena. Transit of Dec. 8, 1874. Transit of Dec. 6, 
1882. 

WHEN an inferior planet is in inferior conjunction, and 
" has a [geocentric] latitude, or distance from the ecliptic, 
less than the Sun's semi-diameter, it will be less distant from the 
Sun's centre than such semi-diameter, and will therefore be 
within the Sun's disc. In this case the planet being between 
the Earth and the Sun, its dark hemisphere being turned towards 
the Earth, it will appear projected upon the Sun's disc as an 
intensely black round spot. The apparent motion of the planet 
being retrograde, it will appear to move across the disc of the Sun 
from E. to W. in a line sensibly parallel to the ecliptic." Such 
a phenomenon is called a transit, and as it can only occur in the 
case of inferior planets it is limited to Vulcan (if there be such a 
planet), Mercury, and Venus. Observations of these planets or 
rather, in practice, of Venus only are available for determining 



338 Eclipses and Associated Phenomena. [BOOK II. 

the parallax of the Sun, from which may be found the distance of 
the Earth from that luminary a . 

The rationale of the process is thus popularly set forth by Lord 
Grimthorpe : " If two men stand before a post with a wall behind 
it, they will see different places on the wall eclipsed or hidden 
by the post; and if the post is as far from the two eclipsed 
places as it is from the men, the two eclipses will be exactly as 
far apart as the two men are ; if the wall is twice as far from the 
post, the two eclipses will be twice as far apart, and so on. 

" Therefore two people on the Earth, as far apart as they can 
conveniently get for them both to see the transit of Venus from 
beginning to end, will see at the same time the two transit spots 
twice and a half as far apart in real distance on the Sun as the 
observers are distant from each other. Suppose they are 7200 
miles apart (measuring through the Earth the shortest way) then 
the two transit spots will be 1 8,000 miles apart on the Sun ; and 
we have only one step more to take in order to find the diameter 
of the Sun in miles ; and that is, to get an accurate map made of 
the disc of the Sun with the exact positions of the two spots at 
the same time ; for then we can measure their distance on the 
map and see what proportion it bears to the diameter, and we 
know that 1 8,000 miles bears that same proportion to the real 
diameter of the Sun, and the business is done. 

" The real difficulty is to get this Sun-map made accurate 
enough to measure from, or to get the exact distance of the spots 
at the same moment, remembering that the two observers are 
nearly half way round the Earth from each other. For that 
purpose the following contrivance is adopted. Instead of ob- 
serving the transit at one moment only, each man observes the 
whole path of Venus across the Sun ; or rather in reality he 
observes the exact time it takes ; for they can observe the first 
and last contact of the spot far more accurately than they can 

For a somewhat full account of the Sc., vol. xxii. p. 375, Nov. 1881 ; also an 

principles which underlie the various Address by the same, Proceedings of the 

methods and of the scientific value of the American Association for the Advance- 

various results hitherto accomplished see ment of Science, vol. xxxi. Aug. 1882. 
a paper by W. Harkness, Amer. Journ. 



CHAP. X.] Transits of the Inferior Planets. 339 

measure distances on the bright face of the Sun ; and it is not 
necessary that they should see anything but the beginning and 
the end of the transit. The places on the Earth are so chosen 
that the paths may appear not only parallel, but at the widest 
distance possible apart, forming two chords across the Sun, 
parallel to the diameter which Venus would pass along if she 
was exactly in the ecliptic and seen from the centre of the Earth. 
The two paths may be on different sides of the Sun's centre if 
Venus is exactly at a node, but they are more likely to be on 
the same side, in which case their difference of length is greater, 
and the observations more likely to give an accurate result. 

" For the accuracy of the map depends on this : you have a 
circle of known diameter to start with, because the time Venus 
would take to cross the middle of the Sun is known from the 
proportion which his diameter bears to the orbit of Venus, and 
the time she takes to perform it. So if that time were known to 
be 6 hours we might draw a circle of 6 inches diameter for the 
Sun; and if one observer reported his transit to have lasted 5 
hours we should find the place where a chord 5 inches long will 
exactly fit ; and if the other transit lasted 5^ hours, we should 
put in another chord 5^ inches long, parallel to and near the 
former. (The real lengths could not be exactly these, but that 
does not signify.) The distance between two chords of 5 and 5^ 
inches in a circle 6 inches wide can be calculated with the utmost 
accuracy, and also the proportion of that distance to the diameter, 
which is the proportion of the 18,000 miles to the real diameter 
of the Sun, the thing we wanted. 

" I have said nothing about the rotation of the Earth during 
the time the transit lasts ; but of course due allowance has to be 
made for that by methods known to astronomers b ." 

James Gregory (the inventor of the " Gregorian " Telescope) 
seems to have been the first to point out this application of 
planetary transit observations c . 

b Astron. without Mathematics, 3rd account of the method see Airy's Lectures 
ed., p. 185. on Astronomy, p. 145. 

c Optica Promota, p. 130. For a lucid 

z 



340 Eclipses and Associated Phenomena. [BOOK II. 



The transits of the inferior planets are phenomena of very rare 
occurrence, especially those of Venus, which occur only at inter- 
vals of 8, 105^, 8, iaif, 8, 105!, &c. years. Transits of Mercury 
usually happen at intervals of 13, 7, 10, 3, 10, 3, &c. years. 
This, however, is not altogether a correct expression of the 
intervals ; for, owing to the considerable inclination of Mercury's 
orbit, it requires a period of about 217 years to bring the transits 
round in a completely regular cycle. 

The following are the dates of the transits of Mercury and 
Venus from the beginning of the I9th century onwards d : 





Mercury. 






Venus. 








d. h. 






d. h. 


1802 


November ... 


8 20 


1874 


December . . . 


8 16 


1815 


November ... 


ii 14 


1882 


December . . . 


6 4 


1822 


November . . . 


4 14 


2004 


June , 


7 21 


1832 


May 


5 o 


2OI2 


June 


5 i3 


1835 


November ... 


7 7 


2117 


December . . . 


10 15 


1845 


May 


8 8 


2125 


December . . . 


8 3 


1848 


November ... 


9 i 


2247 


June 


II 


1861 


November . . . 


II IQ 


22*s; 


June 


8 16 


1868 


November ... 


y 

4 18 


*)D 
2360 


December . . . 


12 13 


1878 


May 


6 6 


2368 


December . . . 


IO 2 


< 

1881 


November ... 


7 2 


2490? 


June 


12 3 


1801 


May 


14 


24Q8 


June 


o 20 


y 
1894 


November . . . 


y w 
10 6 


^y 
2603 


December . . . 


y 

15 12 



The transits of Mercury, owing to the heliocentric position of 
the nodes, always happen in May or November. When the 
transit occurs in May, the planet is passing through the descend- 
ing node, and when in November, through the ascending node. 
Similar remarks apply to the transits of Venus, the only 
difference being that the months are June and December. 






d Lalande, Astron., vol. ii. pp. 457-61. 
Lalande's original Table gives for Venus 
the transits up to A.D. 2984 some time 
hence! For transits of Mercury 1891- 
2108 see Astron. Papers for use of 
American Nautical Almanack, Ed. by 



S. Newcomb, vol. i. part vi. Washington, 
1882. This memoir contains an ex- 
tremely exhaustive discussion of all the 
mathematical questions which arise in 
connection with Transits of Mercury, 
based on past records and on theory. 



CHAP. X.] Transits of the Inferior Planets. 341 

The shortest transit of Mercury yet observed was that of 
Nov. 12, 1782. It lasted only i h i4 m . The longest, that of May 6, 
1878, lasted for 7 h 47 m . The average duration is about 4 h . 

The first observed transit of Mercury occurred on November 7, 
1631, and was predicted by Kepler 6 , whose surmise was verified 
by Gassendi at Paris. The latter remarks : 

" The crafty god had sought to deceive astronomers by passing over the Sun a little 
earlier than was expected, and had drawn a veil of dark clouds over the Earth, in 
order to make his escape more effectual. But Apollo, acquainted with his knavish 
tricks from his infancy, would not allow him to pass altogether unnoticed. To be 
brief, I have been more fortunate than those hunters after Mercury who have sought 
the cunning god in the Sun ; I found him out, and saw him where no one else had 
hitherto seen him V 

The second observed transit of this planet happened on Nov. 3, 
1651. It is chiefly interesting to us from the fact that it was 
observed by a young Englishman, Jeremiah Shakerley; who, 
having found by calculation that the phenomenon would not be 
visible in England, went out to Surat in India for the purpose of 
witnessing it g . 

The third observed transit took place on May 3, 1661. It was 
observed in part by Huygens, Street, and Mercator in London, 
and by Hevelius at Dantzic. The last-named astronomer was 
astonished to find that the angular diameter of the planet was 
so small h : his determination of it agrees well with modern 
results. 

The fourth observed transit occurred on Nov. 7, 1677, an( ^ ^ 8 
noticeable from the fact that it was the first which was watched 
throughout (by Halley) from ingress to egress. 

The transits at which anything of particular interest was 
noticed are the following : 

Transit of Nov. 3, 1697. Wurzelbau, at Erfurt, perceived a 
strange greyish-white spot on the dark body of the planet. 

Transit of Nov. n, 1736. Plantade remarked that the disc of 
the planet appeared surrounded by a luminous ring. 

Transit of May 7, 1799. Schroter and Harding observed the 

Admonitio ad Astronomos, &c. & Wmg,AstronomiaBritannica,p.^2' 

f Opera Omnia, vol. ii. p. 537. h Mercurius in Sole visus, p. 83. 



342 Eclipses and Associated Phenomena. [BOOK II. 

luminous halo seen by Plantade in 1736, and they likewise saw 
two greyish spots on the planet when on the Sun. They ascribed 
to them a motion corresponding to the rotation they subsequently 
inferred from other observations. The halo or ring was of a 
darkish tinge, approaching to violet. 

Transit of Nov. 9, 1802. Fritsch and others saw a greyish 
spot. 

Transit of May 5, 1832. Moll, of Utrecht, saw a ring encircling 
the planet when on the Sun, and also a spot on the planet's disc. 
The ring had something of a violet tinge. Two spots were seen 
by Harding, and Gruithuisen thought he saw one. 

Concerning the transit of Nov. 8, 1 848, Dawes, who observed 
it at Cranbrook in Kent, says : 

" Nothing remarkable was noticed till Mercury had advanced on the Sun's disc to 
about three-quarters of its own diameter, when the cusps appeared much rounded off, 
giving a pear-shaped appearance to the planet. The degree of this deformity, how- 
ever, varied with the steadiness and definition of the Sun's edge, being least when 
the definition was best. A few seconds before the complete entrance of the planet, 
the Sun's edge became much more steady, and the cusps sharper, though still 
occasionally a little broken towards their points by the undulations. At the instant 
of their junction, the definition was pretty good, and they formed the finest con- 
ceivable line, Mercury appearing at the same time perfectly round. . . . No difference 
is recognised in the Nautical Almanac between the polar and equatorial diameters of 
this planet; yet my observations, both with the 5 -foot achromatic and the Gregorian, 
shew a perceptible difference, and nearly to the same amount. . . . The compression 
would appear to be about ^ '." 

Forster observed the transit at Bruges. He remarked the 
extreme blackness of the planet compared with the spots : the 
ratio of the intensities he estimated at 8:5. He also stated that 
the planet had rather the appearance of a globe than of a disc, 
and the difference of blackness between the planet and the 
spots was less remarkable when he used a reflector with a red 
shade k . 

A transit happened on Nov. n, 1861. In England few 
observations were made, owing to unfavourable weather. Mr. 
Baxendell, of Manchester, remarked the excessive blackness of 
the planet as compared with the nuclei of certain solar spots, and 

1 Month. Not., vol. ix. p. 21. Dec. 1848. 
k Month. Not., vol. ix. p. 4. Nov. 1848. 



CHAP. X.] Transits of the, Inferior Planets. 



that the planet's contour became pear-shaped immediately before 
the egress 1 . 

The transit of Mercury which happened on Nov. 5, 1868, was 
visible in England. Important observations were made by 
Huggins m . An aureola of light around the planet and a luminous 
point of light on the body of the planet " nearly in the centre " 
were seen, and thus previous observations were fully confirmed. 
The breadth of the luminous annulus was about of the planet's 
apparent diameter. There was no fading off at the margin, the 

Fig. 159. 





MEBCUEY DURING ITS TRANSIT, NOV. 5, 1 868. 

brightness being everywhere about the same, and only slightly 
in excess of that of the general surface of the Sun. Both the 
aureola and the luminous spot were visible throughout the whole 
transit. 

Huggins's account of what he saw towards the end of the 
phenomenon is as follows : 

"The following appearance was noticed almost immediately after the planet's disc 
came up to the Sun's limb. The spot appeared distorted, spreading out to fill up 
partly the bright cusps of the Sun's surface between the planet's disc and the Sun's 



1 Month. Not., vol. xxii. p. 43, Dec. 
1861. 



m Month. Not., vol. xxix. p. 25. NOT. 
1868. 



344 Eclipses and Associated Phenomena. [BOOK II. 

limb. This appearance increased as the planet went off the Sun, until when the disc 
of the planet had passed by about one-third of its diameter, it presented the form 
represented in the diagram, in which the margin of the disc from points at the end of 
a diameter parallel to the Sun's limb, instead of continuing its proper curve, appeared 
to go in straight lines up to the limb, thus entirely obliterating the cusps of light, 
which would otherwise have been seen between the planet and the limb. In the 
diagram the aureola and the bright spot are not repeated in the figure of the planet 
on the Sun's limb." 

The transit of May 6, 1878 was observed under such dis- 
couraging circumstances of weather that a very brief allusion to 
the results n will suffice. Several observers saw a minute bright 
spot or patch on the planet, and several observers saw no such 
spot or patch. Some saw what they described as a " ring " ; 
some saw what they described as a " halo " encompassing the 
disc of the planet ; others detected no such phenomenon. Some 
who noticed one or both of these things confess to a suspicion 
that spot and ring were merely optical effects, or effects of 
contrast. 

The transit of Nov. 7, 1881 was well seen at several 
stations in Asia and Australia . Tebbutt at Windsor, New 
South Wales, saw at intervals a faint whitish spot which at one 
instant lengthened out into a streak across the disc. He con- 
sidered the phenomenon an optical one not in any way connected 
with the planet itself. He looked for but failed to see any halo 
or ring. On the other hand Dr. Little at Shanghai states that 
the planet was "always surrounded by a darkish halo, which 
seemed well defined, extending to a distance about equal to the 
planet's semi-diameter. With no power could any spots on the 
planet be detected." 

Jenkins, collecting and comparing all the results recorded up 
to 1868, considered himself justified in advancing the following 
propositions : 

ist. That in the May transits, when Mercury is near its aphelion, 
the luminous spot is in advance of the planet, preceding the 
centre ; in the November transits, when Mercury is near its 
perihelion, the luminous spot follows the planet. 

n Month. Not., vol. xxxviii. p. 397. Month. Not., vol. xlii. p. 101, &c. 
May 1878. Jan. 1882. 



CHAP. X.] Transits of the Inferior Planets. 345 

2nd. The luminous spot has never been seen at the centre, but 
always south of it, and therefore cannot be due to diffraction. 

3rd. Sometimes in the same transit two spots have been seen 
close together, where shortly before only one was observed. 

4th. In the May transits the rings round the planet are dark 
or nebulous and of a violet tinge ; in the November transits 
they are bright. 

5th. If we take the two transits which have received the most 
careful observation, May 1832, observed by Moll, and November 
1868, observed by Huggins, we find the contrast very great 
and very typical : in the one case a diffused spot preceding the 
centre, with dark ring surrounding the planet ; in the other a 
sharply defined spot following the centre, with bright ring 
surrounding the planet p . 

The annulus round Mercury and the white spot on Mercury 
during transits across the Sun may now be regarded as regular 
concomitants of the phenomenon, but there is no agreement 
amongst astronomers as to the cause of these appearances. The 
white spot has been regarded by some as indicative of Volcanic 
action, but this seems mere fancy. Prof. Powell, with more 
show of reason, suggested that diffraction of light had something 
to do with the matter, but it is an objection to this theory that 
it presupposes the invariable centrality of the white spot ; now 
the white spot, though often, is not always coincident in position 
with the centre of the planet's disc, and therefore Huggins rejects 
the hypothesis. It might conceivably have its origin in the 
internal reflection of light in a Huygenian Eye-piece. 

We now come to the transits of Venus q , which are more 
important and more rare. In the year 1627 Kepler completed 
the Eudolphine Tables, and being thus in a position to calculate 
the motions of the planets with far more certainty than had ever 
been attained before, he betook himself diligently to the work. 

P Month. Not., vol. xxxviii. p. 337. Notes and Suggestions to Observers. 

Ap. 1878. These should be consulted by persons 

i Previous to the transit of 1878 Lord proposing to conduct observations of 

Lindsay put forth in conjunction with future transits, but this will be a matter 

Dr. U. Copeland an exhaustive paper of for many generations hence. 



346 Eclipses and Associated Phenomena. [BOOK II. 

The first result was, that he ascertained that during 1631 both 
Mercury and Venus would traverse the Sun's disc, the former on 
Nov. 7 and the latter on Dec. 6 ; which information he published 
in a little tract in 1629'. Of the transit of Mercury I have 
already spoken. With reference to that of Venus, Gassendi 
made preparations for observing it ; and though Kepler's cal- 
culations were to the effect that the ingress would not take 
place till near sunset, the French astronomer, anticipating the 
possibility of the calculated times being too late, (as had been 
the case with Mercury a few weeks previously,) prepared to 
commence his watch on Dec. 4, though bad weather prevented 
him seeing the Sun till the 6th. He sought unsuccessfully for 
the planet both on that and on the following day, and it is 
now well known that the transit took place on the night of 
Dec. 6 7. 

The next transit of Venus (the first actually observed) took 
place on Nov. 24, 1639 (o. s.) Kepler did not anticipate it, for 
he said that none would take place between 1631 and 1761, and 
so the honour both of predicting and of observing it rests with a 
young English amateur, the Rev. Jeremiah Horrox, curate of 
Hoole, a village in Lancashire, 20 miles N. of Liverpool. Horrox 
had been engaged in computing the places of the planets by the 
aid of Lansberg's Tables. Finding that these gave very 
erroneous results he discarded them for Kepler's, from which he 
found that on the above named Nov. 24, Venus, in passing its 
inferior conjunction, would cross the heavens a little below the 
Sun. As Lansberg's Tables indicated that the planet would 
cross the upper part of the solar disc, he hoped that a mean of 
the two results, so to speak, might be looked for, and that he 
should see the planet actually on the Sun, towards the lower 
extremity of its disc : further calculation assured him that his 
anticipation would turn out to be correct. Owing to the short- 
ness of the interval that would elapse previous to the actual 
occurrence of the transit he was unable to give much publicity 

r Admonitio ad Astronotnos rerumqtte anni 1631 Phcenomenig, Veneris puta et 
celestium studiosos, de miris rarisque Mercuriiin Solemincitrsu. Lipsise, 1629. 



CHAP. X.] Transits of the Inferior Planets. 347 

to the result at which he had arrived ; indeed all that he seems 
to have done was to inform his brother Jonas of Liverpool 
and his friend William Crabtree, an enthusiastic amateur like 
himself, who resided at Broughton, near Manchester, not many 
miles distant from Hoole. 

Horrox prepared to watch for the planet by transmitting the 
image of "the Sun through a telescope on to a screen in a 
darkened room. His final calculations gave 3 h P.M. on Nov. 24 
as the time of conjunction of the centres of the Sun and planet ; 
but fearing to be too late, he commenced his scrutiny of the Sun 
on Nov. 23. On the following day he began his observations at 
Sunrise, and continued them till the hour of Church service. 
(It was Sunday.) As soon as he was again at leisure that is to 
say at 3 h 15 P.M. he resumed his labours, and, to quote his 
own words, "At this time an opening in the clouds, which 
rendered the Sun distinctly visible, seemed as if Divine Pro- 
vidence encouraged my aspirations ; when, O most gratifying 
spectacle ! the object of so many earnest wishes, I perceived a 
new spot of unusual magnitude, and of a perfectly round form, 
that had just wholly entered upon the left limb of the Sun, so 
that the margin of the Sun and spot coincided with each other, 
forming the angle of contact." Owing to the near approach of 
Sunset, Horrox was unable to observe the planet longer than 
half an hour ; but at any rate he had seen it, and had been able 
to take some measurements 8 . 

Crabtree had also made arrangements for observing the phe- 
nomenon. The Sun was, however, obscured during the whole of 
the day, and he had given up in despair all hope of seeing the 
transit, when, just before Sunset, the clouds broke up, and, 
hastening to his observing chamber, he saw, to his infinite 
delight, Venus depicted on the Sun's disc transmitted on to a 
screen. He was, according to his own account, so entranced by 
the spectacle that ere he recovered his self-possession the clouds 
had again enshrouded the Sun, and he saw the planet no more. 

5 Whatton, Memoir of Horrox, pp. 109 135. See also an article in the Obser- 
vatory, vol. vi. p. 318, Nov. 1883. 



Fig. 1 60. 



348 Eclipses and Associated Phenomena. [BOOK II. 

He subsequently found that a rough diagram, which he drew 
from memory, agreed well with one drawn by Horrox. 

No other transit occurred till June 5, 1761 : this was observed 
in many parts of the world for the purpose of ascertaining, in 
accordance with the special suggestion of Halley, the solar 
parallax. But the results of the different observations were not 
satisfactory. 

Extensive preparations were made for observing the transit of 
June 3, 1769, and King George III. despatched, at his own ex- 
pense, a well-equipped expedition to Tahiti under the command 

of the celebrated navigator 
Cook, then a Lieut., R. N. 
Many of the Continental 
Powers followed the example 
of England, and astronomers 
were sent out to the most 
advantageous points for ob- 
servation. The chief of these 
were St. Petersburg, Pekin, 
Orenburg, lakutsk, Manilla, 
Batavia, for the egress ; and 
Cape Wardhus, Kola and 
Kajeneburg in Lapland, Point Venus in Tahiti, and Fort Prince 
of Wales and St. Joseph in California, for the entire phenomenon. 
The observations were long looked upon as trustworthy, but 
astronomers eventually came to the conclusion that an im- 
portant correction in the final result must be accepted *. Accord- 
ingly, the transits of Dec. 9, 1874 and Dec. 6, 1882 were 
awaited with special eagerness. 

Some phenomena were seen in connexion with the transits of 
1761 and 1769 which require a passing mention. It was noticed 
on both occasions, and by numerous observers, that the interior 
contact of the planet with the Sun did not take place regularly 
at the ingress, but that the planet appeared for a short time after 




VENUS DURING ITS TRANSIT IN 1769. 



See p. 2 ante. 



CHAP. X.] Transits of the Inferior Planets. 



349 



Fig. 161. 



it had entered upon the disc of the Sun to be attached to the 
Sun's limb by a dark ligament. A similar phenomenon was 
noticed at the egress. It was also found that even after the 
planet had got wholly clear of the Sun's limb it did not acquire 
circularity for several seconds 11 . Lalande suggested x that irradi- 
ation was the cause of these phenomena, and this is doubtless the 
true explanation. 

It was remarked by several observers of the transits of 1761 
and 1769, that, both at the ingress and egress, the portion of the 
limb of the planet which was not then projected on the Sun was 
rendered perceptible by reason of a faint 
ring of light which surrounded it. More 
than one observer noticed a ring round 
Venus when it was entirely within the 
disc of the Sun, similar, it would seem, 
to that which has been seen to surround 
Mercury when in the same situation. 
Dunn stated that this annulus had a 
breadth of 5" or 6", that it was some- 
what dusky towards the limb of the 
planet, and that its outer margin was 
slightly tinged with blue. Hitchins described it as excessively 
white and faint, and brightest towards the body of the planet. 
Nairne spoke of it as brighter and whiter than the body of the 
Sun. A comparison of the different accounts seems to shew that 
the above-described rings are not identical, but no sufficient 
explanation has been offered to account for either, though the 
latter has been supposed to indicate the existence of an atmo- 
sphere around the planet y . 

One observer of the transit of 1769 is stated to have seen a 
light on the disc possibly similar to that occasionally noticed on 
Mercury during its transits z . 




VENDS DURING ITS TRANSIT 
IN 1769. 



u See Phil. Trans., 1761, 1768, 1769, 
1770: also HI em. A cad. dts Sciences for 
the same years. 

* Mem. Acad. <1es Sciences, 1770, p. 
409. 



y For references for all these state- 
ments, see Grant's Hist, of Phys. Ast., 

P- 431- 

z Append. Ad. Ephem. Astron., 1766, 

p. 62. 



350 Eclipses and Associated Phenomena. [BOOK II. 



Fig. 162. 



A ring of light was seen by many observers round Venus 
during the transit of Dec. 8, 1874, which the engraving above, 
dated 1769, would seemingly represent equally well 8 . 

Figs. 163 to 1 68 are 6 views of Venus at the transit of 1874, 
drawn by E. J. Stone, who used the 7-inch refractor of the Cape 
Observatory : their large scale renders them of great interest, but 
it does not seem necessary to transcribe his notes on each, which 
are however very brief b . 

It will be remembered that transits of Venus are of importance 
in two senses; firstly, as affording a means of ascertaining the 
Earth's distance from the Sun ; and secondly, for what they dis- 
close respecting the physical circumstances of the planet. In 

this place we are dealing only 
with the second subject, the 
first having been handled in 
Book I. Chapter I. (ante). 

In anticipation of the transit 
of 1882 very extensive prepar- 
ations were made by all the 
leading European Govern- 
ments, and the American 
Government c . And many 
amateurs joined in the work. 
In England a part only of the 
transit was visible, and bad 
weather generally prevailed 
which interfered with such 
part as otherwise might have 
been seen. Shortly before the 
planet entered on the Sun's 
disc that portion of its limb which was outside the Sun appeared, 
according to Prince, " to be illuminated by a brilliant silver line 




VENUS JUST BEFORE THE COMMENCE- 
MENT OP ITS TRANSIT, 1 88 2. (Prince.') 



ft Month. Not., vol. xxxv. p. 133 (Jan. 
l8 75); P- 3 10 (March 1875). For a full 



c The American Government issued a 
very important and exhaustive series of 



account of the observations of 1874 see Instructions. (4to. Washington, U.S. 



Mem. R.A.S., vol. xlvii. p. 31, 1883. 
b Mem. R.A.S., vol. xlvii. p. 101. 1883. 



1882.) 



Figs. 163-168. 



Plate XXII. 




h. in. s. 

At 19 7 12 

FIRST FORMATION OF LIGAMENT. 



1 




h. ra. s. 
At 19 8 5 

APPARENT CONTACT. 




h. in. g. 

At 19 14 20 

THE LIGAMENT BROADER. 




h. m. 8. 
At 19 7 29 

APPARENT CONTACT NOT PERFECT. 




h. m. s. 
At 19 9 39 

THE LIGAMENT BROAD. 




h. m. g. 
At 19 19 2O 
THE LIGAMENT BROADEST. 



VENUS DURING ITS TRANSIT IN 1874. 

(Drawn by E. J. Stone.} 



CHAP. X.] Transits of the Inferior Planets. 



353 



of light, which most distinctly marked the limb of that portion 
of the planet, and which was doubtless produced by the refrac- 
tion of sunlight passing through the planet's atmosphere. The 
effect was very beautiful d ." 

This illuminated streak, but far less sharply defined than 

Figs. 169 171. 






At 14 55 3 
Loc. Sid. Time. 



J 4 55 56 
Loc. Sid. Time. 



14 56 ii 
Loc. Sid. Time. 



VENUS DURING ITS TRANSIT, 1882. 



Prince saw it, was also observed in America by Prof. S. P. 
Langley, who says : 

"It was therefore watched by me, with occasional interruptions, for about 7. 
Owing to the boiling of the limb, it was not easy to determine how much of this 
light lay without, how much within, the planet's contour. When first seen, it 
suggested for a moment the appearance of Baily's Beads, but the writer's very strong 
final impression was that it at any rate extended to some degree within the planet, 
and was brightest on the outside, with a slight gradation toward the planet's centre. 
Its greatest width was estimated at one-fourth of the planet's radius. Every pre- 
caution was taken against instrumental error. The spot was successively examined 
in different parts of the field, the eye-piece was rotated, and the amount of light 



lt Month. Not., vol. xliii. p. 64. Dec. 1882. 

A a 



354 Eclipses and Associated Phenomena. [BOOK II. 

from the reflectors was varied. It was beyond any question a real, if a most 
unexpected and unintelligible phenomenon, and it seems to me that it points to a 
real local cause on the planet. It does not appear to be at all assimilable to the 
concentric spots which some observers have believed they saw both on Venus and on 
Mercury in transit, nor to the alleged phosphorescence on the dark side e ." 

This phenomenon, with variations of detail, was seen by 
Brodie f (by whom it was assumed to be a twilight effect 
resulting from an atmosphere on Venus), by Horner 8 , and 
probably by others. 

Figs. 169-171 were drawn byM. Hatt atChubut, and represent 
the phenomena seen at ingress. The observer seems to have 
been much struck with the appearance presented by the fringe 
of light which surrounded the planet just before the end of the 
internal contact. 

Month. Not., vol. xliii. p. 72. Jan. 1883. 
' Ibid., p. 76. B Ibid., p. 277. 



CHAP. XI.] Occultations. 355 



CHAPTER XI. 



OCCULTATIONS. 



How caused. Table annually given in the "Nautical Almanac." Occultation by a 
young Moon. Effect of the Horizontal Parallax. Projection of Stars on the 
Moons disc. Occultation of Jupiter, January 2, 1857. Occultation of Saturn, 
May 8, 1859. Occultation of Saturn, April 9, 1883. Historical notices. 



WHEN any celestial object is concealed by the interposition 
of another, it is said to be " occulted," and the pheno- 
menon is called an " occupation." Strictly speaking, an eclipse 
of the Sun is an Occultation of that luminary by the Moon, but 
usage has given to it the special name of " eclipse." The 
most important phenomena of this kind are the occultations of 
the planets and larger stars by the Moon, but the Occultation of 
one planet by another, on account of the rarity of such an oc- 
currence, is exceedingly interesting. Inasmuch as the Moon's 
apparent diameter is about ^, it follows that all stars and planets 
situated in a zone extending j on each side of her path will 
necessarily be occulted during her monthly course through the 
ecliptic, and parallax will have the effect of further increasing 
considerably the breadth of the zone of stars subject to occulta- 
tion. The great brilliancy of the Moon entirely overpowers the 
smaller stars, but the disappearances of the more conspicuous 
ones can be observed with a telescope, and a table of them is 
inserted every year in the Nautical Almanac. 

It must be remembered that the disappearance always takes 
place at the limb of the Moon which is presented in the direction 

A a 2 



356 Eclipses and Associated Phenomena. [BOOK II. 

of its motion. From the epoch of its New to that of its Full 
phase the Moon moves with the dark edge foremost, and from 
the epoch of its Full to that of its New phase with the illumi- 
nated edge foremost: during the former interval, therefore, the 
objects occulted disappear at the dark edge, and reappear at the 
illuminated edge ; and during the latter period they disappear at 
the illuminated, and reappear at the dark edge. If the occul- 
tation be watched when the star disappears on the dark side 
of the Moon, that is to say during the first half of a lunation, and 
preferably when the Moon is not more than 2 or 3 days old, the dis- 
appearance is extremely striking, inasmuch as the object occulted 
is suddenly extinguished at a point of the sky where there seems 
nothing to interfere with it. Wargentin relates that on May 18, 
1761, he saw an occultation of a star by the Moon during a total 
eclipse of the latter. He says that the star disappeared " more 
quickly than the twinkling of an eye a ." In consequence of the 
effect of parallax, the Moon, as seen in the Northern hemisphere, 
follows a path different from that which it appears to take as seen 
in the Southern hemisphere ; it happens, therefore, that stars 
which are occulted in certain latitudes are not occulted at all in 
others, and of those which are occulted the duration of invisi- 
bility, and the moment and place of disappearance and reappear- 
ance, are different. 

I must not omit a passing allusion to a circumstance occa- 
sionally noticed by the observers of occultations ; namely, the 
apparent projection of the star within the margin of the Moon's 
disc. 

Admiral Smyth gives an instance, under the date of October 15, 
1829. He says: 

" I saw Aldebaran approach the bright limb of the Moon very steadily ; but, from 
the haze, no alteration in the redness of its colour was perceptible. It kept the same 
steady line to about of a minute inside the lunar disc, where it remained, as pre- 
cisely as I could estimate, 2% seconds, when it suddenly vanished. In this there 
could be no mistake, because I clearly saw the bright line of the Moon outside the 
star, as did also Dr. Lee, who was with me b ." 

" Phil. Trans., vol. li. p. 210. 1761. the projection, though F. Baily and others 
b Mem. R.A.S., vol. iv. p. 642. 1831. didnot see it. 
Other observers, Maclear included, saw 






CHAP. XI.] Occultations. 357 

Sir T. Maclear saw the same thing happen to the same star on 
October 23, 1831 : 

" Previous to the contact of the Moon, and star nothing particular occurred ; but 
at that moment, and when I might expect the star to immerge, it advanced upon the 
Moon's limb for about 3 seconds, and to rather more than the star's apparent diameter, 
and then instantly disappeared c ." 

" This phenomenon seems to be owing to the greater propor- 
tionate refrangibility of the white lunar light, than that of the 
red light of the star, elevating her apparent disc at the time and 
point of contact d ." 

In 1699 La Hire endeavoured to explain the apparition of stars 
on the Moon's disc by supposing that the true disc is accompanied 
by a parasitic light, or, as it was formerly termed, a circle of 
dissipation, which enlarges the star's apparent diameter, and 
through which it shews itself before passing behind the opaque 
part of the lunar globe. Arago accepted this theory with the ex- 
planation that the observer's eye-piece must be in imperfect 
focus, and that so the false disc is caused. The fact that some 
have and some have not seen the phenomenon he considered 
confirmatory of this explanation e . 

The present state of the question is that we do not possess any 
certain explanation of the phenomenon. 

A remarkable occurrence was noticed by Mr. Ralph Copeland, 
on the occasion of the occultation of K Cancri on April 26, 
1863:- 

" About three-fourths of the light disappeared in the usual instantaneous manner ; 
and after an interval of (as near as I can judge) rather more than half a second, the 
remaining portion disappeared." 

Dawes regarded this as a decisive indication that the star was 
double, though he failed to verify this surmise f . On Oct. 30, 



c Mem. E.A.S., vol. v. p. 373. 1833. by Stevelly discussing the Diffraction 

d Smyth. hypothesis in Brit. Assoc. Hep., 1845 ; 

. e Pop. Ast., vol. ii. p. 348, Eng. ed. Transactions of the Sections, p. 5. Also 

For other remarks on this phenomenon, one by Plummer in Month. Not., vol. 

see papers by Airy in Mem. E.A.S., vol. xxxiii. p. 345 (March 1873). 

xxviii. p. 173, 1860, and Month. Not., f Month. Not., vol. xxiii. p. 221 (May 

vol. xix. p. 208 (April 1859), an d one 1863). 



358 Eclipses and Associated Phenomena. [BOOK II. 

1 863, I watched the emersion of x^ 1 Orionis, and it was unques- 
tionably not instantaneous. 

An occultation of the planet Jupiter took place on January 2, 
1857. A dark shadowy streak which appeared projected on the 
planet, from the edge of the Moon, was seen by several observers. 

Fig. 172. 




OCCDLTATION OF JUPITER BY THE MOON : January 2, 1857. (Lastell^ 
Mr. W. Simms, Sen. thus described it : 

"The only remarkable appearance noticed by me during the emersion was the 
very positive line by which the Moon's limb was marked upon the planet ; dark as 
the mark of a black-lead pencil close to the limb, and gradually softened off as the 
distance increased 8 ." 

A representation of this appearance, from a drawing by Lassell, 
is annexed [Fig. 172]. 

An occultation of the planet Saturn by the Moon took place 
on May 8, 1859. Dawes thus described it : 

" At the disappearance, the dark edge of the Moon was sharply denned on the 
rings and ball of the planet, without the slightest distortion of their figure. There 
was no extension of light along the Moon's limb. Even the satellites disappeared 
without the slightest warning, and precisely at the edge which was faintly visible. 

" At the reappearance I could not perceive any dark shading contiguous to the 
Moon's bright edge, such as was seen by myself and several other observers on 
Jupiter on January 2, 1858 [Qy. 1857]. The dark belt south of the planet's equator 
was clearly defined up to the very edge ; and there was no distortion of any kind, 
either of the rings or ball. 

"The very pale greenish hue of Saturn contrasted strikingly with the brilliant 
yellowish light of the Moon h ." 

" Month. Not., vol. xvii. p. 81 (Jan. 1859). Other observations will be found 
1857). a * P- 3 3^ of the same volume. 

h Month. Not., vol. xix. p. 241 ^ 



CHAP. XI.J 



Occultations. 



359 



Mr. W. Simms, Jun. did see a dark shading on the planet 
contiguous to the Moon's bright edge ; but in 1857 he failed to 
notice it. 

The occultation of Saturn on April 9, 1883, was observed 
by Mr. L. W. Loomis, who remarked on the impression being 
vividly conveyed that the Moon was very much nearer to the 
eye than Saturn. The successive disappearance of the rings was 
an extremely interesting phenomenon. 

Fig- 173. 




OCCULTATION OF SATURN BY THE MOON : April 9, 1883. (L. W. 

In an occultation of Saturn on Oct. 30, 1825, Messrs. R. 
Cornfield and J. Wallis plainly saw both one ansa and the ball 
flattened *. 

The earliest record which we have of an occultation is that of 
an occultation of Mars by the Moon, mentioned by Aristotle k . 
Kepler found that it occurred on the night of April 4, 357 B,c. l 

Instances are on record of one planet occulting another, but 
these are of very rare occurrence. Kepler states that he watched 
an occultation of Jupiter by Mai's on January 9, 1591. He also 



1 Mem. R.A.S., vol. ii. p. 457. 1826. k De Coelo, lib. ii. cap 12. 

1 Ad Vitell. Paralifom., p. 307. 



360 Eclipses and Associated Phenomena. 

mentions that Moestlin witnessed an occultation of Mars by 
Venus on October 3, 1590. Mercury was occulted by Venus on 
May 17, i737 m . As these observations, with the exception of 
the last, were made before the invention of the telescope, it is 
possible that the one planet was not actually in front of the 
other, but only that they were so close together as to have had 
the appearance of being one object : as was the case with Venus 
and Jupiter on July 21, 1859. 

Sometimes stars are occulted by planets. J. D. Cassini men- 
tions the occultation of a star in Aquarius by Mars on October i , 
1672". 

m Phil. Trans., vol. xl. p. 394. 1738. Twining in Amer. Journ. of Science, and 
11 See a paper on Occultationa by A. C. Ser., vol. xxvi. p. 15. July, 1858. 



BOOK III. 

PHYSICAL AND MISCELLANEOUS 
ASTRONOMICAL PHENOMENA. 



CHAPTEB I. 

THE TIDES. 



' O ye seas and floods, bless ye the LORD : praise Him, and magnify 
Him for ever." Benedicite. 



Introduction. Physical cause of the Tides. Attractive force exercised by the 
Moon. By the Sun. Spring Tides. Neap Tides. Summary of the principal 
fact*. Priming and Lagging. Diurnal Inequality. 

MANY inhabitants of a maritime country like Great Britain 
have some acquaintance with the phenomena now to come 
under consideration, but beyond a vague notion that the Moon 
has something to do with the tides, very few people have an 
intelligent idea of the way in which the tides are produced a . 

These phenomena are very frequently attributed to the attrac- 
tion of the Moon, whereby the waters of the ocean are drawn 
towards that side of the Earth on which our satellite happens 
to be situated ; in fact, that it is high water when the Moon is 
on or near the meridian, of the place of observation. 

This, though to a great extent true, by no means adequately 

a See a paper by the late Sir J. Lub- by Sir G. B. Airy, in Encycl. Metrop.. 

bock, in the Companion to the Almanac vol. v. p. 241. There are maps of co-tidal 

for 1 830, p. 49. And reference should lines around the British Isles, and over 

also be made to an important and ex- the World generally, which will be found 

haustive Memoir on "Tides and Waves " of interest. 



362 Miscellaneous Astronomical Phenomena. [BOOK III. 

represents the facts of the case, for high water is not only pro- 
duced on the side of the Earth immediately under the Moon, but 
also on the opposite side at the same time. The coincident tides 
are therefore separated from each other by 180, or by half the 
circumference of the globe. Since the diurnal rotation of the 
Earth causes every portion of its surface to pass successively 
under the tidal waves in about 24 h , it follows that there are 
everywhere 2 tides daily, with an interval of about 1 2 h between 
each ; whereas, if the common supposition were correct, there 
would be only one. 

Such being the observed facts, and it being admitted that the 
attraction of the Moon gives rise to the upper tide, some further 
explanation must be sought to account for the lower one. The 
solution is extremely simple as an elementary conception : it is 
only necessary to bear in mind that not only does the Moon 
attract the upper mass of water, but also the solid globe itself, 
which is consequently compelled to recede from the waters 
beneath, leaving them behind, and in a sense heaped together. 

Besides the influence of the Moon in elevating the waters of 
the ocean, that of the Sun is to some extent concerned, but it is 
much more feeble than that of the former, on account of the 
much greater distance of the solar globe. The mean distance of 
the Sun from the Earth is 309-144 times that of the Moon; its 
attractive power is consequently (309- 144) 2 , or 95,570 times less ; 
but inasmuch as the mass of the Sun exceeds that of the Moon 
in the ratio of 25,916,280 to i, which is much greater than 
95,570 to i, it will naturally be said that surely the attraction 
exercised by the Sun exceeds that of the Moon in the same 
proportion that 25,916,280 exceeds 95,570 b . This, however, is 
not the case, for a reason which will now be stated. It must be 
borne in mind that the tides are due solely to the inequality of 
the attraction in operation on different sides of the Earth, and 
that the greater that inequality is the greater will be the 
resulting tide, and vice versa. The mean distance of the Sun from 

b To avoid complicating the obviously crude argument in the text certain thing? 
are left out of consideration. 






CHAP. I.] The Tides. 363 

the Earth is 11,720 diameters of the latter, and consequently 
the difference between its distance from the one side of the 
Earth and from the other will be only TTTTTT of the whole dis- 
tance, while in the case of the Moon, whose mean distance is 
only 30 terrestrial diameters, the difference between the distances 
from one side and from the other, reckoned from the Moon, will 
be ^V of the whole distance. The inequality of the attraction 
(upon which the height of the tidal wave depends) is therefore 
much greater in the case of the Moon than of the Sun ; the ratio, 
according to Newton, being 58 : 23, or about 1\ : i. 

We thus see that there are 2 kinds of tides, lunar and solar. 
When therefore the Sun, Moon, and Earth are in the same 
straight line with each other, that is to say, when it is either 
New or Full Moon, the attractions of the two former bodies act in 
the same line, and we have the highest possible tidal elevations, 
and what are known as "Spring tides ;" but when the Moon is 
in quadrature, or 90 from the Sun, its attraction acts along a 
line which is perpendicular to that along which the attraction 
of the Sun acts, the two tidal elevations are 90 apart, and we 
have the tides which are called " Neap" 

It may be convenient to state here a few general facts relating 
to the tides : 

1. On the day of New Moon, the Sun and Moon cross the 
meridian at the same time, i. e. at noon, and at an interval after 
their passage (varying according to the place of observation, but 
unchangeable or nearly so for each place) high water occurs. 
The water, having reached its maximum height, begins to fall, 
and after a period of 6 h 1 2 m attains a maximum depression ; it 
then rises for 6 h 1 2 m , and reaches a second maximum ; falls for 
another interval of 6 h 1 2 m , and rises again during a 4 th interval 
of 6 h I2 m . It has therefore 2 maxima and 2 minima in a period 
of 24 h 48 m , which is called a Tidal Day. 

2. On the day of Full Moon, the Moon crosses the meridian 

c Practically this is somewhat incor- place at the mean moment between the 
rectly expressed, for it is found that the two tides, the waters usually taking a 
intermediate low water does not take shorter time to rise than they do to fall. 



364 Miscellaneous Astronomical Phenomena. [BOOK III. 

1 2 h after the Sun, i. e. at midnight, and the tidal phenomena are 
the same as in (i). 

3. As time is reckoned by the apparent motion of the Sun, the 
solar tide always happens at the same hour at the same place, 
but the lunar tide, which is the greater, and thereby gives a 
character to the whole, happens 48 m 44 s later every day ; it 
therefore separates Eastwards from the solar tide, at that rate, 
and gradually becomes later and later, till at the periods of the 
I 8t and 3 rd quarters of the Moon it happens at the same time as 
the low water of the solar tide : then the elevation of the high, 
and the depression of the low water, will be the difference of the 
solar and the lunar tides, and the tide will be neap. 

4. The difference in height between the high and low water is 
called the Range of the tide. 

5. The spring tides are highest, especially those which happen 
36 h after the New, or Full Moon. 

6. The neap tides are the lowest, especially those which 
happen 36 h after the Moon is in quadrature. 

7. The interval of time from Noon to the time of high water 
at any particular place is the same on the days both of New and 
Full Moon. This interval is technically known as the "Establish- 
ment of the port." 

The reason why an interval of time elapses between the Moon's 
meridian passage and the time of high water is, that the waters 
of the ocean have to overcome a certain peculiar effect of friction, 
which cannot immediately be accomplished ; it thus happens 
that the lunar tidal wave is not found immediately under the 
Moon, but follows it at some distance. Similar results ensue in 
the case of the solar wave. The tidal wave is also affected in 
another way, by the continued action of both these luminaries, 
and at certain periods of the lunar month is either accelerated 
or retarded in a way which will now be described : " In the I st 
and 3 rd quarters of the Moon, the solar tide is Westward of the 
lunar one ; and consequently the actual high water (which is the 
result of the combination of the 2 waves) will be to the West- 
ward of the place it would have been at if the Moon had acted 



CHAP. I.] The Tides, 365 

alone, and the time of high water will therefore be accelerated. 
In the 2 nd and 4 th quarters, the general effect of the Sun is, for 
a similar reason, to produce a retardation in the time of high 
water. This effect, produced by the Sun and Moon combined, 
is called the Priming and Lagging of the tides. The highest spring 
tides occur when the Moon passes the meridian about i | h after 
the Sun ; for then the maximum effect of the 2 bodies coincides." 
The "priming" and "lagging" effect deranges the average 
retardation, which from a mean value of 48 m may be augmented 
to 6o m or be reduced to 36 m . 

The 2 tides following one another are also subject to a 
variation, called the Diurnal Inequality, depending on the daily 
change in declination of the Sun and Moon ; the laws which 
govern it are, however, very imperfectly known. 

Guillemin writes : " The height of the tides again varies with 
the declinations of the Moon and Sun ; it is by so much greater 
as the two bodies are nearer the equator. Twice a year, towards 
March 21 and Sept. 22, the Sun is actually in the equator. If, 
at the same time, the Moon is near the same plane the tides 
which occur then are the highest of all. These are the Equinoctial 
Spring Tides, because the Earth is then at the vernal or autumnal 
equinox. On the other hand, the smallest tides take place 
towards the solstices, if the Moon attains its smallest or its 
greatest meridional height at the same time as the Sun. Lastly, 
the distances of the Moon and Sun from the Earth have also 
their influence on the height of the tides Other things being 
equal, the height of a tide is greater or less, according as the 
attracting bodies are nearer to or farther from the Earth. Thus 
the tides of the winter solstice are higher than those of the 
summer one d ." 

ll The Heavens. Eng. ed., p. 461. 



366 Miscellaneous Astronomical Phenomena. [BOOK III. 



CHAPTER II. 



LOCAL TIDAL PHENOMENA. 



Local disturbing influences. Table of Tidal ranges. Influence of the Wind. 
Experiment of Smeaton. The Tides in the Severn at Chepstow. Tidal phe- 
nomena in the Pacific Ocean. Remarks by Beechey. Velocity of the great 
Terrestrial Tidal wave. Its course round the Earth, sketched by Johnston. 
Effects of Tides at Bristol. Instinct of animals. Tides extinguished in rivers. 
Instances of abnormal Tidal Phenomena. The " Mascaret" on the Seine. 
Historical notices. 



WE have hitherto been considering the tidal wave, on the 
supposition of the Earth being a perfect sphere covered 
with water to a uniform depth ; but inasmuch as this is not the 
case, it follows that the actual phenomena of the tides are 
widely different and of a much more complicated character, 
owing to the irregular outline of the land, the uneven surface 
of the ocean bed, the action of winds, currents, friction, &c. 
The effects of these disturbing influences are rendered especially 
manifest in the difference of the range of the tide at different 
places on the Earth's surface. If the surface of our globe were 
entirely covered with water, the height of a solar tide would be 
i ft HsV n j an d of a lunar tide 4 ft o in ; but the differences 
in the level of the water of the ocean brought about by tidal 
influences are often far in excess of these figures. For instance, 
in deep estuaries or creeks, open in the direction of the tidal 



CHAP. II.] Local Tidal Phenomena. 367 

wave, and gradually converging inward, the range is very much 
greater than elsewhere, as at : 

Feet. 

Bay of Fundy * ... ... ... ... ... ... ... 50 

Gallegos River (Patagonia) ... ... ... ... ... 46 

Mouth of the Avon ... ... ... ... ... ... 42 

St. Malo ... ... ... ... ... ... 40 

Bristol Channel (off Chepstow a ) ... ... ... ... 38 

Milford Haven ... ... ... ... ... ... ... 36 

On the other hand, where promontories or headlands jut out into 
the sea, the tidal range is frequently small ; thus : 

Feet. 
Wicklow ... ... ... ... ... ... ... ... 4 

Weymouth ... ... ... ... ... ... ... 7 

The Needles ... ... ... ... ... ... ... 9 

Cape Clear ... ... ... . . ... ... ... 1 1 

In very large open waters, like the Atlantic or the Pacific Oceans, 
and in confined seas, like the Baltic, the Mediterranean, &c., the 
elevation of the tidal wave is inconsiderable ; thus : 

Feet. Inches. 

Toulon ... ... ... ... ... ... ... i o 

Antium ... ... ... ... ... i 2 

Porto Rico (S. Juan) ... ... ... ... ... i 6 

South Pacific i 8 

St. Helena ... ... ... ... ... ... ... 3 o 

The usual range of the tides at any particular place is also 
affected by certain conditions of the atmosphere. At Brest, a 
depression of i in in the barometric column causes a difference 
of i6 in in the elevation of the high-water mark ; at Liverpool, 
corresponding to the depression of i in , the difference is about 
io in ; and at the London Docks about 7 in : thus when the 
barometer is low, an unusually high tide may be expected, and 
vice versa. And the influence of the wind also is frequently very 
considerable, so much so that during a violent hurricane, Jan. 8, 
1839, there was no tide at all at Gainsborough on the river 
Trent, a circumstance never before recorded. Smeaton found 
experimentally in a canal 4 miles long, that the water-level at 
one end was 4 in higher than at the other, owing to the force 
of the wind acting on the surface of the water. 

See Nature, vol. xix. p. 432. March 13, 1879. 



368 Miscellaneous Astronomical Phenomena. [BOOK III. 

Concerning the tides at Chepstow, Mr. A. Miller, the lessee of 
the Fisheries there, wrote to me thus, under date of June 7, 
1888: 

"The rise and fall of the spring tides at Chepstow, New 
Passage on Severn, and Clevedon piers, is 45 to 46 ft , taken 
as the highest spring tide. There is scarcely 6 in difference at 
either of these points. I have had careful measurements taken 
for several years. Four years ago [October 17, 1883], the tide 
rose to 48 ft or 49". This was caused by a gale of wind and a very 
exceptional high flood from the hills, the result of unusually heavy 
rain. The houses in the lower part of the town were flooded 2 ft 
deep, and the river overflowed its banks in the Bristol Channel. 
The same thing occurred in 1854. These measurements were 
taken from low-water mark to high- water mark, not from the bed 
of the river or channel. The tidal wave or bore on the Severn 
begins at the Lyde rock just below Beachley and immediately 
above the mouth of the Wye. I have known it go up the Wye 
for about 4 miles in the shape of an unbroken wave i8 in high." 

The tides in the Pacific Ocean present great anomalies. The 
following remarks respecting them are by a missionary: 

" It is, to the missionaries, a well-known fact that the tides in Tahiti and the 
Society Islands are uniform throughout the year, both as to the time of the ebb and 
flow, and the height of the rise and fall, it being high water invariably at noon and 
at midnight, and consequently the water is at its lowest point at 6 o'clock in the 
morning and evening. The rise is seldom more than 1 8 inches or 2 feet above low- 
water mark. It must be observed that mostly once, and frequently twice in the 
year, a very heavy sea rolls over the reef, and bursts with great violence upon the 
shore. But the most remarkable feature in the periodically high sea is, that it 
invariably comes from the W. or S.W., which is the opposite direction to that 
from which the Trade wind blows. The eastern sides of the island are, I believe, 
never injured by these periodical inundations. I have been thus particular iu my 
observations, for the purpose in the first place of calling the attention of scientific 
men to this remarkable phenomenon, as 1 believe it is restricted to the Tahitian and 
Society Island Groups in the South Pacific, and the Sandwich Islands in the North. 
I cannot, however, speak positively respecting the tides at the islands eastward of 
Tahiti ; but all the islands I have visited in the same parallel of longitude south- 
wards, and in those to the westward in the same parallel of latitude, the same 
regularity is not observed, but the tides vary with the Moon, both as to the time and 
the height of the rise and fall, which is the case at Raratonga b ." 

b J. Williams, Narrative of Missionary Enterprises in the South Seas, p. 201. 






CHAP. II.] Local Tidal Phenomena. 369 

The late Admiral Beechey is, so far as I know, the only 
person who ever attempted any solution of the question, and 
he proposed as a simile, a basin to represent the harbour, 
over the margin of which the sea breaks with considerable 
violence, thereby throwing in a larger body of water than 
the narrow channels can carry off in the same time, and con- 
sequently the tide rises, and as the wind abates the water 
subsides. 

The writer above quoted objects to this explanation, and he 
brings forward several arguments, and states several facts, of 
which the following is an abstract : 

1. The undeviating regularity of the tide is so well under- 
stood by the natives that they distinguish the hours of the day 
by terms descriptive of the state of the tide, such as the 
following: "Where is the tide 1 ?" instead of, as we should say, 
"What o'clock is it?" 

2. There are many days during the year when it is perfectly 
calm, and yet the tide rises and falls in the same way, and very 
frequently there are higher tides in calms than during the pre- 
valence of the Trade wind. 

3. The tides are as regular on the West side of the island, 
where the Trade wind does not reach, as on the East, from which 
point it blows. 

4. The Trade wind is most powerful from noon till 4 or 5 
o'clock P.M., during which time the water ebbs so fast that it 
reaches its lowest level by 6 o'clock P.M., instead of in the 
morning, as Admiral Beechey states, at which time it is again 
high water. 

Admiral Beechey's explanation does not seem very satisfactory, 
but we are not in possession of any other. 

The velocity of the tidal wave is subject to much variation, 
and we are not yet in a position to lay down the laws which 
govern it. If the whole globe were uniformly covered, the 
velocity would be rather more than 1000 miles per hour (7926 x 
3-141 6-^- 24*8). It is probably, however, nowhere equal to this, 
unless perhaps in the Antarctic Ocean. 

Bb 



370 Miscellaneous Astronomical Phenomena. [BOOK III. 
The following table of velocities is given by Whewell c : 

Miles. 

In latitude 60 S. 670 

In the Atlantic ... ... ... ... ... ... 700 

Azores to Cape Clear ... ... ... ... ... ... 500 

Cape Clear to Duncansby Head ... ... ... ... 160 

Buchan Ness to Sunderland ... ... ... 60 

Scarborough to Cromer ... ... ... ... ... 35 

North Foreland to London ... ... ... ... ... 30 

London to Richmond ... ... ... ... ... ... 13 

Concerning the general character of the great terrestrial tidal 
wave, I cannot do better than quote the following description by 
a well-known eminent geographer : 

" The Antarctic is the cradle of tides. It is here that the Sun and Moon have 
presided over their birth, and it is here, also, that they are, so to speak, to attend on 
the guidance of their own congenital tendencies. The luminaries continue to travel 
round the Earth (apparently) from East to West. The tides no longer follow them. 
The Atlantic, for example, opens to them a long, deep canal, running from North to 
South, and after the great tidal elevation has entered the mouth of this Atlantic 
canal, it moves continually Northward ; for the second 1 2 hours of its life it travels 
north from the Cape of Good Hope and Cape Horn, and at the end of the first 
24 hours of its existence, has brought high water to Cape Blanco on the West 
of Africa, and Newfoundland on the American continent. Turning now round to 
the Eastward, and at right angles to its original direction, this great tidal wave 
brings high water, during the morning of the 2 nd day, to the Western coasts of 
Ireland and England. Passing round the Northern cape of Scotland, it reaches 
Aberdeen at noon, bringing high water also to the opposite coasts of Norway and 
Denmark. It has now been travelling precisely in the opposite direction to that 
of its genesis, and in the opposite direction, also, to the relative motion of the Sun 
and Moon. But its erratic course if not yet complete. It is now travelling from the 
Northern mouth of the German Ocean Southwards. At midnight of the 2 nd day it is 
at the mouth of the Thames, and wafts the merchandise of the world to the quays of 
the port of London. In the course of this rapid journey the reader will have noticed 
how the lines [on the map] in some parts are crowded together closely on each other, 
while in others they are wide asunder. This indicates that the tide- wave is travelling 
with varying velocity. Across the Southern Ocean it seems to travel nearly 1000 miles 
an hour, and through the Atlantic scarcely less ; but near some of the shores, as on 
the coast of India, as on the East of Cape Horn, as round the shores of Great Britain, 
it travels very slowly ; so that it takes more time to go from Aberdeen to London 
than over the arc of 120 which reaches from 60 of Southern latitude to 60 North of 
the Equator. These differences have still to be accounted for ; and the high velocities 
are invariably found to exist where the water is deep, while the low velocities occur 
in shallow water. We must therefore look to the conformation of the shores and 
bottom of the sea as an important element in the phenomena of the tides d ." 

c Phil. Trans., vol. cxxiii. p. 212. 1833. 
d Johnston, Phys. Atlas. 



CHAP. II.] Local Tidal Phenomena. 371 

Tidal effects on rivers are often very striking. Especially 
is this the case with the Avon at Bristol : when the tide is at its 
ebb, the river is little better than a shallow ditch, but when the 
waters have risen to the maximum height, an insignificant stream 
is converted into a broad and deep channel, navigable by the 
largest Indiaman. 

The instinct of animals in respect of the tides is often very 
remarkable. A Scotch writer observes : " The accuracy with 
which cattle calculate the times of ebb and flow, and follow the 
diurnal variations, is such, that they are seldom mistaken, even 
when they have many miles to walk to the beach. In the same 
way they always secure their retreat from these insulated spots 
in such a manner that they are never surprised and drowned." 

In their passage up rivers, tides are gradually extinguished, 
as will be seen from the following table relating to the Thames 6 : 

Height. Distance from Mouth. 

London (Dock ) ... ... ... i8ft. loin. ... 60 m. 

Putney JO 2 ... 67^ 

Kew 7 i ... 73 

Richmond ... ... ... ... 3 10 ... 76 

Teddington ... ... ... ... i 4^ ... 79 

At certain places on the coast of Hampshire and Dorsetshire 
the waters of the ocean ebb and flow twice in 1 2 hours instead 
of only once, as is usual elsewhere. Southampton, Christchurch, 
Poole, Weymouth, and the Firth of Forth, may be mentioned as 
places where this singular phenomenon has been observed f . 

Macculloch, the Scotch writer just quoted, says that in the 
strait between the island of Isla and the islets of Chenzie and 
Oersa the time of the ebb is lof hours, and that of the flood only 
ij hour 8 . 

Another abnormal tidal phenomenon, presenting some re- 
markable features, occurs once a year in the rivers Severn, 
Humber 11 , and Loire, and in some other rivers 1 of the same 

e Phil. Trans., vol. cxxiii. p. 204. 1833. ' The river Dordogne in France is oc- 

f Phil. Trans., vol. cxxiii. p. 226. 1833. casionally the scene of a natural pheno- 

g J. Macculloch, Description of the menon which would appear to present 

Western Islands of Scotland., 1824, vol. some analogy to the "Bore "of the Severn. 

ii. p. 225. And I believe that the Dee at Chester 

h White, Eastern England, \o\.ii. ch.3. furnishes another instance. 

B b 2 



372 Miscellaneous Astronomical Phenomena. [BOOK III. 

character as regards the formation of their banks. This is the 
" hygre," or " bore," and is due to the fact that a wide estuary at 
the mouth of the river suddenly contracts like a funnel. The 
result is, that the estual spring tide rushes up with an over- 
powering force, carrying all before it. This further peculiarity 

Fig. 174. 




THE "MASCABET" ON THE SEINE, FRANCE. 

likewise subsists : namely, that there is no " slack-water," as is 
ordinarily the case in other rivers, between the ebb and flow of 
the tide. The approach of the bore on the Severn may be heard 
at a considerable distance roaring, as it were, in its upward 
progress. The head is about 3 ft high, and it frequently does 



CHAP. II.] Local Tidal Phenomena. 373 

a good deal of mischief to property. The maximum effect is at 
the 4 th tide after the Full Moon. 

Fig. 174, represents the tidal phenomenon known as the 
" Mascaret " on certain French rivers, especially the Garonne 
and the Seine, which corresponds with the "Bore" of the Severn. 

An inspection of the engraving coupled with the remarks 
made above will sufficiently indicate the general character of 
the phenomenon 11 . 

The evident connexion between the periods of the tides and 
those of the phases of the Moon led to the tides being attributed 
to the Moon's action long before their true theory was understood. 
Aristotle x and Py theas of Marseilles m are both said to have 
pointed out the connexion. Julius Csesar adverts to the con- 
nexion existing between the Moon and spring tides n . 

Pliny says : "^Estus maris accedere et reciprocare, maxime 
mirum : pluribus quidem modis : verum causa in sole lundque ." 
Kepler clearly indicated that the principle of gravitation is con- 
cerned 1 * an opinion from which Galileo strongly dissented q . 
Wallis, in 1666, also published a tidal theory 1 '. Before Sir Isaac 
Newton turned his attention to this subject, the explanations 
given were at best but vague surmises. " To him was reserved 
the glory of discovering the true theor} r of these most remarkable 
phenomena, and of tracing, in all its details, the operation of the 
cause which produces them." 

h For further particulars in florid n De Bello Q-allico, lib. iv. cap. 29. 

detail, see a paper by Flammarion in Pliny, Hist. Nat., lib. ii. cap. 99. 

L' Astronomic, vol. v. p. 281, Aug. 1885. '' Epist. Ast., p. 555. 

1 Tlfpl Koa/jLov, i Dialoffhi. 

m Plutarch, DePlacitis,lib.m. cap. 17. r Phil. Trans., vol. i. p. 263. 1666. 



374 Miscellaneous Astronomical Phenomena. [BOOK III. 



CHAPTER III. 

PHYSICAL PHENOMENA. 

Secular Variation in the Obliquity of the Ecliptic. Precession. Its value. Its 
physical cause. Correction for Precession. History of its discovery. Nutation. 
HerscheVs definition of it. Connexion between Precession and Nutation. 



Variation in the Obliquity of t/te Ecliptic. Although 
it is sufficiently near for most purposes to consider the 
inclination of the plane of the ecliptic to that of the equator as 
invariable, yet this is not strictly the case, inasmuch as it is 
subject to a small but appreciable change of 46-45" (C. A. F. 
Peters) per century. This phenomenon has long been known to 
astronomers, on account of the increase it causes in the latitude 
of all stars in some situations, accompanied by a corresponding 
decrease in the opposite regions. Its effect at the present time 
is to diminish the inclination of the planes of the equator and 
the ecliptic to each other ; but this diminution will not go on* 
beyond certain very moderate limits, after which it will again 
increase, and thus oscillate backwards .and forwards through 
an arc of 1 21', the time occupied in one oscillation being 
about 10,000 years. One effect of this variation of the plane 
of the ecliptic that which causes its nodes on a fixed plane to 
change is associated with the phenomena of the precession of 
the equinoxes, and cannot be distinguished from it, except in 
theory b . 

Precession. The precession of the equinoxes is a slow but 

a Compare Genenis viii. 22. the epoch of January i, 1890, is 23 27' 

h The inclination of the ecliptic for i2"j<)". 



CHAP. III.] Precession. 375 

continual shifting of the equinoctial points from East to West c . 
Celestial longitudes and right ascensions are reckoned from the 
vernal equinox, and if this were a fixed point, the longitude of 
a star would never vary, but would remain the same from age 
to age as does its latitude (sensibly}. Such, however, is not the 
case ; as it has been found that apparently all the stars have 
changed their places since the first observations were made by 
the astronomers of antiquity* 1 . Two explanations only can be 
given to account for this phenomenon : we must either suppose 
that the whole firmament has advanced, or that the equinoctial 
points have receded. And as these points depend on the Earth's 
motion, it is far more reasonable to suppose that the phenomenon 
is owing to some perturbation of our globe rather than that the 
starry heavens should have a real motion relative to these points. 
The latter explanation is accordingly adopted, namely, that the 
equinoxes have a periodical retrograde motion from Ea$t to West, 
thereby causing the Sun to arrive at them sooner than it other- 
wise would had these points remained stationary. The annual 
amount of this motion is, however, exceedingly small, being only 
equal to 50-2" e ; and since the circle of the ecliptic is divided 
into 360, it follows that the time occupied by the equinoctial 

c It may be well to mention that the shall see hereafter have very consider- 

equinoxes are the two points where the able proper motions, 
ecliptic cuts the equator ; and are so e Bessel, by a careful discussion of the 

called because when the Sun in its annual most reliable observations, fixed the value 

course arrives at either of them, day and of general precession for the epoch of 

night are equal throughout the world. 1750 at 50-21129", and the value of luni- 

The point where the Sun crosses the solar precession at 5O'3757 2 "- For the 

equator, going north, is known as the epoch of 1800 he gave for the value of 

vernal equinox ; and the opposite point, the latter 50-36354". The lunar preces- 

through which the Sun passes going sion is about 2\ times the solar preces- 

south, as the autumnal equinox. These sion, just as the lunar tide is 2\ times 

intersecting points are also termed nodes, the solar tide, and for much the same 

and an imaginary line joining the two, reason, namely, the difference of the at- 

the line of nodes. The ascending node tractions. Dreyer's value for 1800 for 

( 8 ) answers to the vernal equinox, and the general precession is 50-2365", and 

the descending ( ?S ) to the autumnal. for the luni-solar precession 50-3752" 

d By " change of place " is here meant (Copernicu*, vol. ii. p. 155. 1882). And 

change of position of the Sphere as a see a paper by L. Struve, Mem. de I'Acad. 

whole to certain fixed co-ordinates, not de St. Petersboury, ;th Ser.. vol. xxxv. 

change of place of the stars inter se, so as p. 3, cited Observatory, vol. xi. p. 200. 

to alter the figures of the Constellations ; April 1888. 
although many individual stars as we 



376 Miscellaneous Astronomical Phenomena. [BOOK III. 

points in making a complete revolution of the heavens is 
25,817 years. It is owing to precession that the Pole-star varies 
from age to age, and also that whilst the sidereal year, or actual 
revolution of the Earth round the Sun, is $6$* 6 h 9 i r-o 8 , the 
equinoctial, solar, or tropical year is only 365* 5 h 48 m 46-05" 
(Airy). The successive returns of the Sun to the same equi- 
noctial points must therefore precede its return to the same 
point on the ecliptic by 2o m 2495 8 of time, which corresponds 
to about 50-27" of arc. It is also on account of the precession 
of the equinoxes that the signs of the ecliptic do not now corre- 
spond with the constellations of the same name, but lie about 28 
Westward of them. Thus, that division of the ecliptic known as 
the sign of Taurus lies in the constellation Aries, the sign of Aries 
having passed into Pisces. It should be remarked, however, 
that the signs and constellations coincided with one another 
about 100 B.C. In recent times, the attempts that have been 
made to establish the motion of the solar system through space 
have rendered an accurate knowledge of precession indispensable ; 
and the elaborate labours of C. A. F. Peters and O. Struve 
have led to a slight modification in the value of the constants 
of precession adopted by Bessel f . Their new value for the 
general precession is, for 1800, 50-241 i" + 0-0002268" . 

" The cause of precession is to be found in the combined action 
of the Sun and Moon^ upon the protuberant mass of matter 
accumulated at the Earth's equator, the attraction of the planets 
being scarcely sensible h . The attracting force of the Sun and 
Moon upon this shell of matter is of a two-fold character ; one 
parallel to the equator, and the other perpendicular to it. The 
tendency of the latter force is to diminish the angle which the 
plane of the equator makes with the ecliptic ; and were it not for 
the rotatory motion of the Earth, the planes would soon coincide ; 
but, by this motion, the planes remain nearly constant to each 
other. The effect produced by the action of the force in question 

1 Tabula, Regiomontance. precession, given at any time, includes 

t Called hence, luni-solar precession. the variation caused by the planets, it is 
h When the value of the constant of called the constant of gen era I precession. 



CHAP. III.] Precession and Nutation- 377 

is, however, that the plane of the equator is constantly, though 
slowly, shifting its place in the manner we have endeavoured to 
describe/' 

In the reduction of astronomical observations the correction to 
be applied for precession in right ascension is almost always 
additive ; increasing in the regions round the poles of the 
heavens, but becoming very small near the poles of the ecliptic. 
It is in the space included between these poles in each hemisphere 
that the correction becomes subtractive ; in the northern hemi- 
sphere, this small space comprehends the constellations lying 
near the XVIII th hour of R.A., that being the R.A. of the North 
ecliptic pole ; and in the southern hemisphere, the constellations 
lying near the VI th hour, that being the R.A. of the South ecliptic 
pole. The remarks I have just made apply only to those stars 
whose declination North or South exceeds 67. The annual preces- 
sion in declination, however, depends on the star's right ascension 
only, both as to amount and direction. At VI and XVIII hours 
it is at ,zero ; at XII hours it reaches the Northern maximum of 
20" ; and at XXIV it reaches a similar Southern maximum. From 
XVIII to XXIV hours, and from XXIV to VI hours, the pre- 
cession is N., consequently additive to stars of North declination, 
but subtractive from those of South decimation : but from VI to 
XVIII, the precession being S., it is additive to Southern, and 
subtractive from Northern stars 1 . 

The discovery of precession dates from about 1 25 B. c., when it 
was detected by Hipparchus, by means of a comparison of his 
own observations with those of Timocharis and Aristyllus, made 
about 178 years previously: its existence was afterwards con- 
firmed by Ptolemy k . It was Copernicus, however, who first gave 
the true explanation of the phenomenon, and Newton who 
discovered its physical cause. 

Nutation 1 . It must be borne in mind that the effect of preces- 
sion varies according to the time of year, on account of the 
ever-varying distance of the Earth from the Sun. Twice a 

' A useful table of precessions will be given in a later volume of this work. 
k Almagest, lib. vii. l Nutatio, nodding. 



378 Miscellaneous Astronomical Phenomena. [BOOK III. 

year, (at the equinoxes,) the influence of the Sun is at zero ; and 
twice a year also, (at the solstices,) it is at its maximum. On no 
two successive days is it of exactly the same value, and con- 
sequently the precession of the equinoctial points is uneven, and 
the obliquity of the ecliptic is subject to a half-yearly variation ; 
since the Sun's force which changes the obliquity is constantly 
varying, while the rotation of the Earth is continuous. This then 
gives rise to a small oscillating motion of the Earth's axis, termed 
the solar nutation : of a far more considerable amount, however, is 
the value of the nutation arising from the agency of the Moon ; 
so much so that it was detected by Bradley before even its 
existence had been inferred from theory m . 

The nature of nutation cannot be better explained than in 
nearly the words of Sir J. Herschel, who says : " The nutation 
of the Earth's axis is a small and slow gyratory movement, by 
which, if subsisting alone, the pole would describe among the 
stars, in a period of i8| years, a minute ellipse having its longer 
axis equal to 18-5", and its shorter to 13-74" (the longer being 
directed towards the pole of the ecliptic, and the shorter of course 
at right angles to it) ; the semi-axis major is, therefore, equal to 
9-25", which quantity is called the 'coefficient of nutation.' The 
consequence of this real motion of the pole is an apparent ad- 
vance and recess of all the stars in the heavens to the pole in the 
same period. Since, also, the place of the equinox on the ecliptic 
is determined by the place of the pole in the heavens, the same 
agency will cause a small alternating motion to and fro of the 
equinoctial points, by which, in the same periods, both the longi- 
tudes and the right ascensions of the stars will be alternately 
increased and diminished. 

" Precession and nutation, although for convenience here con- 
sidered separately, in reality exist together ; they are, in fact, con- 
stituent parts of the same general phenomenon : and since, while 
in virtue of this nutation, the pole is describing its little ellipse 

m Phil. Trans., vol. xlv. p. i. 1748. 9-2231", is the value finally adopted 

n Other values are: Busch's 9'232o", by Peters. (Ntimcrus constans Xi'tci- 

Lundahl's 9-2361", C. A. F. Peters's tionis, 4to. Petropoli, 1842: see p. 5 of 

9-2164". A mean of these, namely W. Strnve 1 * Rapport on Peters's Memoir.) 



CHAP. III.] Precession and Nutation. 379 

of 18-5" in diameter, it is carried on by the greater and regularly 
progressive motion of precession over so much of its circle round 
the pole of the ecliptic as corresponds to i8| years that is to 
say, over an angle i8| times $0-1" round the centre (which, in a 
small circle of 23 28' in diameter, corresponds to 6' 20", as seen 
from the centre of the sphere) ; the path which it will pursue in 
virtue of the joint influence of the 2 motions will be neither an 
ellipse nor an exact circle, but a slightly undulating ring. 

" These movements of precession and nutation are common to 
all the celestial bodies, both fixed and erratic ; and this circum- 
stance makes it impossible to attribute them to any other cause 
than the real motion of the Earth's axis, as we have described. 
Did they only affect the stars, they might, with equal plausibility, 
be considered as arising from a real rotation of the starry heavens 
as a solid shell around our axis, passing through the poles of the 
ecliptic in 25,868 years, and a real elliptic gyration of thai axis 
in rather more than 1 8 years : but since they also affect the Sun, 
Moon, and planets, which, having motions independent of the 
general body of the stars, cannot without extravagance be sup- 
posed to be attached to the celestial conclave, this idea falls to the 
ground ; and there only remains, then, a real motion of the Earth 
by which they can be accounted for ." 

Treatise on Ast., p. 172. 1833. In the original version strikes me as being 
his Outlines of Astronomy Sir John the better of the two, and therefore I 
altered this statement of nutation, but retain it here. 



380 Miscellaneous Astronomical Phenomena. [BOOK III. 



CHAPTEE IV. 

ABERRATION AND PARALLAX. 

Aberration. The constant of Aberration. Familiar illustration. History of the 
circumstances which led to its discovery by Bradley. Parallax. Explanation 
of its nature. Parallax of the heavenly bodies. Parallax of the Moon. Im- 
portance of a correct determination of the Parallax of an object. Leonard 
Digges on the distance of the Planets from the Earth. 

/ABERRATION. The aberration of light is another im- 
-^-^- portant phenomenon which requires to be taken into 
consideration in the reduction of astronomical observations. 
Although light travels with the enormous velocity of 1 86,660 a 
miles per second a speed so great, that for all practical 
terrestrial purposes we may consider it to be propagated 
instantaneously; yet the astronomer, who has to deal with 
distances of millions of miles, is obliged to be more precise. 
A simple illustration will shew this : if we take the mean 
distance of our globe from the Sun at 92,890,000 miles, and 
consider that light travels at the rate of 186,150 miles 
per second, we may ascertain by a simple arithmetical process 
that the time occupied by a ray of light in reaching us from 
the Sun is 8 m 19", so that in point of fact, in looking at the Sun 
at a given moment, we do not see it shining as it is, but as it was 
gm j^s previously. If the Earth were at rest, this would be a 

A. Cornu (Proceedings of the Roy. Young and Forbes, 301,382 kilometres. 

Inst., vol. vii. p. 472, May 1875) makes For a comprehensive review, historical 

it 186,660 miles, but it is probably some- and practical, of the whole subject of the 

what less. Other values obtained ex- velocity of light, see a Memoir by New- 

perimentally are : Helmert, 299,990 kilo- comb in Astron. Paper* prepared for 

metres; Michelson, 299,910 kilometres; American Naut. Aim., vol. ii. part III. 



CHAP. IV.] 



Aberration. 



381 



trivial matter; but as the Earth is in motion, it follows that 
when the solar ray enters the eye of a person on its surface, he 
will be some way removed from the point in space at which he 
was situated when the ray left the Sun ; he will consequently 
see that luminary behind the true place it actually occupies when 
the ray enters his eye. In the course of 8 m 19* the Earth will 
have advanced in its orbit 20-49 a" ; this quantity is called the 
Constant of Aberration b . Aberration may be defined to be a phe- 
nomenon resulting from the combined effect of the motion of 

Fig. 175- 
,p 




B - 
ABERRATION. 



light and of the motion of the Earth in its orbit . Suppose 
a ball let fall from a point P above the horizontal line AB, and 
a tube, of which A is the lower extremity, placed to receive it ; 
if the tube were fixed the ball would strike it on the lower side, 
but if the tube were carried forwards in the direction AB, with 
a velocity properly adjusted at every instant to that of the ball 
while preserving its inclination to the horizon, so that when the 
ball in its natural descent reached B the tube would have been 
carried into the position BQ, it is evident that the ball through- 
out its whole descent would be in the tube ; and a spectator 

b Baily's value is 20-419" ; W. Struve's Struve's was long considered the best, but 

is 20-445"; C. A. F. Peters's, 20-425", Nyren's is now accepted as such. 
20-503", and 20-481"; Lindenau's, c See a paper by Challis in Phil. Mag., 

20-448"; Lundahl's, 20-550"; Maclear's, 4th ser., vol. ix. p. 430. June 1855. 
20-53"; Main's, 20-335"; NyreVs, 20-492". 



382 Miscellaneous Astronomical Phenomena. [BOOK III. 

referring to the tube the motion of the ball, and carried along 
with the former, unconscious of its motion, would fancy that the 
ball had been moving in an inclined direction and had come 
from Q. The following similes are frequently used to exemplify 
aberration: a shower of rain descending perpendicularly will 
appear to fall in its true direction to a person at rest, but if he 
move rapidly through it, it will meet him in a slanting direction : 
in other words, it will have an apparent as well as a real motion. 
A cannon-ball fired from a shore- battery at a vessel passing up a 
river will not pass through the ship in a line coincident with the 
direction of the ball, but will emerge on the other side at a point 
differing more or less from this line ; the amount of the variation, 
however, will depend on the relative velocities of the ball and 
ship at the time. If we suppose the cannon-ball to represent 
light, and the movement of the ship the motion of the Earth in 
its orbit, we have an excellent illustration of the phenomenon 
of aberration d . 

This unquestionably grand discovery resulted more immediately 
from an attempt to detect stellar parallax. Although the facts 
revealed by the invention of the telescope and the discovery of 
gravitation had the effect of establishing beyond doubt the truth 
of the Cppernican theory of the Universe, still it was much to be 
desired that some more direct proof should be adduced. The 
absence of any appreciable change in the positions of the fixed 
stars when examined from opposite sides of the Earth's orbit, was 
one of the earliest, and at the same time one of the most serious, 
arguments brought against the system of Copernicus ; as it was 
always considered that the detection of such a change would 
furnish an irresistible proof that the Earth was not at rest, and 
consequently was not the centre of the system. The first obser- 
vation which ultimately led to the discovery of aberration was 
made by Hooke, who selected the star y Draconis as suitable for 
the detection of annual parallax e . After observing it carefully 



d See Airy's Lectures on Astronomy, serve stars as near the zenith as possible, 

p. 1 88. in order to avoid the effects arising from 

8 Hooke considered it desirable to ob- any uncertainty as to the value of re- 



CHAP. IV.] Parallax. 383 

at different seasons of the year, he came to the conclusion that it 
had a sensible parallax. It was soon found, however, that the 
star was subject to a displacement in a direction contrary to 
that which ought to have resulted had the star been affected 
by parallax only ; and it was for the purpose of endeavouring to 
ascertain the physical cause of this strange phenomenon that 
Bradley was led to provide himself with an instrument, that he 
might more conveniently study the subject of parallax and 
anything that might arise connected therewith. His observations 
completely confirmed those of Hooke, and " at length the happy 
idea occurred to him, that the phenomenon might be completely 
accounted for by the gradual propagation of light combined with 
the motion of the Earth in its orbit." 

Parallax " is the apparent change of place which bodies 
undergo by being viewed from different points." This is the 
general signification of the word ; but with the astronomer 
it has a conventional meaning, and implies the difference be- 
tween the apparent positions of any celestial object when viewed 
from the surface of the Earth and from the centre of either the 
Earth or the Sun, to one or other of which centres it is usual to 
refer all astronomical observations. The position of a heavenly 
body, as seen from the Earth's surface, is called its apparent place ; 
and that in which it would be seen, were the observer stationed 
at the Earth's centre, is known as the true place. It is plain, 
therefore, that the altitudes of the heavenly bodies are depressed 
by parallax, which is greatest at the horizon f , and decreases 
as the altitude of the object increases, until it disappears al- 
together at the zenith. In Figure 176, Z is the zenith, C P the 
visible horizon, A B the rational horizon, O the position of an 
observer, and R the centre of the Earth. From O the observer 
will see the stars projected on the sky at P, P', and P", (apparent 

fraction ; and -y Draconis happened to be ' This is the case because imaginary 

the only bright star passing within a few lines, drawn from the object to the 

minutes of the zenith of Gresham Col- observer, and to the centre of the 

lege, where his instrument was erected. Earth respectively, will then have the 

(Attempttoprovethe Motion of the Earth, greatest possible inclination to each 

p. 7-) other. 



384 Miscellaneous Astronomical Phenomena. [BOOK III. 

placet) ; but, referred to the centre of the Earth, the points of 
projection will be Q, Q', and Q" (geocentric places). The general 
nature of parallax may be readily understood by supposing 
2 persons placed each at the end of a straight line, to look at 
a carriage standing in front of a house at the distance (say) of 
50 yards from each station. It is evident that the carriage will 
appear to each spectator projected upon different parts of the 
house. The angle which this difference of position gives rise to, 
that is to say the angle formed by the 2 lines of direction, is 




PARALLAX. 



the angle of parallax. Let us suppose the 2 observers (still 
at the same distance from each other) to recede from the carriage ; 
the angle of parallax will become more and more acute, until at 
length it will become insensible. The example here adduced 
may be applied to the heavenly bodies 8 . 

Of all the heavenly bodies, the Moon is that of which the hori- 
zontal parallax is the most considerable, because that luminary 
is the nearest to the Earth. It is found in the following way : 

* A very good popular exposition of be found in Guillemin's Soleil, pp. 84-9, 
the principles involved in the measure- 2nd French Edition, 
ment of parallaxes by astronomers will 



CHAP. IV.] Parallax. 385 

Suppose that 2 astronomers take their stations on the same meri- 
dian, one South of the equator, as at the Cape of Good Hope, and 
the other North of the equator, as at Berlin, which 2 places lie 
nearly on the same meridian : the observers would severally refer 
the Moon to different points on the face of the sky the Southern 
observer carrying it farther to the North, and the Northern 
observer farther to the South, than its true place as seen from 
the centre of the Earth. The observations thus made at the 
2 places furnish the materials for calculating, by means of trigo- 
nometry, the value of the horizontal parallax of the Moon, from 
which we can deduce both its distance and real magnitude. The 
parallax thus obtained is called the diurnal, or geocentric, a term 
used to distinguish such parallax from annual, or heliocentric, 
parallax. And in general it may be stated that these terms 
express the angular displacement of a celestial object according 
as it is viewed from the Earth or the Sun respectively: in par- 
ticular, however, it denotes half the angle formed by a imaginary 
lines drawn from each extremity of the diameter of the 
Earth's orbit to a fixed star. But this angle is generally too 
small to be appreciable. It was this fact of the non-detection 
of annual parallax which for a long period of time prior to 
the invention of the telescope formed a great obstacle to the 
progress of Copernican opinions relative to the system of the 
universe. 

The Sun, Moon, and planets, though separated from us by 
millions of miles, are affected by parallax to a small but never- 
theless appreciable amount. With but a few exceptions, however, 
this is not the case with the fixed stars ; for in only a very 
few instances has parallax been detected, and, so far as is yet 
known, the star nearest to us is a Centauri, whose parallax is 
equal to only 07 5", which is equivalent to many billions of 
miles, as will appear hereafter b . 

We may obtain some idea of the importance attaching to a 

h As illustrating the delicacy of obser- inch in diameter would be seen at the 

vations of this kind, the following remark distance of a mile. This is [that of] the 

of Airy's is instructive: "An angle of star which shows the greatest parallax of 

2" is that in which a circle T 6 U of an all." Lectures on Ast,, p. 196. 

C C 



386 Miscellaneous Astronomical Phenomena. [BOOK III. 

correct determination of the parallax of an object by an inspection 
of the following table : 

If the Sun's horizontal parallax were 1 i", the mean distance of the following planets 
from the Sun in miles would be : 

The Earth. Mart. Jupiter. Saturn. 

75,000,000 114,276,750 390,o34>5 00 7 I 5>54>5 

If the Sun's parallax were 10", the above distance would become : 

82,000,000 124,942,580 426,478,720 782,28^,920 

Errors arising from a mistake of only i" : 

7,000,000 10,665,830 36,444,220 66,780,420' 

If the Sun's parallax be taken at 8 - 8o, the distances will be : 

92,890,000 141,536,000 483,288,000 886,065,000 

It is only within comparatively the last few years that the 
efforts of astronomers to detect stellar parallax have been 
attended with any amount of success. The discovery of planetary 
parallax is of course of older date. Pliny considered such in- 
vestigations to be but little better than madness, and Riccioli 
remarks, " Parallaxis et distantia stellarum fixarum, non potest 
certa et evident! observatione humanitus comprehendi." Leonard 
Digges, an old English writer, however, seems to have found 
no difficulty in the matter ; he gives the following table of 
distances, which, however, unfortunately for his reputation, has 
turned out to be seriously incorrect. He adds, " Here dernon- 
stracion might be made of the distaunce of these orbes, but that 
passeth the capacity of the common sort." These are his 
results k : 

Myles 

" From the Earth to the Moone ... ... ... ... 15,750 

From the Moone to Mercury ... ... ... ... 12,812 

From Mercury to Venus ... ... ... 12,812 

From Venus to the Sunne ... ... ... ... 23,437^ 

From the Sunne to Mars ... ... ... ... ... 15,725 

From Mars to Jupiter ... ... ... ... ... 78,721 

From Jupiter to Saturne ... ... ... ... ... 78,721 

From Saturne to the Firmament ... ... ... ... 1 20,485." 

Whence it follows, according to Digges, that the distance from 
London to the stars is exactly 358,463^ miles ! 

1 Ferguson's Astronomy, p. 76, 2nd Edition, London, 1757. 
k Prognostication Euerlastinge, 2nd ed. 1576, fol. 16. 



CHAP. V.] Refraction and Twilight. 387 



CHAPTER V. 

REFRACTION AND TWILIGHT. 

Refraction. Its nature. Importance of a correct knowledge of its amount. Table 
of the correction for Refraction. Effect of Refraction on the position of objects 
in the horizon. Sistory of its discovery. Twilight. How caused. Its 
duration. 

EFR ACTION. Besides the change of place to which the 
heavenly bodies are subjected by the effects of parallax, 
atmospheric refraction gives rise to a considerable displacement ; 
and it is this power which the air, in common with all trans- 
parent media, possesses, which renders a knowledge of the 
constitution of the atmosphere a matter of importance to the 
astronomer. " In order to understand the nature of refraction 
we must consider that an object always appears in the direction 
in which the last ray of light comes to the eye. If the light 
which comes from a star were bent into 50 directions before it 
reached the eye, the star would nevertheless appear in a line 
described by the ray nearest the eye. The operation of this 
principle is seen when an oar, or any stick, is thrust into the 
water. As the rays of light by which the oar is seen have their 
direction changed as they pass out of water into air, the ap- 
parent direction in which the body is seen is changed in the 
same degree, giving it a bent appearance the part below the 
water having apparently a different direction from the part 
above a ." 

Olmsted, Mechanism, of the Heavens, ueq.) there will be found a useful sum- 
p. 94. Edinburgh edition. In Sir J. mary of information concerning refrac- 
Herschel's Outlines of Ast. (pp. 27 et tion. 

C C 2 



388 Miscellaneous Astronomical Phenomena. [BOOK III. 

The direction of this refraction is determined by a general 
law in optics, that when a ray of light passes out of a rarer 
into a denser medium e.g. out of air into water, or out of 
space into the Earth's atmosphere it is bent towards a perpen- 
dicular to the surface of the medium; but when it passes out 
of a denser into a rarer medium, it is bent from the perpen- 
dicular. Inasmuch then as we see any object in the direction 
in which the rays emanating from it reach the eye, it follows 
that the effect of refraction is to make the apparent altitude 
of a heavenly body appear greater than the true altitude ; so 




REFRACTION. 



that for example any object situated actually in the horizon 
will appear above it. Indeed, some objects that are actually 
below the horizon, and which would be otherwise invisible 
were it not for refraction, are thus brought into sight. It was 
in consequence of this that on April 20, 1837, the Moon rose 
eclipsed before the Sun had set ; and other like instances may 
be conceived. 

In Fig. 177, Z is the zenith, C D the visible horizon, A B a 
parallel of latitude, A E B the boundary of the Earth's atmo- 
sphere. Then the light of the star Q will, to the observer at O, 
seem to come from the point P. 



CHAP. V.] Refraction and Twilight. 389 

A correct determination of the exact amount of atmospheric 
refraction, or the angular displacement of a celestial object at 
any altitude, is a very important, but a very difficult subject of 
inquiry, owing to the fact that the density of any stratum of air 
(on which its refractive power depends) is affected by the opera- 
tion of meteorological phenomena with which we are at present 
but very imperfectly acquainted. Thus, the amount of refraction 
at any given altitude depends not only on the density but also 
on the thermometric and hygrometric conditions of the air 
through which the visual ray passes. And although we know 
the general fact that the barometric pressure b and the tempera- 
ture constantly diminish as we rise from the Earth's surface, yet 
the law of this diminution is not fully ascertained. In conse- 
quence of our ignorance on these points, some degree of un- 
certainty is introduced into the determination of the amount of 
refraction, which affects astronomical observations involving 
extremely minute quantities. Nevertheless it must be re- 
membered that inasmuch as the total amount of refraction 
is never considerable, and in most cases very small, it can be 
so nearly estimated as to offer no serious impediment to the 
astronomer. 

Tables are in use d , constructed partly from observation and 
partly from theory, by means of which we can ascertain approxi- 
mately the mean refraction at any given altitude ; additional 
rules being given by which this average refraction may be 
corrected according to the state of the air at the time of observa- 
tion. At the zenith, or at an altitude of 90, there is no refraction 
whatever, objects being seen in the position which they would 

b Since the barometer rises with an causes a decrease of density, it follows 

increase in the weight and density of the that the rise of the thermometer dimin- 

air, its rise is coincident with an augmen- ishes the effect of refraction, the baro- 

tation, and its fall with a decrease, of meter remaining stationary. We may 

refraction. It will be tolerably near the assume that the refraction at any 

truth if we assume that the refraction at given altitude is increased or diminished 

any given altitude is increased or dimin- by -f^ of its mean amount for each 

ished by ^5-^ of its mean amount for degree by which the thermometer ex- 

every io th of an inch by which the baro- ceeds or falls short of the mean tem- 

meter exceeds or falls short of 30 inches. perature of 55 Fahr. 

c Also as an increase of temperature d See Vol. II, potf. 






390 Miscellaneous Astronomical Phenomena. [BOOK III. 

have were the Earth devoid of any atmosphere at all. In 
descending from the zenith towards the horizon, the refraction 
constantly increases, objects near the horizon being displaced in 
a greater degree than those at high altitudes. Thus the re- 
fraction, which at an altitude of 45 is only equal to 57", at the 
horizon increases to nearly 35'. The rate of the increase 
at high altitudes is nearly in proportion to the tangent of the 
apparent angular distance of the object from the zenith ; but 
in the vicinity of the horizon this rule ceases to hold good, 
and the law becomes much more complicated in its expression. 
Since the mean diameter both of the Sun and Moon is about 
32', it follows that, when we see the lower edge of either 
of these luminaries apparently just touching the horizon, in 
reality its whole disc is completely below it, and would be alto- 
gether hidden by the convexity of the Earth were it not for 
refraction. 

It is under these circumstances that one of the most curious 
effects resulting from atmospheric refraction may often be 
noticed, namely the somewhat oval outline presented by the 
Sun and Moon when near the horizon. This arises from the 
unequal refraction of the upper and lower limbs. The lower 
limb being nearer the horizon, is more affected by refraction, 
and consequently is raised in a greater degree than the upper 
limb, " the effect being to bring the two limbs apparently closer 
together by the difference of the two refractions. The form of 
the disc is therefore affected as if it were pressed between two 
forces, one acting above and the other below, tending to com- 
press its vertical diameter, and to give it the form of an 
ellipse, the lesser axis of which is vertical and the greater 
horizontal." 

The dim and hazy appearance of objects in the horizon is not 
only occasioned by the rays of light having to traverse a greater 
thickness of atmosphere (because their direction is oblique), but 
also from their having to pass through the lower and denser 
part. "It is estimated that the solar light is diminished 1300 
times in passing through these lower strata, and we are thereby 



CHAP. V.] Refraction and Twilight. 391 

enabled to gaze upon the Sun, when setting, without being 
dazzled by his beams." Or, as Bouguer put it, the Sun's 
brilliancy at 40 above the horizon is 1000 times greater than 
it is at i. 

" The dilated size (generally) of the Sun or Moon when seen 
near the horizon beyond what they appear to have when high 
up in the sky, has nothing to do with refraction. It is an 
illusion of the judgment, arising from the terrestrial objects 
interposed, or placed in close comparison with them e . In that 
situation we view and judge of them as we do of terrestrial 
objects in detail, and with an acquired habit of attention to 
parts. Aloft we have no associations to guide us, and their 
insulation in the expanse of the sky leads us rather to under- 
value than to over-rate their apparent magnitudes. Actual 
measurement with a proper instrument corrects our error, 
without however dispelling our illusion. By this we learn 
that the Sun, when just on the horizon, subtends at our eyes 
almost exactly the same, and the Moon a materially less angle 
than when seen at a great altitude in the sky, owing to its 
greater distance from us in the former situation as compared 
with the latter f ." Guillemin remarks that if the Moon, when 
in the horizon, be looked at through a tube, the illusion will 
disappear. 

Claudius Ptolemy was the first who remarked that a ray of 
light proceeding from a star to the Earth undergoes a change 
of direction in passing through the atmosphere 8 . He more- 
over stated that the displacement is greatest at the horizon, 
diminishes as the altitude increases, and finally vanishes altogether 

e This explanation of Sir J. Herschel's Moon when low down towards the horizon 
has been disputed, but its general correct- has much to do with the phenomenon, 
ness is rendered highly probable by the but that it is mainly due to some physio- 
fact that the apparent size of a balloon logical cause, connected with the direction 
varies in precisely the same way, accord- of vision, which is worthy of further and 
ing as it is high up in the air or near the special study. 

horizon. See some remarks by Stroobant ' Sir J. Herschel, Outlines of Ast., 

quoted in Observatory, vol. viii. p. 130, p. 35. 

April, 1885. This writer thinks that the * Almay., lib. vii. cap. 6. 
loss of brilliancy suffered by the Sun and 



392 Miscellaneous Astronomical Phenomena. [BOOK III. 

at the zenith- an assertion which we have already seen to be 
perfectly correct. In the 1 6 th century Tycho Brahe also inves- 
tigated the subject of refraction ; and his results, though by no 
means so accurate as those of Ptolemy, are interesting from the 
fact that they were the first which were reduced to the form of 
a Table. Since this period many astronomers have devoted their 
attention to the matter, and the Tables now in most general use 
are those of Bessel. 

Twilight. This is another phenomenon depending on the 
agency of the atmosphere with which the Earth is surrounded. 
It is due partly to refraction and partly to reflection, but chiefly 
to the latter cause. After sunset the Sun still continues to 
illuminate the clouds and upper strata of the air, just as it may 
be seen shining on the tops of hills long after it has disappeared 
from the view of the inhabitants of adjacent plains. The air and 
clouds thus illuminated reflect back part of the light to the 
surface beneath them, and thus produce, after sunset and before 
sunrise, in a degree more or less feeble according as the Sun is 
more or less depressed, that which we call " twilight." Immedi- 
ately after the Sun has disappeared below the horizon all the 
clouds in the vicinity are so highly illuminated as to be able to 
reflect an amount of light but little inferior to the direct light of 
the Sun. As the Sun, however, sinks lower and lower, less and 
less of the visible atmosphere receives its light, and consequently 
less and less of it is reflected to the Earth's surface surrounding 
the position where the observer is stationed, until at length, 
though by slow degrees, all reflection is at an end, and night 
ensues. The same thing occurs before sunrise ; the darkness of 
night gradually giving place to the faint light of dawn, until 
the Sun appears above the horizon and produces the full light 
of day. 

The duration of twilight is usually reckoned to last until the 
Sun's depression below the horizon amounts to 1 8 : this, 
however, varies: in the Tropics a depression of 16 or 17 is 
sufficient to put an end to the phenomenon, but in England a 
depression of 17 to 2i c is required. The duration of twilight 



CHAP. V.] Refraction and Twilight. 393 

differs in different latitudes ; it varies also in the same latitude 
at different seasons of the year, and depends in some measure on 
the meteorological condition of the atmosphere. Strictly speak- 
ing, in the latitude of Greenwich there is no true night from 
May 22 to July 21, but constant twilight from sunset to sunrise. 
Twilight reaches its minimum 3 weeks before the vernal equinox 
and 3 weeks after the autumnal equinox, when its duration is 
i h 5o m . At midwinter it is longer by about 17, but the 
augmentation is frequently not perceptible, owing to the greater 
prevalence of clouds and haze at that season of the year, which 
intercept the light and hinder it from reaching the Earth. The 
duration is least at the equator (i h I2 m ), and increases as we 
approach the Poles, for at the former there are 2 twilights every 
24 hours, but at the latter only 2 in a year, each lasting about 
50 days. At the North Pole the Sun is below the horizon for 6 
months h ; but from January 29 to the vernal equinox, and from 
the autumnal equinox to Nov. 1 2, the Sun is less than 1 8 below 
the horizon : so that there is twilight during the whole of these 
intervals, and thus the length of the actual night is reduced to 
i\ months. The length of the day in these regions is about 6 
months, during the whole of which time the Sun is constantly 
above the horizon. The general rule is, that to the inhabitants of 
an oblique sphere the twilight is longer in proportion as the place is 
nearer the elevated pole 1 . 

Under some circumstances a secondary twilight may be noticed, 
" consequent on a re-reflection of the rays dispersed through the 
atmosphere in the primary one. The phenomenon seen in the 
clear atmosphere of the Nubian Desert, described by travellers 
under the name of the ' After-glow,' would seem to arise from 
this cause k ." 

The "Astronomical" Twilight is that Twilight which has 
reference to the visibility and extinction of the smaller stars. 

h This is not quite literally 6 months An abstract of it is given in the Intell. 

o'wing to the operation of refraction. Obt., vol. vii. p. 135, March 1865. 

' A valuable memoir on twilight, by k Sir J. Herschel, Outlines of Ait., 

J. F. J. Schmidt, will be found in Ast. p. 34. 
Xach., vol. Ixiii. No. 1495, Oct. 14, 1864. 



394 Miscellaneous Astronomical Phenomena. 



The following is a table of its duration for different seasons and 
latitudes : 



Latitude, 

N. or 8. 


Duration. 


Winter Solstice. 


Equinoxes. 


Summer Solstice. 




h. in. 


h. in. 


h. in. 


O 


I 19 


I 12 


I 19 


5 


I I 9 


I 12 


I 20 


10 


I 19 


I 13 


I 21 


15 


I 20 


I 15 


I 2 4 


20 


I 23 


I I? 


I 28 


25 


I 26 


I 20 


i 33 


30 


i 3 


I 24 


i 41 


35 


i 35 


I 29 


i 52 


40 


i 43 


i 35 


2 9 


45 


1 53 


i 44 


2 39 


50 


2 6 


i 55 




55 


2 26 


2 10 


*" ^ 


60 


2 57 


2 33 


i ^-3 !! 


65 


4 3 


3 8 


; H * 



BOOK IV, 

COMETS. 



CHAPTER I. 

GENERAL REMARKS. 



Comets always objects of popular interest, and sometimes of alarm. Usual pheno- 
mena attending the development of a Comet. Telescopic Comets. Comets 
diminish in brilliancy at each return. Period of revolution. Density. Mass. 
Lexell's Comet. General influence of Planets on Comets. Special influence 
of Jupiter. Comets move in I of 3 kinds of orbits. Element of a Comet's 
orbit. For a parabolic orbit, 5 in number. Direction of motion. Eccen- 
tricity of an elliptic orbit. The various possible sections of a cone. Early 
speculations as to the paths in which Comets move. Comets visible in the 
daytime. Breaking up of a Comet into parts, Instance of Sielas Comet. 
Liais's observations of Comet Hi. 1860. Comets probably self-luminous. 
Existence of phases doubtful. Comets tvith Planetary discs. Phenomena 
connected with the tails of Comets. Usually in the direction of the radius 
vector. Secondary Tails. Vibration sometimes noticed in tails. Olbers's 
hypothesis. Transits of Comets across the Sun's disc. Variation in the appear- 
ance of Comets exemplified in the case of that of 1769. Transits of Comets 
across the Sun. 



THE heavenly bodies which will now come under our notice 
are amongst the most interesting with which the astronomer 
has to deal. Frequently appearing suddenly in the nocturnal sky, 
and often having attached to them tails of immense size and 
brilliancy, comets were well calculated in the earlier ages of the 
world to attract the attention of all, and to excite the fear of 
many. It is the unanimous testimony of history, during a period 
of upwards of 2000 years, that comets were always considered to 



396 Comets. [BOOK IV. 

be peculiarly " ominous of the wrath of Heaven, and as harbingers 
of wars and famines, of the dethronement of monarchs, and the 
dissolution of empires." I shall hereafter examine this question 
at greater length. Suffice it for me here to quote the words of 
the Poet, who speaks of 

" A Blazing Star, 

Threatens the World with Famin, Plague, and War; 
To Princes, death ; to Kingdoms, many crosses ; 
To all Estates, ineuitable Losses ; 

To Heard-men, Rot ; To Ploughmen, haplesse Seasons ; 
To Saylors, Storms ; to Cities, ciuil Treasons ." 

However little attention might have been paid by the ancients 
to the more ordinary phenomena of nature (which, however, 
were very well looked after), yet certain it is that comets and 
total eclipses of the Sun were not easily forgotten or lightly 
passed over ; hence the aspects of remarkable comets seen in 

Fig. 178. Fig. 179. 





TELESCOPIC COMET TELESCOPIC COMET 

WITHOUT A NUCLEUS. WITH A NUCLEUS. 

olden times have been handed down to us, often with circum- 
stantial minuteness. 

A comet usually consists of 3 parts, developed, it may be, 
somewhat in the following manner : A faintly luminous speck 
is discovered by the aid of a good telescope ; the size increases 
gradually ; and after some little time a nucleus appears that is, 
a part which is more condensed in its light than the rest, and is 
sometimes circular, sometimes oval, and sometimes (but very 

a Du Bartas, trans. .T. Sylvester, 1621, p. 33. 



Plate XXIII. 



COMPARATIVE SIZES OF THE EARTH, THE MOON'S ORBIT 
AND CERTAIN COMETS, NAMED. 






CHAP. I.] General Remarks. 399 

rarely) presents a radiated appearance. Arago remarked that 
this nucleus is generally eccentrically placed in the head, lying 
towards the margin nearest the Sun. Eddie noticed that the 
nucleus of Fabry's comet of 1886 was of a ruddy brown colour b . 
Both the size and the brilliancy of the object progressively 
increase ; the coma, or cloud-like mass around the nucleus, 
becomes less regular ; and a tail begins to form, which becomes 
fainter as it recedes from the body of the comet. This tail 
increases in length so as sometimes to spread across a large 
portion of the heavens ; sometimes there are more tails than one, 
and occasionally the tail is much narrower in some parts than in 
others. The comet approaches the Sun in a curvilinear path, 
which frequently differs but little from a right line. It generally 
crosses that part of the heavens in which the Sun is situated so 
near the latter body as to be lost in its rays ; but it emerges 
again on the other side, frequently with increased brilliancy and 
increased length of tail. The phenomena of disappearance are 
then not unlike those which marked the original appearance but 
in the reverse order. 

In magnitude and brightness comets exhibit great diversity : 
at rare intervals one appears which is so bright as to be visible 
in the daytime ; but the majority are quite invisible to the naked 
eye and need more or less optical assistance. These latter are 
usually called telescopic comets. The appearance of the same comet 
at different periods of its return is so varying that we can never 
certainly identify a given comet with any other by any mere 
physical peculiarity of size or shape until its "elements" have 
been calculated and compared. It is now known that " the same 
comet may, at successive returns to our system, sometimes appear 
tailed, and sometimes without a tail, according to its position 
with respect to the Earth and the Sun ; and there is reason to 
believe that comets in general, for some unknown cause, decrease 
in splendour in each successive revolution c ." 

Fig. 1 80 represents the comparative diameters of the heads of 4 
well-known comets as measured on particular occasions. The 

b Month. Not., vol. xlvi. p. 456, June 1886. c Smyth, Cycle, vol. i. p. 235. 



400 Cvmets. [BOOK IV. 

woodcut is drawn to scale, but it must not be inferred that there 
is any permanence in the sizes here indicated. 

The periods of comets in their revolutions vary greatly, as also 
do the distances to which they recede from the Sun. Whilst the 
orbit of Encke's comet is contained within that of Jupiter, the 
orbit of Halley's extends beyond that of Neptune. Some 
comets indeed proceed to a much greater distance than this, 
whilst others are supposed to move in curves which do not, like 
the ellipse, return into themselves. In this case they never come 
back to the Sun. These orbits are either parabolic or hyperbolic. 

The density, and also the mass, of comets is exceedingly small, 
and their tails consist of matter of such extreme tenuity that 
even small stars are visible through them a fact first recorded 
by Seneca. That the matter of comets is exceedingly rare is 
sufficiently proved by the fact that they have at times passed 
very near to some of the planets without disturbing in any 
appreciable degree the motions of the said planets. Thus the 
comet of 1770 (Lexell's) in its advance towards the Sun, became 
entangled amongst the satellites of Jupiter, and remained near 
them for 4 months, without in the least affecting them so far as 
we know. It can therefore be shown that this comet's mass 
could not have been so much as -5-5^ that of the Earth. The 
same comet also came very near to the Earth on July i its 
distance from it at 5 h on that day being about 1,400,000 miles 
so that had its matter been equal in quantity to that of the Earth 
it would, by its attraction, have caused our globe to move in an 
orbit so much larger than it does at present that it would have 
increased the length of the year by 2 h 47, yet no sensible 
alteration took place. The comet of 837 remained for 4 days 
within 3,700,000 miles of the Earth without any untoward con- 
sequences. Very little argument, therefore, suffices to show the 
absurdity of the idea of any danger happening to our planet from 
the advent of any of these wandering strangers. Indeed, instead 
of comets exercising any influence on the motions of planets, 
there is the most conclusive evidence that the converse is the 
case that planets influence comets. This fact is strikingly 



CHAP. I.] General Remarks. 401 

exemplified in the history of the comet of 1770, just mentioned. 
At its appearance it was found to have an elliptical orbit, requir- 
ing for a complete revolution only 5! years; yet although this 
comet was a large and bright one, it had never been observed 
before, and has moreover never been seen since ; the reason being 
that the influence of the planet Jupiter, in a short period, com- 
pletely changed the character of its path. " Du Sejour has 
proved that a comet, whose mass is equal to that of the Earth, 
which would pass at a distance of 37,500 miles only, would extend 
the length of the year to $6^ i6 h 5, and could alter the obliquity 
of the ecliptic to the extent of a. Notwithstanding its enormous 
mass and the smallness of its distance, such a body would then 
produce upon our globe only one kind of revolution, that of the 
calendar 3 ." 

Fig. 1 8 1 will illustrate, almost without the necessity of any 
written description, the influence of Jupiter on the group of 
periodical comets which have come within its reach. These 
comets, arranged in the order of their aphelion distances, are as 
follows : 

Radii of Earth's orbit. 

Encke ... ... ... ... ... ... ... 4-1 

Tempel's Second (1873, ii.) ... ... ... ... 4-7 

Tempel's First (1867, ii.) 4-8 

JUPITER 4.9 to 5-5 

Tempel-Swift (1869, iii.) 5.1 

Brorsen ... ... ... ... ... ... ... 5-6 

Winnecke ... ... ... ... ... ... ... 5-6 

D' Arrest ... ... ... ... ... ... ... 5-8 

Faye 5.9 

Biela ... ... ... ... ... ... ... 6-2 

And it is probable that some other comets ought now to be added 
to this list; e.g., Finlay's (1886, vii.), Wolfs (1884, iii.), and 
Denning's (1881, v.). 

A comet may move in either an elliptic, parabolic, or hyper- 
bolic orbit ; but for reasons with which mathematical readers are 
acquainted, no comet can be periodical which does not follow an 
elliptic path. In consequence, however, of the comparative 

d Arago, Pop. Axt,, vol. i. p. 642, Eng. ed. 

D d 



402 



Comets. 



[BOOK IV. 



facility 6 with which the parabola can be calculated, astronomers 
are in the habit of applying that curve to represent first of all 
the orbit of any newly-discovered body. Parabolic "elements" 
having been obtained, a search is then made through a catalogue 
of comets, to see whether the new elements bear any resemblance 




Z1Q 
DIAGRAM ILLUSTRATING THE INFLUENCE OP JUPITER ON COMETS. 

to those of any object which has been previously observed ; if so, 
calculations for an elliptic orbit are undertaken, and a period 
deduced. 

When a comet is discovered the first question asked about it 
by the amateur astronomer is, " When and where can we see it, 
and how long will it last ? " and by the professional astronomer, 



* To compute elliptic elements for a 
comet or a planet will take, even an 
experienced calculator, several days of 



very hard work. An approximation may 
however be obtained by a graphical process 
such as that described in Chap. VI (poxf). 



CHAP. I.] General Remarks. 403 

"What are its elements?" The answer to be given to the first 
question always depends upon the answer given to the last 
question. To the majority of amateurs these elements are almost 
unintelligible, and even to adepts they often convey but a vague 
idea of the true form and position of the orbit. The best way to 
realize their exact import is by making a model f . 

The orbits of all comets, planets, and binary stars are conic 
sections whose size, form, and position in space are defined by 
quantities called "elements," which, for brevity, are usually 
designated by the following symbols : 

T = Moment of the body's Perihelion Passage or nearest ap- 
proach to the Sun . 
A = Longitude at an Epoch given. 

TT = Longitude of the Perihelion or the longitude of the body 
when it reaches that point. In the case of a comet 
(or planet), this is measured along the ecliptic from the 
vernal equinox to the comet's ascending node, and thence 
along the comet's (or planet's) orbit to its perihelion; 
in the case of the Earth, it is measured along the ecliptic 
from the vernal equinox to the perihelion. 
Q = Longitude of the Ascending Node of the body's orbit as seen 
from the Sun (or Primary) ; measured on the ecliptic, from 
the vernal equinox to the ascending node of the orbit. 
i = Inclination of the plane of the orbit to the plane of the 

ecliptic. 

f Eccentricity of the orbit, sometimes given in parts of 
radius of the Earth's orbit, sometimes in seconds of arc, 
and sometimes as an angle, $. Parts of radius are most 
convenient, and seconds of arc may be reduced to that 
unit by dividing them by 206,265". When $ is given, 
then it is to be understood that e = sin. </>. 

f For instructions how to do this see 8 In the case of a binary star, of the 

an article by Professor Harkness in the nearest approach of the companion star 

Sidereal Messenger, vol. vi. p. 329, Dec. to the principal star, in such case called, 

1887. From the introduction to that not the perihelion, but the peri-astron 

article the next few paragraphs are taken passage, 
with verbal alterations. 

D cl 2 



404 Comets. [BOOK IV. 

q = Perihelion distance of the body ; expressed in terms of 
the mean radius of the Earth's orbit as unity. 

For a parabolic orbit e is always i-o (or unity), and in that 
case the elements are frequently given by stating T, ta, Q, i, and 
log. q. Here TT has been replaced by 

0) = IT 8, (l) 

which is counted on the comet's orbit, backward, from the peri- 
helion to the ascending node ; and the perihelion will lie on 
the northern or southern side of the ecliptic according as o> is 
less or greater than 1 80. 

As TT and. Q are counted from the vernal equinox, and t is 
measured from the plane of the ecliptic, these quantities neces- 
sarily refer to a particular equinox, and this is always specified. 

It was long customary to measure longitudes in comets' orbits 
in the direction of the Earth's motion, to limit i to the first 
quadrant, and to specify the direction of the comet's motion, 
whether direct or retrograde ; but many foreign astronomers 
now follow Gauss in regarding retrograde motion as a result of the 
inclination passing into the second quadrant, and in accordance 
with that view they measure a comet's longitude always in the 
direction of its own motion, and permit i to take any value between 
o and 1 80. The circumstance that i is measured at the ascending 
node limits its range to the first and second quadrants, for if it 
were to pass into the third or fourth quadrant the ascending node 
would be converted into a descending one. For a comet having 
direct motion the numerical values of the elements are the same 
in Gauss's system as in the old system, but for a comet having 
retrograde motion they are different, and in that case, if their 
values according to the old system are designated by a subscript 
o, the equations requisite for passing from the old to the Gaussian 
system are : 

i = 180 i o to = 360 <0 o = o) o 

2 = S3 IT = 28 TT O . 

There is frequently much confusion respecting the angles TT 
and o), and it is important to have a clear understanding of the 
relations of co to 77 and Q . In the old system of elements TT is 



CHAP. I.] General Remarks. 405 

measured from the vernal equinox, along the ecliptic in the 
direction of the Earth's motion, to the ascending node of the 
comet, and thence along the comet's orbit, still in the direction of 
the Earth's motion, to the comet's perihelion. In Gauss's system TT 
is measured from the vernal equinox, along the ecliptic in the 
direction of the earth's motion, to the ascending node of the 
comet, and thence along the comet's orbit, in the direction of the 
comet's motion, to the comet's perihelion. These definitions may 
perhaps be elucidated by the following statement. Imagine a 
perpendicular to the plane of the ecliptic, erected from the Sun. 
Then to an observer situated North of the ecliptic in that perpen- 
dicular, the motion of the Earth will be contrary to the hands of 
a clock, and longitudes in the Earth's orbit will increase in that 
direction. Now consider a comet's orbit ; imagine a perpen- 
dicular affixed to it in such a way that when the inclination of 
the orbit to the plane of the ecliptic is t, the inclination of the 
perpendicular shall be (i + 90), and suppose an observer so situated 
in the perpendicular that when i = o he shall be North of the 
ecliptic. Then, according to the old system of elements, for all 
possible values of i the observer will remain North of the ecliptic, 
and the motion of the comet will appear to him as contrary to 
the hands of a clock when direct, and with the hands of a clock 
when retrograde ; but according to Gauss's system he will be 
North of the ecliptic when i is less than 90, South of it when t is 
greater than 90, and to him the apparent direction of the comet's 
motion will always be contrary to the hands of a clock. Which- 
ever system is adopted, from this point of view TT will always 
increase contrary to the clock, and to find the intersection of the 
plane of the comet's orbit with the plane of the ecliptic, or, in 
other words, the line of the nodes, he must set off o> in the 
direction of the hands of a clock, from the perihelion of the 
orbit. 

The motion of a comet is said to be " direct " (or + ) when it 
moves in the order of the signs of the zodiac ; and " retrograde " 
(or ) when it moves contrary to the signs of the zodiac. 

In the case of an elliptic orbit given q and e we can ascertain 



406 



Comets. 



[BOOK IV. 



Fig. 182. 



the length of the major axis (a), and consequently the periodic 
time. 

Given the mean daily motion (ju), we obtain the period in days 
by dividing 1,296,000 (the number of seconds of arc in a circle) 

by M- 

Astronorners are accustomed to perform all these calculations 
by logarithms because of the ease and convenience of doing so. 
Be it remembered that the eccentricity is not the linear distance 

of the centre of the ellipse from 
either focus, but the ratio of that 
quantity to the semi-axis major. 

Up to the present time the 
orbits of more than 300 comets 
have been calculated h : a Table 
of these will be given hereafter. 

Fig. 182 represents the various 
possible sections of a right cone, 
and will convey a better idea of 
the orbits of comets than could 
be given by description. When a 
right cone is cut at right angles 
to its axis, the resulting section 
A B will be a circle ; no comet, 
however, revolves in a circular 
or even nearly circular orbit. 

When a cone is cut obliquely, so that the inclination of the 
cutting plane to the axis of the cone is greater than the constant 
angle formed by the generating line of the cone and the axis, as 
C D, the resulting section will be an ellipse, the shape of which 
will vary from almost a circle on the one hand to almost a 
parabola on the other according to the amount of the obliquity. 




THE VARIOUS SECTIONS OF A CONE. 



h Gauss's Theoria Motus Corporum 
Ccelestium, 4to. Hamburg, 1809, was 
long reckoned the standard work on the 
subject of orbits, but it has in some 
degree been superseded by Oppolzer's 
Lehrbuch zur Bahnbestimmuny der 
Kometen und Planeten, 2nd ed., 2 vols. 



8vo., Leipzic, 1882. A French translation 
by a Belgian, M. E. Pasquier, was pub- 
lished at Paris, 1886, under the title of 
Traitt de la determination des orbites 
des cotnetes et des planetes. See also a 
paper by Airy, in Memoirs R.A.S., vol. 
xi. p. 1 81. 1840. 



CHAP. I.] 



General Remarks. 



407 



When a cone is cut in a direction, so that the inclination of the 
cutting plane to the axis of the cone is less than the constant 
angle formed by the generating line of the cone and the axis, as 
E F, the resulting section will be a hyperbola. When a cone is 
cut in a direction so that the inclination of the cutting plane to 
the axis of the cone is equal to the constant angle formed by the 
generating line of the cone and the axis, as G H, the resulting 
section will be a parabola. 

To the early astronomers the motions of comets gave rise to 
great embarrassment. Tycho Brahe thought that they moved in 
circular orbits ; Kepler, on the other hand, suggested right lines. 
Hevelius seems to have been the first to remark that cometary 
orbits were much curved near the perihelion, the concavity being 
towards the Sun. He also threw out an idea relative to the 
parabola, as being the form of a comet's path, though it does not 
seem to have occurred to him that the Sun was likely to be the 
focus. Borelli suggested an ellipse or a parabola. Sir William 
Lower was probably the first to hint that comets sometimes 
moved in very eccentric ellipses ; this he did in his letter to his 
" especiall goode friend, Mr. Thomas Harryot," dated Feb. 6, 1610. 
Db'rfel, a native of Upper Saxony, was the first practical man ; 
he showed that the comet of 1680 moved in a parabolic orbit. 
Sir I. Newton also gave his attention 
to the subject. Confirming Dorfel, Sir 
Isaac further showed that the motion 
of the comet was in accordance with 
the general Theory of Gravitation. 

History informs us that some comets 
have shone with such splendour as to 
have been distinctly seen in the day- 
time. The comets of B.C. 43, A.D. 575 (?), 
1106, 1402 (i.), 1402(11.), 1472, 1532, 
1577, 1618 (ii.), 1744, 1843 (i-)> l8 47 (i-)> 
1853 (iii.), and 1882 (i.) are the prin- 
cipal ones which have been thus observed. 

There are some well-established instances of the separation of 



Fig. 183. 




THE 1 st COMET OF 1847, VISIBLE 
AT NOON ON MARCH 30. 

(Hind.) 



408 



Comets. 



[BOOK IV. 



a comet into 2 or more distinct portions. Seneca mentions, on 
the authority of Ephorus, a Greek author, that the comet of 
371 B.C. separated into 2 parts which pursued different paths 1 . 
Seneca seems to distrust the statement he repeats, but Kepler 
accepted it after what he had himself seen in regard to the great 
comet of 1618. In the case of this comet Cysatus noticed an 
evident tendency to break up. When first seen this comet was 
a nebulous object, but some weeks afterwards it appeared to 
consist of a group of several small nebulosities. But the best 
authenticated instance of this character is that of Biela's comet 
in 1845-6. When first detected, on November 28, it presented 
the appearance of a faint nebulosity, almost circular, with a slight 

Fig. 184. 




BIELA'S COMET, FEB. 19, 1846. (0. Struve.} 

condensation towards the centre: on Dec. 19 it appeared some- 
what elongated, and by the end of the month the comet had 
actually separated into two distinct nebulosities which travelled 
together for more than 3 months : the maximum distance between 
the parts (157,240 miles) was attained on March 3, 1846, after 
which it began to diminish until the comet was lost sight of in 
April. At its return in 1852 the separation was still maintained, 
but the interval had increased to 1,250,000 miles. As we shall 
have to speak of Biela's comet again in a later chapter no more 
need be said about it here. 



1 QucBst. Nat., lib. vii. cap. 16. But he says however of the writer he quotes : 
" Ephorus vero non est religiosissimse fidei ; ssepe decipitur, saepe decipit." 



CHAP. I.] General Remarks. 409 

Biela's comet does not as regards its duplicity stand alone 
amongst modern comets. A comet seen in February and March 
1 860, only by M. Liais in Brazil, is said to have consisted of a 
principal nebulosity accompanied at a short distance by a second 
nebulosity. It is to be regretted that this object remained visible 
for so short a time as a fortnight, and that our knowledge of it 
depends on the authority of but one observer, and he a French- 
man^ The 2nd comet of 1881 according to the testimony of 
2 observers threw off a fragment which became virtually an 
independent comet, and lasted as such for some days until all 
trace of it was lost 1 . 

The question whether or not comets are self-luminous seems 
now satisfactorily settled; it cannot be doubted that they are 
self-luminous, as indeed the spectroscope tells us. The high 
magnifying power that may sometimes be brought to bear on 
them tends to show that they shine by their own light. Sir W. 
Herschel was of this opinion from his observations of the comets 
of 1807 and 1811 (i.) m It is manifest, however, that if the 
existence of phases could be certainly known, this would furnish 
an irrefragable proof that the comet exhibiting such shone by 
reflected light. It has been asserted from time to time that such 
phases have been seen, but none of the statements ever made 
seem to deserve attention. Delambre mentions that the registers 
of the Royal Observatory at Paris exhibit undoubted evidence of 
the existence of phases in the comet of 1682 : but neither Halley 
nor any other astronomer who observed this comet has given the 
slightest intimation that any phase-phenomena were visible. 
James Cassini mentions the existence of phases in the comet of 
1 744 n ; on the other hand, Heinsius and Che'saux, who paid 
particular attention to this comet, positively deny having seen 
anything of the kind. More recently Cacciatore, of Palermo, 
expressed a decided conviction that he had seen a crescent in the 



k Ast. Nach., vol. Hi. No. 1248. April 342, Aug. n, 1881. 

14, 1860. m Phil. Trans., vol. cii. p. 115. 1812. 

1 Bone, Month. Not., vol. xlii. p. 105, n Mem, Acad. des Sciences, 1744, p. 

Jan. 1882 : Gould, Nature, vol. xxiv. p. 303. 



410 Comets. [BOOK IV. 

comet of 1819. Arago sums up the matter by saying that the 
observations of M. Cacciatore prove only that the nuclei of comets 
are sometimes very irregular . Sir W. Herschel states that he 
could see no signs of any phases in the comet of 1807, although 
he fully ascertained that a portion of its disc was not illuminated 
by the Sun at the time of observation p . The general opinion 
is against the existence of phases, and thus we must consider 
that comets shine by their own inherent light ; nevertheless the 
observations of Airy and others on Donati's comet in 1858 point 
to exactly the opposite conclusion, at least as regards the fail of 
that comet* 1 , but then the tails of comets are strange ethereal 
structures, and if we know little about the heads we know less 
still about the tails. Pons's comet of 1812 was found at its 
return in 1883 to be brighter than the theory of its orbit led one 
to expect. Niersten suggested that this fact was a proof that 
the comet in question was endued with some inherent light of its 
own. 

Some comets have been observed with round and well-defined 
planetary discs. Seneca relates that one appeared after the death 
of Demetrius, king of Syria, but little inferior to the Sun [in 
size ?] ; being a circle of red fire, sparkling with a light so bright 
as to surmount the obscurity of night. The comet of 1652, seen 
by Hevelius, was almost as large as the Moon, though not nearly 
so bright. The comets of 1665 and 1682 are described as having 
been as well defined in their outlines as the planet Jupiter. It 
will be remarked that all these instances were before the days 
of good telescopes. I am not aware of any modern observations 
to the same effect. 

There are several curious phenomena connected with the tails 
of comets which require notice. It was observed by Peter 
Apian that the trains of 5 comets, seen by him between the years 
1531 and 1539, were turned from the Sun, forming more or less a 
prolongation of the radius vector, the imaginary line joining the 
Sun and the comet ; as a general rule, this has been found to be 

Pop. Ast., vol. i. p. 627, Eng. ed. P Phil. Trans., vol. xcviii. p. 156. 1808. 

i Green, Ob*,, 1858, p. 90. 



CHAP. I] 



General Remarks. 



411 



the case r , although exceptions do occur. Thus the tail of the 
comet of 1577 deviated 21 from the line of the radius vector. 
Valz has stated that the tails of comets iv. and v. of 1 863 deviated 
from the planes of the orbits, and that only 2 other comets are 
known the tails of which did the same s . In some few instances, 
where a comet has had more than one tail, the 2 nd has extended 
more or less towards the Sun ; this was the case with the comets 
of 1823, 1851 (iv.), 1877 (ii.), and 1880 (vii.). Although comets 
usually have but one tail, yet 2 is by no means an uncommon 
number ; and indeed the great comet of 1825 had 5 tails (Duiilop), 




DIAGRAM ILLUSTRATING CHANGES IN THE DIRECTIONS OF THE TAILS OF COMETS. 

and that of 1744 as many as 6, or more*. The tails of many 
comets are curved, so as to resemble in appearance a sabre ; such 
was the case with the comets of 1844 (iii.), and 1858 (vi.), amongst 
others. The comet of 1769 had a double curved tail, thus --" 
according to La Nux, who observed it at the Isle of Bourbon. 
The great comet of 1882 exhibited a striking and uncommon 



r The researches of M. E. Biot shew 
that this fact was noticed by the Chinese 
long before the time of Apian, to wit, in 
837. Comptes Sendus, vol. xvi. p. 75 f- 
1843. 

s Comptes Rendus, vol. Iviii. p. 853. 
1864. 



* This statement long depended on the 
unconfirmed authority of De Cheseaux, but 
it is now certain that this comet did ex- 
hibit a complete fan of separate tails. (See 
a paper by Dreyer, with an engraving of 
the tails, Copernicus, vol. iii. 1883.) 



412 Comets. [BOOK IV. 

form of tail, some account of which will be given in a later 
chapter. 

Occasionally a comet exhibits besides its principal tail a 
secondary one usually less bright and shorter than the main tail. 
For instance, Pons's long-period comet of 1812 at its apparition 
in 1886 had on Dec. 29 a primary tail 8 long and a secondary 
one very faint and only 3 long. But the secondary tail is not 
always the shorter of the two. Swift noted a secondary tail in 
the case of the comet ii. of 1881, which was some 55 long, the 
longest secondary tail on record u . 

The trains of some great comets have been seen to vibrate in 
a manner somewhat similar to the Aurora Borealis. The tails 
of the comets of 1618 (ii.) and 1769 may be cited as instances: 
the observer in the latter case was Pingre', whose great knowledge 
of comets adds weight to his testimony. The vibrations com- 
menced at the head, and appeared to traverse the whole length 
of the comet in a few seconds. It was long supposed that the 
cause was connected with the nature of the comet itself, but 
Olbers has pointed out that such appearances could only be fairly 
attributed to the effects of our own atmosphere, for this reason : 
" The various portions of the tail of a large comet must often be 
situated at widely different distances from the Earth ; so that it 
will frequently happen that the light would require several 
minutes longer to reach us from the extremity of the tail than 
from the end near the nucleus. Hence, if the coruscations were 
caused by some electrical emanation from the head of the comet, 
even if it occupied but one second in passing over the whole surface, 
several minutes must necessarily elapse before we could see it 
reach the tail. This is contrary to observation x , the pulsations 
being almost instantaneous." Instances of this phenomenon are 
not very common. The most recent case is that of Coggia's 
comet of 1874. An English observer at Hereford named With 
noticed an "oscillatory motion of the fan-shaped jet upon 
the nucleus as a centre which occurred at intervals of from 

u Work of Warner Observatory, vol. * Mem. Acad. des Sciences, 1775, p. 
i. p. 22. 302. 






CHAP. I.] General Remarks. 413 

3 to 8 sees. The fan seemed to ' tilt over ' from the preceding to 
the following side, and then appeared sharply defined and fibrous in 
structure, then it became nebulous, and all appearance of structure 
vanished y ." A flickering of the tail of this comet was observed 
also by Newall z . 

Respecting the physical constitution of the tails of comets it 
may be said that probably in many cases they are hollow cones. 
This theory would accord with the observed fact that single 
tails usually increase in width towards their extremities and are 
divided in the middle by a dark band, the brilliancy of the 
margins exceeding that of the more central portions. Similarly, 
comets with tails of tolerably uniform width throughout may be 
regarded as hollow cylinders a . 

The following is an excellent instance of the ever-changing 
appearance of comets; it relates to that of 1769. On Aug. 8, 
Messier, whilst exploring with a 3-foot telescope, perceived a 
round nebulous body, which turned out to be a comet. On the 
1 5th the tail became visible to the naked eye, and appeared to be 
about 6 in length ; on the 28th it measured 15; on Sept. 2, 36; 
on the 6th, 49; and on the loth, 60. The comet having now 
plunged into the Sun's rays, ceased to be visible. On Oct. 8, the 
perihelion passage took place ; on the 24th of the same month it 
reappeared, but with a tail only 2 long; on Nov. i the tail 
measured 6 ; on the 8th it was only 2| ; on the 3th it was i : 
the comet then disappeared. 

Transits of comets across the Sun no doubt occasionally happen, 
but only one such spectacle has ever been witnessed, and even 
then the nature of the sight was not understood till afterwards. 
The German Sun-spot observer, Pastorff, noticed on June 26, 
1819, a round dark nebulous spot on the Sun ; it had a bright 

y Ast. Reg., vol. xiv. p. 13. Jan. 1876. been broached respecting Comets. For 

z Month. Not., vol. xxxvi. p. 279. some particulars as to these see a paper 

March 1876. by Huggins, Proc. Roy. Inst., vol. x. p. 8, 

* This work is a record of facts rather 1882 ; a paper by Bredichin, Remarques 

than of theories, and is too bulky already. ginirales sur les queues des combtes; 

Otherwise I might have given it a great also an article by Ranyard in Ast. Reg., 

expansion by embarking on a review of vol. xxi. p. 58, March 1883. 

some of the chief theories which have 



414 Comets. [Boox IV. 

point in its centre. Subsequently when the orbit of couiet ii., 
1819 came to be investigated, Olbers pointed out that the comet 
must have been projected on the Sun's disc between 5 h and 9 h A.M. 
Bremen M.T. Pastorff asserted that his " round nebulous spot " 
was the comet. Olbers, and with him Schumacher, disputed the 
claim, and the matter seems not free from doubt b . Comet v. 
of 1826 was calculated to cross the Sun on Nov. 18, 1826, but 
owing to the general prevalence of bad weather in Europe, only 
2 observers were fortunate enough to be able to see the Sun 
on that day, and neither of them could obtain a glimpse of the 
comet. 

Sir J. Herschel once watched Biela's comet pass in front of a 
cluster of stars, but no obliterating effect was noticed, the 
several stars being all clearly visible through the comet's ethereal 
body. 

b For some further particulars as to Month. Not., vol. xxxvi. p. 309, May 1876. 

this controversy see Webb's Celest. Olij., Hind seems to have the idea of there 

4th ed., p. 40, where there is also a fac- being either error or fraud involved in 

simile of Pastorff s original sketch. See Pastorff 's narrative, 
also an important paper by Hind in 



CHAP. II.] 



Periodic Comets. 



415 



CHAPTER II. 



PEKIODIC COMETS*. 



Periodic Comets conveniently divided into three classes. Comets in Class I. Encke's 
Comet. The resisting medium. Table of periods of revolution. TempeVs 
second Comet. WinnecJce's Comet. Br or sen's Comet. Tempel's First Comet. 
Swiff s Comet. Barnard's Comet. D' 'Arrest's Comet. Finlay's Comet. Wolfs 
Comet. Faye's Comet. Denning 's Comet. Mechain's Comet of 1790. Now 
known as Tuttle's Comet. Stela's Comet. Di Vim's Comet of 1844. List of 
Comets presumed to be of short periods but only once observed. Comets in 
Class II. Westphal's Comet. Pans' s Comet of 1812. Di Vim's Comet of 
1846. Olbers's Comet of 1815. Srorsen's Comet of 1847. Halley's Comet. 
Of special interest. Besume of Halley's labours. Its return in 1759. Its 
return in 1835. Its history prior to 1531 traced by Hind. Comets in Class III 
not requiring detailed notice. 

THE comets which I propose to treat of in the present chapter 
may be conveniently divided into 3 classes : 

1 . Comets of short periods. 

2. Comets revolving in about 70 years. 

3. Comets of long periods. 

The following are the comets belonging to Class I, with which 
we are best acquainted : 



Name. 


Period. 


Next Return. 


i. Encke's 


Years. 
V2Q 


1891 Oct. 


2. Tempel's Second (1873, ii.) 
3. Winnecke's 


5-15 

5.54 


1894 Feb. 
1891 Dec. 


4. Brorsen's 


i-i8 


1890 April 


5. Tempel's First (1867, ii.) 
6. Swift's (1880, v.) 


5-98 
6-00 


1891 April 
1892 Oct. 


7. Barnard's (1884, ii.) 
8. D' Arrest's 


+ 6 

6-64 


1890 
1890 Sept. 


9. Finlay's 


6-67 


1893 


10. Wolf s (1884, iii.) 


6-76 


1891 Aug. 


ii. Faye's 


7-4-4. 


1895 Dec. 


12. Denning's 


8-86 


1890 July 


13. Tuttle's . . 


M-66 


1899 March 









If it should be suggested that I have 
given too much space here to the 



Periodic Comets, I would answer by 
way of excuse that they are, historically 



416 Comets. [BOOK IV. 

ENCKE'S COMET. 

No. i is by far the most interesting comet in the list, and I 
shall therefore review its history somewhat in detail. 

On Jan. 17, 1786, Mechain, at Paris, discovered a small tele- 
scopic comet near the star /3 in the constellation Aquarius. On 
the following day he announced his discovery to Messier, who, 
owing to unfavourable weather, did not see it till the I9th, on 
which night it was also observed by J. D. Cassini, Jun., and the 
original discoverer. It was tolerably large and well-defined, and 
had a bright nucleus, but no tail. 

On Nov. 7, 1795, Miss Caroline Herschel, sister of Sir W. 
Herschel, discovered a small comet, about 5' in diameter, without 
a nucleus, but yet having a slight central condensation of light. 
Olbers observed it on Nov. 21, when it was too faint to allow of 
the field being illuminated, and he was obliged to compare it with 
stars in the same parallel by noting the times of transit across 
the field of view. It was round, badly defined, and about 3' in 
diameter. The orbit greatly perplexed the calculator, and Pros- 
perin declared that no parabola would satisfy the observations. 

On Oct. 19, 1805, Thulis, at Marseilles, discovered a small 
comet, which was faintly visible to the naked eye. Huth stated 
that on the aoth it was very bright in the centre, though without 
a nucleus, and 4' or 5' in diameter. On Nov. i the same 
observer saw a tail 3 long. Several parabolic orbits were 
calculated, and one elliptic one by Encke, to which a period of 
1 2' 1 27 years was assigned. 

On Nov. 26, 1818, the indefatigable Pons, of Marseilles, dis- 
covered a telescopic comet in Pegasus, which was very small and 
ill-defined. As it remained visible for nearly 7 weeks, or till 
Jan. 12, 1819, a rather long series of observations was obtained ; 
and Encke, finding that under no circumstances whatever would 



and physically, very interesting objects ; Earth ; and that, consequently, they are 

that scarcely a year ever passes that objects which furnish many instructive 

some of them do not return to the Sun chances to the class of students for whom 

and therefore to visibility as regards the this work is mainly intended. 



CHAP. II.] Periodic Comets. 417 

a parabolic orbit fairly represent them, determined rigorously to 
investigate the elements according to the method of Gauss, then 
but little practised. Having done this, he found that the true 
form of the orbit was elliptical, and that it had a period of about 
3 1 years. On looking over a catalogue of all the comets then 
known, he was struck with the similarity which the elements 
obtained by him bore to those of the comets of 1786 (i.), 1795, and 
1 805, and he was strongly impressed with the idea that the comet 
whose movements were then under investigation was identical 
with those comets, more particularly as, on the assumption of 
a 3^-year period, it might be expected to have been in perihelion 
at about those epochs. This question could only be settled by 
calculating backwards the effects of planetary perturbation, 
which Encke by an extraordinary effort did in 6 weeks. He was 
accordingly able to assure himself of the identity of the comet of 
1818 with the 3 above-mentioned ones, and also that between 
1786 and 1818 it had passed through perihelion 7 times without 
being seen. 

Encke then proceeded to calculate its next return, and he an- 
nounced that the comet would arrive at perihelion on May 24, 
1822, after being retarded about 9 days by the influence of the 
planet Jupiter. 

" So completely were these calculations fulfilled, that astrono- 
mers universally attached the name of ' Encke ' to the comet of 
1819, not only as an acknowledgment of his diligence and success 
in the performance of some of the most intricate and laborious 
computations that occur in practical astronomy, but also to mark 
the epoch of the first detection of a comet of short period one of 
no ordinary importance in this department of science." 

It unfortunately happened that at its return in 1822 the 
position of the comet in the heavens was such as to render it 
invisible in the Northern hemisphere. It was therefore systema- 
tically watched by only one observer, M. Rumker, who discovered 
it on June 2, at the private observatory of Sir T. M. Brisbane, at 
Paramatta, New South Wales, and he was able to follow it for 
only 3 weeks. Riimker's observations were, however, so far 

E e 



418 Comets. [BOOK IV. 

valuable, that besides showing that the comet actually did 
come back, they furnished Encke with the means of predicting 
with greater certainty its next return, which he found would 
occur on Sept. 16, 1825. 

On this occasion it was first seen by Valz, on July 13, but was 
discovered independently by more than one other astronomer. 
Cacciatore, of Palermo, described it as being round, with a faint 
nebulosity, and about 1 30' in diameter. 

The next return to perihelion took place on Jan. 9, 1829. 
Struve, at Dorpat, found it on Oct. 13, 1828: Harding, at 
Gottingen, and Gambart, at Marseilles, both saw it for the first 

Fig. 1 86. 




ENCKE'S COMET: NOV. 30, 1828. (W. Struce.) 

time on the same day, Oct. 27, the former having been on the 
look-out since Aug. 19, and it was very generally observed till 
the end of December in the same year. On Nov. 30 it was 
visible to the naked eye as a star of the 6 th magnitude, and a 
week afterwards it had become as bright as a star of the 5 th 
magnitude. The outline of the coma was slightly oval, with 
the minor axis (on one occasion at least) pointing towards the 
Sun. 

The 4th of May, 1832, was calculated as the epoch of the next 
perihelion passage. The comet was discovered by Mossotti, at 
Buenos Ayres, on June i, and by Henderson, at the Cape of 
Good Hope, on the following night. Harding, at Gottingen, who 
saw it on Aug. 2 1 , was the only European observer who caught 



CHAP. II.] Periodic Comets. 419 

a glimpse of it, owing to its path lying chiefly in the Southern 
heavens. 

The next return to perihelion was fixed for Aug. 26, 1835. 
The comet was seen both in Europe and at the Cape of Good 
Hope. 

Dec. 9, 1838, was the epoch of the next perihelion passage; 
and as the comet's apparent path would be such as to allow 
observations to be made in Europe under very favourable con- 
ditions, it was looked for with much interest. Boguslawski dis- 
covered it on Aug. 14 ; but Galle, at Berlin, did not see it till 
Sept. 1 6 ; and it was not generally seen till the middle of October. 
At about the end of the first week in November it was visible to 
the naked eye in Draco ; with a telescope a rather bright nucleus 
was seen, and the general form of the coma was that of a broad 
parabola. 

The account of this return would be incomplete were I not 
to refer to a peculiarity connected with the comet's motion, which, 
though it attracted Encke's attention as far back as 1818, may be 
said not to have been brought into special prominence till the 
return of 1838. He found that, notwithstanding every allowance 
being made for planetary influences, the comet always attained 
its perihelion distance about i\ hours sooner than his calculations 
led him to expect. In order to account for this gradual diminu- 
tion of the period of revolution, which in 1789 was nearly 
I2i3 d , but in 1838 was scarcely i2ii- r V d Encke conjectured 
the existence of a thin ethereal medium, sufficiently dense to 
produce an effect on a body of such extreme tenuity as the 
comet in question, but incapable of exercising any sensible 
influence on the movements of the planets. " This contraction of 
the orbit must be continually progressing, if we suppose the 
existence of such a medium ; and we are naturally led to inquire, 
What will be the final consequence of this resistance 1 Though 
the catastrophe may be averted for many ages by the powerful 
attraction of the larger planets, especially Jupiter, will not the 
comet be at last precipitated on the Sun ? The question is full 
of interest, though altogether open to conjecture." 

E e 2 



420 



Comets. 



[BOOK IV. 



The following table, published by Encke b , will more clearly 
illustrate the changes in the comet's periodic time : 



Year of PP. Period, Days. 
1786 

(I7 8 9) 121279 

(1792) 1212-67 

1795 1212-55 

(1799) 1212-44 

(1802) 1212-33 

1805 I2T2-22 

(1809) 1212-10 

(l8l2) I2I2-OO 

(1815) 1211 89 

1819 I2II-78 

1822 . . 1211-66 



Year of PP. Period, Days. 

1825 I2II-55 

1829 1211-44 

1832 1211-32 

1835 1211-22 

1838 I2II-II 

1842 1210-98 

1845 1210-88 

1848 1210-77 

1852 1210-65 

1855 121055 

1858 1210-44 



The propriety of this explanation of a resisting medium has 
been warmly canvassed at different times, and it cannot be said 
yet to command universal assent. One strong point against it is, 
that, with the exception perhaps of Winnecke's, none of the other 
short-period comets (all of them of small size and, presumably, 
unimportant mass) yield any indications that they experience a 
like influence c . On the other hand, Von Asten, who worked at 
the problem with great perseverance, thought there ought to be 
no hesitation in accepting the idea, subject to the limitation that 
the medium does not extend beyond the orbit of Mercury. 

The 1838 return is also noticeable for an important discovery 
in physical astronomy which it, indirectly, was the cause of 
evolving. In Aug. 1835 the comet passed very near the planet 
Mercury so near, in fact, that Encke showed that if Laplace's 
value of Mercury's mass were correct, the planet's attractive 
power would diminish the comet's geocentric R.A. on Nov. 2, 
1838, by 58', and increase its Declination by 17'. As the obser- 
vations indicated no such disturbance of the comet's orbit, it was 
obvious that the received mass of the planet was far too great, 
and a much lower value has since been adopted d . 

b Month. Not., vol. xix. p. 70. Dec. in Month. Not., vol. xxxiii. p. 239. Feb. 
1858. 1873. 

c See a notice of a paper by A. Hall d In Hind's Comets, p. 65 et seq., the 



CHAP. II.] Periodic Comets. 421 

Passing over the returns of 1843 and 1845, as offering no 
features of particular interest, we find that in 1848, on Sept. 24, 
the diameter of the comet's head was 8', and that it was just 
visible to the naked eye on Oct. 6, and for some weeks sub- 
sequently. Early in November it had a tail about i long, 
turned from the Sun, and another and smaller one directed 
towards that luminary. On Nov. 22, at midnight, the comet was 
distant but 3,600,000 miles from Mercury. The frontispiece to 
this volume will convey a good idea of the appearance of the 
comet at this apparition. 

Passing over also the returns of 1852, 1855, and 1858, we 
arrive at that of 1862, the 17 th on record. The passage through 
the perihelion took place on Feb. 6, but the comet was discovered 
by Forster, at Berlin, as early as Sept. 28, 1861. It was then 
very faint, and difficult of observation. The same character 
applies to the return of 1865, which was observed only in the 
Southern hemisphere. In 1868 the comet was unfavourably 
placed and was seen by only a few observers. 

In 1871, on the other hand, the comet was well seen and 
numerous observations of it were made. For a day or two in 
November, it was within the reach of telescopes of small dimen- 
sions. Some physical peculiarities were noted at this apparition 
which deserve mention. When first discovered in August, the 
comet was a nearly round and faint nebulosity, without apparent 
condensation in any part. By the beginning of November, it 
had acquired a remarkable fan - like form, but the precise 
character of the exterior outline differed a good deal according 
to the power of the telescope employed. 

Mr. Carpenter said e : 

" I was able to make out a considerable extension of the illumination beyond the 
bright fan-shaped condensation, but on one side (the spreading side) only. On the 
opposite side this diffused illumination appeared to be cut off nearly in a straight line 
immediately behind (following) the apex of the fan." 



general principles upon which these in- astronomer is noted in the treatment of 

quiries are conducted are laid down with difficult matters. 

that clearness of language for which that e 3fow/A.2V r o#.,vol.xxxii.p.26.Nov.i87i. 



422 



Comet*. 



[BOOK IV. 



The Rev. H. C. Key, speaking in the first instance of what he 
saw on December 3, said f : 

" The train following the comet was quite broad in my telescope, and could not be 
termed a 'ray.' You will observe two rays on the preceding side; these I have 
drawn as you see, but I am not perfectly certain that the effect was not in my own 
eye and not a reality. I took every precaution to find out ; and at the time (as well 

Fig. 187. 




ENCKE'S COMET: NOV. 9, 1871. (J. Carpenter.) 

as now) felt pretty well convinced that it was no illusion. Four or five times I left 
the telescope, and upon returning there were the rays in exactly the same spot and 
direction. I feel pretty confident of their reality (they were extremely faint), but, as 
I say, am not quite certain, as I sometimes see dark lines in the field when first 
going to the telescope. The comet never seemed to me to lose its elliptical form 
from the first night I saw it, Oct. 2Oth. I detected a nucleus for the first time on 

' Month. Not., vol. xxxii. p. 217. March 1872. 



CHAP. II.] Periodic Comets. 423 

Nov. 7th. The train I mentioned before was much fainter than the main body of 
the comet, and I was able to trace it to a distance of about 32' from the nucleus. I 
saw nothing like the drawing of the comet made at Greenwich." 

The return of 1871 was also important by reason of the fact 
that it was found not to have been accelerated, in accordance 
with the Resisting Medium theory, as all previous returns had 
been. Von Asten's conjecture as to this is that in 1869 the 
comet might have come into collision with some unknown minor 
planet which violently deranged its orbit and modified the orbit 
in some degree 8 . 

Encke's comet returned to perihelion again in April 1875, but 
no observations were made calling for notice. 

In 1878 the comet was best seen in the Southern hemisphere. 
Its diameter on August 10 was about 2', and it resembled 
generally a star of the 8 th magnitude, according to the account 
given by Gould. In the Northern hemisphere it was observed 
with extreme difficulty by Winnecke at Strasburg on Aug. 20 
and by Tempel at Arcetri on Aug. 21. O. Struve, even with the 
great 1 5-inch refractor at his command, did not catch sight of it 
till Aug. 24. 

In 1 88 1 the comet passed through perihelion on Nov. 18. It 
was noted by Common, using a 3-ft. reflector, as about 2' in 
diameter, very faint, and with slight indications of an increased 
brightness in the centre. Tacchini found the spectrum to exhibie 
bright bands in the yellow, green, and blue respectively, coin- 
ciding with the 3 principal bands seen in the spectra of the 
hydro-carbons. As in some other comets, the bands were shaded 
off to the blue. A faint continuous spectrum was also detected 11 . 
The spectrum was considered to have undergone no change since 
the previous examination in 1878. 

In 1884 the comet was observed by Tempel on Dec. 13, but it 
was extremely faint. In 1888 it was seen only in the Southern 
hemisphere, being first detected by Tebbutt at Windsor, N.S.W., 
on July 8, about 10 days after passiDg perihelion. 

s Bulletin de PAcad. deSt.Petersbourff,vo\.v. Observatory, vol.i. p. 21. Aprili87J r . 
h Comptes Rendus, vol. xciii. p. 947. 



424 Comets. [BOOK IV. 

M. Berberich has written an interesting historial paper on the 
brightness of Encke's comet at its many successive apparitions 1 . 

TEMPEL'S SECOND PERIODICAL COMET. 

No. 2. On July 3, 1873, Tempel at Milan discovered a small 
faint comet. It was described as being somewhat elongated, with 
an eccentric condensation, and a granular appearance. The 
diameter was at least 2'. It quickly became evident that the 
comet moved in an elliptic orbit of short period. Hind pointed 
out that soon after passing its ascending node and when near 
aphelion the comet passes close to the orbit of Jupiter, in which 
fact is to be found the cause of its periodicity. 

This comet returned again to perihelion in August 1878. It 
was seen at Oxford with difficulty in the 1 2-inch refractor of the 
University Observatory, and resembled a faint round nebula i' in 
diameter, with a very slight central condensation. 

At the return of 1883 (PP. on Nov. 20) the comet was not seen 
owing to its unfavourable position. 

. WINNECKE'S COMET, 

No. 3, was discovered by M. Pons, on June 12, 1819. Encke 
assigned to it a period of 5| years, which, as the table will show, 
was a very close approximation to the truth. It was not, 
however, seen from that time till March 8, 1858, when it was 
detected by Winnecke, at Bonn, and by him regarded as a new 
comet; but he soon ascertained the identity of the two objects. 
It must have returned in 1863, but was not on that occasion 
favourably placed for observation. The next return to perihelion 
occurred in June 1869. The comet was viewed by Winnecke 
himself on April 9 of that year, and is described by him as 
being faint, but not less than 6' or 8' in diameter. Winnecke's 
comet was again visible in 1875 passing through perihelion on 
March n. 

Some calculations by Oppolzer led him to think that 
this comet was observed previous to the occasion which has 

1 Ast. Nach.,vol. cxix., No. 2836, Ap. 24, 1888. 



CHAP. II.] Periodic Comets. 425 

usually been considered its first discovery (namely its detection 
by Pons in 1819), and that it is identical with the comet dis- 
covered by Pons in Feb. 1808. (See the Catalogue of "Un- 
calculated" Comets, post, p. 585.) 

It was due again in the Autumn of 1880, but escaped notice. 
In 1886 however it was seen in the Southern hemisphere after 
perihelion passage. It passed its perihelion 1 2 days earlier than 
it was predicted to do, and according to Oppolzer its movements 
cannot be completely explained by the theory of gravitation alone, 
but the existence of some resisting medium seems indicated. 

BRORSEN'S COMET, 

No. 4, was detected by M. Brorsen, at Kiel, on Feb. 26, 1846. 
The observations showed an elliptic orbit, and the epoch of the 
ensuing arrival at perihelion was fixed for Sept. 26, 1851, but 
its position then was not very favourable, owing to its proximity 
to the Sun, and it escaped observation. Bruhns again discovered 
it on March 18, 1857. I saw it on March 23; it possessed the 
usual nebulous appearance common to these objects, and had a 
diameter of about 2', though it was unfavourably placed in the 
morning twilight, which probably marred its brilliancy. This 
comet again returned to perihelion in April 1868, Oct. 1873, and 
March 1879. Spectroscopic observations on the last-named 
occasion by Konkoly in Hungary and C. A. Young in America 
tended to show that the spectra of this and of Encke's comet 
were identical with one another, and with a hydro-carbon 
spectrum J. Brorsen's comet escaped notice at its return in 
Sept. 1884. 

The period of Brorsen's comet has been gradually diminishing 
owing to the effect of planetary perturbation. Thus : 
In 1 846 ; period = 2034 days. 
l8 57; =2022 

1868; = 2002 



= J 999 
l8 795 = T 994 

Observatory, vol. iii. pp. 56, 105, June, August, 1879. 



426 Comets. [BOOK IV. 

It was missed, as stated above, at the returns of 1851 and 1862 
owing to its unfavourable position. The present orbit was due 
to the action of Jupiter in 1842, and, according to D' Arrest, 
serious disturbances from the same cause will happen in 1937 k . 

TEMPEL'S FIRST PERIODICAL COMET. 

No. 5. On April 3, 1867, Tempel at Milan discovered a small 
telescopic comet. It had a nucleus which was eccentrically placed 
in an oval coma, and Talmage, on May 3, thought that the 
nucleus appeared to have a division across the centre. The 
comet remained visible for about 4 months, which time sufficed 
to make it evident that its orbit was an ellipse of short .period. 
Searle's value of the period was 2064 days ; Bruhns's slightly 
greater, 2074 days. 

On July 3, 1873, Tempel discovered a comet which in his 
telegram he described as " schwach " (faint). Several computers 
obtained elliptic elements of its orbit, but, strangely enough, 
some time elapsed before the comet's identity with comet ii. of 
1867 was found out. It returned to perihelion in May 1879, 
and is now recognised as a permanent addition to the List of 
Short-period Comets. But it escaped detection at its return in 
the Spring of 1885. 

SWIFT'S COMET. 

No. 6. On Oct. 10, 1880, Prof. Swift at Rochester, New Jersey, 
U. S., found a small comet with a very diffused and ill-defined disc 
several minutes in diameter. It was soon ascertained by Chandler 
that the orbit was elliptic with a period of 6 years, and the 
comet identical with comet iii. 1869, discovered by Tempel on 
Nov. 27 of that year. The comet had been very unfavourably 
circumstanced for observation at the return of 1874, and had 
escaped detection. It was also unfavourably placed at its return 
in 1886. It is a peculiarity of this comet that it is well situated 
for observation only at alternate returns to perihelion. 

k Nature, vol. xxx. p. 301, July 24, 1884. 



CHAP. II.] Periodic Comets. 427 

BARNAED'S COMET. 

No. 7. On July 16, 1884, Mr. E. E. Barnard, at Nashville, 
Tennessee, U. S., using a 6-inch refractor, discovered a nebulous 
object which he thought had a suspicious appearance. Some days 
however elapsed ere it was found to be in motion and its cometary 
character ascertained beyond a doubt. Perrotin described the 
comet as exhibiting on Aug. 15 an ill-defined nebulosity about 
iY in diameter, and having a granular structure towards its 
centre. There is no doubt that the orbit is elliptical ; the period 
is at present somewhat uncertain ; but it is probably about 6 years. 
If Berberich's period of 5^49 years is correct, the comet must 
have approached very near indeed to Mars between April 5 
and 10, 1868, and have had its orbit perturbed by that planet. 

D' ARREST'S COMET. 

No. 8. On June 27, 1851, D' Arrest, at Leipzig, discovered a 
very faint telescopic comet in the constellation Pisces. Within a 
fortnight of its discovery the observations appeared irreconcile- 
able with a parabolic orbit, and it was soon placed beyond a 
doubt that its true path was an ellipse. The comet was visible 
for more than 3 months ; but notwithstanding this, the results 
of the calculations of the orbit were very discordant, and the 
predicted return of the comet in the winter of 1857-8 must be 
regarded rather in the light of a successful guess than anything 
else. Sir T. Maclear, at the Royal Observatory, Cape of Good 
Hope, was the only observer of this apparition. 

M. Villarceau communicated to the Academy of Sciences at 
Paris, on July 22, 1861, an interesting memoir on the orbit of 
this comet, which may be usefully placed on record (in an 
epitomised form) as it will serve to give some insight into the 
nature of the mathematical investigations which the calculators 
of cometary orbits are called upon to conduct : 

The perturbations experienced by this comet are owing chiefly to the action of 
J upiter, to which it is so near, that during the month of April of the present year [ 1 86 1 ] 
its distance was only 0-36, or little more than one-third of the Earth's distance from the 
Sun. Before and after this epoch, Jupiter and the comet have continued, and will 



428 Comets. [BOOK IV. 

continue, so little distant from one another, as to produce the great perturbations to 
which the comet is at present subject. 

From a table of the elements of the perturbations produced by Jupiter, Saturn, and 
Mars, in the interval between the appearance of the comet in 1857-8 and its return 
to its perihelion in 1864, M. Villarceau obtained the following results: 

(i) The longitude of the perihelion will have diminished 4 35' to Aug. 1863, and 
will remain sensibly stationary for about a year from that epoch. (2) The longitude 
of the node will have continually diminished to the amount of 2 8'. (3) The inclina- 
tion will have increased i 49' to the middle of 1862, and will diminish 6' during 
a year, continuing stationary during the year following. (4) The eccentricity, after 
having increased to the middle of 1860, will diminish rather quickly, and will remain 
stationary from 1863-5 * 1864-6. " But of all these perturbations," says M. Villar- 
ceau, " the most considerable are those of the mean motion and the mean anomaly. 
After having increased from 5" to July, 1860 the mean motion diminishes 9" in one 
year, and nearly 1 2" in the year following, remaining stationary in the last year, and 
with a value 15", 5" less than at its origin. The perturbations of the mean anomaly, 
after having gradually increased till 1860, will increase rapidly till 1861, when they 
will amount to 10 28'; and setting out from this, they will increase 9', and in 1863 
and 1864 they will have resumed the same value which they had in 1861." 

The effect of the first of these perturbations will be to increase the time of the 
comet's revolution by about 69 days ; and of the second, to hasten by 49 days the 
return of the comet to its perihelion in 1864. It will pass its perihelion on Feb. 26, 
whereas without the influence of these perturbations it would have passed it on 
April 15. 

As was anticipated, the comet escaped notice altogether at its 
return to perihelion in 1864. But in 1870, astronomers were 
more fortunate, and were able to follow it for 4 months. 
Winnecke has pointed out that D'Arrest's comet is undoubtedly 
the faintest of all the known periodic comets 1 . It came back 
again to perihelion in 1877? but was not seen at its return in the 
winter of 1883. 

FINLAY'S COMET, 

No. 9. On Sept. 26, 1886, a small tailless comet 1'in diameter 
was discovered at the Royal Observatory, Cape of Good Hope. It 
was at first thought to be possibly identical with the lost comet 
of Di Vico, but subsequent investigation negatived this theory : 
it is however certainly a short-period comet, and its next return 
will be looked forward to with interest. 

1 Ast. Nuch., vol. Ixxv. No. 1824, Oct. 12, 1870. 



CHAP. II.] Periodic Comets. 429 

WOLF'S COMET. 

No. 10. On Sept. 17, 1884, Dr. Wolf of Heidelberg discovered 
a small telescopic comet which Col. Tupman described a week 
later as about 2' in diameter and possessing a stellar nucleus 3" 
in diameter. It soon proved to be a short-period comet revolving 
round the Sun in about 6^ years. 

FAYE'S COMET, 

No. u, was discovered by M. Faye, at the Paris Observatory, on 
Nov. 22, 1843, it being then in the constellation Orion. It ex- 
hibited a bright nucleus, with a short tail, but was never 
sufficiently brilliant to be seen by the unaided eye. That the 
comet's path was an ellipse seems to have been suspected in- 
dependently by more than one observer. To Le Verrier, however, 
is due the credit of having completely investigated its elements. 
That astronomer showed that the comet came into our system at 
least as far back as the year 1747, when it suffered much per- 
turbation from Jupiter m ; and, further, that its next perihelion 
passage would occur on April 3, 1851. 

It was rediscovered by Challis, at Cambridge, on Nov. 28, 1850. 
O. Struve described it, under the date of Jan. 24, 1851, as having 
a diameter of 24". During the whole time it was observed it had 
scarcely any nucleus or tail. This comet returned in due course 
to perihelion on Sept. 12, 1858, having been detected 4 days 
previously by Bruhns, at Berlin. It was also seen in 1866, 1873, 
i88o n , and 1888, and next after Halley's and Encke's comets may 

m The intelligent reader may wonder no means friendly, with the colossal 
why Jupiter is so constantly called to planet. It is, moreover, an incidental 
account as the great bugbear of these indication of the potency of Jupiter's 
short-period comets. The reasons are influence over comets, that so many short- 
two in number: (i) The immense mass period comets have periods amounting to 
of Jupiter compared with that of any of between 5 and 6 years, being about the 
the other planets ; and (2) the fact that time occupied by Jupiter in traversing 
the. aphelia of all these comets lie very half its orbit. (See Fig. 181, on p. 402, 
close to the orbit of Jupiter; so that ante.) 

when at their greatest distance from the n For a fuller history of this comet 

Sun, they are constantly liable to ren- see Month. Not., vol. xli. p. 246, Feb. 

contres more or less intimate, though by 1881. 



430 Comets. [BOOK IV. 

be regarded as the best-known cometary member of the Solar 
system. 

DENNING'S COMET. 

No. 12. On Oct. 4, 1881, Mr. W. F. Denning at Bristol dis- 
covered a bright telescopic comet in the constellation Leo. It was 
circular in form, about i' in diameter, and showed a slight central 
condensation. The ellipticity of its orbit soon became known to 
those who undertook the computation of its elements, and there 
is no doubt that it constitutes an interesting addition to our 
list of short-period comets, and the first made by an Englishman. 

The elements bear some resemblance to those of the comet of 
1819 (iv.), discovered by Blainpain. Winnecke thinks that the 
comet seen at Paris in 1855 by Goldschmidt, and then regarded 
as perhaps Di Vice's, and Hind's comet of 1846 (ix.), may both 
have been apparitions of Denning' s comet. The further con- 
sideration of these suggestions must stand over till after the 
next return of this object to perihelion, which will be awaited 
with much interest by astronomers, the more so as it is known 
that it must come much under the influence of several of the 
major planets. 

TUTTLE'S COMET, 

No. 13, was detected by Mechain, on Jan. 9, 1790. It was only 
followed for a fortnight. On Jan. n Messier could see but a 
confused nebulosity, without any indications of a nucleus. It 
was not re-observed vmtil its return at the commencement of 
1858, on Jan. 4 of which year it was detected by H. P. Tuttle, at 
Harvard College Observatory, Cambridge, U.S. It returned 
again to perihelion in Nov. 1871 and Aug. 1885, and is now 
accepted as a regular member of the group of short-period 
comets. On the last occasion it was very faint, and was only 
followed in the morning twilight for about a fortnight. 

BIELA'S COMET. 

Besides those enumerated in the Table, there is another very 
remarkable periodic comet, even more interesting than Encke's. 



CHAP. II.] Periodic Comets. 431 

but for altogether a different reason : I shall therefore give its 
history at some length. 

On March 8, 1772, Montaigne, at Limoges, discovered a comet 
in Eridanus, which, from want of suitable instruments, he was 
unable properly to observe, or to see at all after the 2oth ; 
Messier, however, saw it four times between March 26 and 
April 3. 

On Nov. 10, 1805, Pons discovered a comet, which was found 
also by Bouvard on the i6th. It had a nucleus, and the diameter 
of the coma on Nov. 23 was 6' or 7'. On Dec. 8 it was at its 
nearest point to the Earth, and Olbers saw it without a telescope. 
Bessel and others calculated elliptic elements, and its identity 
with the comet of 1772 was suspected, though no predictions as 
to its next return were ventured on. 

On Feb. 27, 1826, M. Biela, at Josephstadt, Bohemia, discovered 
a faint comet in Aries, which Gambart found on March 9. The 
observations extended altogether over a period of 8 weeks, and it 
was soon made evident that the orbit was an ellipse of moderate 
eccentricity; and further, that the comet was the same as that 
which had already been observed in 1772 and 1805. 

In anticipation of its next return in 1832 investigations into 
the orbit of the comet and the perturbations by which it would 
be affected were undertaken by Santini, Damoiseau, and Olbers. 
Santini found that its period in 1826 was 2455 days, but that the 
attraction of the Earth, Jupiter, and Saturn would accelerate its 
next return by rather more than 10 days, which he accordingly 
fixed for Nov. 27, 1832. Damoiseau's investigations gave a 
similar result. Early in 1828 Olbers called attention to the 
fact that in 1832 the comet would pass within 20,000 miles of 
the Earth's orbit ; but that as the Earth would not reach that 
particular point till one month after the comet had passed it, 
no danger was to be apprehended. Astronomers were quite 
satisfied as regards this matter, 'but their confidence was not 
shared by the unscientific many, who were greatly alarmed lest 
a collision should take place, and our globe become a sufferer 
thereby. 



432 Comets. [BOOK IV. 

Punctually at the time appointed the comet returned to peri- 
helion, through which it passed within 1 2 hours of the time fixed 
by Santini five years previously. It was first seen at Borne on 
Aug. 23, but, owing to its excessive faintness, it was not 
generally observed till two months later. 

The next return was calculated to take place on July 23, 1839, 
but in consequence of its close proximity to the Sun, the comet 
was not detected. 

Continuing his researches, Santini fixed on Feb. 1 1 , 1 846, as 
the epoch of the next perihelion passage ; and as it would be 
visible for a considerable period, much interest was excited 
amongst astronomers, who anticipated that a remarkably good 
opportunity would be afforded for correcting the theory of its 
motion. 

Di Vico, at Rome, with the powerful telescope at his command, 
discovered it on Nov. 28, 1 845, and Galle, at Berlin, saw it two 
days later ; but by the generality of observers it was not seen 
till the second or third week in December. I have already 
adverted to the very curious phenomenon which took place at 
this apparition of Biela's comet. (See ante, p. 408.) 

The comet returned again to perihelion in Sept. 1852, and was 
visible for three weeks. The same reason which prevented it 
from being seen in 1839 also caused it to pass undetected in 
May 1 859 ; so that we were obliged to await its next return to 
perihelion in Jan. 1866 for further information relative to its 
physical condition. This return was looked forward to with 
much interest ; as it was important to know what changes had 
occurred during the preceding 13 years in the relative position 
of the two portions so strangely rent asunder, as already narrated 
whether they still travelled through space in company or not. 
That between 1846 and 1852 they had become, for all practical 
purposes, two complete comets, seemed indisputable ; and in the 
sweeping Ephemerides issued from the Nautical Almanac office, 
by Mr. Hind, for facilitating their rediscovery in 1859, two 
independent sets of elements and positions were given. 

It was calculated that the comet would have been seen in 



CHAP. II.] Periodic Comets. 433 

1865-6 under very favourable circumstances, and search was 
systematically made for it at numerous European Observatories, 
but without success. Much disappointment was felt by astrono- 
mers : and startling as such a suggestion may appear , even the 
continued existence of the comet seemed so open to uncertainty 
that all hopes of seeing it again were given up. At least one 
man, however, did not despair. M. Klinkerfues of Gb'ttingen 
kept the subject before him, and as the result of his labours, he 
sent, on Nov. 30, 1872, to Pogson at Madras, a telegram as 
follows : "Biela touched Earth on zyth : search near 6 Centauri" 
The search was made, and a comet found, and observations of it 
were obtained on Dec. 2 and 3, 1872. Bad weather and the 
advance of twilight prevented further success p . Here the matter 
rests : it was however the opinion of Bruhns that the comet seen 
by Pogson could not possibly have been Biela's, but was, by a 
remarkable coincidence, some other. 

The further consideration of the question " Why has Biela's 
comet disappeared ? " seems now to belong to the subject not of 
Cometic but of Meteoric Astronomy. Accordingly we shall have 
more to say about it in Book V (post}. 

Di Vice's COMET. 

On Aug. 22, 1844, M. Di Vico, at Rome, discovered a tele- 
scopic comet, which, towards the end of the following month, 
became perceptible to the naked eye. With a telescope a bright 
stellar nucleus and a short tail were seen. It some became evident 
that the observations could not be reconciled with any parabolic 
orbit, and elliptic elements were calculated by several computers. 
The most complete investigation is due to Briinnow, who found 
that the comet's periodic time was 1993 days. Carrying on his 
researches to the next return to perihelion, which was calculated 

I assume that we are required to spring of 1866. (See Month. Not., vol. 

ignore certain alleged observations of xxvi. pp. 241 and 271.) 

"something" which formed a topic of P Month. Not., vol. xxxiii. p. 116. 

discussion at several meetings of the Dec. 1872. 
Koyal Astronomical Society, in the 

Ff 



434 Comets. [BOOK IV. 

to occur in the spring of 1850, he found that "when the comet 
was near enough to the Earth to be otherwise discerned, it was 
always lost in the Sun's rays, the geocentric positions of the Sun 
and comet at perihelion being nearly the same, and continuing so 
for some months, on account of the apparent direct movement of 
both bodies." 

Its next return to perihelion was fixed for Aug. 6, 1 855 ; and 
as it would be favourably situated for observation, hopes were 
entertained that it would again be detected. Such, however, 
was not the case; nor was it seen in 1861, 1866, 1872, or 1877 
and therefore we are no longer justified in including it in the list 
of " known " short-period comets, but its size and brilliancy 
(considerable for a short-period comet) render its non-appearance 
since 1844 a remarkable fact. Certain computations by Le 
Verrier render it probable that this comet is identical with that 
of 1678. 

On Sept. 26, 1886, Finlay discovered a small comet the 
elements of whose orbit were found to resemble closely those 
assigned to Di Vice's comet by Briinnow ; but the resemblance 
appears to be fortuitous : that is to say, that they are 2 distinct 
comets moving in orbits similar in many respects but not in all. 

Another instance of this sort of thing seems to be exhibited 
by the comets of 1843 (i.), 1880 (i.), and 1882 (ii.). 

Short periods have also been assigned to the following comets ; 
but too much uncertainty prevails with respect to them, to 
justify their being included with the foregoing q : 

Clausen (1743, i.) Blainpain (1819, iv.) 

Burckhardt (1766, ii.) Peters (1846, vi. 

Lexell (1770, i.) Coggia (1873, vii.). 
Pigott (1783) 

The last-named of these comets (1873, vii.) was the subject of 
an elaborate investigation by Weiss, who thought it a return of 
the comet of 1818 (i.), but he could not satisfy himself whether 

i The reader will find a few brief par- body interested in this branch of as- 

ticulars in the notes to the 1 st catalogue tronomy ought to possess. Those who 

(posf) ; but for further information he read German will find P. Carl's Reper- 

must consult Hind's Comets, or Cooper's torium der Cometen-Astronomie, pub- 

Cometic Orbits two works which every- lished at Munich in 1864, a useful book. 



CHAP. II.] 



Periodic Comets. 



435 



its period was 55'8, 18-6, or 6*2 years, though he gives the pre- 
ference to 6'2 years. 

In Class II. we have the following comets : 



Name. 


Period. 


Probable next 
Return. 


i. Westphal (1852, iv.) ... 


Years. 

67.77 


IQ2O 


2. Pons (1812) 


70-68 


IQ54 


3. Di Vico (1846, iv.) 


72.35 


IQIQ 


4. Gibers (1815) 


74-of; 


1061 


5. Brorsen (1847, v.) 


74.07 


IQ22 


6. Halley 


76-78 


IQIO 









It has been suggested (I know not by whom) that 4 of the 
above may have originally constituted a single comet. Inde- 
pendently of this, Kirkwood has given reasons why some 
connection may exist between Nos. 2 and 3 in the above Table. 

No. 2 was discovered by the indefatigable Pons on July 20, 
1812, being the i6th comet found by him in 10 years. It had 
an irregular nebulous form without tail or beard, and was only 
visible through a telescope. Encke having assigned a period of 
707 years, the return of the comet was anticipated about 1883, 
and accordingly it was sighted on Sept. 3 by Brooks in America 
by the aid of a sweeping ephemeris computed by Schulhof and 
Bossert. It appears to have exhibited in 1883-4 physical 
characteristics differing altogether from anything recorded in 
1812. Chandler in America and Schiaparelli in Italy saw it on 
several occasions in Sept. 1883, first as a nebulosity, then as a 
star, then as a nebulosity again ; whilst Miiller at Potsdam on 
Jan. i, 1884 observed changes up and down both in magnitude 
and brightness to the extent of T V of a magnitude in if hour. 

Trdpied observed it daily from Jan. 13 to 18 without noticing 
anything very remarkable, but on Jan. 19 the aspect of the 
nucleus had so changed that it was difficult to realise that the 
same object was being scrutinised. The head then exhibited 
3 zones, as in Fig. 188. 

F f 2 



436 



Comets. 



[BOOK IV. 



" The interior and most brilliant zone was almost circular, and remarkable owing 
to its milky aspect : it stood out sharply from the adjoining zone and was of a leaden 
hue : outside this second zone came the ordinary nebulosity of the tail, having on the 
south-west side a parabolic outline. 

The nucleus had undergone a considerable lengthening : it consisted of two distinct 
parts of very different brilliancy united by a very well marked twisted link (itrangle- 
menf) which occupied almost the centre of the inner circular zone. The Southern 

Fig. 1 88. 




PONS'S COMET: Jan. 19, 1884. (Trepied.) 

part of the nucleus, which was by far the brightest, was terminated by an elliptic arc 
very sharply denned and tangential to the circumference of the zone ; the Northern 
part on the contrary was suddenly cut off at the extremity of the diameter, whose 
direction coincided with that of the axis of the nucleus. This direction was 
almost exactly identical with that of the axis of the tail. On January 20 the 
nucleus and the nebulosity which surrounded it had resumed their accustomed aspect. 
I observed the comet up till the end of the ist week in February without being able 
to detect any changes like that which happened on Jan. 19. It follows therefore 



CHAP. II.] Periodic Comets. 437 

that the transformations in question must have run their course in a few hours; and 
herein consists the remarkable character of the whole phenomenon." 

Trepied's observations accord generally with those of Perrotin, 
Thollon and Rayet, which apply however to the date of Jan. 13. 
It would appear therefore to follow that the changes in this 
comet, whatever their nature, were in some sense periodic a 
circumstance additionally remarkable. 

No. 4, Olbers's comet, came back to perihelion in 1 887. It was 
discovered by Brooks in America on Aug. 24. 

No. 6. The comet which has the most interesting pedigree is 
undoubtedly that which bears the name of our illustrious 
countryman Halley; and as its history will, moreover, serve to 
exemplify various remarks made in previous pages on the nature 
and appearance of comets, I cannot do better than give a 
summary of the said history from the time of the comet's last 
appearance, in 1835, back to the earliest ages. 

A few years after the advent of the celebrated comet of 1680, 
Sir I. Newton published his Principia, in which he applied to the 
orbit of that comet the Theory of Gravitation first promulgated 
in that work. He explained the method of determining, by 
geometrical construction, the visible portion of the path of a 
body of this kind, and invited astronomers to apply these prin- 
ciples to the various recorded comets. He considered that it 
was very probable that some comets might move in elongated 
ellipses which near perihelion would be scarcely distinguishable 
from parobolas, and even thought that the comet of 1680 might 
be moving in one which it took about 575 years to complete. 
The illustrious Halley (to whose solicitations and exertions the 
publication of the Principia is in great measure due, for he bore 
the whole labour and undertook the whole expense of its editing 
and publication) also took this view. But although we now 
know that the period of that comet is far longer and is in fact 
measured by thousands of year, Halley 's investigations in a 
subsequent instance led him to a conjecture which was fully 
substantiated. He undertook the labour of examining the cir- 
cumstances attending all the comets previously recorded with a 



438 



Comets. 



[BOOK IV. 



view to ascertain whether any, and, if so, which, of them, 
appeared to follow the same path. Careful investigation soon 
proved that the orbits of the comets of 1531 and 1607 were 
similar, and that they were, in fact, the same as that followed 
by the comet of 1682, seen by himself. He suspected therefore 

Fig. 189. 




HALLEY'S COMET, JAN. 9, 1683 (N.S.), SHEWING LUMINOUS SECTOB DRAWN BY HEVELius 1 ". 

(and rightly too, as the sequel showed) that the appearances at 
these 3 epochs were produced by the 3 successive returns of one 
and the same body, and that consequently its period was some- 
where about 751 years. There were nevertheless a circumstances 
which might be supposed to offer some difficulty, inasmuch as it 
appeared that the intervals between the successive returns were 
not precisely equal, and that the inclination of the orbit was not 
exactly the same in each case. Halley, however, " with a 
degree of sagacity which, considering the state of knowledge 
at the time, cannot fail to excite unqualified admiration, observed 

Fig. 190. 




PLAN OF THE OKBIT OP HALLEY S COMET COMPARED WITH THE 
ORBITS OF CERTAIN PLANETS. 

that it was natural to suppose that the same causes which dis- 
turbed the planetary motions would likewise act on comets ; " 
in other words, that the attraction of the planets would exercise 

r A a, > us climactericus, p. 139. 



CHAP. II.] Periodic Comets. 439 

some influence on comets and their motions. The truth of this 
idea we have already seen exemplified in the case of the comet 
of 1770. In fine, Halley found that in the interval between 
1607 and 1682 the comet passed so near Jupiter that its velocity 
must have been considerably increased, and its period con- 
sequently shortened ; he was, therefore, induced to predict its 
return about the end of 1 758 or the beginning of 1 759. He thus 
plaintively wrote on the subject : " Wherefore if it should return 
according to our prediction about the year 1758 impartial 
posterity will not refuse to acknowledge that this was first 
discovered by an Englishman." Although Halley did not sur- 
vive to see his prediction fulfilled, yet, as the time drew near, 
great interest was manifested in the result, more especially as 
Clairaut had named April 13, 1759, as the day on which the 
perihelion passage would take place. It was not destined, 
however, that a professional astronomer should be the first 8 to 
detect the comet on its anticipated return ; that honour was 
reserved for a farmer near Dresden, named Palitzsch, who was 
also a student of Nature and who saw it on the night of 
Christmas-day, 1758. But few observations were made before 
the perihelion passage (on March 12), owing to the comet's 
proximity to the Sun ; during the months of April and May, 
however, it was seen throughout Europe, although to the best 
advantage only in the Southern hemisphere. On May 5 it had a 
tail 47 long. 

Previously to the last return of this comet, in 1835, numerous 
preparations were made to receive it. Early in that year Rosen- 
berger, of Halle, published a memoir, in which he announced 
that the perihelion passage would take place on Nov. n, though 
Damoiseau and Ponte'coulant both fixed upon a somewhat earlier 
period. 

Let us now see how far these expectations were realised. The 
comet was seen at Rome on Aug. 5 ; as it approached the Sun 

s It was stated by Prof. R. Grant in a early date, but was ordered to hold his 
lecture at the Royal Institution in 1870, tongue. I do not know what authority 
that Messier detected this comet at an there is for this statement. 



440 Comets. [BOOK IV. 

it gradually increased both in magnitude and brightness, but did 
not become visible to the naked eye till Sept. 20. On Oct. 1 9 
the tail had attained a length of fully 30. The comet was soon 
afterwards lost in the rays of the Sun, and passed through its 
perihelion on Nov. 15, or within 4 days of the time named by 
Rosenberger. It reappeared early in Jan. 1836, and was ob- 
served in the South of Europe and at the Cape till the middle of 

Fig. 191. 




HALLEY'S COMET, 1835, OCT. n. (Smyth.} 

May, when it was finally lost to view, not to be seen again till 
the year 1910*. 

We have seen above that Halley traced his comet back to the 
year 1531 ; we must now, therefore, briefly review its probable 
history prior to that date, as made known by the labours of 
modern astronomers. Halley surmised that the great comet of 

1 Drawings by Bessel will be found in the Ast. Nach., vol. xiii. Nog. 300-2. 
Feb. 20, 1836. 



CHAP. II.] 



Periodic Comets. 



443 



1456 was identical with the one observed by him in 1682, and 
Pingre converted Halley's suspicion into a certainty. The pre- 
ceding return took place, as Laugier has shown, in 1378, when 
the comet was observed both in Europe and China ; but it does 
not appear to have been so bright in that year as in 1456. In 
Sept. 1301 a great comet is mentioned by nearly all the historians 
of the period. It was seen as far North as Iceland. It exhibited 
a bright and extensive tail, which stretched across a considerable 

Fig- 193- 




HALLEY'S COMET, 684. (From the Nuremberg Chronicle.') 

part of the heavens. This was most likely Halley's comet. The 
previous apparition is not so well ascertained, but it most likely 
occurred in July 1223, when it is recorded in an ancient 
chronicle that a wonderful sign appeared in the heavens shortly 
before the death of Philip Augustus of France, of which event 
it was generally considered to be the precursor. It was only 
seen for 8 days. Although but little information is possessed 
about it, and that of a very vague character, yet it seems 
probable that this was Halley's comet. In April 1145 a great 
comet is mentioned by European historians, which is one of the 
most certain of our series of returns. In April 1066 an im- 
portant comet became visible which astonished Europe. It is 



444 Comets. [BOOK IV. 

minutely, though not very clearly, described in the Chinese 
annals ; and the path there assigned to it is found to agree with 
elements which bear a great resemblance to those of Halley's 
comet. In England it was considered the forerunner of the 
victory of William of Normandy, and was looked upon with 
universal dread. It was equal to the Full Moon in size, and its 
train, at first small, increased to a wonderful length. Almost 
every historian and writer of the II th century bears witness to 
the splendour of the comet of 1066, and there can be but little 
doubt that it was Halley's. Previous to this year the comet 
appeared in 989, 912, 837, 760, 684, 608, 530, 451, 373, 295, 218, 
141, 66 A.D., and n B.C., all of which apparitions have been 
identified by Hind u . 

Concerning the comets belonging to Class III. (comets of long 
period), it is not necessary to notice them further here ; they will 
be found in the Catalogue, passim. 

Flammarion, making use of some previous labours in this field 
by Kirkwood and others, has worked out the idea of particular 
comets being associated with particular planets in a way which 
has yielded some results too curious and interesting to be passed 
over. In addition to the I st or Jupiter group to which reference 
has already been made*, he finds that every major planet beyond 
Jupiter seems to have a group of comets attached to it; and 
moreover, as there is a group of comets without a known 
planetary leader, he makes bold to speculate that this fact is a 
proof that a Trans-Neptunian planet exists and will one day be 
found. 

The following are Flammarion's groups, the figures appended 
representing in Radii of the Earth's orbit the mean distances of 
the respective planets and the aphelion distances of the re- 
spective comets : 

2ND GBOUP. 

SATUBN 9-0 to 10-1 

Tattle's Comet 10-5 

u Month. Not., vol. x. p. 51. Jan. 1850. * See pp. 401, 402, ante. 



CHAP. II.] Periodic Comets. 445 

3RD GROUP. 

URANUS 18-3 to 20-1 

Comet of 1866 (^i.) and November Meteors ... ... 19-7 

Comet of 1867 (i.) 19-3 

4TH GROUP. 

NEPTUNE 29-8 to 30-3 

Comet of 1852 (iv.) (Westphal) 29-32 

Comet of 1812 (Pons) 33 

Comet of 1846 (iv.) (Di Vico) 34 

Cometof 1815 (Gibers) 34 

Comet of 1847 (v.) (Brorsen) 35 

Halley's Comet 35 

5TH GROUP. 

Trans-Neptunian planet ? ... ... ... ... 47 to 48 ? 

Comet of 1862 (iii.), and August Meteors ... ... 49 

Comet of 1532 and 1661... ... ... ... ... 48 

Flammarion finally hints at the speculation that the undis- 
covered planet must, if it be related to the comets of the 5 th 
group, revolve at somewhere about twice the distance of Neptune, 
say, in a period of 300 years y . 

y L 'Astronomic, vol. iii. p. 89, March portant mistakes or misprints in the 
1884. I have corrected several im- French original. 



446 Comets. [BOOK IV. 



CHAPTER III. 



REMARKABLE COMETS. 

The Great Comet of iSn.The Great Cornel of 1843. The Great Comet of 1858. 
The Comet of 1860 (iii.). The Great Comet of iS6i.The Comet of 
1862 (iii.)- The Comet of 1864 (ii.). The Comet of 1874 ("*) The Comet 
of 1882 (iii.). 

THE comets which might be included under the above head 
are so numerous as to make it impossible that all should 
receive full attention. I must therefore limit myself to some 
few of the most interesting, premising that Grant includes the 
following comets under the designation " remarkable " : 

1066 1531 1682 1823 

1106 1556 1689 1835 

"45 1577 I7 2 9 I8 43 

1265 1607 1744 1858 

1378 1618 1759 1861 

1402 1661 1769 

1456 1680 1811 

The Comet of 1811 (i.) is one of the most celebrated of modern 
times. It was discovered by Flaugergues, at Viviers, on March 26, 
18 1 1, and was last seen by Wisniewski at Neu-Tscherkask, on 
Aug. 17, 1812. In the autumnal months of 1811 it shone very 
conspicuously, and its considerable Northern declination caused 
it to remain visible throughout the night for many weeks. The 
extreme length of the tail at the beginning of October was about 
25, and its breadth about 6. Sir W. Herschel paid particular 
attention to this comet, and the observations which he made are 



CHAP. III.] 



Remarkable Comets. 



447 



very valuable. He states that it had a well-defined nucleus, the 
diameter of which he found by careful measurement to be 428 
miles ; further, that the nucleus was of a ruddy hue, though the 
surrounding nebulosity had a bluish-green tinge a . This comet 

Fig. 194. 




THE GREAT COMET OF l8ll. 

undoubtedly is a periodical one. Argelander, whose investigation 
of the orbit is the most complete that has been carried out, assigned 
to it a period of 3065 years, subject to an uncertainty of only 43 
years b . The aphelion distance is 14 times that of Neptune, or, 
more exactly, 40,121,000,000 miles. 

The Comet of 1 843 (i.) was one of the finest that has appeared 
during the present century. It was first seen in the Southern 
hemisphere towards the end of the month of February, and during 

a Phil. Trans., vol. cii. pp. 118, 119, 121. 
b Berlin. Ast. Jahrbuch, 1825, p. 250. 



448 Comets. [BOOK IV. 

the first fortnight in March it shone with great brilliancy. It was 
not visible in England until after the 1 5 th , when its splendour had 
much diminished ; but the suddenness with which it made its 
appearance added not a little to the interest which it excited. 
The general length of the tail during March was about 40, and 
its breadth about i. The orbit of this comet is remarkable for 
its small perihelion distance, which did not exceed, according to 
the most reliable calculations, 538,000 miles ; and the immense 
velocity of the comet in its orbit, when near the perihelion, 
occasioned some extraordinary peculiarities. Thus between 
Feb. 27 and 28 it described upon its orbit an arc of 292. 
Supposing it to revolve in an ellipse, this would leave only 68 
to be described during the time which would elapse before its 
next return to perihelion. 

It has been thought by some that this comet was identical 
with those of 1668 and 1689, but so little is known for certain 
about this latter that we are not yet in a position to admit 
or deny the identity of the 3 bodies. In the work to which 
reference is made in the note the question is discussed with great 
ability c . 

The Comet of 1858 (vi.). On June 2 in that year, Dr. G. B. 
Donati, at Florence, descried a faint nebulosity slowly advancing 
towards the North, and near the star A. Leonis. Owing to its 
immense distance from the Earth (240,000,000 miles), great diffi- 
culty was experienced in laying down its orbit. By the middle 
of August, however, its future course and the great increase in 
its brightness which would take place in September and October 
were clearly foreseen. Up to this time (middle of August) 
it had remained a faint object, not discernible by the unaided eye. 
It was distinguished from ordinary telescopic comets only by the 
extreme slowness of its motion (in singular contrast to its sub- 
sequent career), and by the vivid light of its nucleus : " the latter 
peculiarity was of itself prophetic of a splendid destiny." Traces 
of a tail were noticed on Aug. 20, and on Aug. 29 the comet was 

c E. J. Cooper, Cometic Orbits, pp. the supposed identity of this comet see 
159-69. For something more concerning post, under the head of Comet iii., 1882. 



Fig. 195. 



Plate XXV. 




DONATI'S COMET: October 5, 1858. 

(Drawn by Pape.) 

fir 



Fig, 196. 



Plate XXVI. 




DONATI'S COMET: October 9, 1858. 

(Drawn by Pape.} 
Gg 2 



CHAP. III.] 



Remarkable Comets. 



453 



faintly perceptible to the naked eye ; for a few weeks it occupied 
a Northern position in the heavens, and it was therefore seen both 
in the morning and evening. On Sept. 6 a slight curvature of 
the tail was noticed, which subsequently became one of its most 
interesting features. On Sept. 17 the head equalled in bright- 
ness a star of the 2 nd magnitude, the length of the tail being 4. 

Fig. 197. 




DONATl's COMET, 1858, SEPT. 30. (Smyth.) 

The comet passed through perihelion on Sept. 29, and was at its 
least distance from the earth on Oct. 10. Its rapid passage to 
the Southern hemisphere rendered it invisible in Europe after the 
end of October, but it was followed at the Santiago-de-Chili and 
Cape of Good Hope Observatories for some months afterwards, 
and was last seen by Sir T. Maclear at the latter place on March 

4, i859- 

"Its early discovery enabled astronomers, while it was yet 
scarcely distinguishable in the telescope, to predict, some months 



454 Comets. [BOOK IV. 

in advance, the more prominent particulars of its approaching 
apparition, which was thus observed with all the advantage of 
previous preparation and anticipation. The perihelion passage 
occurred at the most favourable moment for presenting the comet 
to good advantage. When nearest the earth, the direction of the 
tail was nearly perpendicular to the line of vision, so that its 
proportions were seen without foreshortening. Its situation in 

Fig. 198. 




DONATI'S COMET, 1858, PASSING ARCTUEU8 ON Oct. 5. 

the latter part of its course atforded also a fair sight of the 
curvature of the train, which seems to have been exhibited with 
unusual distinctness, contributing greatly to the impressive effect 
of a full-length view." 

This comet, though surpassed by many others in size, has not 
often been equalled in the intense brilliancy of the nucleus, which 
the absence of the Moon, in the early part of October, permitted 
to be seen to the very best advantage. There is no doubt that 
the comet of Donati revolves in an elliptic orbit with a period 



Fiys. 199-203. 



Plate XXVII. 






I 

8 



00 
IO 
00 



o 

O 





O 

n 



CHAP. III.] 



Remarkable Comets. 



457 



of about 2000 years (Stampfer, 2138^; Lowy, 2O4o y ; Von 
Asten, 1879?). 

The following is a table of the dimensions of the comet's 
nucleus and tail, at the undermentioned dates d : 



Date. 


Diameter of Nucleus. 




Length of Tail. 


1858. 
July 19 


Miles. 
5 5600 


o 


Miles. 


Aug. 30 


6 4660 


2 


= 14,000,000 


Sept. 8 


3 = 1980 


4 


= 16,000,000 


12 




6 


= 19,000,000 


23 


1 1280 


5 


= I2,OOO,OOO 






ii 


= 17,000,000 


27 




13 


= l8,OOO,OOO 


28 




10 


= 26,000,000 


30 




22 


= 26,000,000 


Oct. 2 




25 


= 27,000,000 


- 


i. 5 = 400 


33 


= 33,OOO,OOO 


6 


3-0 = 800 


5 


= 45,000,000 


8 


4-4 = 1 1 20 


5 


= 43,OOO,OOO 


10 


2< 5 ~ 630 


60 


= 5I,OOO,OOO 


12 




45 


= 39,000,000 











The Comet of 1860 (iii.). In the latter end of June 1860, 
a comet of considerable brilliancy suddenly made its appearance 
in the Northern circumpolar regions. Bad weather prevented it 
from being generally observed in England, but in the South of 
Europe it was well seen ; copies of some drawings made at Rome 
are annexed. [Plate XXVIII.] 

Few comets created greater sensation than the Great Comet of 
1861 (ii. of that year). It was discovered by Mr. J. Tebbutt, an 
amateur observer in New South Wales, on May 13, previous to 
its perihelion passage, which took place on June 1 1 . Passing 
from the Southern hemisphere into the Northern, it became 



d G. P. Bond, Math. Month. Mag., 
Boston, U.S., Nov. and Dec. 1858. Mr. 
Bond subsequently published a magni- 



ficent memoir on this comet in vol. ii. of 
the Annals of the Harvard College Ob- 
servatory. Cambridge, Mass., 1862. 



458 Comets. [BOOK IV. 

visible in this country on June 29, though it was not generally 
seen till the next evening. So many accounts of it were pub- 
lished that selection is difficult, but the following pages will 
be found to contain an epitome of the most noticeable features e . 
Sir J. Herschel observed it in Kent. He says : 

"The comet, which was first noticed here on Saturday night, June 29, by a 
resident in the village of Hawkhurst (who informs me that his attention was drawn 
to it by its being taken by some of his family for the Moon rising), became con- 
spicuously visible on the 3O th , when I first observed it. It then far exceeded in 
brightness any comet I have before observed, those of 1811 and the recent splendid 
one of 1858 not excepted. Its total light certainly far surpassed that of any fixed 
star or planet, except perhaps Venus at its maximum. The tail extended from its 
then position, about 8 or 10 above the horizon, to within 10 or 12 of the Pole-star, 
and was therefore about 30 in length. Its greatest breadth, which diminished 
rapidly in receding from the head, might be about 5. Viewed through a good 
achromatic, by Peter Dollond, of 2f -inches aperture and 4-feet focal length, it 
exhibited a very condensed central light, which might fairly be called a nucleus ; 
but, in its then low situation, no other physical peculiarities could be observed. On 
the I st instant it was seen early in the evening, but before I could bring a telescope 
to bear on it clouds intervened, and continued till morning twilight. On the 2 ud 
(Tuesday), being now much better situated for observation, and the night being 
clear, its appearance at midnight was truly magnificent. The tail, considerably 
diminished in breadth, had shot out to an extravagant length, extending from the 
place of the head above o of the Great Bear at least to ir and p Herculis ; that is to 
say, about 72, and perhaps somewhat further. It exhibited no bifurcation or lateral 
offsets, and no curvature like that of the comet of 1858, but appeared rather as a 
narrow prolongation of the Northern side of the broader portion near the comet than 
as a thinning off of the latter along a central axis, thus imparting an unsymmetrical 
aspect to the whole phenomenon. 

" Viewed through a 7-feet Newtonian reflector of 6-inches aperture the nucleus 
was uncommonly vivid, and was concentrated in a dense pellet of not more than 4" 
or 5" in diameter (about 315 miles). It was round, and so very little woolly that it 
might almost have been taken for a small planet seen through a dense fog ; still so 
far from sharp definition as to preclude any idea of its being a solid body. No 
sparkling or star-light point could, however, be discerned in its centre with the power 
used (96), nor any separation by a darker interval between the nucleus and the 
cornetic envelope. The gradation of light, though rapid, was continuous. Neither on 
this occasion was there any unequivocal appearance of that sort of fan or sector of 
light which has been noticed on so many former ones. 

" The appearance of the 3 rd was nearly similar, but on the 4 th the fan, though 
feebly, was yet certainly perceived ; and on the 5 th was very distinctly visible. It 
consisted, however, not in any vividly radiating jet of light from the nucleus of any 
well-defined form, but in a crescent-shaped cap formed by a very delicately graduated 
condensation of the light on the side towards the Sun, connected with the nucleus, 

e By far the most complete account is that by the Rev. T. W. Webb in the Month, 
Not., vol. xxii. p. 305. 1862. 



Figs. 204-209. 



Plate XXVIII. 




June 26. 



June 30. 



July 6. 



June 28. 




July i. 




July 8. 



COMET III: 1860. 

{Drawn by Cappelletti^and Rosa.) 



CHAP. III.] Remarkable Comets. 461 

and what may be termed the coma (or spherical haze immediately surrounding it), by 
an equally delicate graduation of light, very evidently superior in intensity to that 
on the opposite side. Having no micrometer attached, I could only estimate the 
distance of the brightest portion of this crescent from the nucleus at about 7' or 8', 
corresponding at the then distance of the comet to about 35,000 miles. On the 4 th 
(Thursday) the tail (preserving all the characters already described on the 2 nd ) passed 
through a Draconis and T Herculis, nearly over rj and e Herculis, and was traceable, 
though with difficulty, almost up to a Ophiuchi, giving a total length of 80. The 
northern edge of the tail, from a Draconis onwards, was perfectly straight, not in 
the least curved, which, of course, must be understood with reference to a great 
circle of the heavens. 

" Viewed, on the 5 th , through a doubly refracting prism well achromatised, no 
certain indication of polarisation in the light of the nucleus and head of the comet 
could be perceived. The two images were distinctly separated, and revolved round 
each other with the rotation of the prism without at least any marked alternating 
difference of brightness. Calculating on Mr. Hind's data, the angle between the Sun 
and earth and the comet must then have been 104, giving an angle of incidence 
equal to 52, and obliquity 38, for a ray supposed to reach the eye after a single 
reflection from the cometic matter. This is not an angle unfavourable to polarisa- 
tion, but the reverse. At 66 of elongation from the Sun (which was that of the 
comet on the occasion in question), the blue light of the sky is very considerably 
polarised. The constitution of the comet, therefore, is analogous to that of a cloud ; 
the light reflected from which, as is well known, at that (or any other) angle of 
elongation from the Sun, exhibits no signs of polarity." 

Hind stated that he thought it not only possible, but even 
probable, that in the course of Sunday, June 30, the Earth passed 
through the tail of the comet at a distance of perhaps two-thirds 
of its length from the nucleus. The head of the comet was in the 
ecliptic at 6 P.M. on June 28, at a distance from the Earth's orbit 
of 13,600,000 miles on the inside, its longitude, as seen from the 
Sun, being 279 i'. The earth at that moment was 2 4' behind 
that point, but would arrive there soon after 10 P. M. on Sunday, 
June 30. The tail of a comet is seldom an exact prolongation of 
the radius vector, or line joining the nucleus with the Sun ; 
towards the extremity it is almost invariably curved ; or, in other 
words, the matter composing it lags behind what would be its 
situation if it travelled with the same velocity as the nucleus. 
Judging from the amount of curvature on the 3O th , and the direc- 
tion of the comet's motion, Hind thought that the Earth very 
probably encountered the tail in the early part of that day, or, at 
any rate, that it was certainly in a region which had been swept 
over by the cometary matter but a short time previously. 



462 Comets. [BOOK IV. 

In connexion with this subject, he added that on the evening of 
June 30, while the comet was so conspicuous in the northern 
heavens, there was a peculiar phosphorescence or illumination of 
the sky, which he attributed at the time to an auroral glare ; it 
was remarked by other persons as something unusual, and, con- 
sidering how near we must have been on that evening to the tail 
of the comet, it may perhaps be a point worthy of consideration 
whether such an effect might not be attributable to this proximity. 
If a similar illumination of the heavens had been remarked gener- 
ally on the Earth's surface it would have been a very significant 
fact. 

Mr. Lowe, of Highfield House, confirmed Mr. Hind's state- 
ment of the peculiar appearance of the heavens on June 30. 
The sky, he says, had a yellow auroral glare-like look, and the 
Sun, though shining, gave but feeble light. The comet was 
plainly visible at a quarter to 8 o'clock (during sunshine), while 
on subsequent evenings it was not seen till an hour later. In 
confirmation of this, he adds that in the Parish Church the vicar 
had the pulpit candles lighted at 7 o'clock, which proves that a 
sensation of darkness was felt even while the Sun was shining. 
Though he was not aware that the comet's tail was surrounding 
our globe, yet he was so struck by the singularity of the appear- 
ance, that he recorded in his day-book the following remark : 
" A singular yellow phosphorescent glare, very like diffused Aurora 
Borealis, yet, being daylight, such Aurora would scarcely be 
noticeable." The comet itself, he states, had a much more hazy 
appearance than at any time after that evening. 

De La Rue attempted to photograph the comet. After 3 
minutes' exposure in the focus of his 1 3-inch reflector the comet 
had left no impression upon a sensitised collodion plate, although 
a neighbouring star, TT Ursse Majoris close to which the comet 
passed on the night of July 2 left its impression twice over, 
from a slight disturbance of the instrument. De La Rue also, 
at that time, fastened a portrait camera upon the tube of his 
telescope, and, with the clock motion in action, exposed a 
collodion plate for 15 minutes to the open view of the comet with- 



Figs. 210-213. 



Plate XXIX. 




JulyS. (Webb.) 




July 2. (Brodie.) 




July 2. (Brodie.} 




July 2. {Chambers.} 



THE GREAT COMET OF 1861. 



Fig. 214. 



Plate XXX. 




O 

CO 



CD 
CO 



EH 
I 



H 
K 

EH 



Hh 



CHAP. III.] Remarkable Comets. 467 

out any other effect than the general blackening of the surface by 
the skylight, together with impressions of several fixed stars in 
the neighbourhood. 

Respecting the polarisation of the light of the comet, Secchi 
said : 

" The most interesting fact I observed was this : the polarisation of the light of the 
comet's tail and of the rays near the nucleus was very strong, and one could even 
distinguish it with the band polariscope ; but the nucleus presented no trace of 
polarisation, not even with Arago's polariscope with double coloured image. On the 
contrary, on the evenings of July 3, and following days, the nucleus presented 
decided indications, in spite of its extreme smallness, which, on the evening of July 7, 
was found to be hardly i". 

" I think this a fact of great importance, for it seems that the nucleus on the 
former days shone by its own light, perhaps by reason of the incandescence to which 
it had been brought by its close proximity to the Sun. 

" During the following days the tail has been constantly diminishing, but it is 
remarkable that*it has always passed near to a Herculis, and that it reached to the 
Milky Way up to July 6. It would seem that the two tails were nearly independent, 
and that on July 5 the length and straightness had gone off from the large one, and 
that this bent itself to the southern side. Last night (July 7) the long train was 
hardly perceptible. The light was polarised in the plane of the tail." 

Observations on the polarisation of the light of the comet were 
also made by M. Poey, at Passy. This gentleman observed the 
polarisation in Donati's comet at Havannah in 1 858, in which case 
the light was polarised in a plane passing through the Sun, the 
comet, and the observer ; but, in the case of the present comet, 
" the plane of polarisation seemed to pass sensibly perpendicular 
to the axis of the tail," which, he thought, might have been owing 
to atmospheric refraction. 

The comet of 1862 (iii.), though not one of first-class brilliancy, 
was nevertheless a very interesting object, particularly on account 
of the fact that a jet of light, frequently altering in form, was ob- 
served for a long time to emanate from its nucleus. Annexed are 
some views drawn by the late Prof. Challis of Cambridge. This 
comet had a tail, which, on Aug. 27, was 20 long. 

The comet of 1864 (ii.), visible in August, had a head unusually 
large, scarcely less than in diameter. To the naked eye it 
resembled on the 4 th of that month a dull blurred star of the 
3 rd magnitude, but in the telescope it appeared as a circular mass 
of nebulous matter with a central condensation very similar to the 

H h 2 



468 



Comets. 



[BOOK IV. 



well-known planetary nebula in Virgo. There was a faint tail, 
but it presented no special feature of interest. 

The comet of 1874 (iii.), discovered by Coggia at Marseilles on 
April 17, was one of considerable interest. The drawing from 
which Plate XXXII has been engraved (and of which figure 215 
is a skeleton outline), was made with an achromatic telescope 
of 8^ inches aperture and n\ feet local length, on July 13, the 

Fig. 215. 




COGGIA'S COMET OP 1874. 
Skeleton outline on July 13. (Srodie.) 

a,y, a. Undefined outline of nebulous head. 

6, c, b. Fairly denned outline of second envelope. 

d, d. Sharply defined outline of first envelope, semicircular, and very bright. 

e, e. Very sharply defined clear dark space between bifurcation of tail, free from nebu- 

losity. 

f, /. Singular eccentric envelopes, sharply defined, fading away at and into 6 b. The centres 

of those envelopes were at d. 

g, c. Between these two points several envelopes concentric with d d were traceable. 

most favourable night during its appearance, when its position 
in the heavens, its contiguity to the Earth, and the absence 
of twilight are jointly taken into consideration. The Southward 
motion of the comet was so rapid that on July 14 the presence 



Figs. 216-221. 



Plate XXXI. 




Aug. 7. 




Aug. 18. 




Aug. 18. 




Aug. 19. 




Aug. 22. 




Aug. 29. 



COMET III, 1862. 

(Drawn by Challis.) 



Fig. 222. 



Plate XXXII. 




COGGIA'S COMET, 1874: on July 13. 

(Th'fnvn by Srodie."} 



CHAP. III.] Remarkable Comets. 473 

of twilight greatly interfered with the details shown in the 
drawing. The following description is from the pen of Mr. 
F. Brodie : 

" The head of the comet presented the great peculiarity of having two eccentric 
envelopes in addition to the ordinary bright envelope immediately surrounding the 
nucleus. The first envelope was a bright and sharply defined semicircle surrounding 
the nucleus : the two eccentric envelopes were nearly as bright, and also very sharply 
defined, also semicircular, having their centres placed (about) on the edge of the first 
envelope, and intersecting each other. The second centrical envelope just embraced 
both these eccentric envelopes, and was about half the width of the nebulous head of 
the comet. Between this second envelope and the ill-defined outline of the head (that 
is, between c and g} there were faintly marked outlines of other concentric envelopes. 
The nucleus, which, according to Hind, was 4000 miles in diameter, appeared to be 
somewhat flattened on the side opposite to the Sun. From this side also the head of 
the comet divided itself into two distinct parts forming the commencement of the tail : 
for some distance this bifurcation was remarkably sharply defined, suggesting an 
intense repulsive force acting upon the nucleus of the comet ; and the space enclosed 
between this bifurcation was strikingly free from nebulous matter, until at some little 
distance away from the nucleus the sharp definition faded into the general nebulosity 
of the tail." 

The following remarks f on this comet are by two French ob- 
servers, MM. Wolf and Rayet : 

" After having maintained for many days a great sameness of form, on June 22 a 
series of changes in the shape of the head of the comet commenced. On that day the 
comet, viewed with a Foucault telescope of 40 centimetres, appeared to be enclosed in 
the interior of a very elongated parabola. Starting from the nucleus, which was 
placed as it were at the focus of the curve, the brightness decreased gradually 
towards the summit : but in the interior of the parabola the diminution of the bright- 
ness was sudden, and the boundary-line exhibited another parabola a little more open 
than the first, and having at its own summit the brilliant nucleus itself. The outline 
of the parabola which passed through the nucleus was prolonged so as to form the 
lateral boundaries of the tail, the edges of which were well denned and were much 
brighter than the interior parts. This tail had then the appearance of a luminous 
envelope hollow in the inside. The nucleus was always very sharp. On July I the 
general form of the comet remained the same ; it appeared always to possess a para- 
bolic outline at its exterior edge. The nucleus however jutted out into the interior 
of the second parabola, and the opposite margins of the tail were not strictly 
symmetrical. The West side, that is to say the side which had the greatest R.A., was 
very sensibly brighter than the other. . . . From July 5, the want of symmetry spoken 
of above became more and more marked, and near the head the decrease of the 
brightness became less regular. On July 7, the contrast between the two branches 
was striking, the Western branch of the tail being about twice as bright as the 
Eastern. At the same time the nucleus appeared to be becoming diffused, and 
it seemed to fade away in the direction of the head of the comet, although still 

f Translated for this work from Guillemin's ComMes, p. 293. 



474 Comet*. [BOOK IV. 

sharply defined on the side nearest the tail ; one could not fail to remark its resem- 
blance to an open fan. . . . Our last observation of the comet was made on July 14 at 
9. 30 P.M. : important changes in the aspect of the head had manifested themselves. 
The fan of light had disappeared on the West side, and was replaced by a long spur 
of light which was traceable for a considerable distance across the head ; on the West 
side the remnant of the fan terminated abruptly, and the boundary-line there made 
but a small angle with the main axis of the comet. On this same occasion two rays 
of light were visible two jets as they might be deemed thrown forwards, the one 
to the right and the other to the left ; these luminous rays seemed to have their origin 
at the edge of the fan of which they formed a sort of prolongation. The ray which 
pointed towards the East projected well forwards, and being bent round towards the 
tail soon reached the preceding edge of the comet ; it was faint and hardly surpassed 
the nebulosity in brilliancy. The ray projected towards the West was much more 
brilliant, and was similarly bent round towards the tail, which it assisted in providing 
with a bright exterior edge." 

On July 13, the comet was 35,000,000 miles from the Earth, 
and although it approached to within 26,000,000 miles on July 21 , 
it was then too nearly in Conjunction with the Sun to be seen. 
The tail was calculated by Hind to have increased in actual 
length from 4,000,000 miles on July 3 to 25,000,000 miles 
on July 19, augmenting in angular length from 4 to upwards of 
43. On the evening on which Mr. Brodie's sketch was taken 
the tail appeared to be rather arched towards the western horizon, 
and could be traced by the naked eye for nearly 20. This 
comet certainly revolves in an elliptic orbit, but the period 
is long. Geelmuyden's value is io,445 y ; Seyboth's, 57ii y . In 
either case the semi-axis major must be some 300 or 400 times 
the Earth's mean distance from the Sun. 

The comet of 1882 (iii.) was in some respects one of the most 
remarkable of modern times. It was conspicuously visible to 
the naked eye for some weeks in September, and altogether 
remained in sight for the long period of 9 months ; but these 
facts, though noteworthy, would not have called for any special 
remark had not other peculiarities been forthcoming to distin- 
guish this comet from almost all others. Briefly stated, its 
special features were, that the head underwent changes in the 
nature of disruptions ; that the tail may have been tubular ; 
that the extremity of the tail was not only bifid but totally 
unsymmetrical ; and that on one occasion the comet seems to have 



CHAP. III.] Remarkable Comets. 475 

thrown off a mass of matter which became, and for several days 
was observed as, a distinct comet. 

Many observers noticed the changes which took place in the 
nucleus and head. Prince said : 

" Oct. 13. I could notice, however, that there was a decided change in the 
appearance of the nucleus. Instead of being of an oval shape, it had become a long 
flickering column of light in the direction of the tail." 

" Oct. 20. I noticed, however, at once, that a still further change had occurred in 
the nucleus since the I3th, which amounted, in fact, to its disruption into at least 
3 portions." 

"October 23. The disruption of the nucleus which I had noticed on the 2oth was 
now fully apparent. The nucleus proper had become quite linear, having upon 
it the 4 distinct points of condensation which I have endeavoured to represent in 
the subjoined sketch. 

Fig. 223. 




THE GREAT COMET OF 1 882. FORMATION OP THE NUCLEUS. (C. L. Prince.) 

" It must be understood that the accompanying woodcut is to be considered rather 
as a diagram of the head of the comet than as a view of what I actually observed, 
and that the points in question are somewhat exaggerated in size, as well as the linear 
character of the nucleus itself. I found it was very difficult to represent, by means 
of a wood-block, such a nebulous object ; but I think it will serve to illustrate the 
nature of the wonderful disruption, and the relative distance of the several portions 
inter se : a was the most difficult portion to discern ; b was by far the brightest of 
all ; c was considerably less bright than b ; and d was nearly as faint an object as a, 
and not quite so large. The linear nucleus, with these points of condensation upon 
it, was surrounded by a distinct oblong coma, which was rounded off at the lower 
extremity, while the upper portion, following the direction of the tail, terminated 
more decidedly in a point. Mr. G. J. Symons, F.R.S., was with me in the observa- 
tory, and his impression was that there were Jive points of condensation, and he 
remarked that ' the nucleus was like a string of beads.' At intervals I thought 
there was another point of light between b and c, but as I could not absolutely 
satisfy myself of its objective existence, I have only represented the four portions, of 



476 Comets. [BOOK IV. 

the presence of which I entertained no doubt whatever. Both Mr. Symons and 
myself particularly noticed the frequent flickering of the light of the nucleus, which 
was quite apparent both to the naked eye and in the telescope g ." 

J. F. J. Schmidt published a sketch of the nucleus, as seen by 
himself, which is not unlike Prince's, and having seen the latter 
he refers to it as a good representation of what he saw himself. 
He noticed a vibratory motion in the fan h . 

The tubular character of the comet's tail was suggested by 
Tempel, who brought out the idea in some striking sketches sub- 
mitted by him to the Royal Astronomical Society, accompanied, 
for comparison's sake, by a drawing of the appearance of two 
hollow glass cylinders as seen in the focus of an eye-piece '. 

The peculiar conformation of the extremity of the tail of this 
comet will be sufficiently indicated by the accompanying woodcut k . 

Fig. 224. 




THE GREAT COMET OF 1882. NAKED-EYE VIEW ON NOV. 14. (. J. Hopkins.) 

Most observers noticed this feature, which though rare as respects 
the comets of the last half century may be conceived to be the 
shape meant by old writers when they speak (as they often 
do) of having seen a comet resembling in form a "Turkish 
scy miter." 

Mr. Hopkins himself likened the general form of the tail to 
the Greek letter y. 

E Jlfon^.JV r o/.,vol.xliii.p.85, Jan. 1883. MoMh. Not., vol. xliii. p. 322, April 

11 Ast. Nacli., vol. cv. No. 2499, March 1883. 

19, 1883; Observatory, vol. vi. p. 157, k Month. Not., vol. xliii. p. 90, Jan. 

May 1883. 1883. 



CHAP. III.] 



Remarkable Comets. 



477 



The last physical peculiarity of the great comet of 1882, to be 
referred to, its throwing off a mass of matter which became a 
satellite comet, was recorded by Schmidt at Athens and Barnard 
and Brooks in America. Perhaps it is going beyond the legiti- 

Fig. 225. 




THE GREAT COMET OP 1 88 2, ON OCT. 9 AT 4 h A.M. (frlammarion.) 

mate limits of the available evidence to speak quite as plainly 
as this, but the fact is clear that Schmidt saw on Oct. 9 and 
on 2 or 3 later days a nebulous mass in the neighbourhood 
of the comet, which calculation indicated was cometary matter 
moving round the Sun in an orbit considerably resembling the 



478 Comets. [BOOK IV. 

orbit of the comet. Brooks's observation was made on Oct. 21 : 
what he saw was a nebulous mass on the opposite side of the 
comet to Schmidt's mass 1 . With the evidence before us of what 
happened in 1 846 in the case of Biela's comet it is impossible not 
to draw the inference that the nebulous mass (or masses) was or 
had been a part of the comet itself; and this theory becomes 
much strengthened when read in the light of the disruptive 
changes which the condition of the nucleus underwent, according 
to the testimony of Prince and others, as above mentioned. 

Even the orbit of the comet of 1882 has greatly puzzled astro- 
nomers. It was found (see Catalogue I., post] that the elements 
thereof closely resembled those of the comet of 1880 (i.), often 
spoken of as the " great Southern comet of 1880." This in turn 
was considered to be a comet moving in an elliptic orbit with a 
period of about 37 years and to be in fact a return of the celebrated 
comet of 1843 which caused such a sensation in the March of 
that year. It remains still a moot point what is the inter- 
pretation to be put upon these orbital resemblances. The 
question is a very speculative one, and it does not seem profitable 
to discuss the matter more fully at present, except to record the 
suggestion that the 4 great comets of 1 843, 1 880, 1 882, and 1887 (i.) 
had at some past time a common origin, but by some process of 
disintegration the original mass has yielded fragments, which 
pursuing slightly different paths, arrive at perihelion at irregular 
intervals m . 

Gen. G. H. Willis observed the comet at sea 70 miles E. 
of Gibraltar on Oct. 19 at 5 A.M., with the air extremely clear 
and the wind calm. He says that in appearance the comet was 
so "extremely delicate, light and airy that it would be almost 
impossible to depict it on paper." The engraving [Plate XXXIII] 
is a French reproduction of the original English lithograph n . 

1 Sidereal Messenger, vol. ii. p. 149, 2535, Aug. 31, 1883 (Hartwig) ; vol. cvii. 

Aug. 1883. No. 2550, Oct. 31, 1883 (Peters); Month. 

m Month. Not., vol. xliii. p. 108, Feb. Not., vol. xliii. p. 288, March 1883 

1883. Month. Not., vol. xlviii. p. 199, (Brett). 

Feb. 1888. For various drawings of the Month. Not., vol. xliv. p. 86, Jan. 

comet of 1882 see Ast. Nach., vol. civ. No. 1884. 
2489,Feb. 5, 1883 (Barnard); vol.cvi.No. 



Fig. 226. 



Plate XXXIII. 




THE GREAT COMET OP 1882: Oct. 19. 



CHAP. Ill ] 



Remarkable Comets. 



481 



With reference to Holden's sketches dated October 13 and 
October 17, it may be remarked that 2 of the nuclei seen by 
Holden were seen by Cruls at Rio de Janeiro, at the inter- 
mediate date of October 15. Cruls found these nuclei to resemble 



Fig. 227 



Fig. 228. 




Oct. 13. (Holden.} Oct. 17. (Holden.} 

THE COMPOUND NUCLEUS OP THE GREAT COMET OP 1882. 

stars of the 7 th and 8 th magnitudes respectively, the distance 
between them being 6f". He was further led to regard the 
peculiar appearance of the tail as being really due to 2 tails, one 
superposed upon the other, each connected with a nucleus of its 
own, independent of the other. 

Sawerthal's comet of 1888 exhibited on March 27 a triple 
nucleus not unlike that of the great comet of 1882 . 

Letter of M. Cruls in Ait. Nach., vol. cxix. No. 2842, May 26, 1888. 



I 1 



482 



Cwriets. 



[BOOK IV. 



CHAPTER IV. 

CERTAIN STATISTICAL INFORMATION RELATING 
TO COMETS. 



Dimensions of the Nuclei of Comets. Of the Comce. Comets contract and expand 
on approaching to, and receding from, the Sun. Exemplified by Encke's in 
1838. Lengths of the Tails of Comets. Dimensions of Cometary orbits. 
Periods of Comets. Number of Comets recorded. Duration of visibility of 
Comets. Unknown Comet found recorded on a photograph of the Eclipse of the 
Sun of May 17, 1882. 



nnHE following are the real diameters a , in English miles, of 
the nuclei of some of the comets which have been satis- 
factorily measured 1 * within the last hundred years: 



Examples of a Large Nucleus. 

Miles. 

The Comet of 1845 (iii.) 8000 

Donati's Comet, 1858 5600 

The Comet of 1815 5300 

The Comet of 1825 (iv.) 5100 



Examples of a Small Nucleus. 

The Comet of 1 798 (i.) 

The Comet of 1806 

The Comet of 1 798 (ii.) 

The Comet of 181 1 (i.) 



Miles. 
28 
3 

"5 

428 



l All the dimensions in miles in this 
chapter depend on the old value of the 
Sun's parallax. They need to be aug- 
mented by about fa to accommodate them 
to what is now regarded as the probable 
amount of the Sun's parallax. This has not 
however been done because all cometary 
measures are so uncertain that to give 
precise values in miles is affectation. 



b This is in truth a very ambiguous 
expression, for when one considers the 
erratic motions of comets, the difficulty 
of ascribing definite boundaries to them, 
and the risk of error on the part of 
observers owing to peculiarities of tele- 
scope and weather, it will be readily 
understood how easy it is to make serious 
mistakes. 



CHAP. IV.] 



Cometary Statistics. 



483 



The dimensions of the coma, or heads, of comets also vary 
greatly, thus : 



Examples of a Large Coma. 

Miles. 

The Comet of 1811 (i.) ... 1,125,000 
Halley's Comet, 1835 .. 357,000 
Encke's Comet, 1828 ... 312,000 



Examples of a Small Coma. 

Miles. 

The Comet of 1847 (v.) ... 18,000 
The Comet of 1847 (i.)... ... 25,500 

The Comet of 1849 (ii.) ... 51,000 



It should be remarked that the real dimensions of comets are 
found to vary greatly at different periods of the same apparition, 
for there is no doubt that many of these bodies contract as they 
approach the Sun, and expand again as they recede from it a 
fact first noticed by Kepler in the case of the great comet of 161 1. 

The following measurements of Encke's comet in 1838, when 
approaching the Sun, will illustrate this : 



Date. 


Diameter. 


Distance 
from Q. 


1838. 
Oct. q 


Miles. 
281 ooo 


1-42 


2X. . 


120,500 


I-IQ 


Nov. 6 


7Q OOO 


I-OO 


ia 


74 OOO 


0-88 


16 . ... 


63,000 


0-83 


,, 2O 


KK.KOO 


O'76 


, 23 


^8-fiOO 


o-yi 


24 


3O.OOO 


0-60 


Dec. 12 


6 600 


O-3Q 


14 


i;,4Oo 


0-36 


16 


4.2SO 


o-m 


17 


3,000 


<M4 









Another point of considerable interest in regard to the dimen- 
sions of comets is raised by the question, ' Do they waste away 1 ? ' 
and it seems that the answer to this must be in the affirmative. 
It has been supposed that Halley's comet as described by con- 
temporary writers 1500 or more years ago was possessed of a 
much larger and more brilliant tail than it has exhibited during 
the last 2 centuries. And probably there is some significance 
in the fact that none of the well-known short-period comets are 

i i 2 



484 Comets. [BOOK IV. 

noted for tails or ever exhibit more than what may be called 
apologies for tails. 

The tails of comets, more especially of those visible to the 
naked eye, are often of stupendous length, as the following table 
will show : 

Greatest Length. Miles. 

The Comet of 1 744 ... ... ... 24 = 19,000,000 

The Comet of 1860 (Hi.) ... ... ... 15 = 22,000,000 

The Comet of 1 86 1 (ii.) ... ... ... 105 = 24,000,000 

The Comet of 1 769 ... ... ... 97 = 40,000,000 

The Comet of 1858 (vi.) 50 = 42,000,000 

The Great Comet of 1618 ... ... 104 = 50,000,000 

The Comet of 1680 ... ... ... 60 = 100,000,000 

The Comet of 1811 (i.) ... .. ... 25 = 100,000,000 

The Comet of 1811 (ii.) .. ... 37 = 130,000,000 

The Comet of 1843 (i.) ... ... ... 65 = 200,000,000 

Cometary orbits are usually of immense extent. Thus : 
i. As to Perihelion Distance. 



Greatett Known. Miles. 



Leatt Known. Mile 



The Comet of 1729 ... 383,800,000 The Comet of 1843 (i.) ... 538,000 

2. As to Aphelion Distance. 



Greatett Known. Miles. 

The Comet of 1844 (ii.)4o6,i 30,000,000 



Least Known. Miles. 

The Comet of Encke ... 388,550,000 



We have already seen that the period of the shortest comet 
yet known is but little more than 3 years: this is in striking 
contrast to the periods exhibited in the following table, which are 
however so vast as to deserve little reliance : 

Years. 

The Comet of 1882 (i.) 400,000 

The Comet of 1844 (ii.) 102,050 

The Comet of 1 780 (i.) 75,3 '4 

The Comet of 1877 (Hi.) 28,000 

The Comet of 1680 . ... 15,864 

The Comet of 1847 (Hi.) J 3,9 l8 

The Comet of 1840 (ii.) 13,864 

A significant fact with respect to the periods of the known 
periodical comets has already been mentioned , namely that 
there seems some disposition on the part of these comets to 
become associated with particular planets. It is not improbable 
that, as our knowledge becomes enlarged, some very interesting 
facts may come to light, which are at present hidden. 

c See pp. 401, and 444, ante.. 



CHAP. IV. Cometary Statistics. 485 

TABLE OF NUMBER OF COMETS RECORDED. 



Period. 



Before A.D. 79 

Century o 100 22 

101 200 22 

201300 39 

3OI 400 22 

401 500 19 

501 600 25 

601 700 29 

701 800 '7 

801 900 .... 41 

901 1000 30 

1001 noo 37 

noi 1200 28 

12011300 29 

13011400 34 

14011500 43 

1501 1600 39 

1601 1700 32 

17011800 72 

18011888 (December) 270 



Comets 
Observed. 



929 



Orbits 
Calculated. 



4 

I 

2 

3 
o 
i 

4 
o 

2 
1 

2 

4 
o 

3 

7 

12 

13 

2O 

6 4 

2 49 



392 



Comets 
Identified. 



3 

3 

i 

4 

5 

8 

68 



109 



From the earliest period up to the present time, the number 
of comets of which there is any trustworthy record is somewhat 
over 900 ; but as it is only within the last 100 years that optical 
assistance has been made generally available in a systematic 
search for them, the real number of those that have appeared is 
probably not less than several thousands, especially when we 
consider that there have doubtless been many, visible only in 
the Southern hemisphere. 

Comets remain visible for periods varying from a few days to 
more than a year, but the most usual time is a or 3 months. 
Much depends on the apparent position of the comet with respect 



486 



Comets. 



[BOOK IV. 



to the Earth and the Sun, and much on its own intrinsic lustre. 
Among the comets which remained longest in sight, are the 
following : 

Months. 

TheComet of 1811 (i.) ... ... 17 

The Comet of 1825 (iv.) ... ... ... ... ... ... 12 

The Comet of 1861 (ii.) ... 12 

The Comet of 1835 (iii.), (Halley's) 9^ 

The Comet of 1847 (iv.) 9^ 

The Comet of 1858 (vi.) 9 

The Comet of 1882 (iii.) 9 

The Comet of 1884 (i.) 9 

There are some few comets which have only been seen on one 
or two occasions, unfavourable weather preventing more extended 
observation of them. Fig. 229 is a case in point. It represents 
a comet seen during the totality of the solar eclipse of 1882, 
which was never seen again, and as to whose history and fate we 
know nothing. 

Fig. 229. 




ECLIPSE OP THE SUN OP MAY If, 1882, SHOWING AN UNKNOWN COMET. (Eanyard.) 



CHAP. V.] Historical Notices. 487 



CHAPTER V. 

HISTORICAL NOTICES. 

Opinions of the Ancients on the nature of Cornels. Superstitious notions associated 
with them. Extracts from ancient Chronicles. Pope Cnlixtus III. and the 
Comet of 1456. Extracts from the writings of English, authors of the i6th and 
ifth centuries. Napoleon and the Comet of 1769. Supposed allusions in the 
Bible to Comets. Conclusion. 

GOING back to the early ages of the world, we find that the 
Chaldseans considered comets to be permanent bodies 
analogous to planets, but revolving round the Sun in orbits so 
much more extensive, that they were therefore only visible 
when near the Earth. This opinion, which, by the by, is the 
earliest hint that we have of the existence of periodical comets, 
was also held by philosophers of the Pythagorean school. Yet 
Aristotle, who records this, insists that comets are merely 
mundane exhalations, carried up into the atmosphere, and there 
ignited. 

Anaxagoras, Apollonius, Democritus, and Zeno considered that 
these bodies were aggregations of many small planets. 

It is a somewhat remarkable fact, that Ptolemy, so celebrated 
for his varied astronomical attainments, should nowhere have 
made any mention of comets ; his omission is, however, atoned 
for by Pliny, who seems to have paid much attention to them. 
He enumerates 12 kinds, each class receiving its name from 
some physical peculiarity of the objects belonging to it. 

Seneca considered that comets must be above [i.e. beyond] the 
Moon, and he judged from their rising and setting, that they had 
something in common with the stars. 



488 Comets. [BOOK IV. 

Paracelsus gravely insisted that comets were celestial mes- 
sengers, sent to foretell good or bad events an idea which, even 
in the present day, has by no means died out. The ancient 
Romans did not trouble themselves much about astral phe- 
nomena ; they nevertheless looked upon the comet of 43 B.C. 
as a celestial chariot carrying away the soul of Julius Csesar, 
who had been assassinated shortly before it made its appear- 
ance. 

In an ancient Norman Chronicle there occurs a curious ex- 
position of the divine right of William I. to invade England : 
" How a star with 3 long tails appeared in the sky ; how the 
learned declared that stars only appeared when a kingdom 
wanted a king, and how the said star was called a comette." 
Another old chronicler, speaking of the year 1060, says : " Soon 
after [the death of Henry, King of France, by poison], a comet 
denoting, as they say, change in kingdoms appeared, trailing 
its extended and fiery train along the sky. Wherefore, a certain 
monk of our monastery, by name Elmer, bowing down with 
terror at the sight of the brilliant star, wisely exclaimed, ' Thou 
art come ! a matter of lamentation to many a mother art thou 
come ; 1 have seen thee long since ; but I now behold thee 
much more terrible, threatening to hurl destruction on this 
country*.' " 

The superstitious dread in which comets were held during the 
Middle Ages is well exemplified in the case of the comet of 1456 
(Halley's). We find that the then Pope, Calixtus III., ordered 
the Church bells to be rung daily at noon, and extra Ave Marias 
to be repeated by everybody. Whilst the comet was still visible 
Hunniades, the Hungarian general, gained an advantage over 
Mahomet II., and compelled him to raise the siege of Belgrade, 
the remembrance of which the Pope preserved by ordering the 
Festival of the Transfiguration, the anniversary of which was 
kept a few days after the battle, to be observed throughout 
Christendom with additional solemnities. " Thus was established 
the custom, which still exists in Romish countries, of ringing the 
* Will. Malmes., De gestis Regwn Anglice, lib. ii. cap. 225. 



CHAP. V.] Historical Notices. 489 

bells at noon ; and perhaps it is from this circumstance that the 
well-known cakes made of sliced nuts and honey, sold at the 
Church-doors in Italy on Saints' days, are called comete b ." 

Leonard Digges says that " comets signify corruptions of the 
ay re. They are signs of Earthquakes, of warres, of chaungyng 
of kingdomes, great dearth of corne, yea a common death of man 
and beast"." 

One John Gadbury says that " Experience is an eminent evi- 
dence, that a comet like a sword, portendeth war ; and an hairy 
comet, or a comet with a beard, denoteth the death of kings." 
He also gives us a register of cometary announcements for 
upwards of 600 years, and adds in large Roman capitals, " as if 
God and nature intended by comets to ring the knells of princes, 
esteeming bells in Churches upon Earth not sacred enough for 
such illustrious and eminent performances." 

Shakespeare speaks of 

" Comets importing change of times and states 
Brandish your crystal tresses in the sky, 
And with them scourge the bad revolting stars 
That have consented unto Henry's death d ." 

Milton says: 

" Satan stood 

Unterrified, and like a comet burned, 
That fires the length of Ophiuchus huge 
In th' Arctic sky, and from its horrid hair 
Shakes pestilence and war 6 ." 

The last comet employed in an astrological character was that 
of 1769, which Napoleon I. looked upon as his protecting genie. 
Indeed, as late as 1 808 Messier published a work on it, of which 
the title is given below f . 

During the visibility of Donati's comet in 1 858, the question 
was mooted whether the Bible contained any reference to these 



b Smyth, Cycle, vol. i. p. 231. A friend ed., London, 1576, fol. 6. 

suggests a derivation which certainly & Henry VI., First Part, Act I. Scene I. 

appears much more rational ; namely, Paradise Lost, Book II. 

comedere, to eat. f La Grande Comete qui a paru d la 

c Prognostication Euerlantinge, 2nd Naiseance de Napoleon le Grand. 



490 Comets. [BOOK IV. 

objects : the following passages were adduced in support of the 
idea : 

1. In Leviticu* xvii. 7 it is said, "They shall no more offer 
their sacrifices unto Seirim," or Shoirim, which is rendered in 
the Authorised Version "devils," and in other versions "goats." 
Maimonides states that the Sabian astrologers worshipped these 
seirim, which seerns to confirm the idea that they were celestial 
bodies. 

2. In Isaiah xiv. i a we find, " How art thou fallen from heaven, 
O Lucifer, son of the morning ! how art thou cut down to the 
ground, which didst weaken the nations ! For thou hast said in 
thy heart, I will ascend into heaven, I will exalt my throne 
above the stars of God." In this passage a certain Hillel is said 
to have fallen from heaven ; but it is unknown what Hillel 
means. Some interpreters derive the word from Hebrew verbs 
signifying to glory, boast, agitate, howl, &c. Hillel may therefore 
signify a comet, for it answers to the ideas of brightness, swift 
motion, and calamity. 

3. In the General Epistle of St. Jude, verse 13, certain impious 
impostors are compared to "wandering stars, to whom is re- 
served the blackness of darkness for an aeon [age]." In all 
probability the passage may be taken to refer to comets g . 

4. The last quotation which I make is from the Revelation of 
St. John the Divine, xii. 3 : " There appeared another wonder in 
heaven; and behold a great red dragon, .... and his 
tail drew the third part of the stars of heaven." Satan is here 
likened to a comet, because a comet resembles a dragon (or 
serpent) in form, and its tail frequently does compass or take 
hold of the stars. 

These ideas are given for what they are worth, and that is 
probably not much. 

8 See Alford's New Test, for English Readers. In loco. 



CHAP. VI.] Determination of Orbits. 491 



CHAPTER VI. 

DETERMINATION OF THE ELEMENTS OF THE ORBIT 
OF A COMET BY A GRAPHICAL PROCESS*. 



SECTION I. Preliminary. 

ri\HE first and most important step to be taken in applying 
-- the following graphical process for the investigation of the 
orbit of a comet consists in working out the projection of the 
orbit on the ecliptic, which involves finding such an inclination of 
the plane of the orbit and such position of the node as shall be 
at once consistent with the longitudes and latitudes reduced from 
the observations available, and shall also satisfy Kepler's law of 
equal (or proportional) areas being described round the Sun in 
equal (or proportional) times ; and afterwards to compare the 
developed orbit with one of the varieties of Conic Sections with 
which it must necessarily be in accord. This in practice means 
rinding the proper parabola, for leaving out of consideration a 
few well-known elliptical comets of comparatively short period, 
the curve, whether elliptical or hyperbolic, approximates almost 
always so closely to the parabola that, until observations have 
been multiplied and all corrections for parallax and aberration 
have been applied, it is useless to attempt to discriminate between 
them. Moreover, the graphical method is scarcely available to in- 
dicate the course of a comet from only a few days' observations. 
Let a scale, divided into 100 parts, be made, on card or stout 

a This chapter has been specially of a paper contributed by him to the 
written for this work by Mr. F. C. Royal Astronomical Society in 1881. 
Penrose, F.R.A.S., and is an extension (Month. Not., vol. xlvi. p. 68. Dec. 1881.) 



492 Comets. [Boox IV. 

paper (as it may have to be bent round a curve), to represent the 
Sun's mean distance ; and inasmuch as many tentative proportions 
will have to be tried, the slide rule will be found a valuable 
auxiliary ; but as the standard lines which represent the longitudes 
of the different observations used should be laid down very 
accurately, and are found once for all, it is better in the transfor- 
mations of R. A.'s and Declinations into Longitudes and Latitudes 
to use logarithms. The Nautical Almanack gives for every day at 
noon the Sun's longitude and distance from the Earth. Inter- 
polating these for the times of each observation we shall obtain 
with sufficient accuracy (neglecting parallax) the relative places 
of the observer and the Sun. Let the plane of the paper repre- 
sent the ecliptic and lay down very carefully these terrestial 
places, and through them draw straight lines in the directions 
of the longitudes of the comet, already supposed to have been 
worked out. These lines should be drawn in ink, that they may not 
be erased in rubbing out the trial pencil-lines which will have 
to be drawn between them. It will also be convenient to mark 
down at this stage some subdivisions of the longitude lines 
where the heights above the ecliptic are as the numbers 20, 30, 
40, &c. ; these points being given by the co- tangents to the 
latitude. These marks will of course be confined to those parts 
of the longitude lines where the projection seems likely to pass. 
Theoretically 3 observations suffice to determine the path of a 
comet, but for the graphical investigation 4 are much better. 

If the conversions from the equator to the ecliptic are performed 
by calculation the following remarks may be found useful. 

(i) In using the formula below and referring to Fig. 230 in 
which P represents the North Pole and E the North Pole of the 
ecliptic, and C being any place of the comet b , it should be 
observed that when the comet's R. A. is between 12 hours and 
24, the angle at P is acute ; and in the formula : 

cos E C = cos P E cos P C + sin E P sin P C cos E P C 
the latter value (cos E P C) will be positive, but for all other 
hours of R. A. it will be negative. 

b Supposed in the diagram to be in R. A. 2o h and N. P. D. 84. 



CHAP. VI.] 



Determination of Orbits. 



493 



(2) When the comet's R. A. is between 6 hours and 18, the 
supplement of the angle included between E P and E C must be 
deducted from 270; to give the proper longitude, but for the 
other hours of R. A. the supplement of the said angle must be 
added to 270. 

Fig. 230. 




RELATION OF THE EQUATOB TO THE ECLIPTIC. 

The general formula referred to gives the latitude only. The 
longitude has to be derived from it and from the previous data 
by the formula : 

sin PEC_sinEPC 
sin PC : : sin EC ; 

and, as observed just above, the angle CES is to be added to 
or subtracted from 270 according to circumstances. 

Also, before proceeding to the graphic work it is desirable to 
make a careful inspection of the longitude lines and the latitude 
numbers just described, as from the relation of these numbers to 
one another a sound hypothesis may usually be made of the 
course of the comet as it passes the different longitude lines 
by considering the connection between the heights above the 
ecliptic and their distance from the Earth. Without this help 
some doubt might at first arise in some cases as to which was 
the direction of the comet ; that is, whether it was direct or 



494 Comets. [BOOK IV. 

retrograde. A few minutes devoted to this inspection may 
save much time in the end. In addition to the above, any 
information given in the recorded observations of variation in 
brightness or development of the comet's tail should be taken 
into account. 

The first step now will be to take into consideration the lengths 
on the projection of the arcs traversed between the observations. 
These are not strictly proportional to the time-intervals either 
on the orbit or on the projection, but unless the observations 
record places very distant from one another the time-intervals 
may be used at the first start, and a table of these should be 
formed giving different numerical equivalents. For instance 
suppose the time-intervals were 7, 8, 9. 

Form a table such as the following, viz. : 

7 =8:9, 
8-75 : 10 : 11-25, 
10.5 : 12 : 13.5; 

which may be extended either by addition or interpolation as 
may be required when the circumstances of the case indicate 
which are likely to be the numbers most in request. The examples 
will show how these are applied, and in Section 5 Rules are given 
for correcting them for a second approximation. 

The next step will be the adjustment of the areas and of the 
latitudes. A few preliminary remarks on these heads will be 
found useful. 



SECTION 2. On the proportioning of the Areas in the different 
Segments of the Projection. 

Let the plane of the paper be that of the orbit, and let A BCD 
be the places under examination. If there is no great amount 
of deflection from the straight line as between A B and B C, the 
subtended areas are to each other nearly as the triangles formed 
by the chord with the radius vector ; and if C N be a straight 
line drawn through K at right angles to SB, and if AN be 
parallel to S B, the area subtended by A B is to that subtended 



CHAP. VI.] 



Determination of Orbits. 



495 



by B C very nearly as N K : K C ; but if the question lies between 
such arcs as B C and C D, the difference in the areas inclosed 
between the chords and the arcs cannot be neglected in the 
comparison. In that case we may proceed thus : Produce S C 
to E, make EF = EB, and following the previously described 
method cut off the arc C G, which approximately subtends the 
same area as BC, and by similar construction the remaining 
area subtended by G D can be measured. 

Thus area B C S : area C D S : C H : C J. 

Lines such as A N, F G, J D used in this construction may be 
conveniently called area-measurers or pediometric lines. 

Fig. 231. 




SCHEME FOR ADJUSTING THE SUBTENDED AREAS. 



In the figure given above (Fig. 231) the curve of the orbit has 
been supposed, but the same holds good on the projection as 
respects the areas, although the arcs are not in simple proportion. 
A more exact rule for measuring the areas will be given in Sect. 5 
(post], but the method just explained is sufficient for the purposes 
of approximation, and is very rapidly performed graphically. 



490 



Comets. 



[BOOK IV. 



SECTION 3. The Latitudes and the Inclination of the Plane 
of the Orbit. 

In Fig. 232 let the plane of the paper represent the ecliptic, 
and let E be the position of the observer. Let E L be the direc- 
tion of the comet's longitude, S Q the node, and p P p' an arc of 

Fig. 232. 




DIAGRAM FOR FINDING POINTS OF PROJECTION WHEN NODE AND 
INCLINATION ARE GIVEN. 



the projection ; and the dotted line q Q q' an arc of the developed 
orbit : that is, the plane of the orbit is supposed to revolve on its 
node through the angle i until it coincides with the ecliptic, and 
let / be the observed latitude. 

The height of the point P above the ecliptic can be measured 
either by P E tan I or P N tan i. 

Let K be a point on E L, which for the sake of accuracy it is 



CHAP. VI.] Determination of Orbits. 497 

convenient to take at some distance from E. Through K draw 
K D perpendicular to S Q , and make K D = K E tan I cot i. Join 
E D, and at the point N where it cuts the node draw a straight 
line perpendicular to the node. This line, if the angles and work 
are correct, will pass through P, because from similar triangles 
NP KD 



_ T _. _. , 7 T,-^ , 

_P=, , . . N P = P E - - ; , or PE tan I PN tan , as 

P E K E tan * 

above. 

Thus with the node, latitude and inclination given, the point 
P is found by the intersection of E D with S 8 . K may be any 
point on E L, but it is convenient to take it at some definite 
value of cot/; for instance (our scale being the Earth's mean 
distance divided into 100 parts), K E tan / may be 100, 50, 25, &c. 
according to circumstances, as will be seen in the examples, 
post. When the projection has been found the developed orbit 
is easily obtained by making N Q = N P sec i. 

The above method, which can be constructed very rapidly, offers 
a convenient plan for testing the accuracy of any given solution 
of the elements of the orbit of a comet, but for the purpose of 
the graphical working the process is as follows : The direction of 
the node and the inclination of the orbit having been previously 
obtained, the method explained in this Section is used to bring 
the whole work together and to average the individual obser- 
vations. After this has been done, and the different points so 
amended have been marked down, the work at this stage ought 
to be tried by the rule of the areas, and if it stands this test 
also, the small discrepancies which may still remain between 
the developement and the proper conic section (presumably a 
parabola) will be still further reduced by its comparison with 
that curve, and it will be seen what are the slight modifications 
which have to be made in the node or in the inclination, or 
in both, in order to reduce the outstanding errors. Whatever 
corrections are applied to these should be made by small instal- 
ments, and to each separately, and the effect noted down. 



Kk 



498 Comets. [BOOK IV. 

SECTION 4. To find a Parabola having its Focus at 8 and which 
shall coincide with two Points of the Orbit. 

In Fig. 233 let Q and P be the two points ; usually the 
extremities of the developement. With the centre Q and at the 
distance Q S, describe the arc S N F ; and with the centre P and 
at the distance PS, describe the arc S MG. The straight line which 

Fig- 333- 




DIAGRAM FOB FINDING THE PEBIHELION FROM GIVEN POINTS ON THE ORBIT. 

is tangent to the two circles at M and N will be the directrix of the 
proper parabola, and from this all the other parts can be found. 
The curve when drawn may be conveniently applied on tracing- 
paper, keeping the focus on the place of the Sun and turning it 
about until it best fits all the points of the developement. 

SECTION 5. The Measurement of the Areas in a Parabola. 
Fig. 233 may also be used to illustrate the exact rule for the 
measurement of the areas in a parabola. Let A be the vertex, 
A S = 0, and let P H and Q I be perpendiculars drawn from the 
principal axis A X. 



CHAP. VI.] Determination of Orbits. 499 

If PSQ be the area of the space bounded between SP, SQ, and 
the curve, then 

= PSQ. 



SECTION 6. TJie Relations between the Time-intervals and the 
Longitude Lines. 

At the first opening of the enquiry, except the help given by the 
latitude numbers, as mentioned in Sect, i, there is usually little 
to guide the student beyond the time-intervals and the longitude 
lines. It is important therefore to consider their relation to one 
another. Proportions founded on the time-intervals may gene- 
rally be used as a useful first approximation unless the inclination 
of the orbit is very steep or there is a great change of direction 
in the path of the comet with respect to the node, between the 
different observations. As this may not unfrequently be the 
case, the remarks following should be taken into consideration. 

In comparing the lengths of adjacent arcs in the orbit it can 
easily be shown that they are to each other inversely as the 
square root of the mean radius vector in each arc, and if the arcs 
are of limited extent are practically as the inverse square roots 
of the radii in the middle of each arc. This variation will of 
course affect the projection also, with which we primarily have 
to deal ; but the arc in the projection also depends upon the 
general angle made with the node, which may frequently be 
taken without sensible error to be the angle which the chord of 
the arc makes with the node. Calling this angle, if measured on 
the orbit, a, or if on the projection, ft, we should find that if s 
be a small arc of the projection corresponding to S on the orbit, 

* : S : : Vi sin 2 a sin 2 i : I ; or s : S : : i : Vi + tan 2 ?' sin 2 j8 ; the 
relation between a and j3 being tan /3 = tan a cos /. 

It will be seen that when a or ft are small, and i not very 
great, the projection will have almost the same length as the 
original arc, and when these approach 90 the ratio of the 
projection to the original will be as i to sec i. Also it will be 
observed that when the inclination i is very steep it produces 
great influence on these proportions. 

K k 2 



500 Comets. [BOOK IV. 

It follows from the above considerations that although the 
length of an arc traversed in a given time increases or diminishes 
as the comet approaches or recedes from the Sun, yet when we 
compare the adjacent arcs of the projection, this tendency may be 

Fig. 234. 




DIAGRAM FOB COMPARING ARCS. 



greatly modified by the direction of its course with respect to 
the node. In the first approximations it is not desirable to try 
to calculate these effects minutely, although it will be useful to 
take some account of them when possible. But it may often be 
worth while to obtain a first approximation roughly, and from 



CHAP. VI.] Determination of Orbits. 501 

this to deduce the effect produced by the causes above referred to, 
and then to rub out the first pencillings and proceed afresh with 
an amended table of the intervals. The diagram here given (see 
opposite, Fig. 234), which has been calculated from the formula 



\/i + tan 2 i sin 2 ^ 

gives values of the length of a small arc of the projection com- 
pared with the corresponding arc of the orbit. 

If the orbit has been developed and the angular direction of 
its course a ascertained, /3 is easily obtained from the relation 
tan fi = tan a cos /. As an example of this diagram, if /3 = 30 and 
i= 45, it will be found by the scale that s : S = 9 : 10. Other 
values can be found by interpolation. 

SECTION 7. Checks available^ derived from certain properties of 
Parabolic Orbits. 

When the elements of a comet have been approximately ascer- 
tained, a very useful check may be employed (confining our 
attention to parabolic orbits) from a consideration of the fact 
that the velocity of a comet in such an orbit at perihelion is to 
that of a planet moving in a circular orbit c at the same distance 
as >/2 to i. 

The sine of the daily arc traversed by such a planet at Peri- 
helion would be 1-7213 of our scale. In the comet at the same 
distance it would be 2-43302, and for any other distance this 
number must be divided by the square root of the distance. 

It will often be useful to remember this principle at a pre- 
liminary stage, when a consideration of it may help to point out 
the distance at which the first approaches should be commenced. 

SECTION 8. Examples of the Graphical Process. 
The first example to be given is that of Schaberle's comet of 
1881 (iv). Observations on 5 days will be considered. It is 
proposed to find the elements of the orbit from the first 4 and 
then to try them on the 5 th for a test and final correction. 

c The motion of the earth in its orbit. It will require very little calculation, as 
although not quite circular, may without it has necessarily been laid down graphic- 
serious error be used in this comparison. ally in the course of the work. 



502 Comets. [BOOK IV. 

The observations reduced to longitude and latitude yielded 
the following apparent places : 

G. M. T. Longitude. Latitude. 

o > ii a i n 

Oxford 



July 31-41 


95 19 56 


22 34 17 


(') 


Aug. 4-42 


99 2 24 


25 8 9 


(2-) 


10-44 


108 5 14 


29 49 o 


(3-) 


J 9-53 


137 33 22 


36 7 o 


(40 


Sept. 2-33 


199 18 o 


17 22 


(5-) 



Marseilles 

The Sun's places at these times with respect to the observer 
being 

Longitude. Distance, 
o / it 

(i.) 128 44 43 101-50 

(2.) 132 35 i 101-42 

(3.) 138 25 17 101-35 

(4-) 147 5 43 101-16 

(5.) 160 26 6 100-84 

The plane of the paper represents the ecliptic, S being the 
Sun's centre and S Y the line of the Equinoctial Node. 

It is evident from the inspection of the latitude numbers (see 
Sect, i) that during the first 4 observations the comet was 
passing from left to right, that is with retrograde movement, 
and approaching the Sun ; and when first observed must have 
been a little beyond the point O where the first two longitude 
lines intersect. In this example we seem obliged at first to 
use the time-intervals as the only representatives of the lengths 
of the arcs, for at present no theory can yet be formed 
of modifications of their proportional length, as discussed in 
Section 5. 

The table of time-intervals will be formed of such terms as : 

4-01 6-02 9-09 

8 12 18-10 

10 15 22-60 

1 1-3 17 25-70 

12.6 19 28-6 

16 24 36-2 

If we lay the scale of hundredths of the Sun's mean distance 
across the middle interval, and, allowing for moderate and con- 
tinuous curvature, take in the adjacent spaces on each side, we 



Fig. 235. 




PLATE XXXIV. (faces p. 502). 




CH.AP. VI] Determination of Orbits. 503 

shall find that the two first terms of our table are out of the 
question. Nor will 15 by any sort of arrangement combine with 
10 on one side and 22-6 on the other: but with 17 combined with 
11-3 and 25-7 the case is different, and we may note down its 
points of coincidence, as at A, B, C, and D. It will be well 
however at this stage to proceed further and try another value, 
say 24, in the middle interval, and mark down also the places 
given on the four longitude lines, namely, <z, b, c, A. 

The reason for placing the scale at that particular obliquity to 
L 2 and L 3 so as to fall upon the points a, b, c, d, rather than in an- 
other way, nearer to or further from A, B, C and D, is the condition 
that the curvature of the projection must be fairly continuous. 
Nearer to A B C D it would have made abed too straight, or 
even convex to the Sun, and further from A B C D it would have 
been too abrupt. This consideration generally determines within 
very moderate limits the direction that these trial lines ought to 
take. The distance however will require a different discrimina- 
tion, which we should now apply, namely, the area test. Join 
SB, S C, and draw the pediometric lines as explained in Section 2, 
namely, the offsets from M M' which fall near A, and from N N' 
which fall near D, thus confirming very nearly the points 
already chosen. When we apply a similar test to the other 
trial-curve by joining S , S c &c., we find that the pediometrics 
m m' and n n' are quite discordant, especially n n f . Thus we 
may feel satisfied that to obey this test the projection cannot be 
far from a line passing through ABC and D. We now 
proceed to consider the latitudes and to obtain the inclination 
and the node. The heights above the ecliptic due to points on 
the longitude lines having been marked, show that in this case 
at the points A, B, C and D we have respectively 46-7, 45-7, 43-0 
and 36-6. If these places were exact, the node could now be 
so drawn through S (similar to the node in Sect. 3) that the 
height of each of the above-named points, divided by the cor- 
responding horizontal distance from the node, measured on the 
projection, would give the same value of tan i. Here it is nearly 
so, for we may so choose our base-lines passing through the Sun 



504 Comets. [BOOK IV. 

that the two outside points =~ and rr-^ g* ve 39 C 3'> whilst 
^ give 39 20'; the mean being 39 25', and the 



(jr r> 

approximate node X X'. 

This approximate node might be found tentatively, but a better 
way is to join the two points under consideration, as A and D, and 
draw from A and D offsets equal to, or proportional to, the 
measure of their latitude number (or height above the ecliptic) ; 
join the extremities of the offsets ; and produce as required to 
meet the produced line A D. The point of intersection will lie 
in the approximate node. That due to B and C will be found 
by similar construction. We are now in a position to use this 
approximate node and inclination to improve the figure by 
introducing a modification of the lengths of the arcs of the 
projection as explained in Section 6. If we draw perpendiculars 
to the node to the middle parts of the arcs of the projection, and 
develope them in the ratio of sec i : i , we can obtain approximate 
places on the orbit and get a near value of the radius vector. 
These distances appear to be in AB, B C, and C D respectively 
about 75, 70 and 65, and on this account (as shown in Sect. 6) 
the spaces traversed in equal times in the three arcs would be 
to each other as F T , ^ T , T , but the angles which the chords 
of the arcs on the projection make with the node seem to be 
respectively 8 10', 10 45', and 18. 

Using the diagram of Section 6, and interpolating the values 
above measured for /3 in combination with 2=40, we obtain for 
the proportional lengths in the projection 0-99, 0-985, and 0-965. 

The comparison of equal-time arcs therefore on these three 
sequents will be as : 
99 



The comparative arc intervals thus modified will have for their 
proportions 3-9 ; 6-02 ; 9-2 ; very nearly as u ; 17 ; 26. 

In this example the corrections above found are small because 
the angle /3 of Section 6 is small, and i is not very large, but 



CHAP. VI.] Determination of Orbits. 505 

under certain circumstances they may become very significant. 
It must be borne in mind that these modifications affect only 
the lengths of the arcs, and in comparing the areas the true time- 
intervals must be used. 

In making the adjustments it seems unnecessary to alter D, 
considering the favourable near coincidence of the pediometric 
line with that point, but the other points should be shifted to 
A', B' and C', giving them latitude values of 47-1, 46-3, and 43-8 
respectively. 

The value of i which now results from the corrected numbers and 
the corresponding perpendiculars drawn to the node becomes 
39 45', which we take for the measure of i, and the node takes 
the direction S S3 . 

The next step is to use the method of Section 3, to establish 
points on the various longitude lines in accurate accordance with 
this node and inclination. 

Choosing the latitude points 50, on each of the first four longi- 
tude lines, draw from A' B' C' and D perpendiculars to S 3, 60-1 
in length, which will make |$ = tan i. Draw straight lines from 
the extremities of these perpendiculars to their respective places 
of observation, and where they cut the node, as T F f , U G,' &c., 
erect perpendiculars to their proper longitude lines and produce 
them upwards to form the developed figure by the method of 
Section 3. These points will be Hj H 2 H 3 H 4 . By that of 
Section 4 we can now obtain the parabola which will pass 
through Hj and H 4 . Let this be drawn on tracing-paper and 
applied as therein directed, and it will point out that, with the 
vertex at P, it will produce a very good coincidence with the 
four points of the developement, and will also satisfy very 
closely the rule of the areas. 

We have obtained this parabola from 4 observations only. 
The fifth observation (that of September 2) may now be used to 
test its accuracy. 

We shall find that if this observation be worked out in a 
manner similar to that of the others by obtaining the point Z 
due to latitude mark 25, and finishing the construction, that the 



506 Comets. [BOOK IV. 

point of its developement H 5 will fall at a distance of not more 
than 0-5 from the arc of the parabola produced, and that if the 
axis of the trial parabola 64 be reduced to 63-5 and the vertex 
or perihelion be turned towards H 5 , a very small arc, about 2 20', 
there will be a very near coincidence indeed amongst all the 
points. After these corrections have been made the perihelion 
point measures along the curve 8-2 of the scale beyond the 
point due to Aug. 19-53, a distance traversable in about 2-70 
days. 

This gives the date of the perihelion, August 22-23. 

The longitude of this point is 328 20'- 

The longitude of the node is 97 30'- 

The elements of the orbit, stated in the usual way, are : 

T Aug. 22-23 

W 328 20' 

q 0636 

8 97 3o' 

39 45' 

H Retrograde. 

Stechert's published elements of this comet are : 

T Aug. 22-29 

334 55' 

? 0-633 

8 97 2' 

39 46' 
/* Retrograde. 

These last elements, if tried by the test of Section 3, do not 
in some particulars satisfy the geometrical conditions so well as 
those given above, found by the graphical process. In some of 
them there is very little difference. 

The second example is that of Tebbutt's Comet of 1881 (iii). 
We will make use of 4 observations of apparent places reduced 
to the ecliptic : 

G. M. T. Longitude. Latitude. 

o / // o t 

1. Cape of Hope May 31-21 69 38 6 52 9 

2. June 9-18 74 16 36 38 31 

3. Greenwich June 24-48 86 19 6 + 25 59 
4- Joly i3'58 I02 37 33 +60 36 



Fig. 236. 




PLATE XXXV. (faces p. 506). 




CHAP. VI.] Determination of Orbits. 507 

The Sun's places at these times, with respect to the observer, 
were : 

Longitude. Distance. 

o / 

1. 70 18-9 101-43 

2. 78 54-0 IOI-5I 

3. 93 29-6 101-66 

4. in 42.5 101-64 

As in the previous example the plane of the paper represents 
the ecliptic, S being the Sun's centre, S Y in PI. XXXV, Diagram 
A. is the line of the Equinoctial Node, and Lj L 2 L 3 L 4 represent 
the different longitude lines with some of the heights above the 
ecliptic as derived from the latitude angles marked upon them. 

The time-intervals in this case are : 
9-0 : 15-25 : 19-1. 

It is clear however that at the time of the first and second 
observations the comet was approaching the Sun, and afterwards 
receding, and with a considerable change of angular direction. 
We may therefore fairly assume, although it would be premature 
to speak with exactness, that on the principles of Section 6 the 
first and third values in the three columns of time-intervals will 
be increased as compared with the middle column. Let us 
assume the proportions of the arc-intervals to be 

9 .8 15-25 19-7 

ill-O 17-0 22-O 

12-3 19-0 24-6 

13-6 2I-O 27'2 

Placing 1 7 across the middle space and bending the scale a little 
so as to be slightly concave to the Sun, we find a fair agreement 
with 22 towards L 4 , but the other interval is not well bridged, as 
it extends only to G, considerably short of L r The direction of 
the curve is from near the 60 mark on ^ to the same figure on L 4 . 

With 21 on the central space, 13-6 may be found to agree 
fairly well with L 15 but 27-2 is considerably too long for the 
other space, and it overlaps it towards F, the direction being 
from near 41 on L : to 31 on L 4 . It becomes therefore clear that 
a better result is to be looked for between these two trials. With 



508 Comets. [BOOK IV. 

1 9 on the central space we can place both 1 2-3 on L : and 24-6 on 
L 4 , the curve ranging from about 49 on L 15 26 on L 2 , 14-2 on 
L 3 to 58 on L 4 ; and we may now mark in the points A, B,C, D. 

As in this example we have the opportunity of arriving very 
closely at the direction of the node, it will be convenient to 
obtain it at this stage. As the second observation was taken 
below the ecliptic and the third above, it follows that the node 
lies somewhere between these two. If, as in Diagram B. on Plate 
XXXV, the chords of the three arcs A B, B C, CD be taken as 
abscissae, and ordinates given to the points A, B, C, and D pro- 
portional to the tangents of the latitude angles, we may construct 
a curve which will determine very nearly indeed the point Q where 
the latitude was zero, and we shall thus obtain the distance of the 
node either from C or B. Set off this distance C 8 in the direction 
C B, and join S Q ; this will be the node. It will be at once 
apparent that neither of the two outside trial curves can satisfy 
the condition that tan i = the latitude number divided by that of 
the distance from the node, and it is unnecessary to apply the area 
test to them. We may therefore confine our attention to the 
points A, B, C, D. 

On this curve the pediometrics M M' and N N' very nearly confirm 
the points already chosen. A, however, has to be shifted to A' 
at 47 -5, and D moved from 58 to 60. The combined heights of 
these two, 107.5, divided by the distance between them (perpen- 
dicular to the node), 51, representing tan/, gives for this angle 
64 36', whilst the angle derived from the two inner points B and 
C is 65 8' ; the mean 64 42'. 

At this point of the work it would usually be convenient to 
rub out the first trial pencil lines referred to in Section i, and 
proceed by the method of the latitudes (see Section 3), but as 
that would destroy the previous work of this example, we proceed 
to Diagram C. on Plate XXXV. On Lj at 80, L 2 at 40, L 3 at 30, 
and on L 4 at 80, ordinates are drawn, determined by the ratio 
tan i = tan 64 42'. Draw TF, UG, VI, and W K as in the last 
example, and from the points F, G, I and K develope the orbit by 
making F H x = FA sec /, &c. 



CHAP. VI.] Determination of Orbits. 509 

By the method of Section 5. using Hj and H 4 , we determine the 
perihelion distance and other elements of the parabola which 
would pass through those two points. The distance so determined 
is 72-7. It will be seen by Diagram C. how nearly it coincides 
with H 2 and H 3 . The vertex of the parabola (i. e. the perihelion) is 
at P ; its longitude (TT) measuring 268 55'. The time T may be 
obtained thus : By means of the pediometric line N N' cut off Q, 
making the area subtended by H 2 Q = that subtended by H! H 2 , 
the time being 9 days. It is easy to measure the small arc R P 
as 1-2 1 day. T therefore becomes June 9-18 + 9 1-21 = June 
16-97. The elements of the orbit are now ascertained : 

T June 16-97 

268 55' 
q 0-727 [= log. 9-86153] 

S3 270 48' 

i 64 42' 

The elements calculated by Mr. Hind were : 

T June 16-457 

26 5 15' 44" 

q 0-7346 [= log. 9-8657] 

S 270 57' 46" 

i 63 28' 46" 

If we take the case of the comet dealt with in the last example 
it will appear that the space due to the parabolic orbit between 
the dates June 9 and June 24 should bear the proportion of about 



1-671 : i- (viz. A/VW * ^2) to that traversed by the earth during 
the same period a proportion which it will be found by 
measurement on the diagram has been very nearly obtained. 



SECTION 9. To form an epTiemeru of a Comet. 

If it be desired to form an ephemeris from the elements of a 
comet's orbit the procedure graphically would be as follows : 

Taking the case of Example i, namely given the perihelion 
distance 0-636, and the date of perihelion passage Aug. 22-23 ; 
let it be desired to find the cornet's place in R. A. and Decl. for 
Sept. 2-33. 



510 Comets. [BOOK IV. 

By dividing the normal value of daily motion at perihelion 
(Sect. 7) by Vo-6$6 we obtain in this case in terms of our scale 
3-051 8, and for the subtended area 97-049. This requires for 1 1 i 
days an area of 1077-24. 

The formula given in Section 5, namely, A = ' > would 

suffice for finding the place on the orbit, but would require the 
solution of a cubic equation, and as that might be tedious, it 
would be more convenient to use the formula as a correction of a 
value otherwise obtained. By the pediometric method we should 
obtain 11-1x3-0518 = 33-875 for the ordinate. This however 
would be somewhat too great, as the space inclosed between the 
chord and the arc is too large to be neglected. But the excess 
can be easily calculated. 

From the equation to the parabola we readily obtain the 
quantity 4-5107 as the abscissa due to 33-875, and from the 

formula A = we obtain A = 1 105-25. 

The area in excess, 28-01, is proportional to 0-29 of a day, so 
that instead of the place due to Sept. 2-33 we have that of Sept. 
2-52, which will probably answer the purpose aimed at nearly as 
well : if not, an adjustment could be easily^made. 

If the point H 5 had been at a greater distance from the Peri- 
helion, it would have been requisite to have approximated to it 
by stages by the pediometric method, as shown in Section 2, the 
place so obtained to be corrected by the formula used above. 

The point H 5 on the orbit having been obtained, draw through 
it the straight line H R, perpendicular to the node, and find upon 
it the point J where R J = R H 5 cos i. Join E. J and this will 

give the longitude of the required place. Also - - = tan / 

J 6 J 

gives the latitude. 

The R. A. and Decl. may now either be computed or solved 
graphically. 



CHAP. VII.] Catalogue. No. I. 511 



CHAPTER VII. 

A CATALOGUE OF ALL THE COMETS WHOSE ORBITS HAVE 
HITHERTO BEEN COMPUTED. 

"1TTHEN a new comet has been discovered, the first thing to 
be done is to obtain 3 observations of it, whereby the 
elements of the orbit may be computed. The computer will 
then examine a catalogue of comets to see if he can identify the 
newly-found stranger with any that have been before observed*. 
The value of a good catalogue is obvious ; and therefore I have 
compiled as complete a one as possible. 

In the preparation of the following list, care has been taken 
that only the most reliable orbits that were to be obtained should 
be inserted, the general rule being to prefer the one which was 
derived from the longest arc, other things being satisfactory. 
Among the authorities consulted may be mentioned Piwgre, 
Hussey, Others, Cooper, Hind, Arago, Galle, and many others. 

The Epoch of perihelion passage is expressed in Greenwich Mean 
Time, N.S., since 1582. 

The Longitudes of Perihelion and of the Ascending Node are 
given for the respective epochs, but for any other epoch an 
allowance must be made for the effect of precession. This 
allowance is additive for subsequent dates and subtractive for 
previous ones, as follows: i year = 50"; 100 years = i 23 46"; 
1000 years = 13 56' 50". 

The periods assigned in the column of " Duration of Visibility " 
are subject to much uncertainty, more especially in the case of the 
ancient comets. 

ft In the Annuaire de V Observatoire will be found a catalogue of comets 
Royal de Sruxelles, 1883, at p. 70, there arranged in the order of the Inclinations. 



512 



Comets. 



[BOOK TV. 



No. 


No. 


Year. 


PP.. 


r 


a 


t 


9 


I 


I 


370 B. C. 


d. h. 
Winter 




150210 


O 

270330 


o 
above 30 


very sm. 


2 


2 


I 3 6 


April 29 


230 


22O 


20 


roi 


3 


3 


68 


July 


30033 


150 I 80 


70 


o'8o 


4 


4 


ii 


Oct. 8 19 


280 


28 


10 + 


0-58 










o > 


O / 


o / 




5 


(4) 


66 A. D. 


Jan. 14 4 


325 o 


32 40 


40 30 


0-445 


6 


(4) 


141 


March 29 2 


251 55 


12 50 


17 o 


0720 


7 


5 


178 


Sept. beg. 


290 


190 


18 


0'5 


8 


(4) 


218 


April 6 











9 


6 


240 


Nov. 9 23 


271 o 


189 o 


44 o 


0-372 


10 


(4) 


295 


April i + 










ii 


(4) 


451 


July 3 12 










12 


7 


539 


Oct. 20 14 


3!3 30 


58 or 238 


10 


0'34 l 


13 


8 


565 ii. 


July ii 18 


84 


158 45 


60 30 


0-775 


14 


9 


568 ii. 


Aug. 29 7 


3i8 ?5 


294 15 


4 8 


0-907 


15 


10 


574 


April 7 6 


M3 39 


128 17 


46 3i 


0-965 


16 


(4) 


760 


June 1 1 










17 


ii 


770 


June 6 14 


357 7 


90 59 


61 49 


0-642 


18 


12 


837 i- 


Feb. 28 23 


289 3 


206 33 


IO 12 


0-580 


19 


13 


961 


Dec. 30 3 


268 3 


350 35 


79 33 


0-552 


20 


(4) 


989 ii. 


Sept. ii 23 


264 


84 


'7 


0-568 


21 


M 


1006 


March 2 2 


34 


38 


i? 30 


0-583 


22 


(4) 


1066 


April i o 


264 55 


25 50 


17 o 


0-720 


23 


15 


1092 


Feb. 15 o 


156 20 


125 40 


28 55 


0-928 


2 4 


16 


1097 i. 


Sept. 21 21 


332 30 


207 30 


73 30 


0738 


-5 


17 


1231 


Jan. 30 7 


134 48 


*3 30 


6 5 


0-948 



i. It is said to have separated into two parts. 

3. It had a short but brilliant tail. 

4. An apparition of Halley's comet (?), mentioned by Dion Cassius as having been 
suspended over Home previous to the death of Agrippa. 

5. An apparition of Halleys comet (?). It had a tail 8 long. 

6. An apparition of Halley's comet. 

9. Elements somewhat doubtful. It had a tail 30 long. 

1 1. Undoubtedly an apparition of Halley's comet. 

12. It had a tail 10 feet long ! ! 

13. A mean orbit. It had a tail 10 long. 

14. Elements very reliable. On Sept. 8 it had a tail 40 long. 

15. Elements very uncertain. 



CHAP. VII.] 



Catalogue. No. I. 



513 






/* 


Calculator. 


Date of 
Discovery. 


Discoverer. 


Duration 
of Visibility. 


I'D 




Pingre" 


. . 


Greek obs. 


(?) 


I'O 





Peirce 





Chinese obs. 


5 weeks. 


I'O 


+ 


Peirce 


68, July 23 


Chinese obs. 


j weeks. 


I'O 


- 


Hind 


n, Aug. 26 


Chinese obs. 


9 weeks. 


I'O 


- 


Hind 


66, Jan. 31 


Chinese obs. 


7 weeks. 


I'o 


_ 


Hind 


141, Mar. 27 


Chinese obs. 


4 weeks. 


I'O 


+ 


Hind 








ro 




Hind 


218, April 


.. .. 


6 weeks. 


I'O 


+ 


Burckhardt 


240, Nov. 10 


Chinese obs. 


6 weeks. 







Hind 


295 


.... 


7 weeks. 


I'O 




Laugier 


451, May 17 


Chinese obs. 


13 weeks. 


I'O 


+ 


Burckhardt 


539, Nov. 17 


Chinese oba. 


9 weeks. 


I'O 


- 


Burckhardt 


565, Aug. 4 


Chinese obs. 


15 weeks. 


I'O 


+ 


Laugier 


568, Sept. 3 


Chinese obs. 


10 weeks. 


I'O 


+ 


Hind 


574, May 2 


Chinese obs. 


13 weeks (?). 


I'O 




Laugier 


760, May 1 6 


Chinese obg. 


8 weeks. 


I'O 


- 


Laugier 


770, May 26 


Chinese obs. 


10 weeks. 


ro 


- 


Pingre 1 


837, Mar. 2*2 


Chinese obs. 


5 weeks. 


I'O 


- 


Hind 


962, Jan. 28 


Chinese obs. 


5 weeks. 


I'O 


- 


Burckhardt 


989, July 28 


Chinese obs* 


5 weeks. 


I'O 





Pingre" 


1006, April 


European obs. 


3 or 6 weeks. 


I'O 


- 


Hind 


1 066, April i 


Chinese obs. 


6 weeks or + . 


I'O 


+ 


Hind 


1092, Jan. 8 


Chinese obs. 


17 weeks. 


I'O 


+ 


Burckhardt 


1097, Sept. 30 


Chinese obs. 


4 weeks. 


I'O 


+ 


Pingre- 


1231, Feb. 6 


Chinese obs. 


4 weeks. 



1 6. An apparition of Halley's comet. 

17. It had a tail about 30 long. 

18. Tolerably trustworthy. The maximum length of the tail was 80, but it 
dwindled down to 30 in a fortnight. 

20. Probably an apparition of Halley's comet. Mentioned by several Saxon writers. 

21. These elements appear to have escaped the notice of recent cometographers, 
though given by Pingre ; but has it been confounded with the following ? 

22. Possibly an apparition of Halley's comet. This is the famous object which 
created such universal dread throughout Europe in 1066. In England it was looked 
upon as a presage of the success of the Norman invasion. 

23. Elements satisfactory. 

24. A tail 50 long was seen in China, and much bifurcated. 

I.I 



514 



Comets. 



[BOOK IV. 



No. 


No. 


Tear. 


pp. 


IT 


9 


i 


. 


26 


18 


1254 


d. h. 
July 15 23 


O 1 

272 30, 


1 

'75 30 


30 25 


0-430 


27 


'9 


1299 


March 31 7 


3 20 


107 8 


68 57 


0-318 


28 


(4) 


1301 i. 


Oct. 23 23 


312 


138 


13 


0-640 


29 


20 


13371. 


June 15 i 


2 2O 


93 i 


40 28 


0-828 


30 


21 


1351 


Nov. 25 23 


6 9 


Indeterminate. 


It) 


31 


22 


1362 i. 


March 1 1 4 


219 


249 


21 


0-456 


32 


3 


1366 


Oct. 21 ii 


48 4 


217 25 


27 37 


0-979 


33 


(4) 


1378 


Nov. 8 1 8 


299 31 


47 17 


'7 56 


0-583 


34 


24 


1385 


Oct. 16 6 


101 47 


268 31 


52 15 


0774 


35 


25 


'433 


Nov. 7 1 8- 


267 i 


96 20 


76 o 


0-492 


36 


26 


1449 


Dec. 99 


264 26 


261 18 


24 20 


0-327 


37 


(4) 


'456 


June 8 5 


298 57 


43 56 


'7 37 


0-580 


38 


2.7 


'457 iii- 


Sept. 3 16 


92 50 


256 5 


20 20 


2-103 


39 


2.8 


1462 


Aug. 6 3 


196 


25 


25 


0-31 


40 


-9 


1468 ii. 


Oct. 7 6 


356 3 


61 15 


44 19 


0-853 


4i 


30 


1472 


Feb. 28 5 


48 3 


207 32 


i 55 


0-539 


42 


*- 


1490 | 


Dec. 24 ii 
Dec. 35 21 


58 40 
"3 


288 45 
268 


5i 37 

75 


0-738 
0-755 


43 


32 


1499 


Sept. 6 1 8 


o 


326 30 


21 


0-954 


44 


33 


1500 


May 1 7 


290 


3io 


75 


''4 


45 


34 


1506 


Sept. 3 15 


250 37 


132 50 


45 * 


0-386 


46 


(4) 


I53i 


Aug. 24 21 


3 or 39 


49 25 


17 56 


0-5670 






r 


Oct. 19 14 


135 44 


119 8 


42 27 


0-6125 


47 


35 


J 532 | 


Oct. 19 22 


in 7 


So 27 


32 36 


0-5091 


48 


36 


1533 { 


June 14 21 
June 16 19 


217 40 

IO4 12 


2 99 T 9 
125 44 


28 14 
35 49 


0-3269 

0-2028 



26. One of the grandest comets on record. Its tail is said to have been 100^ long. 
Hoek has published several orbits all differing much from Pingr^'s. 

27. Elements very doubtful. 

28. Probably an apparition of HaHey's comet. 

29. A fine comet. The elements assigned by Halley, Pingre 1 , and Hind differ 
somewhat from those here given. 

30. Very uncertain. No latitudes given. 

31. Uncertain. The tail was 20 feet long, and the head was the size of a wine-glass/ 

32. Very uncertain. 

33. An apparition of H alley's comet. 
34. Tolerably certain. The tail was 10 long. 

37. An apparition of Halley's comet. It had a splendid tail, 60 long. At one 
time the head was round, and the size of a bull's eye, and the tail like that of a 
peacock ! ! {Chinese Obs.) 

38. Only approximate. It had a tail 15 long. 



CHAP. VII.] 



Catalogue. No. I. 



515 



6 


M 


Calculator. 


Date of 
Discovery. 


Discoverer. 


Duration 
of Visibility. 


I'O 


+ 


Pingrd 


1264, July 14 


Chinese & European 


3 months. 


I-O 


- 


Pingre" 


1 299, Jan. 24 


Chinese obs. 


II weeks. 


I-O 


- 


Laugier 


1301, Sept. 16 


Chinese & European 


6 weeks. 


I'O 


- 


Laugier 


1337. May 


Chinese & European 


3 or 4 months. 


ro 


+ 


Burckhardt 


1351, Nov. 24 


Chinese obs. 


i week. 


i-o 





Burckhardt 


1362, Mar. 5 


Chinese obs. 


5 weeks. 


i-o 


- 


Hind 


1366, Aug. 26 


Chinese obs. 


Several days. 


ro 


- 


Laugier 


1378, Sept. 26 


Chinese obs. 


6 weeks. 


I O 


- 


Hind 


1385, Oct. 23 


Chinese obs. 


(?) 


i-o 


- 


Celoria 


1433. Oct. 12 


Chinese obs. 


3 months. 


I-O 


+ 


Celoria 


1450, Jan. 19 


Chinese obs. 


7 weeks. 


0-96 


- 


Celoria 


1456, May 29 


European & Chinese 


i month. 


10 


+ 


Hind 


1457, June 


European obs. 


3 months. 


I'O 


- 


Hind 


1462 


Chinese obs. 




i-o 


- 


Laugier 


1468, Sept. 


European obs. 


2 or 3 months. 


i-o 





Laugier 


1471, Dec. 


Regiomontanus 


3 months. 


i-o 
i-o 


+ 


Hind -i 
Peirce J 


1491, Jan. 


Chinese obs. 


(?) 


I'O 


+ 


Hind 


1499 


Chinese obs. 


(?) 


i-o 


- 


Hind 


1500, April 


European & Chinese 


3 weeks or + . 


i-o 


- 


Laugier 


1506, July 31 


Chinese obs. 


2 weeks. 


i-o 





Halley 


i53i,Aug.i 


P. Apian 


5 weeks. 


i-o 
i-o 


+ 

+ 


Mdchain T 
Halley J 


1532, Sept. 22 


P. Apian 


1 6 weeks. 


i-o 

I'O 


+ 


Olbers -i 
Douwes J 


I533> J n e 


P. Apian 


i\ months. 



40. Uncertain. It had a tail 30 long. 

41. A celebrated comet. When at its least distance from the Earth (3,300,000 
miles), on Jan. 21, it was quite visible in full daylight. It had a fine tail, which the 
Chinese say was as long as a street I 

42. Uncertain. 

43. In the middle of August this Comet seems to have approached very near to 
the Earth. (Hind, MSS. communicated.) 

44. Elements uncertain. It was as large as a ball ! and had a tail from 3 to 

5 lon g- 

46. An apparition of Ualley's comet. It had a tail 7 long. 

47. It had a tail several degrees long. Olbers has computed an orbit which agrees 
well with Halley's, but Me"chain's is considered the best. 

48. According to Olbers, both these orbits will satisfy the observations, and it is as 
yet impossible to decide between them. It had a tail 15 long. 



516 



Camels. 



[BOOK IV. 



No. 


No. 


Tear. 


PP. 


it 


& 


1 


1 


49 


(18) 


1556 


d. h. 
April 22 o 


/ 

274 14 


175 25 


30 12 


0-5049 


50 


37 


1558 


Aug. 10 12 


329 49 


332 36 


73 29 


0-5773 


5i 


38 


1577 


Oct. 26 22 


129 42 


25 20 


75 9 


0-1775 


52 


39 


1580 


Nov. 28 12 


108 26 


19 6 


64 33 


O'6O23 


S3 


40 


1582 


May 6 1 6 


245 23 


231 7 


61 27 


0-2257 








May 6 10 


256 IS 


229 18 


60 47 


0-1683 


54 


4 1 


1585 


Oct. 8 o 


9 8 


37 44 


6 5 


1-0948 


55 


42 


1590 


Feb. 8 o 


217 57 


165 37 


29 29 


0-5677 


56 


43 


1593 


July 18 13 


176 19 


164 15 


87 58 


0-0891 


57 


44 


1596 


July 25 5 


270 54 


330 20 


5i 58 


0-5671 


58 


(4) 


1607 


Oct. 27 o 


300 46 


48 14 


17 6 


0-584! 


59 


45 


i6i8i. 


Aug. 17 3 


3l8 20 


293 25 


21 28 


0-5I29 


60 


46 


iii. 


Nov. 8 8 


3 5 


75 44 


37 " 


0-3895 


61 


47 


1652 


Nov. 12 15 


28 18 


88 10 


79 28 


0-8475 


62 


(35?) 


1661 


Jan. 26 21 


115 16 


81 54 


33 o 


OH427 


63 


48 


1664 


Dec. 4 12 


130 33 


81 15 


21 18 


1-0255 


64 


49 


1665 


April 24 5 


?i 54 


228 2 


76 5 


0-1064 


65 


50 


i663 


Feb. 24 18 


4 9 


193 26 


27 7 


0-25II 








Feb. 28 19 


277 2 


357 J 7 


35 58 


0-0047 


66 


51 


1672 


March I 8 


46 59 


297 30 


83 22 


0-6974 


67 


52 


1677 


May 6 o 


137 37 


236 49 


79 3 


0-2805 


68 


53 


1678 


Aug. 1 8 7 


322 47 


163 20 


2 52 


I-I453 



49. A rery fine comet, which was expected to return in 1860. 

50. Hoek gives : PP. = Sept. 13 ; IT 2 1 5 ; Q 335 ; i 69 : q = 0-280. 

51. It had a tail 22 long. This comet formed the subject of the observations of 
Tycho Brahe for the detection of parallax. 

52. Elements approximate. Observed also by Tycho Brahe. 

53- Very uncertain. It had a faint tail 3 long, which resembled a piece of silk ! ! 

54. This orbit was computed some years ago, to see whether the comet of 1844 (ii) 
was identical with this one. 

55. It had a tail 7 long. 

56. It had a tail 4^ long. 

57. Discovered also by Tycho Brahe. 

58. An apparition of Halley's comet. It had a tail 7 long. 

59. Somewhat uncertain. Seen at Lintz, Aug. 27, and by Kepler, Sept. I. 



CHAP. VII.] 



Catalogue. No. I. 



517 



6 


ft 


Calculator. 


Date of 
Discovery. 


Discoverer. 


Duration 
of Visibility. 


ro 


+ 


Hind 


1556, Feb. 28 


P. Fabricius 


10 weeks. 


i-o 


- 


Gibers 


1558, July 14 


Landgrave of Hesse 


6 weeks. 


ro 


_ 


Woldstedt 


1577, Nov. i 


In Peru 


12 weeks. 


I'O 


+ 


Schjellerup 


1580, Oct. 2 


Mcestlin 


10 weeks. 


i-o 


- 


Pingre' 


1582, May 12 


Tycho Brahe 


3 weeks. 


I'O 





D'Arrest 








I'O 


+ 


C. A. Peters 


1585, Oct. 19 


Tycho Brahe & 


4 weeks. 






and Sawitsch 




PtOthmann 




I'D 


- 


Hind 


1590, Mar. 5 


Tycho Brahe 


3 weeks. 


i-o 


+ 


La Caille 


1593, July 20 


De Rissen 


6 weeks. 


I'O 


- 


Hind 


1596, July ii 


Mrestlin 


5 weeks. 


0-96708 





Lehmann 


1607, Sept. ii 


Kepler 


9 weeks. 


ro 


+ 


Pingre* 


1618, Aug. 25 


At Caschau 


4 weeks. 


i-o 


+ 


Bessel 


Nov. 30 


Many observers. 


7 weeks. 


i-o 


+ 


Halley 


1652, Dec. 20 


Hevelius 


3 weeks. 


i-o 


+ 


Me"chain 


1661, Feb. 3 


Heveliua 


5 weeks. 


i-o 


- 


Lindelof 


1664, Nov. 17 


In Spain 


17 weeks. 


I'O 


- 


Halley 


1665, Mar. 27 


At Aix 


4 weeks. 


I'D 

ro 


+" 


Henderson i 
Henderson i 


1668, Mar. 5 


Gottignies, etc. 


3 weeks. 


i-o 


+ 


HaUey 


1672, Mar. 2 


Hevelius 


7 weeks. 


i-o 


- 


Halley 


1677, April 27 


Hevelius 


12 days. 


0-62697 


+ 


Le Verrier 


1678, Sept. ii 


La Hire 


4 weeks. 



60. A splendid comet ; it had a tail, according to Longomontanus, 104 long, and 
of a reddish hue. Said to have been visible in the daytime. 

61. Elements only approximate. 

62. By some supposed to be identical with the comet of 1532 ; it was not re- 
observed, however, as was anticipated, about 1791. 

63. It had a tail from 6 to 10 long. 

64. It had a tail 25 long. 

65. Seen chiefly in the southern hemisphere ; both orbits satisfy the observations, 
and it is impossible to say which is the correct one. 

66. It had a tail about i long. 

67. It had a tail about 6 long. 

68. Elements only approximate. 



518 



Comets. 



[BOOK IV. 



No. 


No. 


Year. 


PP. 


IT 


S3 


< 


2 


69 


54 


1680 


d. h. 
Dec. 17 23 


262 49 


o / 

272 9 


60 40 


0-0062 


70 


(4) 


1682 


Sept. 14 19 


301 55 


51 H 


i? 44 


0-5829 


71 


55 


1683 


July 13 2 


85 35 


173 24 


83 13 


Q'5595 


72 


56 


1684 


June 8 10 


238 52 


268 15 


65 48 


0-9601 


73 


57 


1686 


Sept. 1 6 14 


77 o 


350 34 


31 21 


0-3250 


74 


58 


1689 


Nov. 29 4 


269 41 


90 35 


59 4 


0-0189 


75 


59 


1695 


Nov. 9 1 6 


60 


216 


22 


0-8435 


76 


60 


1698 


Oct. 18 16 


270 51 


267 44 


II 46 


0-69 1 2 


77 


61 


1699 i. 


Jan. 13 8 


212 31 


32i 45 


69 2O 


07440 


78 


62 


1701 


Oct. 17 9 


'33 4 


298 41 


4i 39 


0-5926 


79 


63 


1 702 ii. 


March 13 14 


13846 


188 59 


4 24 


0-6468 


80 


64 


1706 


Jan. 30 4 


72 29 


13 ii 


55 4 


0-4258 


Si 


65 


1707 


Dec. ii 23 


79 54 


52 4 6 


88 36 


0-8597 


82 


66 


1718 


Jan. 14 21 


121 39 


I2 7 55 


3i 8 


1-0254 


83 


67 


1/23 


Sept. 27 15 


42 53 


14 14 


50 o 


0-9987 


84 


68 


1729 


June 13 6 


320 31 


3io 38 


77 5 


4^435 


85 


69 


i73/i- 


Jan. 30 8 


325 55 


226 22 


18 20 


O-2-228 


86 


70 


ii. 


June 3 5 


261 58 


132 5 


61 52 


0-8349 


87 


7i 


1739 


June 17 10 


102 38 


207 25 


55 42 


0-6735 


88 


72 


I742i. 


Feb. 8 4 


2'7 35 


185 38 


66 59 


0-7656 


89 


73 


I743i- 


Jan. 8 4 


93 9 


86 54 


i 53 


0-86l5 


90 


74 


ii. 


Sept. 20 21 


247 o 


6 2 


45 37 


0-5-229 


9i 


75 


1744 


March i 8 


197 12 


45 45 


47 8 


O-222O 



69. A splendid comet, whose tail ultimately attained a length of from 70 to 90. 
Halley conjectured that this was a return of the comet of 1106, 531 A.D., and 44 B.C., 
but this has since been shewn to be unlikely. The orbit here given supposes a period 
of 88 1 4 years ; this, however, is subject to much uncertainty, inasmuch as the ob- 
servations might possibly be satisfied by an 805 years' ellipse, or even by a hyper- 
bolic orbit. 

70. An apparition of ff alley's comet. It had a tail from 12 to 16 long. 

71. It had a tail varying from 2 to 4. 

73. Its nucleus was as bright as a ist-magnitude star, and it had a tail 18 long. 

74. Observed very roughly in the East Indies. It had a tail 60 long. Pingrd 
makes the 3 = 323 45'- 

75. Observed still more imperfectly than the last in the southern hemisphere. It 
had a tail 18 long. 

76. Uncertain. 



CHAP. VII ] 



Catalogue. No. /. 



519 






M 


Calculator. 


Date of 
Discovery. 


Discoverer. 


Duration 
of Visibility. 


0-99998 


+ 


Encke 


1680, Nov. 14 


G. Kirch 


18 weeks. 


0-96792 


- 


Eosenberger 


1682, Aug. 15 


Flamsteed 


5 weeks. 


IX) 


- 


Plummer 


1683, July 23 


Flamsteed 


6 weeks. 


l'O 


+ 


Halley 


1684, July I 


Bianchini 


2 weeks. 


1X> 


+ 


Halley 


1686, Aug. 


In India 


I month. 


I'O 


- 


Vogel 


1689, Dec. 10 


Bichaud 


2 weeks. 


TO 


+ 


BurckharJt 


1695, Oct. 28 


Jacob 


3 weeks. 


i-o 





Halley 


1698, Sept. 2 


La Hire 


4 weeks. 


I'O 


- 


La Caille 


1699, Feb. 17 


Fontenay 


2 weeks. 


ro 


- 


Burckhardt 


1701, Oct. 28 


Pallu 


I week. 


I'O 


+ 


Burckhardt 


1702, April 20 


Bianchini 


2 weeks. 


I'O 


+ 


La Caille 


1706, Mar. 1 8 


J. D. Cassini 


4 weeks. 


i-o 


+ 


La Caille 


1707, Nov. 25 


Manfredi 


8 weeks. 


ro 


- 


Argelander 


1718, Jan. 18 


C. Kirch 


3 weeks. 


ro 


- 


Sporer 


1723, Oct. 9 


Uncertain 


9 weeks. 


1-00503 


+ 


Burckhardt 


1729, July 31 


Sarabat 


25 weeks. 


ro 


+ 


Bradley 


1737, Feb. 6 


Tn Jamaica 


4 weeks. 


i-o 


+ 


Hind 


Feb. 


At Pekin 


(?) 


i-o 


- 


La Caille 


1739, May 28 


Zanotti 


ii weeks. 


i-o 


- 


La Caille 


1742, Feb. 5 


Cape of G. Hope 


13 weeks. 


0-72130 


+ 


Clausen 


1 743, Feb. 10 


Grischau 


2 weeks. 


I'O 





D'Arrest 


Aug. 1 8 


Klinkenberg 


4 weeks. 


i-o 


+ 


Betts 


Dec. 9 


Klinkenberg 


4 months (?) 



78. Observed also by Thomas at Pekin. 

79. Very roughly observed ; visible to the naked eye. 
81. Discovered by J. D. Cassini, Nov. 29. 

83. It was seen in Europe, with a faint tail i long. 

84. Scarcely perceptible to the naked eye. The orbit is a hyperbolic one, and 
remarkable for its enormous perihelion distance, the greatest known. 

86. Elements only approximate. 

88. Visible to the naked eye, with a tail 6 or 8 long. 

89. Very imperfectly observed. An elliptic orbit ; period assigned, 5-436 years. 
' 90. Very uncertain. Visible to the naked eye. 

91. The finest comet of the i8th century. On Feb. 15 it had a bifid tail, the 
eastern portion being 7 long, and the western 24. Visible in a telescope in the 
daytime. Euler has calculated an elliptic orbit, to which he assigns a period of 
122,683 years ! ! The statement of this comet having had six tails (at one time dis- 
believed) has been confirmed by the testimony of De Lisle discovered by Winnecke. 



520 



Comets. 



[BOOK IV. 



No. 


No. 


Year. 


PP. 


w 





t 


2 


92 


(17?) 


1746 


d. h. 
Feb. 15 o 


/ 

140 o 


o / 

33S o 


o / 

6 o 


0'95 


93 


76 


1747 


March 3 7 


277 2 


147 18 


79 6 


2-1985 


94 


77 


1748 i. 


April 28 1 8 


215 23 


232 51 


85 28 


0-8404 


95 


78 


ii. 


June 18 at 


278 47 


33 8 


6? 3 


0-6253 


96 


79 


i/57 


Oct. 21 7 


122 58 


214 it 


12 SO 


Q'3375 


97 


80 


1758 


June ii 3 


267 3 8 


230 5 


68 19 


0-2153 


98 


(4) 


1759 i- 


March 12 13 


33 10 


53 5 


17 3<S 


0-5845 


99 


81 


ii. 


Nov. 27 2 


53 24 


139 39 


78 59 


0-7985 


IOO 


82 


iii. 


Dec. 16 21 


138 24 


79 50 


4 5i 


0-9659 


IOI 


83 


1762 


May 28 8 


104 2 


348 33 


85 38 


i -0090 


103 


84 


1763 


Nov. i 20 


8458 


356 24 


72 3i 


0-4982 


103 


85 


1764 


Feb. 12 13 


15 14 


I2O 4 


S^ 53 


0-5552 


I0 4 


86 


1 766 i. 


Feb. 1 7 8 


H3 15 


244 10 


40 50 


0-5053 


105 


87 


ii. 


April 26 23 


251 13 


74 H 


8 i 


0-3989 


106 


88 


1769 


Oct. 7 14 


144 II 


175 3 


4 45 


0-1227 


107 


89 


1770 i. 


Aug. 13 12 


356 16 


131 59 


i 34 


0-6743 


108 


90 


ii. 


Nov. 22 5 


2O8 22 


1 08 42 


31 25 


0-5282 


109 


9i 


1771 


April 19 5 


104 3 


27 5i 


Ii 15 


0-9034 


no 


92 


1772 


Feb. 19 2 


no 14 


254 o 


18 17 


1-0136 


III 


93 


1773 


Sept. 5 14 


75 I0 


121 5 


61 14 


1-1268 


112 


94 


1774 


Aug. 15 19 


3i7 27 


1 80 44 


83 20 


1-4328 


"3 


95 


1779 


Jan. 4 2 


87 14 


25 4 


32 30 


0-7131 


114 


96 


1780 i. 


Sept. 30 22 


346 35 


123 41 


54 23 


0-0963 



92. Elements uncertain, but they strongly resemble those of the comet of 1231. 
It passed very near the Earth, 

93. Observed only during 1 746. 

94. Discovered by J. D. Maraldi, April 30. Visible to the naked eye, with a tail 
2 long. 

95. Very uncertain. 

96. Elements tolerably reliable. It had a small tail. 

98. The first predicted apparition of Halley's comet. On May 5 its tail was 47 
long. 

99. Visible to the naked eye, with a tail 5 long. Elements resemble those of the 
comet of 1449. 

100. This comet came near the Earth, and moved with great rapidity ; it had a 
tail 4 long. 

101. It bad a small tail. 

102. An elliptic orbit ; period assigned, 7334 years. Lexell makes it 1137 years. 



CHAP. VII.] 



Catalogue. No. I. 



521 



1 


V- 


Calculator. 


Date of 
Discovery. 


Discoverer. 


Duration 
of Visibility. 


I'O 


* 


Hind 


1746, Feb. 2 


Kindermans 


4 weeks. 


10 


- 


La Caille 


Aug. 13 


Che"saux 


15 weeks. 


I'D 


- 


Le Monnier 


1 748, April 26 


At Pekin 


9 weeks. 


I'O 


+ 


Bessel 


May 19 


Klinkenberg 


4 days. 


I'O 


+ 


Bradley 


'757. Sept. IT 


Gartner 


5 weeks. 


I'O 


+ 


Pingre" 


1758, May 26 


La Nux 


5 months. 


0-96768 


- 


Rosenberger 


Dec. 25 


Palitzch 


5 months. 


I'O 


+ 


La Caille 


1760, Jan. 25 


Messier 


8 weeks. 


I'O 


- 


La Caille 


Jan. 7 


At Lisbon 


14 weeks. 


I'O 


+ 


Burckhardt 


1762, May 17 


Klinkenberg 


6 weeks. 


0-99868 


+ 


Burckhardt 


1763, Sept. 28 


Messier 


8 weeks. 


I'O 


- 


Pingre* 


1764, Jan. 3 


Messier 


6 weeks. 


I'O 


- 


Pingre" 


1 766, March 8 


Messier 


9 weeks. 


0-8640 


+ 


Burckhardt 


April I 


Helfenzrieda 


6 weeks. 


0-99924 


+ 


Bessel 


1769, Aug. 8 


Messier 


1 6 weeks. 


0-78683 


+ 


Le Verrier 


1770, June 14 


Messier 


15 weeks. 


ro 


- 


Pingre" 


1771, Jan. 10 


La Nux 


8 days. 


1-00936 


+ 


Encke 


April i 


Messier 


15 weeks. 


0-90314 


+ 


Bessel 


1772, Mar. 8 


Montaigne 


3 weeks. 


I'O 


+ 


Burckhardt 


1773, Oct. 12 


Messier 


27 weeks. 


1-02829 


+ 


Burckhardt 


17 74, Aug. ii 


Montaigne 


II weeks. 


I'O 


+ 


Zach 


1779, Jan. 6 


Bode 


19 weeks. 


0-99994 





Cliiver 


1780, Oct. 26 


Messier 


5 weeks. 



103. Visible to the naked eye, with a tail i\ long. 

105. Discovered by Messier, April 8. An elliptic orbit; period assigned, 5*025 
years. Visible to the naked eye, with a tail 3 or 4 long. 

106. Visible to the naked eye, with a tail from 60 to 80 long. Bessel assigns 
2090 years as the most likely period of revolution. He has shewn that an error of 
5" either may increase the period to 2673 years or diminish it to 1692 years. 

107. The celebrated LexelTs comet. The diameter of the head, July I, was 2. 
It had also a small tail, and approached within 1,400,000 miles of tbe Earth. 

108. It had a faint tail, 5 long. 

109. The orbit of this comet has been found hyperbolic. It had a tale about 2 
long. Recent calculations by Kreuz negative the hyperbola (A. N., 2469). 

1 10. The first recorded apparition of JJiela's comet. 

111. Just perceptible to the naked eye. 

113. Discovered by Messier, Jan. 18. 

114. An elliptic orbit ; period assigned, 75,314 years. 



522 



Comets. 



[BOOK IV. 



NO: 


No. 


Year. 


PP. 


7T 


a 


t 



2 


i'5 


97 


1780 ii. 


d. h. 
Nov. 28 20 


O / 
246 52 


o / 

141 1 


/ 

72 3 


0-5152 


116 


98 


1781 i. 


July 7 4 


239 II 


83 o 


81 43 


07758 


117 


99 


ii. 


Nov. 29 12 


16 3 


77 22 


27 13 


0-9610 


118 


100 


1783 i. 


Nov. 19 13 


49 3i 


55 12 


47 43 


1-4953 


119 


IOI 


1784 i. 


Jan. 21 4 


80 44 


56 49 


5i 9 


0-7078 


120 


102 


ii. 


March 10 o 


137 


35 


84 


0-637 


121 


103 


1785 i. 


Jan. 27 7 


109 51 


264 12 


70 14 


i'i434 


122 


IO4 


ii. 


April 8 8 


297 29 


64 33 


87 31 


o'4273 


123 


I5 


1786 L 


Jan. 30 20 


156 38 


334 8 


13 36 


0-3348 


I2 4 


1 06 


ii. 


July 7 21 


59 25 


194 22 


50 54 


0-4101 


"5 


I0 7 


1787 


May 10 19 


7 44 


106 51 


48 15 


0-3489 


126 


108 


1788 i. 


Nov. 10 7 


99 8 


156 56 


12 27 


1*0630 


127 


109 


ii. 


Nov. 20 7 


22 49 


352 24 


64 30 


0-7573 


128 


no 


1790 i. 


Jan. 15 5 


60 14 


176 ii 


3i 54 


0-7581 


129 


III 


ii. 


Jan. 28 7 


ill 44 


267 8 


56 58 


1-0632 


130 


112 


iii. 


May 21 5 


273 43 


33 " 


63 52 


0-7979 


131 


"3 


1792 i. 


Jan. 13 13 


36 29 


190 46 


39 46 


1-2930 


132 


114 


ii. 


Dec. 27 6 


'35 59 


283 15 


49 i 


0-9662 


133 


"5 


1793 i- 


Nov. 4 20 


228 42 


108 29 


60 21 


0-4034 


*34 


116 


ii. 


Nov. 20 5 


7i 54 


2 


51 31 


I-495I 


135 


(105) 


1/95 


Dec. 21 10 


156 4 i 


334 39 


13 4* 


0-3344 


136 


117 


1796 


April 2 19 


192 44 


17 2 


64 54 


1-5781 


137 


118 


1/97 


July 9 2 


49 2 7 


329 IS 


50 40 


0-5266 


138 


119 


1798 i. 


April 4 n 


i4 59 


122 9 


43 52 


0-4847 


139 


I2O 


ii. 


Dec. 31 13 


34 27 


249 3 


42 26 


'7795 

















115. Discovered by Olbers on the same day. 

116. Visible to the naked eye, Nov. 9, with a tail 3 long. It came very near the 
Earth. 

118. An elliptic orbit ; period assigned, 5*613 years. 

119. Visible to the naked eye, with a tail 2 long. 

1 20. Not only are the elements uncertain, but it is doubtful whether the comet 
ever existed. 

122. Visible to the naked eye, with a tail 8 long. 

1 23. The first recorded apparition of Encke 1 * comet. 
126. Visible to the naked eye, with a tail 2| Ion?. 

128. Imperfectly observed on four occasions. Elements only approximate. 



CHAP. VII.] 



Catalogue. No. I. 



523 



6 


M 


- Calculator. 


Date of 
Discovery. 


Discoverer. 


Duration 
of Visibility. 


I'D 


- 


Gibers 


1780, Oct. 1 8 


Montaigne 


3 da ys. 


I'D 


H- 


Me"chain 


1781, June 28 


Me"chain 


3 weeks. 


I-O 


- 


Me"chain 


Oct. 9 


Me"chain 


II weeks. 


0-6784 


H- 


Burckhardt 


1783, Nov. 19 


Pigott 


4 weeks. 


I'O 


- 


Me"chain 


Dec. 15 


La Nux 


23 weeks. 


ro 


+ 


Burckhardt 


1784, April 10 


D'Angos 


5 days. 


I O 


+ 


Me"chain 


1785, Jan. 7 


Messier 


5 weeks. 


I'O 


- 


Me"chain 


Mar. II 


Mechain 


5 weeks. 


o 84836 


+ 


Encke 


1786, Jan. 17 


Me"chain 


3 days. 


i-o 


+ 


Me"chain 


Aug. l 


Miss Herschel 


12 weeks. 


10 


- 


Saron 


1787, April 10 


Mdchain 


7 weeks. 


i-o 


_, 


Me"chain 


1788, Nov. 25 


Messier 


5 weeks. 


I'0 


+ 


Mechain 


Dec. 21 


Miss Herschel 


4 weeks. 


10 


- 


Saron 


1 790, Jan. 7 


Miss Herschel 


2 weeks. 


I'D 


+ 


Mechain 


Jan. 9 


Me"chain 


3 weeks. 


10 


- 


M(5chain 


April 1 8 


Miss Herschel 


10 weeks. 


I'O 





Mdchain 


1791, Dec. 15 


Miss Herschel 


6 weeks. 


I O 


- 


Prosperi n 


1793, Jan. 8 


Gregory 


6 weeks. 


I'D 


- 


Saron 


Sept. 27 


Messier 


15 weeks. 


0-97342 


+ 


D'Arrest 


Sept. 24 


Perny 


10 weeks. 


0-84888 


+ 


Encke 


1 795, Nov. 7 


Miss Herschel 


3 wesks. 


I'O 


_ 


Gibers 


1796, Mar. 31 


Olbers 


2 weeks. 


i-o 


- 


Gibers 


1/97, Aug. 14 


Bouvard 


3 weeks. 


I'O 


+ 


Burckhardt 


1798, April 12 


Messier 


6 weeks. 


i-o 





Burckhardt 


Dec. 6 


Bouvard 


I week. 



130. Visible to the naked eye, with a tail 4 long. 

132. Discovered by Mechain and Piazzi, Jan. 10. There was a trace of a tail to 
be seen. 

134. Discovered by Miss Herschel, Oct. 7. An elliptic orbit ; period assigned, 422 
years. 

135. An apparition of Encltes comet. It was just visible to the naked eye. 

136. Very faint. 

137. Discovered by Miss Herschel and Lee on the same evening ; by Riidiger, 
Aug. 15, and by Kecht, Aug. 16. 

139. Discovered by Olbers, Dec. 1 8. Elements only approximate. 



524 



Comets. 



[BOOK IV. 



No. 


No. 


Year. 


PP. 


ir 


a 


t 


2 


140 


121 


1799 i. 


d. h. 

Sept. 7 5 


I 

3 39 


99 3' 


o / 

5 56 


0-8399 


141 


(61) 


ii. 


Dec. 25 21 


190 20 


326 49 


77 I 


0-6258 


142 


122 


1801 


Aug. 8 13 


182 41 


42 28 


20 45 


0-2564 


43 


123 


1802 


Sept. 9 21 


332 9 


31 *5 


57 o 


1-0941 


144 


124 


1804 


Feb. 13 15 


148 53 


176 49 


56 44 


1-0772 


US 


(I0 5 ) 


1805 


Nov. 21 12 


156 47 


334 20 


13 33 


0-3404 


146 


(92) 


1806 i. 


Jan. i 23 


109 32 


251 15 


13 38 


0-9068 


M7 


125 


ii. 


Dec. 28 22 


97 2 


322 19 


35 2 


1-0815 


148 


126 


1807 


Sept. 1 8 17 


2 7o 54 


266 47 


63 10 


0-6461 


149 


127 


1808 ii. 


May 13 22 


69 12 


322 58 


45 43 


0-3898 


ISO 


128 


iii. 


July 12 4 


252 38 


24 ii 


39 l8 


0-6079 


'51 


12 9 


1810 


Oct. 5 i 


64 56 


308 35 


63 5 


0-9685 


152 


13 


1811 i. 


Sept. 12 6 


75 o 


140 24 


73 2 


i'0354 


^53 


131 


ii. 


Nov. 10 23 


47 27 


93 i 


3i i? 


1-5821 


154 


I 3 2 


1812 


Sept. 15 7 


92 18 


253 i 


73 57 


0-7777 


155 


133 


1813 i. 


March 4 12 


69 56 


60 48 


21 13 


0-6991 


156 


134 


ii. 


May 19 10 


197 43 


42 40 


8l 2 


1-2-161 


157 


i3S 


1815 


April 25 23 


149 2 


83 28 


44 29 


1-2128 


158 


136 


1816 


March I 8 


267 35 


323 '4 


43 5 


0-0485 


159 


137 


1818 i. 


Feb. 3 5 


76 18 


256 i 


34 ii 


0-6959 


160 


138 


ii. 


Feb. 25 23 


182 45 


70 26 


89 43 


1-1977 


161 


139 


iii. 


Dec. 4 22 


ioi 55 


89 59 


63 5 


0-8550 


162 


(105) 


1819 i. 


Jan. 27 6 156 59 


334 33 


13 36 


0-3352 



140. Discovered by Olbers, Aug. 26. At first faint, but afterwards visible to the 
naked eye, with a tail 10 long. 

141. Probably a return of the comet of 1699. "Visible to the naked eye, with a 
tail from i to 3 long. 

142. Discovered at Paris, July 12. Elements resemble those of the comet of 1462. 

143. Discovered by Me'chairi, Aug. 28, and by Olbers, Sept. 2. 

144. Discovered by Bouvard, March 10, and by Olbers, March 12. 

145. An apparition of Encke's comet. Discovered by Pons, Huth, and Bouvard, 
Oct. 20. Visible to the naked eye, with a tail 3 long. 

146. An apparition of Biela's comet. Discovered by Bouvard, Nov. 16, and by 
Huth, Nov. 2 2. Visible to the naked eye. 

148. Discovered by Pons, Sept. 20. It wr.s visible to the naked eye, with a tail 5 
long. An elliptic orbit; period assigned, 1714 years, which may, however.be ex- 
tended to 2157 years or reduced to 1403 years. 

149. Discovered by Wisniewski, March 29. 

150. Elements only approximate. 



CHAP. VII.] 



Catalogue. No. I. 



525 



e 


M 


Calculator. 


Date of 
Discovery. 


Discoverer. 


Duration 
of Visibility. 


I'D 


- 


Burckhardt 


1 779, Aug. 7 


Me'chain 


3 weeks. 


I'O 





Me"chain 


Dec. 26 


Me"chain 


lo days. 


ro 


- 


Doberck 


1801, June 30 


Reissig 


3 weeks. 


I'O 


+ 


Gibers 


1802, Aug. 26 


Pona 


6 weeks. 


I'D 


+ 


Bouvard 


1804, Mar. 7 


Pons 


3 weeks. 


0-84617 


+ 


Encke 


1805, Oct. 19 


Thulis 


3 weeks. 


074578 


+ 


Gambart 


Nov. 10 


Pona 


4 weeks. 


I'O 





Burckhardt 


1806, Nov. 10 


Pona 


14 weeks. 


0-99548 


+ 


Bessel 


1807, Sept. 9 


Paris! 


28 weeks. 


I'O 


- 


Encke 


1808, Mar. 25 


Pons 


I week. 


I'O 


- 


Bessel 


June 24 


Pona 


lo days. 


I'O 


+ 


Thraeix 


1810, Aug. 22 


Pons 


6 weeks. 


0-99509 


- 


Argelander 


1811, Mar. 26 


Flaugergues 


17 months. 


0-98271 


-t- 


Nicolai 


Nov. 16 


Pons 


13 weeks. 


'95454 


+ 


Encke 


1812, July 20 


Pona 


10 weeks. 


I'O 


- 


Nicollett 


1813, Feb. 4 


Pons 


5 weeks. 


I'O 





Encke 


Mar. 28 


Pona 


6 weeks. 


0-93121 


+ 


Bessel 


1815, Mar. 6 


Gibers 


25 weeks. 


I'O 


+ 


Burckhardt 


1816, Jan. 22 


Pons 


II days. 


i-o 


+ 


Hind 


1818, Feb. 23 


Pona 


4 days. 


I'O 


+ 


Encke 


1817, Dec. 26 


Pons 


1 8 weeks. 


I'O 


- 


Rosenberger 


1818, Nov. 28 


Pons 


9 weeks. 


0-84858 


+ 


Encke 


Nov. 26 


Pons 


7 weeka. 



152. A very celebrated comet, conspicuously visible in the evenings of the autumn 
of 1811. It had a tail 25 long and 6 broad. The most reliable computations assign 
a periodic term of 3065 years, subject to an uncertainty of not more than 43 years. 
The orbit of this comet is liable to much planetary perturbation. 

153. An elliptic orbit ; period assigned, 875 years. Visible to the naked eye. 

154. An elliptic orbit; period assigned, 70-68 years. Visible to the naked eye, 
with a tail 2 long. 

156. Discovered also by Harding, April 3. Visible to the naked eye. 

157. An elliptic orbit; period assigned, 70-049 years. Bessel anticipated that 
planetary perturbation would bring it back to perihelion, 1887, Feb. 9. It had 
a short tail. 

158. Elements only approximate. 

159. The observations were few and indifferent. 

161. Discovered by Bessel, Deo. 22. It moved very rapidly. Rosenberger has 
computed a hyperbolic orbit. 

162. An apparition of Enche's comet, the periodicity of which was now discovered. 



526 



Comets. 



[BOOK IV. 



No. 


No. 


Year. 


PP. 


K 


S3 


i 


2 


163 


140 


1819 ii. 


d. h. 
June 27 17 


/ 

287 5 


273 42 


o / 

80 44 


0-3410 


164 


141 


iii. 


July 18 21 


274 4 o 


113 10 


10 42 


0-7736 


165 


142 


iv. 


Nov. 20 5 


67 18 


77 13 


9 * 


0-8925 


166 


H3 


1821 


March 21 12 


239 29 


48 40 


73 3 


0-0918 


167 


144 


1822 i. 


May 5 14 


I9 2 43 


177 26 


53 37 


0-5044 


1 68 


('05) 


ii. 


May 23 23 


157 " 


334 25 


13 20 


0-3459 


169 


MS 


iii. 


July 15 20 


"9 59 


97 44 


36 18 


0-8473 


170 


146 


iv. 


Oct. 23 18 


271 4 o 


92 44 


52 39 


i 1450 


171 


M7 


1823 


Dec. 9 10 


274 34 


303 3 


76 ii 


0-2265 


172 


148 


1824 i. 


July ii 12 


260 16 


234 19 


54 34 


0-5912 


'7.< 


149 


ii. 


Sept. ig i 


4 31 


279 !5 


54 36 


1-0501 


174 


(112) 


1825 i. 


May 30 13 


273 55 


20 6 


5 6 4 1 


0-8891 


175 


150 


ii. 


Aug. 18 17 


10 14 


192 56 


89 41 


0-8834 


176 


(-05) 


iii. 


Sept. 1 6 6 


157 H 


334 27 


13 21 


0-3448 


177 


151 


iv. 


Dec. 10 16 


318 4 6 


215 43 


33 32 


1-2408 


178 


(9*) 


1826 i. 


March 1 8 9 


109 45 


251 28 


13 33 


0-9025 


179 


152 


ii. 


April 21 23 


116 54 


197 38 


40 2 


2-OJII 


1 80 


153 


iii. 


April 29 o 


35 48 


40 29 


5 i? 


0-1881 


181 


154 


iv. 


Oct. 8 22 


57 48 


44 6 


25 57 


0-8528 


182 


155 


V. 


Nov. 1 8 9 


315 31 


235 7 


89 22 


0-0268 


183 


156 


1827 i. 


Feb. 4 22 


33 3 


184 27 


77 35 


0-5065 


184 


'57 


ii. 


June 7 20 


97 3i 


318 10 


43 38 


0-8081 



163. A very brilliant comet, with a tail 7 long. 

164. An elliptic orbit ; period assigned, 5*618 years. Considered by Clausen as a 
return of the comet of 1766 (ii). 

165. Discovered by Pons, Dec. 4. An elliptic orbit ; period assigned, 4-810 years. 
Clausen thought this comet might be identical with that of 1 743 (i). 

166. Discovered by Nicollet on the same day, and by Blainpain, Jan. 25. Visible 
to the naked eye, with a tail 2% long. 

167. Discovered by Pons, May 14, and by Biela, May 17. 

1 68. The first predicted apparition of Encke's comet. Seen only in New South 
Wales. 

169. Its apparent motion was very rapid. 

170. Discovered by Gambart, July 16. An elliptic orbit; period assigned, 5444 
years. Visible to the naked eye, with a tail I % long. 

171. Discovered by Pons, Dec. 29; by Kohler, Dec. 30 ; and by Santini, Jan. 3. 
This comet had, in addition to the usual tail turned from the Sun, another turned 
towards it. 



CHAP. VII.] 



Catalogue. No. I. 



527 






/* 


Calculator. 


Date of 
Discovery. 


Discoverer. 


Duration 
of Visibility. 


I'O 


+ 


Bouvard 


1819, July I 


Tralles 


1 6 weeks. 


0-755I9 


-f 


Encke 


June 12 


Pons 


5 weeks. 


0-68674 


+ 


Encke 


Nov. 28 


Blainpain 


8 weeks. 


I'O 




Rosenberger 


1821, Jan. 21 


Pons 


15 weeks. 


I'O 


- 


Nicollet 


1822, May 12 


Gambart 


7 weeks. 


0-84446 


+ 


Encke 


June 2 


Riimker 


3 weeks. 


I'O 


- 


Hind 


May 31 


Pons 


2 weeks. 


0-99630 


- 


Encke 


July 13 


Pons 


17 weeks. 


I'O 


_ 


Encke 


1823, Dec. i 


In Switzerland 


13 weeks. 


I'O 


- 


Riimker 


1824, July 15 


Riimker 


4 weeks. 


1-00173 


+ 


Encke 


July 23 


Scheithauer 


22 weeks. 


I'O 


- 


Clausen 


1825, May 19 


Gambart 


8 weeks. 


I'O 


+ 


Clausen 


Aug. 9 


Pons 


3 weeks. 


0-84488 


+ 


Encke 


July 13 


Valz 


8 weeks. 


Q'9953 6 


- 


Han sen 


July 15 


Pons 


12 months. 


0-74657 


+ 


Santini 


1826, Feb. 27 


Biela 


8 weeks. 


I'O 


+ 


Clausen 


1825, Nov. 6 


Pons 


22 weeks. 


I'O 





Clttver 


1826, Mar. 29 


Flaugergucs 


9 days. 


I'O 


+ 


Argelander 


Aug. 7 


Pons 


15 weeks. 


I'O 


- 


Cliiver 


Oct. 22 


Pons 


ii weeks. 


I'O 


- 


Heiligenstein 


Dec. 26 


Pons 


5 weeks. 


I'O 





Heiligenstein 


1827, June 20 


Pons 


4 weeks. 



172. Seen only in the southern hemisphere. 

173. Discovered by Pons, July 24, and afterwards by Gambart and Harding. 

174. It had a tail i long. Elements resemble those of 1790 (iii). 

175. Discovered by Harding, Aug. 23. Orbit remarkable for its great inclina- 
tion. 

176. An apparition of Encke's comet. Discovered by Plana, Aug. 10, and by Pons, 
Aug. 14. _ 

177. Discovered by Biela, July 19. Very conspicuous early in October, with a 
bifid tail 15 long. An elliptic orbit ; period assigned, 4386 years. 

178. An apparition of Biela' s comet, whose periodicity was now discovered. Found 
by Gambart, March 9. 

1 80. Elements uncertain. 

181. The path of this comet crosses the ecliptic near the Earth's orbit. 

182. Discovered by Clausen, Oct. 26, and by Gambart, Oct. 28. Visible to the 
naked eye, with a tail | long. 

i8.j. Discovered also by Gambart. Elements resemble those of the comet of 1500. 



528 



Comets. 



[BOOK IV. 



No. 


No. 


Year. 


PP. 


IT 


S3 


< 


1 


185 


I 5 8 


1827 iii. 


d. h. 

Sept. 1 1 6 


O / 

250 57 


/ 

*49 39 


/ 

54 4 


0-1378 


186 


(105) 


1829 


Jan. 9 17 


157 17 


334 2 9 


13 20 


C'3455 


187 


59 


1830 i. 


April 9 7 


212 II 


2o6 21 


21 l6 


0*9214 


1 88 


1 60 


ii. 


Dec. 27 15 


3io 59 


337 53 


44 45 


0-1258 


189 


(105) 


1832 i. 


May 3 23 


157 21 


334 32 


13 22 


0-3434 


190 


161 


ii. 


Sept. 25 12 


227 55 


72 27 


43 18 


1-1839 


191 


(90 


iii. 


Nov. 26 2 


no o 


248 15 


13 13 


0-8790 


192 


162 


1833 


Sept. 10 4 


222 51 


323 o 


7 21 


0-4584 


'93 


163 


1834 


April 2 15 


276 33 


226 48 


5 56 


0-5I50 


194 


164 


1835 i- 


March 27 13 


207 42 


58 19 


9 7 


2-0413 


'95 


(105) 


ii. 


Aug. 26 8 


157 23 


334 34 


13 21 


o'3444 


196 


(4) 


iii. 


Nov. 15 22 


3>4 31 


55 9 


17 45 


0-5865 


197 


(105) 


1838 


Dec. 19 o 


157 27 


334 36 


13 21 


0*3440 


198 


165 


1840 i. 


Jan. 4 10 


192 ii 


"9 57 


53 5 


0-6184 


199 


1 66 


ii. 


March 13 2 


80 12 


236 50 


59 I2 


1-2204 


200 


(16) 


iii. 


April 2 12 


34 20 


186 4 


79 5i 


07420 


201 


167 


iv. 


Nov. 13 15 


22 31 


248 56 


57 57 


1-4808 


202 


(i5) 


1842 i. 


April 12 o 


157 29 


334 39 


13 20 


0-3450 


203 


1 68 


ii. 


Dec. 15 22 


3 2 7 17 


207 49 


73 34 


0-5044 


2O4 


169 


1843 i. 


Feb. 27 9 


27 8 39 


I 12 


35 41 


0-0055 


205 


170 


ii. 


May 6 i 


281 29 


157 14 


52 44 


1-6163 


2O6 


171 


iii. 


Oct. 17 3 


49 34 


209 29 


II 22 


1-6925 



185. At one time supposed to be a return of the comet of 1780 (i). An elliptic 
orbit ; period assigned, 2611 years. 

186. An apparition of Encke's comet, afterwards visible to the naked eye. 

187. Discovered in the southern hemisphere. Visible to the naked eye, with a tail 
8 long. > 

1 88. Visible to the naked eye, with a tail 2^ long. 

189. An apparition of Encke's comet. Discovered by Henderson, June 2. Only 
one observation was made in Europe. 

190. Discovered by Harding, July 29. 

191. The first predicted apparition of Ida's comet. 
193. Discovered by Dunlop, March 16. 

195. An apparition of Encke's comet. Discovered by Boguslawski. July 30. 

196. The second predicted return oi H alley's comet. It was visible to the naked 
eye during the whole of October, with a tail from 20 to 30 long. 

197. An apparition of Encke's comet. Discovered by Galle, Sept. 16. Perceptible 
to the naked eye, Nov. 7. 

202. An apparition of Encke's comet. 



CHAP. VII.] 



Catalogue. No. I. 



529 



f 


/* 


Calculator. 


Date of 
Discovery. 


Discoverer. 


Duration 
of Visibility. 


0-99927 


- 


Cliiver 


1827, Aug. 2 


Pons 


10 weeks. 


0-84462 


+ 


Encke 


1828, Oct. 13 


Struve 


15 weeks. 


0-99938 


+ 


Hadenkamp 


1830, March 16 


D'Abbadie 


22 weeks. 






and Mayer 








I'O 


- 


Wolfers 


1831, Jan. 7 


Herapath 


9 weeks. 


0-84541 


+ 


Encke 


1832, June I 


Mossotti 


? 


i-o 


- 


C. A. Peters 


July 19 


Gambart 


4 weeks. 


0-75146 


+ 


Santini 


Aug. 25 


Dumouchel 


18 weeks. 


I'O 


+ 


C. A. Peters 


1833, Oct. i 


Dunlop 


2 weeks. 


ro 


+ 


Petersen 


1834, March 8 


Gambart 


6 weeks. 


ro 


- 


W. Bessel 


1835, April 20 


Boguslawski 


5 weeks. 


0-84503 


+ 


Encke 


July 22 


Kreil 


9 weeks. 


0-96739 


- 


Westphalen 


Aug. 6 


Dumouchel 


41 weeks. 


0-84517 


+ 


Encke 


1838, Aug. 14 


Boguslawski 


1 6 weeks. 


1-OOO2O 


+ 


Peters, Struve 


1839, Vec. 3 


Galle 


10 weeks. 


o'99323 


- 


Loomis 


1840, Jan. 25 


Galle 


9 weeks. 


ro 


+ 


Petersen 


March 6 


Galle 


3 weeks. 


0-96985 


+ 


Gotze 


Oct. 27 


Bremiker 


16 weeks. 


0-84479 


+ 


Encke 


1842, Feb. 8 


Galle 


15 weeks. 


ro 


- 


Petersen 


Oct. 28 


Laugier 


4 weeks. 


0-99989 


- 


Hubbard 


1843, Feb. 28 


Many observers. 


7 weeks. 


1-00017 


+ 


Gotze 


May 2 


Mauvais 


21 weeks. 


'5559 6 


+ 


Le Verrier 


Nov. 22 


Faye 


20 weeks. 



198. Perceptible to the naked eye, Jan. 8. 

199. An elliptic orbit ; period assigned, 2423 years. Plantamour, however, makes 
it 13,864 years. 

200. Probably a return of the comet of 1097. It had a tail 5 long. 

201. An elliptic orbit ; period assigned, 344 years, subject to an uncertainty of 
about 8 years. Possibly a return of the comet of 1490. 

202. An apparition of Encke's comet. 

203. Small and faint. 

204. One of the finest comets of the present century. It had a tail 60 long. 
The orbit is remarkable for its small perihelion distance. The period assigned is 
376 years. This may be a return of the comet of 1668, but many others have 
also been supposed to be identical with it. (See Cooper's Cometic Orbits, pp. 
162-9.) 

206. Usually known as Fayes comet. It had a very small tail. Period, 7'44 
years. 

M m 



530 



Comets. 



[BOOK IV. 



No. 


No. 


Year. 


PP. 


IT 


S3 


< 


9 


207 


(53?) 


1844 i. 


d. h. 
Sept. a ii 


/ 

343 30 


o / 

63 49 


O I 

* 54 


1-1864 


208 


172 


ii. 


Oct. 17 8 


1 80 24 


3i 39 


48 36 


0-8553 


209 


173 


iii. 


Dec. 13 1 6 


296 o 


118 23 


45 36 


0-2512 


2IO 


174 


1845 i. 


Jan. 8 3 


91 19 


336 44 


46 so 


0-9051 


211 


175 


ii. 


April 21 o 


I9 2 33 


347 6 


56 23 


1-2546 


212 


(44) 


iii. 


June 5 i 6 


262 i 


337 48 


48 41 


0-4016 


213 


(105) 


iv. 


Aug. 9 15 


*57 44 


334 19 


13 7 


0-3381 


214 


176 


1846 i. 


Jan. 22 2 


89 6 


in 8 


47 26 


1-4807 


215 


(92) 


ii. 


Feb. 10 23 


IO9 2 


245 54 


12 34 


0-8564 


216 


177 


iii. 


Feb. 25 7 


116 28 


102 37 


30 57 


0-6500 


217 


l?8 


iv. 


March 512 


90 27 


77 33 


85 6 


0-6637 


218 


179 


v. 


May 27 21 


82 32 


161 18 


57 35 


1-3/62 


219 


180 


vi. 


June i 5 


240 7 


260 28 


30 24 


1-5287 


22O 


181 


Vii. 


June 5 12 


162 o 


261 51 


29 18 


0-6334 


221 


182 


viii. 


Oct. 29 I? 


98 35 


4 41 


49 41 


0-8306 


222 


183 


1847 i. 


March 30 6 


276 2 


21 42 


48 39 


0-0425 


223 


184 


ii. 


June 4 i 8 


In I 34 


173 56 


79 34 


2-i 161 


224 


185 


iii. 


Aug. 9 8 


21 I 7 


76 43 


32 38 


1-4847 


22 5 


186 


iv. 


Aug. 9 10 


246 41 


338 i? 


83 27 


1-7671 


226 


187 


v. 


Sept. 9 13 


79 I2 


309 48 


19 8 


0-4879 


22 7 


188 


vi. 


Nov. 14 9 


274 14 


190 50 


/i 53 


0-3291 


228 


189 


1848 i. 


Sept. 8 i 


3io 34 


211 32 


84 24 


0-3199 


229 


(105) 


ii. 


Nov. a 6 2 


157 47 


334 22 


13 8 


0-3370 



207. Visible to the naked eye. An elliptic orbit ; period assigned, 5*469 years. 
It has not been observed since. Possibly identical with the comet of 1678. 

208. Discovered by D'Arrest, July 9. Visible to the naked eye, Nov. 10. Period, 
102,050 years, subject to an uncertainty of 3090 years. 

209. First seen in the southern hemisphere. It had a tail 10 long. 

211. Discovered by Faye, March 6. 

212. Discovered by Richter, June 6. A fine comet. Visible to the naked eye, 
with a tail 2^ long. A return of the comet of 1596. Period, 250 years. 

213. An apparition of Encke's comet. Discovered by Di Vico, July 9, and by 
Coffin, July 10. 

214. An elliptic orbit; period assigned, 2721 years. 

215. An apparition of Bielas comet. Discovered by Galle, Nov. 28. It was at 
this return that the comet separated into 2 parts. 

216. An elliptic orbit ; period assigned, 5-58 years. 



CHAP. VII.] 



Catalogue. No. I. 



531 



6 


M 


Calculator. 


Date of 
Discovery. 


Discoverer. 


Duration 
of Visibility. 


0-61765 


+ 


Briinnow 


1844, Aug. 22 


Di Vico 


19 weeks. 


0-99960 


- 


Plantamour 


July 7 


Mauvais 


35 weeks. 


ro 


+ 


Hind 


Dec. 19 


Wilmot 


12 weeks. 


i-o 


+ 


Gotze 


Dec. 28 


D'Arrest 


13 weeks. 


i-o 


+ 


Faye 


1845, Feb. 25 


Di Vico 


9 weeks. 


0-98987 


- 


D' Arrest 


June 2 


Colla 


4 weeks. 


0-84743 


+ 


Encke 


July 4 


Walker 


IO days. 


0-99240 


+ 


Jelinek 


1846, Jan. 24 


Walker 


14 weeks. 


0-75700 


+ 


Plantamour 


1845, Nov. 26 


Walker 


21 weeks. 


0-79446 


+ 


Hind 


1846, Feb. 26 


Brorsen 


8 weeks. 


0-96224 


+ 


Peirce 


Feb. 20 


Di Vico 


10 weeks. 


rb 


- 


Argelander 


July 29 


Di Vico 


II weeks. 


0-72133 


- . 


C. H. Peters 


June 26 


C. H. Peters 


4 weeks. 


0-98836 


- 


Wichmann 


April 30 


Brorsen 


6 weeks. 


i-o 


+ 


Hind 


Sept. 23 


Di Vico 


3 weeks. 


0.99991 


+ 


Hornstein 


1847, Feb. 6 


Hind 


1 1 weeks. 


I'O 


- 


Von Littrow 


May 7 


Colla 


30 weeks. 


I'O 


- 


Schweizer 


Aug. 31 


Schweizer 


13 weeks. 


I'O 


- 


Von Littrow 


July 4 


Mauvais 


41 weeks. 


0-97256 


+ 


D'Arrest 


July 20 


Brorsen 


8 weeks. 


I'O 


- 


D'Arrest 


Oct. i 


Miss Mitchell 


13 weeks. 


I'O 


- 


Sonntag and 


1848, Aug. 7 


Petersen 


3 weeks. 






Quirling 








0-84782 


+ 


Encke 


Aug. 27 


G. P. Bond 


13 weeks. 



217. Discovered by G. P. Bond, Feb. 26. 

218. Discovered by Hind, 2 hours later. 

219. Discovered by Di Vico, July 2. An elliptic orbit ; period assigned, 12-8 years, 
subject to an uncertainty of i year. 

220. Discovered by Wichmann, May i. Visible to the naked eye, May 14. An 
elliptic orbit ; period assigned, 400 years. 

222. Visible in the daytime. It had a tail i| long. The true elements are 
probably elliptical. Hornstein has throughly discussed the orbit of this comet. 

225. A parabolic orbit best satisfies the observations. 

226. Period assigned, 75 years. 

227. Discovered by Di Vico, Oct. 3 ; by Dawes, Oct. 7 ; and by Madame Riimker, 
Oct. ii. 

229. An apparition of Encke's comet. Discovered by Hind, Sept. 13. Perceptible 
to the naked eye, Oct. 6. On Nov. 3 it had a tail more than i long. 

M m 2 



532 



Comets. 



[BOOK IV. 



No. 


No. 


Year. 


PP. 


IT 


a 


< 


2 


230 


190 


1849 i. 


d. h. 
Jan. 19 8 


/ 

63 II 


215 10 


85 4 


0-9599 


231 


191 


ii. 


May 26 ii 


235 43 


202 33 


67 9 


I-I593 


232 


192 


iii. 


June 8 4 


267 3 


30 31 


66 59 


0-8946 


233 


'93 


1850 i. 


July 23 12 


273 24 


92 53 


68 12 


1-0815 


234 


'94 


ii. 


Oct. 19 8 


89 20 


206 o 


40 6 


0-5647 


235 


071) 


1851 ' 


April 3 ii 


49 42 


209 30 


II 21 


1-6999 


236 


J95 


ii. 


July 9 o 


324 10 


149 19 


I 4 I 4 


1-1847 


237 


196 


iii. 


Aug. 26 5 


31 58 


223 40 


38 9 


0-9843 


238 


197 


iv. 


Sept. 30 19 


338 45 


44 28 


74 o 


0-1410 


239 


(105) 


1852 i. 


March 14 18 


157 51 


334 23 


13 7 


0-3374 


240 


198 


ii. 


April 19 13 


280 o 


317 8 


48 52 


0-9050 


241 


(92) 


iii. 


Sept. 23 i 


109 8 


245 52 


12 33 


0-8606 


243 


199 


iv. 


Oct. 12 15 


43 12 


346 i.l 


40 58 


1-2510 


243 


200 


1853 i- 


Feb. 24 6 


153 21 


69 49 


20 19 


1-0938 


244 


201 


ii. 


May 9 16 


201 53 


40 57 


57 44 


0-9044 


245 


202 


iii. 


Sept. i 17 


31 56 


140 31 


61 31 


0-3068 


246 


203 


iv. 


Oct. 16 14 


302 7 


220 4 


61 i 


0-1725 


?47 


204 


1854 i. 


Jan. 4 6 


55 57 


227 3 


66 7 


I-2OO2 


248 


205 


ii. 


March 24 o 


213 47 


3 J 5 26 


82 22 


0-2770 


249 


(13) 


iv. 


June 22 2 


272 58 


347 48 


71 8 


0-6475 


250 


206 


V. 


Oct. 27 9 


94 20 


324 34 


40 59 


O'SOOI 


i 















230. A parabolic orbit satisfies the observation, but a period of 382,801 years has 
been assigned ! ! ! 

231. It had a small tail. 

232. Discovered a few hours later by Bond, and by Graham April 14. Period, 
8375 years. 

233. Visible to the naked eye, with a tail. Carrington has assigned a period of 
about 29,000 years. 

234. Discovered by Brorsen, Sept. 5 ; by Mauvais and Robertson, Sept. 9 ; and by 
Clausen, Sept. 14. 

235. The first predicted apparition of Faye's comet. 

236. Period, 6^441 years. 

237. Discovered by Schweizer, Aug. 21. Period assigned, 5544 years. 

238. It had a tail more than 1 long, and also a shorter one turned towards the Sun. 

239. An apparition of Enckes comet. 

240. Discovered by Petersen, May 17, and by G. P. Bond, May 19. It was very 
email and faiut. 

241. An apparition of Biela's comet. Theoretical elements. 



CHAP. VII.] 



Catalogue. No. I. 



533 



f 


M 


Calculator. 


Date of 
Discovery. 


Discoverer. 


Duration 
of Visibility. 


I'O 


+ 


Pogson 


1848, Oct. 26 


Petersen 


20 weeks. 


ro 


+ 


Goujon 


1849, April 15 


Goujon 


24 weeks. 


0-99783 


+ 


D'Arrest 


April ii 


Schweizer 


20 weeks. 


ro 


+ 


Villarceau 


1850, May I 


Petersen 


17 weeks. 


ro 


+ 


Reslhiiber 


Aug. 29 


G. P. Bond 


9 weeks. 


0-55501 


+ 


Le Verrier 


Nov. 28 


Challis 


14 weeks. 


0-70001 


+ 


D'Arrest 


1851, June 27 


D'Arrest 


17 weeks. 


0-99685 


+ 


Brorsen 


Aug. i 


Brorsen 


8 weeks. 


ro 


+ 


J. Breen 


Oct. 22 


Brorsen 


4 weeks. 


084767 


+ 


Encke 


1852, Jan. 9 


Hind 


8 weeks. 


ro 





Sonntag 


May 15 


Chacornac 


3 weeks. 


0-75625 


+ 


Santini 


Aug. 25 


Secchi 


5 weeks. 


0-92475 


+ 


Marth 


June 27 


Westphal 


24 weeks. 


ro 


- 


D'Arrest 


1853, March 6 


Secchi 


3 weeks. 


ro 


- 


Bruhns 


April 4 


Schweizer 


10 weeks. 


1-00026 


+ 


Krahl 


June 10 


Klinkerfues 


7 months. 


I'O 


_ 


Bruhns 


Sept. 1 1 


Bruhns 


II weeks. 


ro 


- 


Marth 


Nov. 25 


Van Arsdale 


12 weeks. 


ro 


- 


Hornstein 


1854, March 23 


Many observers 


6 weeks. 


ro 


- 


Bruhns 


June 4 


Klinkerfues 


10 weeks. 


ro 


+ 


Bruhns 


Sept. 1 1 


Klinkerfues 


11 weeks. 



242. Discovered also by C. H. Peters. Visible to the naked eye early in October. 
Period, 70 years. 

243. Discovered by Schweitzer and C. W. Tuttle, March 8, and by Hartwig, 
March 10. Elements resemble those of the comet of 1664. 

244. Visible to the naked eye in the beginning of May, with a tail 3 long. 

245. Visible in the daytime, Aug. 31 to Sept. 4. In the south of Europe, a tail 
15 long was seen. 

246. Perceptible to the naked eye about the middle of the month. Elements 
resemble those of the comet of 1582. 

247. Discovered by Klinkerfues, Dec. 2. 

248. First seen in the south of France, when very conspicuous, with a tail 4 long. 
Elements resemble those of the comet of 1 799 (ii). 

249. Discovered also by Van Arsdale. At the time of the PP it was visible to 
the naked eye. The elements strongly resemble those of the comets of 961 anil 
1558. 

250. Discovered also by several other observers. Probably a return of the comet 
of 1845 (i). 



534 



Comets. 



[BOOK IV. 



No. 


No. 


Year. 


PP. 


IT 


a 


i 


2 


251 


207 


1854 vi. 


d. h. 
Dec. 15 17 


I 

165 9 


/ 

238 7 


o / 

14 9 


1-3575 


252 


208 


1855 i- 


Feb. 5 17 


226 33 


189 40 


51 12 


1-2195 


253 


(22) 


iii. 


May 30 5 


2.37 36 


260 15 


23 7 


0-5678 


254 


(105) 


iv. 


July I 5 


157 53 


334 26 


13 8 


o-337i 


255 


209 


L - v 


Nov. 25 15 


85 21 


52 2 


10 16 


1-2248 


2 5 6 


2IO 


1857 i. 


March 21 8 


74 49 


313 12 


87 57 


0-7721 


257 


(i/7) 


ii. 


March 29 5 


115 4 8 


ioi 53 


29 45 


0-6202 


2 5 8 


211 


iii. 


July 17 23 


49 37 


23 4 


58 59 


0-3675 


259 


212 


iv. 


Aug. 24 o 


21 46 


200 49 


32 46 


0-7427 


260 


213 


V. 


Sept. 30 19 


250 21 


14 46 


56 18 


0-5651 


261 


2I 4 


vi. 


Nov. 19 i 


44 15 


139 18 


37 50 


1-1009 


262 


(195) 


vii. 


Dec. 33 o 


323 3 


148 27 


13 56 


1-1696 


26.? 


(III) 


1858 i. 


Feb. 23 8 


115 29 


268 54 


54 32 


1-0274 


264 


(Ml) 


ii. 


May 2 i 


275 38 


H3 32 


10 48 


0-7689 


265 


215 


iii. 


May 2 23 


200 46 175 4 


19 30 


I-I493 


266 


216 


iv. 


June 5 4 


226 6 


324 21 


80 28 


0-5462 


267 


(171) 


V. 


Sept. 12 14 


49 49 


209 45 


II 21 1-6999 


268 


217 


vi. 


Sept. 29 23 


36 13 


165 19 


63 I 


0-5784 


269 


218 


vii. 


Oct. 12 19 


4 13 


159 45 


21 l6 


1-4270 


270 


(105) 


viii. 


Oct. 1 8 8 


157 57 


334 28 


13 4 


0-3407 


271 


219