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The Planetoids, Between Mars and Jupiter. Comet Showing
a Triple Tail.

THE

SCIENCE- HISTORY
OF THE UNIVERSE

FRANCIS ROLT- WHEELER

Managing Editor

IN TEN VOLUMES

THE CURRENT LITERATURE PUBLISHING COMPANY

NEW YORK

1909

INTRODUCTIONS BY

Professor E. E. Barnard, A.M., Sc.D.,
Yerkes Astronomical Observatory.

Professor Charles Baskerville, Ph.D., F.C.S.
Professor of Chemistry, College of the City of New York.

Director William T. Hornaday, Sc.D.,
President of New York Zoological Society.

Professor Frederick Starr, S.B., S.M., Ph.D.,
Professor of Anthropology, Chicago University.

Professor Cassius J. Keyser, B.S., A.M., Ph.D.,
Adrain Professor of Mathematics, Columbia University

Edward J. Wheeler, A.M., Litt.D.,
Editor of 'Current Literature.'

Professor Hugo Munsterberg, A.B., M.D., Ph.D., LL.D.,
Processor of Psychology, Harvard University.

EDITORIAL BOARD

Vol. I — Waldemar Kaempffert,
'Scientific American/

Vol. II — Harold E. Slade, C.E.

Vol. Ill — George Matthew, A.M.,

Vol. Ill— Professor William J. Moore, M.E.,

Assistant Professor of Mechanical Engineering, Brooklyn

Polytechnic Institute.

Vol. IV — William Allen Hamor,

Research Chemist, Chemistry Department, College of the

City of New York.

Vol. V — Caroline E. Stackpole, A.M.,
Tutor in Biology, Teachers' College, Columbia University.

Vol. VI— Wm. D. Matthew, A.B., Ph.B., A.M., Ph.D.,

Assistant Curator, Vertebrate Paleontology, American

Museum of Natural History.

Vol. VI — Marion E. Latham, A.M.,
Tutor in Botany, Barnard College, Columbia University.

Vol. VII — Francis Rolt-Wheeler, S.T.D.

Vol. VII— Theodore H. Allen, A.M., M.D.

Vol. VIII— L. Leland Locke, A.B., A.M.,
Brooklyn Training School for Teachers.

Vol. VIII— Franz Bellinger, A.M., Ph.D.

Vol. IX— S. J. Woolf.

Vol. IX— Francis Rolt-Wheeler, S.T.D.

Vol. X — Professor Charles Gray Shaw, Ph.D.,
Professor of Ethics and Philosophy, New York University.

Leonard Abbott,
Associate Editor 'Current Literature/

THE

SCIENCE - HISTORY
OF THE UNIVERSE

VOLUME I

ASTRONOMY

By WALDEMAR KAEMPFFERT

INTRODUCTION

By PROFESSOR E. E. BARNARD

Copyright, 1909, by
CURRENT LITERATURE PUBLISHING COMPANY

CONTENTS

Introduction by Professor E. E. Barnard

CHAPTER PAGE

I The Evolution of Astronomical Ideas i

II The Evolution of Observational Methods ii

III The Rise of Astrophysics . . 27

IV Celestial Photography 39

V The Law of Gravitation '. 49

VI Planetary Distances 57

VII Planetary Motions 67

VIII The Solar System 71

IX The Sun 91

X Solar Energy 107

XI Mercury 124

XII Venus 134

XIII The Earth 147

XIV The Moon 163

XV Mars 180

XVI Jupiter 194

XVII Saturn 202

XVIII Uranus and Neptune 210

XIX The Planetoids 216

XX Comets, Meteors and Meteorites 227

XXI The Constellations 257

XXII The Motions of the Stars 270

XXIII Variable and Binary Stars 279

XXIV Nebulae and Star Clusters 292

XXV Cosmogony and Stellar Evolution 307

vii

The Editor desires to express his gratitude to the uni-
versities, the learned societies and the libraries which
have placed their facilities at his disposal in connection
with this work. Especial thanks are due to the Columbia
University libraries, not only for the opportunities af-
forded, but also for the interest shown in forwarding
research work from their collections.

Recognition of courtesy is due to the many publishers
who have granted permission for certain quotations from
their copyrighted volumes, among them being Messrs. D.
Appleton & Co., the Macmillan Co., the S. S. McClure Co.
and The McGraw Publishing Co.

To acknowledge the personal indebtedness to the mem-
bers of the Editorial Board, the Contributors, and all who
have assisted with suggestion and advice would make too
long a list ; but mention should be made of Mr. Edward J.
Wheeler, Litt. D., Editor of Current Literature, to whose
scholarly judgment and discrimination is largely due what
merit may be found herein. F. R.-W.

INTRODUCTION

In the present volume there have been covered in a
comprehensive and popular manner the various depart-
ments of Astronomy. Owing to its treatment in a defi-
nitely historical and descriptive manner, however, it may
be possible to supplement the general review by a few
brief statements of some of the results and problems that
confront us in the actual work of the observational as-
tronomy of to-day.

There is frequently brought before the astronomer the
fact that certain subjects that were apparently exhausted
have proved through the more advanced methods of to-
day, or perhaps by chance, to be veritable mines of dis-
covery, richer by far than had been anticipated in all the
previous investigations. A remarkable illustration of this
fact is the splendid work of Professor Hale at the Solar
Observatory of the Carnegie Institution at Mount Wilson,
California. The Sun had almost been relegated to that
limbo from which nothing new can ever come. With the
exception of Hale's development of the spectroheliograph,
which made possible the continuous photographic study of
the surface of the Sun and of the solar prominences, but
little advance had been made in solar research for a very
long period of time. Even with the new instrument the
ix

x INTRODUCTION

work seemed to be confined to the photography of the
prominences and a few other -features of the Sun that
were already observable visually with the spectroscope.
Before this the Sun was somewhat of a curiosity and but
little new information was had concerning it. It only
became really interesting when a total eclipse was immi-
nent, at which time the corona could be seen and studied.
The spectroheliograph was the first great step in the study
of the Sun. Even though this made possible a continuous
photographic record of the prominences and kindred fea-
tures it could not record the more attenuated and delicate
corona. Indeed, we seem to-day as far as ever from any
sight of this mysterious object without the aid of the
friendly Moon, which for a few minutes at long intervals
hides the Sun and gives us our only view of the corona.

But the great work done by Professor Hale and his
associates at Mount Wilson (which was foreshadowed
by his work at the Yerkes Observatory) in the discovery
of the solar vortices and magnetic fields of sun-spots has
revolutionized the study of that body and opened up new
fields of investigation in this direction that are almost
unlimited.

Mr. Abbot, of the Smithsonian Institution, has also
established a permanent station at Mount Wilson for the
investigation of the solar constant and a general study of
the heat of the Sun. The solar investigations, therefore,
that are going on at Mount Wilson are among the most
important that have ever been undertaken. They are not
only of the highest interest, but may ultimately lead to
important results bearing upon the commercial life of the
world by revealing to us some possible means of forecast-

INTRODUCTION xi

ing conditions upon the Earth. Any vagaries in the Sun
must have more or less direct influence on the conditions
of the Earth which owes its every throb of life to the
mighty influence of the Sun.

Much of the ordinary spectroscopic work may be said
to be in its infancy because of the vast fields of research
that are open to it. It is already laying the foundation for
a very accurate determination of the distance of the
visible binary stars where both stars can be observed with
the spectroscope — an accuracy that can never be attained
by the ordinary methods of parallax work. Already this
has given results of precision in the case of Alpha Cen-
tauri, whose distance has been determined by Professor
Wright, of the Lick Observatory, from spectroscopic ob-
servations combined with the known orbit of the star.
Time, however, is an element in this work, and after a
sufficiently long interval a valuable harvest of knowledge
of star distances will result. The spectroscopic material
for such investigations is being specially obtained by Pro-
fessor Frost and his associates at the Yerkes Observatory
(as well as by others elsewhere), where spectrograms of
the various visual binaries that are bright enough to give
a measurable spectrum are being carefully and accurately
accumulated. A possible improvement of the spectro-
scope, whereby a larger percentage of the light can be
utilized, will make possible the extension of this class of
work, for at least 90 per cent, of the available light cannot
at present be utilized. If this can be done, the efficiency
of the spectroscope will be vastly increased and a great
number of objects at present beyond the reach of accurate
spectroscopic study will be investigated and their nature

xii INTRODUCTION

and physical conditions become known. A step in this
direction is the intended erection on Mount Wilson of a
reflecting telescope one hundred inches in diameter. The
great light-grasping power of this instrument will enable
much fainter objects to be studied than can be observed
with the present means.

Only a few years ago our knowledge of comets seemed
to be satisfactory. What we could see with the naked
eye or with the telescope apparently readily agreed with
certain theories that were formulated to explain them.
The tails of various comets were sorted out and assigned
to different classes. This one was a hydrocarbon tail and
that a hydrogen tail, etc. The spectroscope had shown
that comets in general consisted of some form of hydro-
carbon gas (such as cyanogen). Such gas or gases are
evidently mixed up with minutely divided matter which is
disrupted and expelled from the comet's head and thrown
out backward from the comet away from the Sun. This
was shown later by the experiments of Lebedew, Nichols
and Hull to be due to the pressure of the Sun's light upon
the smaller particles of the comet, which drove them away
into space with increasing velocity to form the tail. The
simple phenomena thus seen by the eye were rather easy
of explanation. Photography, however, has revealed such
a mass of strange phenomena in these bodies that the
theories which seemed so satisfactory before are now
seriously questioned, and some of them appear to be
entirely inadequate to explain some of the phenomena
shown by the photographic plates. But little indication
of many of the most extraordinary changes and peculiari-
ties of comets' tails is seen by the eye. In part this is

INTRODUCTION xiii

due to the fact that much of the light of a comet is of a
nature that has but little effect on the human eye, though
it is peculiarly strong in its action on the photographic
plate. The first of these bodies to exhibit these peculiari-
ties was Comet IV, 1893 (Brooks). Some of the phe-
nomena of its tail, as revealed on the photographs, ap-
peared to defy the ordinary theories and seemed to show-
that an influence outside that of the direct action of the
Sun upon the comet had manifested itself in the distortion
and breaking of the tail. The scarcity of active comets
in the succeeding years left this question in abeyance.
Comet C, 1903 (Borrelly), however, gave us much infor-
mation as to the actual velocity of the outgoing particles
of the tail, some of which receded from the comet at the
rate of 29 miles a second. This object also quite clearly
showed that a seat of force of great activity existed in the
comet itself, which enabled it to shoot out streams of
matter at large angles to the main direction of the tail,
which were apparently not bent or affected by the pres-
sure of the Sun's light. The phenomena of Comet IV,
1893, were repeated in Comet C, 1908 (Morehouse). But
a great amount of new phenomena was also shown by
this last body which demands still greater changes in our
ideas of comets and their tails. This object is so recent
and its phenomena so startling that astronomers have not
yet had time to thoroughly discuss the vast amount of
material that exists for its study. Briefly, added to the
already known rapid changes in the tail of a comet, this
object exhibited the most extraordinary freaks. Tails
were repeatedly formed and discarded to drift out bodily
in space until they finally melted away. In several cases

xiv INTRODUCTION

the tail was twisted or corkscrew shaped, as if it had gone
out in a more or less spiral form. Areas of material con-
nected with the tail would become visible at some distance
from the head, where apparently no supply had reached
it from the nucleus. Several times the matter of the tail
was accelerated perpendicularly to its length. At one
time the entire tail was thrown forward and violently
curved perpendicularly to the radius vector in the general
direction of the sweep of the tail through space. This
peculiarity is opposed to the laws of gravitation. There
is no known cause for this freak of the tail. Evidently
we have here, and in many other of the phenomena of this
body, some unknown influence at work in the planetary
spaces. What this is, is one of the great problems for the
future to solve. It has been suggested that many of the
unaccountable phenomena of this comet are electrical and
can be attributed to the same influence that produces our
magnetic storms and auroras on the Earth, and these are
believed to be due to abnormal disturbances on the Sun.
It is to be hoped that the present return of Halley's comet
will add much to a solution of this problem.

The study of the dark or apparently vacant regions of
the sky, especially in the Milky Way, is of paramount
importance. The photographic plate has shown that the
dark regions (the so-called "coal sacks") are generally
connected with masses of nebulosity or gaseous matter.
These are especially remarkable in the regions of the
stars Theta Ophiuchi and Rho Ophiuchi. In the latter
case we find a magnificent nebula in a rich region of the
Milky Way occupying a hole that is apparently devoid of
stars. Some astronomers have attributed the general

INTRODUCTION xv

absence of stars here to absorbing matter — to an opacity
and partial dying out of the nebula that cuts off the light
of the stars which are beyond it. What these apparent
vacant regions really are is, therefore, an unsolved prob-
lem at present. Some of them are evidently due to the
thinning out and actual absence of stars in those parts of
the sky. But the others, which are connected with nebu-
losities, seemingly must have some other explanation. One
fact that appears to be brought out by the great nebula of
Rho Ophiuchi is that the groundwork of the Milky Way
in this region, and by inference elsewhere, may be made
up of stars actually much smaller than the average of
those seen in the general sky. If this were so it would
materially change our ideas of the Milky Way. This
supposition comes from the fact that the great nebula is
connected with some of the brighter stars in this region,
while at the same time there is apparently evidence that it
is connected with the faint stars that form the ground-
work of the Milky Way here. If, however, the dark
regions about and near the nebula are due to the absorp-
tion of light by an opacity of the nebula, the supposition
as to the relative sizes would not hold, for the nebula in
that case might be very much nearer to us than the Milky
Way. It will be evident that an understanding of the nature
of the dark regions of the Milky Way is of the utmost im-
portance to a proper knowledge of our stellar universe.

The great nebulous regions of the sky that photography
has revealed to us are intimately connected with the
Milky Way. They cover very large regions of the heavens
and must be almost inconceivably great. In no case has
it been possible to determine the exact dimensions of these

xvi INTRODUCTION

wonderful objects, because we do not know their distances.
It is possible, however, by assumptions that are justified
by facts to arrive at some idea of their minimum extent.
If they are no further away than the nearest fixed stars,
and from their evident connection with certain stars we
know that they must be much further away, we can form
some idea of their vastness. Our own Sun if removed
to the distance of the nearest fixed stars would present
an apparent diameter of about the hundredth part of a
second of arc. Its known diameter is something like a
million miles (accurately 867,000 miles). Some of these
nebulous regions are many degrees in diameter. The one
connected with the Pleiades is ten degrees in diameter.
It is certainly connected with the cluster whose distance
is much beyond the nearest fixed stars. From this it will
be readily seen that this great nebulous region must be
at least some four million times greater in diameter than
our Sun, or over one hundred thousand times greater than
the entire diameter of our known solar system. These
are figures that appear to be appallingly great. But they
are only relatively so and only shock us because the facts
are new and we are not yet used to them.

What is the ultimate function of these enormous masses
of gaseous matter that we find lying in space? Are we
sure that they are the primitive matter from which worlds
and systems are finally to be evolved ?

These, very briefly, are a few of the problems that we
encounter in astronomy as developed by the subtle means
of research in use at the present time.

E. E. Barnard.

ASTRONOMY

CHAPTER I

THE EVOLUTION OF ASTRONOMICAL IDEAS

Herbert Spencer has stated that evolution is a change
from the indefinite to the definite, from the incoherent to
the coherent. If any proof of that doctrine were re-
quired, it would assuredly be found in the development of
astronomical conceptions. In this chapter an attempt will
be made to outline in a general way the manner in which
the present theories were evolved from the mysticism of
folk-lore and religion. Some of the matter herein pre-
sented is drawn from Arrhenius' "Die Vorstellung vom
Weltgebaude im Wandel der Zeiten."

The astronomical beliefs of prehistoric man were no
doubt similar to those entertained by the Eskimo of the
Arctic regions and the untutored tribes of Argentine
Republic, South Africa and Australia, tribes who, living
only for the day, concern themselves but little with to-
morrow and yesterday and care nothing about the universe.

Somewhat more cultured than these Eskimo and
South American and South African tribes are primitive
nations who have endeavored to account for the origin of
the Earth and the heavens by anthropomorphic theories.
The universe must have been created by some Personal
Being who had at his disposal something to mold. The
idea that the universe was made out of nothing is a philo-
sophical assumption which was introduced by the highly

2 ASTRONOMY

cultured philosophers of the East. The something out of
which the universe was created is usually regarded as
water, an element which to the eye at least is perfectly
homogeneous, shapeless, and chaotic. That the fertilizing
mud was deposited by floods must have attracted the at-
tention of ancient primitive races, for which reason they
may have assumed that all the Earth was slowly and
gradually deposited from water. Thus we find that Thales
(550 B.C.) argued that all things were created from water.
Yet other substances were assumed as primordial matter,
and later Anaximines of Miletus, who also flourished in
the sixth century, called the generative principle of things
air or breath, while Heraclitus, who flourished at Ephesus
near the end of the sixth century, believed that all bodies
were transformations of one and the same element, which
he called fire.

The belief that primordial water is the origin of all
things was deeply rooted in Asiatic races, for it occurs
over and over again in many creation myths, among others
in the Chaldean and in the Hebrew. Instead of water we
sometimes find that an egg may be taken as the primal
unit, no doubt because every organism springs from an
apparently lifeless seed. Thus we find that the egg plays
a most important part in the creation myths of the Japa-
nese as well as in narratives from India, China, Polynesia,
Finland, Egypt and Phenicia.

In many of these creation myths, of which I. Riem has
collected no fewer than sixty-eight, more or less inde-
pendent of one another, deluges are prominent features.
In nearly all of them it is supposed that after the water
subsided the land was exposed, fertilized and made to
bring forth.

All of these creation myths are interwoven and inter-
connected with religious belief. To the savage mind every-
thing that moves is endowed with a Spirit. Accordingly
primitive man endeavors to propitiate the Spirit by magic,
knowledge of which art is given only to the medicine man

EVOLUTION OF IDEAS 3

or to the priest. In a certain sense, therefore, magic is the
precursor of natural science, and the myths and lore upon
which the practice of magic is based are remotely ante-
cedent to our scientific theories. According to Andrew
Lang, myths are based as much upon primitive science,
resting upon superstition, as upon primitive religious con-
ceptions.

In Maspero's "Histoire Ancienne des Peuples de l'Orient
Classique" we find an account of the Chaldean conception
of the universe. Surrounded on all sides by the ocean, the
Earth rises in the middle like a high mountain whose
summit is covered with snow from which the Euphrates
springs. The Earth is encircled by a high wall, and the
abyss between the Earth and the wall is filled by the ocean.
Beyond it is the abode of the immortals. The wall sup-
ports the vault of the firmament, shaped by Marduk, the
Sun god, out of a hard metal, which shines in the daytime
but which at night is like a blue bell set with stars. In
the morning the Sun enters the vault by an eastern en-
trance and at night makes its exit by a western outlet.
Marduk arranged the year according to the course of the
Sun and divided it into twelve months, each of which
counted three periods of ten days. The year, therefore,
numbered three hundred and sixty days. Every sixth year
a special year was intercalated, so that the year had on an
average three hundred and sixty-five days.

As the lives of the Chaldeans were to a high degree
influenced by a change in the seasons, they laid great stress
upon division of time. In the beginning they probably
based their chronology upon the movements of the Moon,
like many another race. Soon they recognised that the
Sun exerted a stronger influence, and accordingly they in-
troduced a solar year whose divisions they ascribed to
Marduk. The stars were observed because their positions
determined the seasons. Since the seasons govern organic
life, a pernicious belief in the influence of the stars took
root, a belief which prevailed for twenty centuries and

4 ASTRONOMY

which crippled the advance of science up to the time of
Galileo. Diodorus Siculus, a contemporary of Julius
Caesar, describes this astrology in the following words, as
given in a translation by Philemon Holland (1700) :

"Therefore from a long observation of the Stars, and
an exact Knowledge of the motions and influences of every
one of them, wherein they excel all others, they (the
Chaldean astrologers) foretell many things that are to
come to pass.

"They say that the Five Stars which some call Planets,
but they Interpreters, are most worthy of Consideration,
both for their motions and their remarkable influences,
especially that which the Grecians call Saturn. The
brightest of them all, and which often portends many and
great Events, they call Sol, the other Four they name
Mars, Venus, Mercury, and Jupiter, with our own Country
Astrologers. They give the name of Interpreters to these
Stars, because these only by a peculiar Motion do portend
things to come, and instead of Jupiters, do declare to Men
beforehand the good-will of the gods; whereas the other
Stars (not being of the number of the Planets) have a
constant ordinary motion. Future Events (they say) are
pointed at sometimes by their Rising, and sometimes by
their Setting, and at other times by their Colour, as may
be experienced by those that will diligently observe it;
sometimes foreshewing Hurricanes, at other times Tem-
pestuous Rains, and then again exceeding Droughts. By
these, they say, are often portended the appearance of
Comets, Eclipses of the Sun and Moon, Earthquakes and
all other the various Changes and remarkable effects in
the Air, boding good and bad, not only to Nations in gen-
eral, but to Kings and Private Persons in particular.
Under the course of these Planets, they say are Thirty
Stars, which they call Counselling Gods, half of whom
observe what is done under the Earth, and the other half
take notice of the actions of Men upon the Earth, and
what is transacted in the Heavens. Once every Ten Days

EVOLUTION OF IDEAS 5

space (they say) one of the highest Order of these Stars
descends to them that are of the lowest, like a Messenger
sent from them above; and then again another ascends
from those below to them above, and that this is their con-
stant natural motion to continue forever. The chief of
these Gods, they say, are Twelve in number, to each of
which they attribute a Month, and one Sign of the Twelve
in the Zodiack. Through these Twelve Signs the Sun,
Moon, and the other Five Planets run their Course."

The Chaldean priests developed a most perfect astrology.
They mapped out the positions of the stars for every day
with such care that they could tell their true positions for
some time in advance. The different stars either repre-
sented deities or were directly identified with them. If,
therefore, a Chaldean king wished to know which gods
ruled over his destiny, he consulted the priests as to the
position of the stars on his birthday and was informed of
the chief events of his career.

This Chaldean belief that the celestial bodies were gods
transformed astronomy into a religion. Hence astronomi-
cal theories were promulgated only by the ruling priest
caste. To doubt the tenets of that caste was to expose
oneself to merciless persecution, an Oriental trait that
passed over to the nations of classic antiquity and to the
semi-barbarous nations of the Middle Ages.

The Jews appropriated the Chaldean conception of the
universe, but modified it, so that it was transformed from
a polytheistic to a monotheistic conception.

No doubt the Chaldaic accounts of the beginning of the
world influenced Egyptian thought. According to Ma-
spero, the Egyptians believed that matter without form
was shaped by a deity, always a different person in differ-
ent parts of the land and by different methods, into the
world as we see it.

The classic nations borrowed much of Egyptian civiliza-
tion and with it Egyptian religion and science. For, the
Greek creation myth, like all the others, assumes that

6 ASTRONOMY

chaos once existed and that out of it Gaa, the mother of
all things, and her son, Uranos,- the god of heaven, were
created.

The Greek cosmogony was adopted by the Romans with-
out noteworthy development. Hence it is that Ovid wrote
on the origin of the universe much as Hesiod had done
seven hundred years before. In that long interval of
seven centuries the study of nature had advanced but little.
Indeed it was not until the invention of the telescope that
astronomy was lifted entirely out of the hands of the
priesthood and placed upon a sure scientific footing. Be-
fore the invention of the telescope, therefore, astronomy
appears merely in the garb of a myth. At its best it was
metaphysical.

The rudiments of astronomical science are to be found
in the efforts of the Chaldeans, Egyptians and Greeks to
devise calendars and to mark time. That effort neces-
sitated a study of the motions of the celestial bodies.
Moreover, exigencies of husbandry rendered necessary
some method of keeping track of the seasons so that seed
time and harvest could be ascertained. The regular occur-
rence of such events as the Nile flood made requisite suit-
able preparations. Hence the early Egyptians so built
their temples that they might know the time of the summer
solstice and hence the time when the flood might be ex-
pected. This was a matter of practical importance,
not merely connected with religion or priestcraft, but
on which the lives and the happiness of the people of
Egypt depended, and might be compared with the modern
time observations made at the great national observatories.
The observation of the stars was carried on with at least
this object in view, and gradually with the development of
civilization time reckoning from the stars became an im-
portant consideration closely connected with the lives of
the people. With the study of the stars for such a purpose
naturally an amount of information as to their positions
and motions was accumulated, and for centuries the practi-

EVOLUTION OF IDEAS 7

cal side of astronomy was the study of the position of the
stars and the motion of the planets. The astrology of the
Chaldeans spreading westward increased rather than di-
minished the interest in the stars, for not only was the
connection of the planets with natural phenomena and the
mere reckoning of time studied, but the mystical element
involving prophecy of future events attracted attention.
In other words, astrology was a pseudo-science, for which
reason it is difficult to estimate its benefits or to exaggerate
its evils. In its scientific aspect it involved the observa-
tion and record of the position of the heavenly bodies with
all the exactness that the mathematical and observational
methods of the time could achieve. It enabled the motions
of the planets to be studied as well as the positions of the
fixed stars and the course of the Sun as it passed through
them. But, on the other hand, when the interpretation of
the appearance of the skies was involved, superstition and
poetic fancy had full sway, in which no doubt cer-
tain elements of self-interest and deception on the part of
the priests or astrologers were not lacking. Hence these
men did not study the sky to interpret phenomena on a
scientific basis. Confined in the narrow limits of super-
stition, they not only made no progress but actually held
back astronomy as they did other sciences.

That the work of the astrologer was mysterious there
can be no doubt, and as no reason was assigned for the
movement of the planets or the position of the stars, it was
a natural assumption on the part of the people that some
supernatural agency was at work, which also was con-
nected with their lives and their future. With the begin-
ning of the development of scientific astronomical theory
proper the power and position of the astrologers began to
wane — slowly, it is true, for when Tycho Brahe was in-
vited to deliver lectures on astronomy at the University
of Copenhagen, the first dealt very largely with astrol-
ogy. Cardan and Kepler among the distinguished astrono-
mers of the Middle Ages, Roger Bacon, Burton and Sir

8 ASTRONOMY

Thomas Brown were among the men of mind who were
interested, at least in part, in the teachings of the underly-
ing basis of the cult. As explanations of the motions of the
heavenly bodies on a rational basis were forthcoming, the
doom of the astrologer, so far as participation in the scien-
tific creed of the day was concerned, was sealed. If there
was a natural explanation that could be accepted, how
could supernatural influences condition the movements of
the planets or the positions of the stars? If then these
movements were natural and made in obedience to natural
laws, how could they affect the future course of life and
future occurrences that obviously had no connection with
natural phenomena? The law of gravitation, which ex-
plained the solar system and the movement of the planets,
corroborated this view and left only the comets as striking
natural phenomena which could not be explained in a
way that the popular mind could grasp. With the rise of
learning and the growth of observation, the explanations
of natural phenomena by astronomers secured acceptance
by the people. Finally, when Halley's prediction of the
return of his comet, first made in 1705, was verified in 1758,
the reign of natural law in the world of the heavens be-
came an accepted fact, from which only the ignorant or
superstitious could dissent.

Distinctly different and apart from astrological influence
was the work of Copernicus, whose researches mark the
beginning of the new and philosophical science of astron-
omy, in which the element of mysticism was gradually dis-
placed and observation and reasoning were depended on.
Copernicus, as will be seen when the development of the-
ories of the solar system is considered in an early chapter,
returned to many of the fundamental ideas of Pythagoras,
and the early Greek philosophers, especially that the Sun
was the center of the universe. He was a thoughtful
student not only of Greek philosophy but of the work of
such later astronomers as Ptolemy and his successors, so
that when he announced a theory of the solar system in

EVOLUTION OF IDEAS 9

which the Earth and other planets revolved around the Sun
as a center, it was based upon the fullest knowledge of
previous reasoning and theory. Nevertheless he was cast-
ing to one side the tradition and the science of the day as
k was then understood and presenting what was a concep-
tion of the heavenly world no less daring than original.
His theory was a natural outcome of the revival of learn-
ing in the Renaissance, foreshadowed by the work of such
men as Leonardo da Vinci and others, in whom the scien-
tific spark had been awakened. With Copernicus the
evolution of his heliocentric theory was a matter of scien-
tific reasoning rather than of direct observation. But it
marked the beginning of a series of epoch-making dis-
coveries presented in a clear and positive form, so that
the theory of the revolution of the planets around the Sun
became one of the fundamental canons of astronomy.
Thus, as will appear in the course of our history, the
Copernican theory in which the revolution of the planets
around the Sun is made clear, Kepler's theory of planetary
motion in which laws are stated to account for this motion,
and finally, Newton's announcement of the great universal
law of gravitation, are the foundation stones on which
modern astronomical science firmly rests.

The invention of the telescope established the similarity
in the bodies of the solar system and revealed facts that
previously had been hidden from observers of the heavens..
Indeed, with the invention of the telescope and the growth
of mathematical- science, there began an era of de-
scriptional astronomy in which exact observation was com-
bined with careful computation and mathematical analysis,,
an era which continued into the nineteenth century with
undiminished vigor. Brilliant discoveries were made pos-
sible by improved and powerful instruments, accompanied
by theoretical work of even greater value. In the middle of
the nineteenth century new instruments were put at the
command of the scientist which had a remarkable effect in
extending the boundaries of the science. The telescope

io ASTRONOMY

had facilitated merely the observation of the stars. The
spectroscope, on the other hand, enabled the astronomer to
ascertain their composition.

With the application of the spectroscope to astronomy
began the welding of physics and chemistry with as-
tronomy and the birth of that modern science of astro-
physics, which has afforded data for the study of the seri-
ous problems connected with the evolution of the universe.
From the soothsaying star-gazer of Chaldean times to the
modern astrophysicist, who works in a laboratory as well
as in an observatory, we have a development that is respon-
sible for the aggregation of knowledge which we now pos-
sess of the vast universe with its suns, planets, stars and
nebulae.

The spectra of distant celestial bodies recorded on the
photographic plate by the spectrographs of large telescopes
are now studied in comparison with the spectra of ter-
restrial substances produced in the physical laboratory.
Not only the nature and composition of the stars can be
ascertained, but also their motion in space which are be-
yond the range of any telescope. The New Astronomy has
become on its astrophysical side almost an experimental
science with the methods and accuracy of the chemical or
physical laboratory. It is from this modern astronomy,
with its breadth and resourcefulness, that modern science
looks not only for advances in its own particular field, but
in the broader and ever interesting problems of cosmogony
as concerned in the evolution of the stars and other bodies
making up the universe.

CHAPTER II

THE EVOLUTION OF ASTRONOMICAL METHODS OF OBSERVATION

The history of astronomical observation is the history
of man's attempt to bring the stars nearer to him. His
own senses are so feeble and so very subject to error that
he has been constrained to devise subtle artificial senses
which take the place of eyes and hands. Thus early he
invented position-finders, which enabled him to determine
with more or less precision a star's direction or position at
a given time and not merely to guess at that position^
great eyes, called telescopes, that see what his eyes can
never see and also determine positions with greater ac-
curacy ; wonderful spectroscopes that analyze a star's com-
position as nicely as if it were a stone picked up in the
road ; and photographic devices that reveal secrets of star
structure that otherwise would never be disclosed by his
unaided senses.

For determining the position of the heavenly bodies the
instruments used have always been comparatively simple.
All are based on certain rudimentary geometric principles.
As geometry was a science fairly well developed among
the ancients, it is not difficult to realize that they had vari-
ous means of measuring angles, both vertical and horizon-
tal. In most ancient cases, however, the observers have
failed to hand down their methods, merely recording the
results without indicating the circumstances in which they
were obtained, so that it is impossible to discuss the values
of the observations and correct them in the light of recent

12 ASTRONOMY

discoveries. It is evident that the instruments of the
ancients were simple, but their precise nature is altogether
uncertain.

The earliest astronomical observations of which there is
record were made by the Chinese. The Shu King, the
oldest known scientific work, states that two thousand
years before the present era the Chinese determined the
seasons — that is to say, the positions of the Sun at the
equinoxes and solstices — by means of four stars which
have since been identified and found to be so suitable that
a modern astronomer could not have made a better choice.
The Chinese also determined, eleven hundred years before
the present era, the obliquity of the ecliptic, which they
found equal to 23 deg. 54 min. The obliquity, which
varies, is now 23 deg. 37 min., and calculation shows that
at the epoch of the Chinese observations it must have been
23 deg. 51 min. Hence the error Of the Chinese determina-
tion was only three minutes of arc.

Among the few astronomical values which have re-
mained constant during the history of man are the times
of revolution of the planets. The Hindus determined the
revolution of Mercury with an error of i/10000 of a day.
For Venus the error was 2S/ 10000 of a day, for Mars
Vicoo of a daY- In the case of Jupiter the error amounts
to one-quarter of a day, but it is to be remembered that
the period of revolution of this planet exceeds 11 years, so
that the same observer could not observe many returns of
the planet to the same point of its orbit. This comment
applies with still greater force to Saturn, the revolution of
which occupies 29 years. Hence it is not astonishing that
in this case the Hindus were six days in error.

Among the ancient Greeks is a measurement of a ter-
restrial meridian made about 200 b.c. by Eratosthenes
(276 B.C. to 195 or 196 B.C.), who found the circumference
of the Earth equal to 250,000 stadia by measuring the an-
gular distance of the Sun from the zenith at the summer
solstice both at Alexandria and at Syene in Upper Egypt

METHODS OF OBSERVATION 13

by means of the length of shadow cast by a vertical pillar
at noon at each place. According to the researches of
Tannery, the stadium as an astronomical unit equals 157.5
meters (516.7 feet), which gives for the Earth's cir-
cumference a length of 39,690 kilometers (24,662 miles)
instead of 40,000 kilometers (24,855 miles) as we know it.
Here the precision is remarkable, especially when it is
remembered that the measurement was effected by count-
ing the paces contained in an arc of the meridian and by
multiplying the number so found by the length of a pace.

The instruments most frequently employed by early
astronomers were divided circles and compasses with sim-
ple sights which allowed the line of vision to be directed
to the star under observation and its direction as com-
pared with some other line of sight to be measured.
Ptolemy's ring or astrolabe, for example, described in the
fifth book of his Almagest, and used to identify the relative
positions of the stars and planets, was composed of two
concentric vertical circles. The outer circle, about 16
inches in diameter, was fixed and graduated. It supported
the interior ring, which was movable and carried the two
sights. There was also a geometric square which was
used in a manner analogous tc that of a table of logarithms.
Various forms of apparatus for the measurement of hori-
zontal and vertical angles were early evolved, and as the
study of the heavenly bodies developed to a point where it
was useful in navigation, the cross-staff or back-staff was
invented, consisting of simple sighting bars with cross-
pieces suitable for the calculation and measurement of
such angles as the heights of the heavenly bodies above
the horizon and their distance from one another. Quad-
rants of one form or another, with a sighting bar and
divided circular scale, and astrolabes, or celestial circles,
also for the direct measurement of angles, were employed.
Many of these, by the Middle Ages, were examples of ac-
curacy of division.

A quadrant designed by Tycho Brahe (1 546-1601), for

14 ASTRONOMY

example, was of 19 feet radius and had its circumference
graduated to single minutes. Various forms of armillary
spheres were constructed in which the stars were placed in
their relative positions on great circles of the celestial
sphere. Such devices served for much of the early astro-
nomical work, taking the place of modern star charts.

Tycho Brahe, like his predecessors, employed wooden
instruments. One of these was a large Ptolemy's ring,
surmounted by a post carrying horizontal arms, by which
it was turned in bearings like a capstan, so that the ring
could be brought into any vertical plane. Tycho Brahe
also constructed a mural circle, by means of which vertical
angles could be measured. Hence it was by using the
naked eye and rudimentary instruments that he accumu-
lated observations of such precision that they served
Kepler as the basis of the researches which led to the dis-
covery of the laws of planetary movement.

The eye can distinguish an object whose diameter is
equal to about x/>000 of its distance, which corresponds
with an angular diameter of about one minute of arc.
This was the measure of the precision of early observa-
tions. Its value may be appreciated by stating that it cor-
responds with the diameter of a lead-pencil seen at a dis-
tance of 70 feet. The telescope, by increasing the distance
at which objects can be distinguished, therefore has been
and is now the chief reliance of the astronomer in deter-
mining position. While the naked eye to-day may be said
to have been very largely supplanted by spectroscopic and
photographic observation, yet the telescope has constantly
met the demands of astronomers as its power has increased
and its scope widened.

By chance or otherwise it was found by a Dutch spec-
tacle maker, Lippershey, about 1608, that two lenses when
placed at some distance apart would act to magnify distant
objects, just as a single lens would enlarge the image of a
near-by object. This action of the lens can be explained
by considering the effect on a prism of transparent material

METHODS OF OBSERVATION 15

placed in the path of a beam of light. When a beam of
light falls on one of the angular faces of the prism at a
direction other than perpendicular to the face it is forced
to change its direction on account of refraction, due to the
change in medium. That is, a ray of light passing ob-
liquely through air into a denser medium, such as glass, is
bent toward the perpendicular, and in passing out from a
denser to a rarer medium is bent away from the perpen-
dicular. A lens may be considered as a collection of
prisms of constantly changing angles, so that the effect
would be to bend parallel rays coming from a point at
infinite distance in such a way that they would all be
brought to a single point known as the focus. Conse-
quently a telescope may be regarded as a light-gatherer.

The importance to astronomy of Lippershey's invention
can be appreciated from the fact that as soon as Galileo
heard of it he constructed such an instrument which,
hardly the size of a small toy spyglass, magnified three
times, or brought the heavenly bodies three times as near.
He applied it to celestial observation in 1609.

The value of the telescope as an astronomical instrument
became apparent immediately. It was from the use of his
"optik tube," as he called it, that Galileo arrived at the
conclusion that Ptolemy was wrong and Copernicus right
— how will become apparent from a consideration of the
discoveries made by Galileo. He did more than this, how-
ever; for by the application of the telescope to the observa-
tion of the stars he became in truth the founder of our
modern science of astrophysics.

Galileo saw hosts of stars never before revealed to the
unaided eye. The six stars in the Pleiades now appeared
as 36, and various nebulous objects of light, such as the
Milky Way, were found to consist of multitudes of fine
stars clustered together. But his crowning achievement
occurred on January 7, 1610, when in turning his telescope
toward Jupiter he discovered four satellites of that planet
and determined that their periods of revolution around*

16 ASTRONOMY

Jupiter ranged from about forty-two hours to sev-
enteen days. Here was a miniature system simi-
lar to that conceived by Copernicus. Was it any won-
der that Galileo abandoned the Ptolemaic teaching? Thus
Galileo was able to strike a serious blow at the infallibility
of Aristotle and Ptolemy, by whom no mention had been
made of the existence of such extra bodies. At this time,
however, others besides Galileo were working with the
telescope, among them Thomas Harriott (1560-1621) in
England, Simon Marius (1570-1624) and Christopher
Scheiner (1575-1650) in Germany. Thenceforth observa-
tional astronomy with the telescope was anchored on a firm
basis.

As was quite natural, telescopes eventually formed an
important part of the equipment of the observatory of
Tycho Brahe and of John Kepler (1571-1630). In one of
Kepler's works on "Optics" is contained a suggestion for
the use of a convex lens for an eye-piece in the construc-
tion of the telescope. Galileo's instrument consisted of a
lead tube containing a large double-convex lens, which
served as an objective, and a small double-concave lens at
the eye-end in order to give an erect image — an arrange-
ment which finds its counterpart in the modern opera
glass.

Kepler's suggested improvement provided a more effi-
cient and fairly modern astronomical telescope. The
actual construction of an instrument of this type, however,
is credited to Scheiner rather than Kepler, who was not-
ably deficient in mechanical skill. After considerable ex-
perimenting by various astronomers and instrument mak-
ers, it was found that a comparatively small objective with
a considerable focal length was most useful and effective.
In 1672 Capani, of Bologna, constructed an instrument of
this kind 136 feet long, while Auzout actually made a tele-
scope 600 feet in length, which, however, failed to work.
These, of course, were skeleton structures not mounted in
tubes. Perhaps the best of them were those of Huygens

METHODS OF OBSERVATION

17

(1629-1695), whose skill in grinding lenses stood him in
such good stead that he was able to construct a telescope
with which he determined the ring of Saturn. Huygens*
telescope had considerable focal length. He placed the
object glass on a tall vertical pole or staff so balanced that

^.^j^^^^-^^^^^-

Fig. 1 — Huygens' Aerial Telescope.

it could be moved in any direction by means of a cord.
The observer on the surface of the Earth was supplied
with an eye-piece which he maintained in a straight line
with the star he was observing by means of a cord.
All these telescopes were "refractors." They were sub-

i8

ASTRONOMY

ject to certain inherent defects, chief among which was the
difficulty of bringing to a single focus all the rays of differ-
ent colors. The seventeenth-century philosophers believed
it impossible to overcome the unequal refrangibility of the
different colored rays of light which produced "chromatic
aberration" and resulted in an image indistinct for the
blurring of various colors. Accordingly they gave up the

r7

i

JFocu*

k

il

To the
Eye

Fig. 2 — Refracting Telescope.

fo<»% i | :

HUA

BiB

idea of perfecting the refracting telescope and directed
their attention to constructing an instrument on a different
principle, using a concave mirror to form the image of the
object observed. Mersenne, in 1639, suggested the employ-
ment of a spherical mirror, but the idea appears to have
been dropped. Quite independently, James Gregory, in
1663, proposed a similar arrangement, using, however, a
parabolic in place of a spherical mirror. At that time he
could not find a workman able to construct such a mirror.

METHODS OF OBSERVATION 19

In the Gregorian instruments the parabolic reflector is
placed at the lower end of the tube, while on its axis and
a short distance beyond its focus is placed a small concave
reflector. The light from the distant object falls upon
the large mirror, fromv which it is reflected back to the
small one, which throws it back through a hole in the cen-
ter of the large reflector. It then passes into the eye-
piece, which, indeed, in Gregory's time had been much
improved by Huygens.

Gregory's efforts turned Newton's attention to reflecting
telescopes. In 1669 he cast his first disk and began to grind
it, but it was not until 1672 that he had real success. Then
he made two small instruments, one of which was only
' about an inch in diameter, with a magnifying power of
about 38. The principle of Newton's telescope differed
from that of Gregory's in that it had a small plane mirror
placed in the cone of light from the reflector at an angle
of 45 degrees. Being placed inside the focus, this mirror
brought the light cone at right angles to its original direc-
tion, thus forming the image outside the tube and obviating
the necessity of a hole in the parabolic reflector.

About the same time that Newton completed his instru-
ment the Cassegrain construction was proposed, which was
similar in construction to the Gregorian telescope, except
for the small mirror. In the Gregorian this mirror was
concave. Cassegrain (1672) proposed the use of a con-
cave mirror inside the focus. It brought the light from
the object to a focus through a hole in the center of the
large parabolic mirror.

Owing to the difficulty in obtaining a suitable mirror
alloy, little progress was made for some time in construct-
ing reflecting telescopes. In 1718, however, Hadley, the
inventor of the sextant, constructed one on the Newtonian
principle, 5 feet in length. The instrument magnified over4
200 times and revealed as much as the old refracting tele-
scopes. Perfect as this Newtonian telescope seemed to be,
the Gregorian type held the field until 1774.

20 ASTRONOMY

By using a small Gregorian telescope Herschel had his
attention directed to the wonders of astronomy. His in-
come being too limited to purchase an instrument, he set
about making one for himself. During his life he is said
to have made upward of 400 telescopes, mostly of the New-
tonian type. Among his earliest efforts was the construc-
tion of a 5-foot reflector, which was a wonderful success.
Then came one 7 feet in length. The largest of his instru-
ments was completed under George III. in 1789. This
telescope surpassed all his previous efforts, for it was
actually 40 feet long and had a reflecting mirror 4 feet in
diameter. The story of Herschel's work with this great
telescope would fill a volume.

The largest telescope of the reflecting type was con-
structed by Lord Rosse, an Irish peer, and used at Par-
sonstown. It had mirror of 54 feet focus and a diameter
of 6 feet, but it could be used only for observations on or
near the meridian. While out of use for many years, it
long held the record for size, which, however, is now
taken by the 100-inch reflector recently completed for the
Mount Wilson Solar Observatory, California.

The case of the refracting telescope, which as we have
seen had been all but abandoned on account of its chro-
matic aberration by the seventeenth-century astronomers
and physicists, was not as hopeless as they believed. Not-
withstanding Newton's dictum that it was useless to try
to improve it, owing to the impossibility of producing re-
fraction without dispersion, Euler read a paper before the
Berlin Academy in 1747 proving mathematically the pos-
sibility of correcting both the spherical and chromatic
aberration of an object glass. Upon reading Klingen-
stierna's paper corroborating Euler's views, John Dollond
made a series of most valuable experiments which led him
to the solution of the problem of the achromatic object
glass — namely, that by properly combining two kinds of
glass, flint and crown, he could unite the colored rays fairly

METHODS OF OBSERVATION 21

well and still have refraction to unite the incident rays to
form an image.

Dollond's discovery occurred in 1758; his work soon
became famous. He was surely master of his subject and
had a clear field for many years. Like other opticians, he
labored under great difficulties in securing glass suitable
for telescopes of any diameter. Fortunately a genius had
taken hold of this problem in the person of Guinand, a
Swiss watchmaker, who, after long experimenting, solved
the problem of making fine disks of optical glass. He as-
sociated himself with the celebrated Fraunhofer in 1805,
and they successfully made optical glass disks up to fifteen
inches aperture. To Fraunhofer are due many of the most
important discoveries in the theory of the achromatic ob-
jective.

With proper optical glass and methods of correction the
refracting telescope soon came into its own. The size of
the objectives was increased so that sufficient amounts of
light were gathered to form a distinct image. The best
makers of Europe gradually developed both lenses and
mountings so that precision of measurement and ease of
adjustment were secured. It was in the United States that
the best work in this field began to be carried on. The
lenses of Alvan Clark gained an international reputation.
An objective 30 inches in diameter was made by him for
the Russian Observatory at Pulkova soon after a 26-inch
telescope had been completed for the U. S. Naval Observa-
tory in Washington. These were succeeded by the 36-inch
instrument of the Lick Observatory and the 40-inch tele-
scope of the Yerkes Observatory, with both of which re-
sults in proportion to their increased size have been
obtained.

The seventeenth century really marks the beginning of
instrumental work and accurate measurements in as-
tronomy. The vernier, which made it possible to sub-
divide linear and circular scales with accuracy, made its
appearance in 163 1. In 1640 the optical axis or line of

22 ASTRONOMY

direction of the telescope was practically defined, and the
micrometer was invented by William Gascoigne (1612-
1644), which was the forerunner of the filar micrometer,
so essential to modern astronomy, where an image at the
focus of a telescope can be measured.

The micrometer is indeed an important adjunct to the
telescope, for, unless angular distances can be measured,
the mere bringing nearer of the celestial bodies would
have but a limited amount of usefulness. In the microme-
ter of William Gascoigne two pointers carried by a single
screw were placed at the focus of a telescope. When these
pointers were parallel they pointed to zero ; but, by revolv-
ing the screw, they could be separated and the number of
revolutions or parts of a revolution could be read from a
divided head. Consequently all that it was necessary to
know was the distance between two successive threads of
the screw in order to obtain an exact value for any dis-
tance which the pointers might separate. Now if it were
desirable to determine the angular distance between two
stars, each pointer was set on a star and the distance
between them was thus gradually measured, so that by sim-
ple mathematics the corresponding angular distance could
be computed.

Micrometers soon became an important part of exact
observation with a telescope. Auzout and Picard made
subsequent improvements, so that finally a micrometer
resulted in which a spider filament was placed on a frame
moved by a screw with graduated head, thus enabling in-
creased precision of observation to be obtained. This is
the fundamental device now used with various improve-
ments and refinements. Roemer, who was the first to
determine the velocity of light, improved the micrometer
in 1672 by adding springs to take up the lost motion. He
also constructed the first meridian telescope in 1689. By.
the middle of the seventeenth century the use of telescopic
sights for determining the position of the stars had become
established. The precision of the observations of that

METHODS OF OBSERVATION 23

epoch may be estimated at 10 seconds of arc, which corre-
sponds to the diameter of a lead pencil seen at a distance
of about 550 feet.

Methods and instruments continued to improve. The
observations of Lalande attained a limit of precision of one
second of arc, corresponding to a pencil at 5,500 feet.
At the beginning of the nineteenth century great improve-
ments were made. In 1875 the limit of precision had been
reduced to one-half a second, which removes the lead pen-
cil to 11,000 feet or more than 2 miles.

For minute measurements one of the most useful devices
has been the heliometer or divided object glass micrometer,
the first really available type of which was constructed by
Fraunhofer for the observatory at Konigsberg. In this
instrument an object glass or lens is used which is divided
along its diameter. The two parts of the glass are mounted
so that they can be moved laterally with respect to each
other. Consequently each half supplies a distinct image
of the same object, but separated by a strictly measurable
amount. Thus, if a double star is under examination, the
two half lenses into which the object glass is divided can
be moved until the upper star in one image is brought into
coincidence with the other star in the lower iamge, so that
the distance apart becomes known by the amount of motion
employed. By using screws with heads of considerable
size to move the halves of the object glass, the heliometer
can be read to the thousandth part of a revolution, and
in the case of the Konigsberg instrument such a division,
equivalent to 1/20 of a second of arc, could be measured
with accuracy. This new instrument, which was not
mounted until 1829, three years after the death of Fraun-
hofer, was at once employed by Bessel to solve the problem
of star distances. His measurement of the parallax of the
star known as "61 Cygni," corresponding with a distance
about 600 times that of the Earth from the Sun, not only
was considered ascertained beyond question, but is spoken
of by Miss Clerke as "memorable as the first published

24 ASTRONOMY

instance of the fathom line so industriously thrown into
celestial space having really and indubitably touched bot-
tom." In 1874 the heliometer was" applied to the observa-
tion of the transit of Venus, and again in 1877, when Mars
came into opposition with the Sun, Sir David Gill, using
the heliometer, made a valuable determination of the solar
parallax, obtaining a value of 8.78 seconds, corresponding
with a distance of 93,080,000 miles. By this time the heli-
ometer had become an accepted method for improving as-
tronomers' knowledge of the Sun's distance. A number of
heliometers were employed in cooperation at different
points of the Earth's surface, the work of Professor Elkins
at Yale in connection with Sir David Gill at the Cape of
Good Hope Observatory being especially notable.

Another modern development of telescopic astronomy
has been the direct measurement of the magnitude and
brightness of a star, thus superseding to a great degree the
judgment of the eye upon which the older astronomers had
depended from the days of Hipparchus. The photometers
(light measurers) used with telescopes for this purpose
consist either of those designed to cut down the amount of
lijht furnished by a measurable amount and thus cause
the star to disappear, or those in which conditions are so
arranged that the light of the star appears just equal to
some standard light. Under the first head is the so-called
"cat's eye," in which a wedge of dark neutral tinted glass
is placed close to the eye, either at the eyeball of the eye-
piece or at the principal focus where the micrometer wires
are usually placed. As the wedge is introduced until the
star just disappears the graduation is read, which gradu-
ation can be reduced to a scale of magnitudes. In the
other class of photometers the light of the star is compared
with an arbitrary artificial star formed by light from an
oil lamp shining through a small aperture.

To Huygens is due the application in 1655 of the pen-
dulum to the practical measurement of time, thus giving
us a clock so regulated that it was possible to make ac-

METHODS OF OBSERVATION 25

citrate time observations. The invention of the pendulum
clock, patented in 1657, therefore marks a distinct epoch in
astronomy.

The most usual and most useful form of mounting for a
telescope is the equatorial, the principal axis of which is
inclined at an angle equal to the latitude of the observatory
and is directed toward the North Pole in the Northern
Hemisphere and toward the South Pole in the Southern
Hemisphere. The axis of the instrument is thus parallel
to the Earth's axis of rotation and is therefore called the
polar axis. It carries a graduated circle which is parallel
to the celestial equator, known as the hour circle, from
which circle may be read the hour angle of the body
upon which the telescope happens to be pointed. The polar
axis also carries the bearings of the declination axis, which
is perpendicular to the polar axis and carries the telescope
itself and the declination circle. When the equatorial tele-
scope is directed toward a star or a planet it is necessary
only to use clock-work machinery to cause the polar axis of
the instrument to turn with a uniform motion in order to
follow any star or planet which otherwise would soon be
carried out of the field of view by the rotation of the Earth.
The equatorial also enables the observer to look at once to
a particular part of the heavens where a given body is
expected to be at a given time.

The mounting for a modern equatorial telescope requires
large and heavy moving parts. Where a solar or stellar
image is desired it does not seem desirable to employ such
a heavy mechanism. To Leon Foucault, about 1868, the
idea occurred to construct a fixed telescope with a mirror,
moving with one-half the angular velocity of the Sun,
deflecting a beam in a fixed direction. Such an instrument
was constructed and was employed with good results, altho
its operation was marred by imperfections in its driving
mechanism. However, the device did not attract much
attention until plans were made in the eclipse expedition
of 1890 for extensive photographs of the phenomenon. It

26 ASTRONOMY

was proposed to use the instrument in connection with a
second mirror to produce an image which would not move.
This device, now called a "ccelostat," was found admirable
for eclipse photography. Experiments were made at the
Yerkes Observatory to construct such an instrument for
solar work. The work was subsequently transferred to the
Carnegie Institution Observatory on Mount Wilson. An
extension of the same principle may be found in the tower
telescope of that institution, where a ccelostat is mounted
on the top of a skeleton tower and a beam of light is re-
flected to a laboratory beneath. To-day the most modern
and efficient reflecting telescope of large size is the ioo-
inch instrument designed for the same observatory by
Prof. G. W. Ritchey.

At the present time both refracting and reflecting tele-
scopes are in use and have been brought to a great degree
of perfection. Just which is the better it would be hard to
say. The old speculum metal reflector has been almost dis-
carded and glass, coated with silver, has been substituted.
The glass is much superior to the metal, as it can be figured
more accurately, and if tarnished the silver can be removed
without changing the figure of the mirror.

Again, much study has been given in France to a form
of telescope known as the equatorial coude, in which the
optical axis of the telescope is parallel to the axis of the
Earth and the light of the star is reflected into it by two
mirrors. Such an instrument, constructed for the Paris
Observatory, has been very convenient for the astronomer
who can sit in his chair and observe the stars as easily as
he can use his microscope. But the loss of light and defini-
tion by the double reflection, as well as the deflection of
the mirrors and the varying temperatures to which the
different parts of the instrument are subjected, render this
construction far from perfect.

CHAPTER III

THE EVOLUTION OF ASTRONOMICAL INSTRUMENTS AND

METHODS THE RISE OF ASTROPHYSICS THE SPECTROSCOPE

AND ITS MODIFICATIONS

To gain a knowledge of the composition and nature of
the celestial bodies is the fundamental problem of astron-
omy. Unable to bring a celestial body or a specimen from
it, except in the rare case of a meteorite, to the chemical
laboratory for study, the astronomer is dependent entirely
on a study of the energy that it emits in the form of light
and heat rays. Strange as it may seem, these rays furnish
as true a record of their birth and life history as if a sam-
ple from the distant star had been tested in the assay fur-
nace or with the reagents of the chemist. The simple
instrument called a spectroscope gives an accurate and
permanent record which affords complete data for the
studies of the astronomer.

White light is composed of various forms of vibration
which, taken by themselves, will supply light of various
colors from red to violet. It was found by Sir Isaac New-
ton in passing a beam of white light through a prism that
not all of the rays are bent equally toward and away from
the perpendicular, but that the amount of bending de-
pended upon their color, or as it is now termed, their wave
length and position in the solar spectrum. Thus, when he
permitted a beam of white light, emerging from a hole in
a shutter, to fall upon a prism in a dark room, he found
that there was produced after its emergence a brightly col-
27

28 ASTRONOMY

ored band with the red at one end, where the waves were
refracted the least, and shading through yellow and green
to violet, where the waves were bent or refracted most.
Consequently, if there were a source of light capable of
furnishing one color and that only, it would be obvious,
wherever that color appears in the bright band produced
by the prism, that it radiated from a particular source.

Before 1753 a young Scotchman, Thomas Melvill, no-
ticed that when various compounds of sodium were intro-
duced into an alcohol flame and viewed through a prism

Fig. 3 — Dispersion of Light by the Prism.

there appeared a particular shade of yellow light, which
was always bent or refracted to a fixed and definite de-
gree. Others repeated these experiments, and finally
Fraunhofer (1787-1826), a great optician of Munich, re-
discovered this deep yellow ray and found its place in the
spectrum. The same phenomenon was noticed by many
other experimenters. Indeed, the omnipresence of the
yellow light was often an embarrassment in spectral re-
search. That this yellow line was due to sodium was
pointed out by William Swan. Finally, it was noted that
the distribution of sodium was so general and the prism
test of its presence so delicate that its absolute exclu-
sion was well nigh impossible.

RISE OF ASTROPHYSICS

29

Before Fraunhofer's experiments, the round hole in the
shutter of Newton had been supplanted by a slit or crevice
about one-twentieth of an inch wide, and the spectral band
thus formed from sunlight was not only continuous but
free from overlapping images, so that the colors were
shown in their purity, crossed by seven dark lines. In
the course of his experiments Fraunhofer not only used
the slit, but added to it the telescope of the modern spec-
troscope. He was surprised to find not merely seven but

Fig. 4 — Newton's Diagram.
Showing spectral band from blue (B.) to red (R.) formed by a
prism from a beam of sunlight coming from the round hole
in a shutter of a darkened room.

thousands of dark transverse lines, many of which he
mapped, counted and designated by the letters of the al-
phabet.

Not only did he examine sunlight in this way, but also
the light of the Moon and planets, and found that stellar
spectra, too, were crossed by the same dark lines. In the
case of certain stars there were even dark bands. He
found that one or rather a pair of solar lines which he
had marked in his map with the letter "D" coincided ex-
actly with the yellow beam which accompanied incandes-
cent sodium vapor. The coincidence was noted by Fraun-
hofer, but the explanation came in 1859 from the distin-

30 ASTRONOMY

guished German physicist, Professor Gustav Robert Kir-
choff (1824-1887). He it was who sent a beam of bright
sunshine through sodium vapor and discovered that the
"D" line of Fraunhofer, instead of being effaced by the
flame, was strengthened. The same held true with iron.
The inference was of course drawn that sodium and iron
were constituents of the glowing atmosphere of the Sun
and that light of the particular wave length in passing
through such an atmosphere was absorbed.

This principle has been formulated by Miss Clerke as
follows : "Substances of every kind are opaque to the pre-
cise rays which they emit at the same temperature — that
is to say, they stop the kinds of light or heat which they
are then actually in a condition to radiate. But it does not
follow that cool bodies absorb the rays which they would
give out if sufficiently heated. Hydrogen at ordinary tem-
perature, for instance, is almost perfectly transparent, but
if raised to the glowing point — as by the passage of elec-
tricity— it then becomes capable of arresting, and at the
same time of displaying in its own spectra, light of four
distinct colors." In these few words we have the essence of
spectroscopic chemistry and astrophysics. Materials of
the Earth when heated to incandescence give a bright line
spectrum characteristic of the individual element, but the
same materials in the Sun show a spectrum marked by
dark lines.

While spectrum analysis was applied to chemistry and
terrestrial materials by Bunsen, Kirchoff worked industri-
ously and made a map of the solar spectrum some eight
feet in length, in which the various lines of the ele-
mentary bodies were represented. The spectroscope,
as constructed by Kirchoff, consisted of a slit placed
at the principal focus of the convex lens to make
the rays parallel for their passage through the prism.
In order to secure greater dispersion, several prisms
were added and the emerging beam was passed through
the telescope to form an image of the spectrum. The

RISE OF ASTROPHYSICS 31

new instrument at once presented an enormous num-
ber of lines for study, not only of the Sun, but of various
other celestial bodies. It was soon applied to the observ-
ing end of an astronomical telescope, so that the celestial
image was formed directly at the slit of the spectroscope.
By increasing the number of prisms the dispersion of
the spectroscope can be increased, and a longer spectral
band produced, in which otherwise closely adjacent lines
are increasingly separated. But in passing through a
number of prisms there is considerable loss of light by

Fig. 5 — S., slit of collimating telescope directed toward source of
light, lens of collimating telescope to render parallel rays
from S. ; P., prism ; B., base on which prism stands ; L., lens
of observing telescope ; E., eyepiece of observing telescope.

reflection and absorption, so that a limit is soon set to the
number of prisms employed. Another form of spectro-
scope, which makes use of a grating or a number of fine
lines ruled very closely together on a transparent or a
reflecting surface, has been found to possess greater dis-
solving power without any accompanying loss of light.
In fact, the resolving power of a perfect grating depends
simply upon the total number of lines it contains, so that
the light efficiency per unit area may be as great for a
large grating as for a small one.

The principle of the grating depends upon the interfer-
ence of the various minute light waves caused by a series
of lines, amounting to from ten to twenty thousand to the
inch, ruled on a transparent or a reflecting surface. The

32

ASTRONOMY

mathematical discussion of the formation of the spectra by
the interference of the light waves in passing through or
being reflected from such a grating can hardly find a place
here. The result, however, is essentially the same as in the
case of the prism. As soon as the dispersion of light was
obtained by this means it was found that it could be stud-
ied quantitatively, and that the grating could be used for

Fig. 6 — Kirchoff's Spectroscope.

astronomical measurements with as great facility as the
prism spectroscope.

Lewis M. Rutherfurd of New York was able to make ex-
cellent gratings about 1864, but it remained for Professor
Henry A. Rowland (1 848-1 901) at the Johns Hopkins
University to construct a dividing engine with a screw,
practically free from error, which would move a small
plate of polished speculum metal by regular intervals of
V«ooo of an inch under a diamond point which traced sharp
and regular lines. This machine not only was remarkably

RISE OF ASTROPHYSICS 33

sensitive in its action, but automatically compensated for
any minute irregularities in the screw. It was made to
work at a constant temperature. It automatically pro-
ceeded with its ruling night and day until a grating of the
desired length was completed. Professor Rowland for the
first time ruled gratings on concave surfaces and used
them in place of the prism of the ordinary spectroscope.
The spectra obtained with these diffraction gratings in
conjunction with special lenses were many feet in length
and could be photographed in sections on photographic
plates, each of about 20 inches in length.

The grating spectroscope has been modified by Pro-
fessor A. A. Michelson of the University of Chicago, who
has devised a new form of grating in which a series of
glass plates precisely equal in thickness are placed one on
another like a flight of steps. A parallel beam of light
when transmitted through them is resolved into spectra of
a very high order, exceeding even those of Rowland's
largest gratings, so that compound lines in the spectrum
can be studied with facility.

The application of the spectroscope has made of as-
tronomy an experimental science, with methods and instru-
ments for research and future progress fully as promising
as may be found in any of the physical or natural sci-
ences. The spectroscope has not only amplified astrono-
my, but it has developed the new science of astrophysics,
in which astronomy is combined with physics. New meth-
ods and instruments for research already have brought to
light striking discoveries which have compelled the modi-
fication of older astronomical and cosmical theories.

In connection with the spectroscope it is possible to
measure the temperature of the radiations sent out from
the Sun and the stars with a high degree of accuracy by
means of the bolometer, a sensitive thermometer, invented
by Professor S. P. Langley. It consists of two very fine
threads of platinum wire about 1/2500 of an inch in thick-
ness, mounted side by side within a constant temperature

34 ASTRONOMY

chamber. On one of these wires the radiation is permit-
ted to fall, while the other is carefully shielded. Any
change in the temperature of tte wire on which the light
or heat waves fall produces a difference in its electrical re-
sistance that can be measured with a high degree of pre-
cision, so that a difference of less than one-millionth of a
degree in the temperature can be clearly indicated.

The spectrum formed by the spectroscope is caused to
move slowly across the exposed platinum wire of the
bolometer and a galvanometer in the circuit reflects from
its mirror a spot of light upon a photographic plate, so
that the deflections of the magnetic needle are photo-
graphed and registered, thus indicating the intensity and
energy at the different parts of the spectrum. This in-
strument was first used by Langley to determine the
amount of heat received from the Sun on the top of Mount
Whitney in 1881, and since that time it has been employed
by him and his successors at the Astrophysical Laboratory
at Washington and also at Mount Wilson. The problem
that the bolometer seems capable of solving is to deter-
mine the atmospheric absorption of light and heat in the
passage of the Sun's rays to the Earth. It has also been
used to measure the heat spectrum of the Moon and some
of the brighter stars; in the case of the former showing
that the Moon is very cold, as there is a considerable quan-
tity of heat radiated having a wave length greater than
that of the heat radiated from a block of ice.

After the fundamental work of Kirchoff in identifying
the spectral lines of the Sun and the stars with various
terrestrial materials it was but natural that the compo-
sition of stars as shown in their spectra should be thoroly
attacked by astronomers. Among the first of these was
Sir William Huggins, who devoted the greater part of a
useful scientific life to research of the heavenly bodies,,
especially as revealed by the spectroscope.

In 1862 Huggins, Secchi and Lewis M. Rutherfurd
began their researches in stellar spectra that enabled them

RISE OF ASTROPHYSICS 35

to classify and compare the spectral bands furnished by
the different stars. It was the spectroscope in the hands of
Sir William Huggins that made possible the solution of the
riddle of the nebulae, the nature of which for long years
had been a vital point of discussion among astronomers. On
August 29, 1864, directing his spectroscope to the planetary
nebulae in Draco, Huggins saw, instead of the bright band
he anticipated, a single line which subsequently was re-
solved into three lines. Thus he proved that the nebula
was not an aggregation of stars or incandescent solid ma-
terials, which would have afforded a continuous spectrum
crossed by dark bands and a luminous gas.

The effect of the spectroscope on astronomical research
is thus summarized by Professor Hale : "In astronomy the
introduction of physical methods has revolutionized the ob-
servatory, transforming it from a simple observing station
into a laboratory. The interest of the student of astrophys-
ics is no longer confined simply to celestial phenomena.
For astrophysics has become, in its most modern aspect, al-
most an experimental science, in which some of the funda-
mental problems of physics and chemistry may find their
solution. The stars may be regarded as enormous crucibles,
in some of which terrestrial elements are subjected to
temperatures and pressures far transcending those obtain-
able by artificial means. In the Sun, which appears to us
not merely as a point of light like the stars, but as a vast
globe whose every detail can be studied in its relation-*
ship to the general problem of the solar constitution, the
immense scale of the phenomena always open to observa-
tion, the rapidity of the changes and the enormous masses
of material involved provide the means for researches
which could never be undertaken in terrestrial labora-
tories. Hence it is that astrophysics may equally well be
regarded as a branch of physics or as a branch of astrono-
my."

The great advantage of the spectroscope over the eye or
the direct image from the photographic plate is its ability

36 ASTRONOMY

to analyze the action of light. While the intensity of light
suffers in its journey through space, yet the nature or
character of the light undergoes practically no change,
so that the light from a distant star, separated from the
Earth by an interval that seems to us almost infinite, can
be received in our spectroscope and be resolved into a
spectral band with difficulty but slightly greater than that
which would be found in the employment of a luminous
source of light at the opposite end of the laboratory table.

The spectroscope, therefore, can be used in astronomy
to determine the composition of a distant body according
to the principles of spectrum analysis. But this is not all.
It also enables the determination whether the light from
a luminous body in the heavens is approaching or reced-
ing, and whether the light emitted from such a body is the
same to-day as it was yesterday or a half century ago,
and whether it comes from one or more bodies which the
eye and perhaps the telescope cannot separate, but which
are distinctly separate. Hence the astronomer is only too
glad to remove the eyepiece from his telescope and put in
its place some spectroscopic device which will analyze
the light into separate colors and give him much valuable
information as to the constitution and motions of even the
most distant star.

The value of the spectroscope is greatly increased by the
application of photography. The general nature of a
spectroscopic investigation can best be indicated by ab-
stracting from Professor Hale his description of solar
spectrum analysis: "Sunlight must be reflected from a
mirror to a heliostat (driven by clockwork, to maintain
the beam in a fixed direction) to the slit. Between the slit
and the heliostat a lens is introduced, for the purpose of
forming an image of the Sun upon the slit. When the illu-
mination is secured in this way, the whole grating is filled
with light from the diverging rays. The grating then pro-
duces an image of the solar spectrum upon the photo-

RISE OF ASTROPHYSICS 37

graphic plate, where it may be recorded by giving a suit-
able exposure.

"To facilitate an accurate comparison, the solar spec-
trum is photographed side by side on the same plate with
the spectrum of the substance whose presence in the Sun
is to be determined. In order to accomplish this, one-
half of the slit is covered, and the sunlight is admitted
through the other half. Thus the solar spectrum is photo-
graphed on one side of the plate. After this exposure is
completed, the sunlight is shut off, and the screen in front
of the slit moved so as to cover the half previously open
and to uncover the other half. The image of the Sun on
the slit of the spectroscope is then replaced by an image
of an electric arc light, burning between two poles of iron.
The spectrum of the iron vapor is thus produced on the
plate, and a strip of this spectrum is photographed beside
the strip of solar spectrum.

"The bright lines of iron are represented in the solar
spectrum by corresponding dark lines which accurately
image them in position. In Rowland's work on the solar
spectra thousands of lines were found to correspond with
the iron lines given by the electric arc.

"The same process can be employed to determine the
presence of other substances in the Sun. In the case of
metals, the electric discharge may be caused to pass be-
tween two metallic rods, or fragments of the metal may be
placed in a hole drilled in one of the carbons of an ordi-
nary electric arc-lamp. In the latter case the spectrum of
carbon, and also of impurities which the carbon poles al-
ways contain, will appear on the plate with the spectrum
of the metal in question. But these extra lines may al-
ways be identified, and usually give no trouble. The iden-
tification of the solar lines, however, is not always so sim-
ple as in the case of iron. Many substances are doubtfully
represented in the Sun by only a small number of lines,
and it is sometimes very difficult to decide whether a few
apparent coincidences are sufficient to warrant one in

38 ASTRONOMY

drawing definite conclusions. The matter is usually de-
termined by ascertaining whether certain well-known
groups of lines, which for various reasons are considered
to be especially characteristic of an element, are actually
represented. If these groups are absent, an apparent co-
incidence with certain less characteristic lines belonging to
the same element should be regarded with suspicion. In
the case of gases, the comparison is effected by the aid
of vacuum tubes, in which the gas, usually at low pressure,
is illuminated by an electric discharge. Thus the lines
given by a hydrogen tube in the laboratory have been
shown to coincide in position with lines ascribed to hy-
drogen in the Sun.

"After many years of study of the solar spectrum by
these methods Rowland reached the conclusion that the
chemical composition of the Sun closely resembles that of
the Earth. Certain elements, such as gold and radium,
iodine, sulphur and phosphorus, chlorine and nitrogen,
have not been detected in the Sun. But this does not
prove that they are certainly absent, as their level in the
solar atmosphere, or the low degree of their absorptive
effects, might prevent them from being represented. On
the other hand, various substances not yet found on the
Earth are shown by many unidentified lines of the solar
spectrum to be present in the Sun. Some if not all of
these will probably be discovered by chemists, just as
helium was found by Ramsay in clevite. Rowland re-
marked that if the Earth were heated to a sufficiently high
temperature it would give a spectrum closely resembling
that of the Sun."

CHAPTER IV

THE EVOLUTION OF ASTRONOMICAL INSTRUMENTS AND
METHODS CELESTIAL PHOTOGRAPHY

After the spectroscope, photography has been the most
useful tool of the astronomer, and to its aid must be cred-
ited some of the most important work of the latter half of
the nineteenth century. For the development of celestial
photography as outlined in the present chapter an interest-
ing paper by Professor E. E. Barnard supplies in large
part the material. According to Professor Barnard, the ap-
plication of photography to astronomy may be said to date
from the very first announcement of Daguerre's wonderful
discovery of the production of a permanent image by the
effect of light upon silver salts. "The celebrated French
astronomer, Arago, quickly foresaw its great possibilities,
especially in the faithful delineation of the surface features
of the Sun and the Moon, for these two objects at least
were bright enough to register themselves with the slug-
gish materials then in use." It was of course obvious to as-
tronomers and physicists that the formation of an image
on a sensitized plate was in no way different from that
produced at its focus by the telescope lens and that the
image of a celestial body could be produced as well as any
other.

It was from America that the first practical work came,
and "within less than one year from the announcement of
Daguerre's discovery, in March, 1840, Dr. John W. Dra-
per of New York city succeeded in getting pictures of the
39

40 ASTRONOMY

Moon which, though not very good, foreshadowed the
possibilities of lunar photography. Five years later, at the
Harvard College Observatory, Bond, with the aid of
Messrs. Whipple and Black, of Boston, succeeded in ob-
taining still better pictures of the Moon with the 15-inch
refractor. These pictures on daguerreotype plates aroused
great interest, especially in England. However, the diffi-
culties encountered led to failures generally,, except in the
case of De la Rue, Dancer and one or two others. In
1858 De la Rue, using a 13-inch metal speculum reflect-
ing telescope, without clockwork, and guiding it by follow-
ing a lunar crater seen through a plate, made the most
important of the early efforts at lunar photography." His
photographs were the best until those made in America,
in i860, by Dr. Henry Draper, son of the illustrious John
W. Draper. He secured excellent photographs of the
Moon, superior to any previously made, and capable of
considerable enlargement. These pictures were the best
taken until Lewis M. Rutherfurd began his remarkable
work about 1865. His admirable photographs of the Moon
were made with a refractor of 11-inch aperture, which,
constructed under his immediate supervision, was the first
telescope corrected especially for the photographic rays.

"The completion of the Lick Observatory in 1888 marked
another decided advance in astronomical photography, es-
pecially of the Moon. The great focal length of this mag-
nificent instrument gave an unenlarged image of the Moon
about six inches in diameter, which in itself was a great
advantage." Good results were also secured with the
Yerkes refractor.

Admirable lunar photographs have been made by MM.
Loewy and Puiseux, with the equatorial coude, at Paris,
and have shown the usefulness of this singular instrument
for such work.

"The first picture of the Sun seems to have been made
on a daguerreotype plate by Fizeau and Foucault in 1845,"
says Professor Barnard. "During the total eclipse of

The Great Nebula in Orion.

CELESTIAL PHOTOGRAPHY 41

the Sun on July 28, 185 1, a deguerreotype was secured
with the Konigsberg heliometer (2.4 inches in diameter
and 2 feet focus) by Dr. Busch, which appears to have-
been the first photographic representation of the corona.
It showed considerable detail quite close to the Moon."

But in the early eclipses photographic work seems to
have been devoted mainly to representations of the so-
lar prominences, which at that time were as rarely seen as
the corona itself. "During the eclipse of 1869, however,
Professor Himes secured a photograph which showed the
brighter structure of the corona. Similar pictures were
also obtained during the same eclipse by Mr. Whipple, of
Boston. The corona was also slightly shown on pictures
made as early as i860 by M. Serrat. None of them, how-
ever, showed more than slight traces of the corona, ex-
tending only for a few minutes of arc from the Moon's;
limb. Nearly all the pictures seem to have been taken
with an enlarging lens, which was doubtless used to get
the prominences on a larger scale."

"The first really successful photographs of the corona
were obtained at the eclipse of December 22, 1870, when it
was shown on the plate to a distance of about half a de-
gree from the Moan's limb. This picture, made by Mr.
Brothers, at Syracuse, Sicily, showed a considerable
amount of rich detail in the coronal structure; the same
can also be said of the photographs of this eclipse taken by
Colonel Tennant and Lord Lindsay's party. These seem
to have been the first pictures that really showed the great
value of photography for coronal delineation. The eclipse
of 1871 was still more successfully photographed, and an
excellent representation of the corona, full of beautiful'
detail, was secured."

"In 1878 extensive preparations were made to observe
the eclipse of July 29 of that year. Photography was to play
an important part, though astronomers did not rely very
strongly upon it; for it appears that all were prepared to
make the customary drawings of the corona. Unfortu-

42 ASTRONOMY

nately each person faithfully carried out that purpose.
A most suggestive illustration of the uncertainty of such
work is found in the large collection of drawings pub-
lished in a volume issued by the United States Govern-
ment relating to the eclipse of 1878. An examination of
these forty or fifty pictures shows that scarcely any of
them would be supposed to represent the same object, and
none of them at all closely resembled the photographs.
The method of free-hand drawing of the corona made un-
der the attending conditions of a total eclipse received its
death blow at that time, for it showed the utter inability
of the average astronomer to sketch or draw what he really
saw under such circumstances."

In the eclipses of 1882, 1886 and 1889 photography]
played a part of increasing importance in the observa-
tions. In the latter year there were a large number of
amateur photographers who took advantage of the eclipse
to make many photographs, which, in a number of cases,
were taken in a systematic and scientific manner. At the
Lick Observatory a beginning was made in eclipse photog-
raphy with an extemporized apparatus and successful ex-
posures were made. During the eclipse of 1896 important
work was done in photographing the flash spectrum or the
momentary reversal of the Fraunhofer lines which occurs
when the edge of the. Sun disappears behind the Moon or
reappears from it and for an instant exposes the revers-
ing layer, which was first seen by Professor Young at the
eclipse of 1870. This photograph was made by a young
Englishman, William Shackleton, who, on exposing a plate
at the critical instant of the reversal of the lines, caught
for the first time the fugitive bright lines which are vis-
ible for only about a minute. This gave a permanent vis-
ible record of the phenomenon which removes it from the
class of hasty visual observations, whose results depend
upon the memory of the observer.

The photographing of such a minute point of light as a
star is quite different from that of a luminous or brilliant

CELESTIAL PHOTOGRAPHY 43

body like the Sun or Moon. Yet it was early essayed, and
from the first photograph of a star by Bond in 1850 to the
present time stellar photography has gradually risen to a
prominence as remarkable as it is important. Indeed, it is
now quite indispensabl 1. The principal reason for the real
increase of importance in this work, however, was the suc-
cesful introduction of the very rapid dry plate. The wet
or collodion process, which astronomers soon pushed to its
limits, was poorly adapted to the photography of the stars,
and of no use whatever for comets and nebulae. "Notwith-
standing the inherent difficulties of the wet plate, the pho-
tographs of the star clusters, etc., of the southern skies,
obtained under the direction of Gould with an 11-inch pho-
tographic refractor by the wet process, were of the high-
est value and showed upon measurement a striking agree-
ment in accuracy with visual work. The same can be said
of Rutherfurd's photographs of the Pleiades, Praesepe,
etc., which were made prior to Dr. Gould's, and which
were the first photographs of this kind."

"As early as 1857 Bond had shown, by measurement of
a series of photographs of the double star Mizar, that the
highest confidence could be placed in measures of star
plates. This was subsequently fully verified by Gill, Elkin
and others. As regards absolute accuracy Dr. Elkin
showed in 1889 that measures of a photograph of the
Pleiades taken by Mr. Burnham, with the great telescope
at Mount Hamilton, had equal value with the heliometer
measures of the same stars."

By 1 881 or 1882, however, dry or gelatine emulsion;
plates were beginning to be used with every promise of
their ultimate value, as was shown by the photographs of
the comet of 1881, which were made by Draper and Jans-
sen. These were the first photographs ever made of a
comet. Efforts had been made to secure pictures of Do-
nates comet in 1858, but without success.

It was quite obvious that as soon as satisfactory pho-
tographs of the stars were secured some earnest effort

44 ASTRONOMY

would be made to make use of them in a quantitative
and systematic way. Previously,, for the production of star
maps and catalogues, elaborate series of observations were
made at the various observatories and the positions of the
stars computed and incorporated in large volumes. At the
Royal Observatory at the Cape of Good Hope Sir David
Gill, in 1882, after making some pictures with a large!
camera of the comet of that year, found that not only
did the plate show the stars visible to the naked eye, but
a number as small as the ninth or tenth magnitude. Ac-
cordingly it occurred to him that such photographs fur-
nished a novel and excellent method of cataloguing the
stars and mapping the heavens, as it was necessary only to
measure on the glass negatives the positions of the various
stars and refer them to certain well-known points of ref-
erence. From 1887 to 1891 the entire southern heavens
from 1 8° south declination to the celestial pole were
duly photographed. The half million stars found on the
negatives were then measured and the magnitude of each
determined by Professor J. C. Kapteyn at the Univer-
sity of Groningen, Holland. Thus in 1899 was finished the
Cape photographic "Durchmusterung," which is published
in three quarto volumes and contains the magnitude and
approximate position of every star photographed, the mag-
nitude of the stars on each plate being reduced to a visual
scale.

At the time when Sir David Gill began his photographic
work, Dr. Barnard states, "the Henry brothers of Paris
were making a chart of the stars along the ecliptic in their
search for planetoids. They had at this time reached the
region of the Milky Way, and the marvelous wealth of
stars they encountered on entering the boundaries of that
vast zone completely discouraged them from carrying their
charts through the rich region traversed by the ecliptic.
While hesitating as to the advisability of continuing their
work, the photographs of the great comet came to their
notice. They were struck with the great number of stars

CELESTIAL PHOTOGRAPHY 45

shown on these pictures together with the image of the
comet. The idea at once occurred to them that they could
use this wonderful process to make their charts. They
began at once the construction, with their own hands, of a
suitable photographic telescope of 13^2 inches diameter
for the photography of the stars. This instrument pro-
duced exquisite star pictures, which were marvels of
definition, as well as photographs of the nebulae, of Saturn
and Jupiter, the Moon, etc."

It was the success of the Henry brothers' work that
led to the International Astro-Photographic Congress,
which met at Paris in 1886. This Congress undertook the
organization of an International Commission engaged in
the preparation of a photographic chart and catalogue of
the heavens, and the work since that time has been actively
in progress. Uniform instruments of the same aperture
and focal length are used at the eighteen observatories
participating in this work and two sets of plates are being
made, one to include all the stars that are capable of be-
ing photographed and the other one those of the eleventh
magnitude. With this photographic map astronomers
anywhere can compile their own catalogues, and portions
of such catalogues by various national observatories have
already been issued. The method of preparing the chart
consists in photographing the whole sky upon glass plates
about 8 inches square. Each observatory has had assigned
to it definitely its part of the sky, and about 11,000 plates of
the size specified will be required to complete the task.
Each plate of course carries one or more well determined
catalogue stars, whose position is known with accuracy, so
that from such points of reference it is possible to deter-
mine exactly the position of any other star on the plate.

"The photographic plate not only did away with the ne-
cessity of making the star charts by eye and hand, so essen-
tial to facilitate the discovery of planetoids, but it also did
away with the necessity of the charts themselves for that
purpose. The little planet, which is moving among the

46 ASTRONOMY

stars, now registers its own discovery by leaving a short
trail — its path during the exposure — on the photographic
plate. The first of these photographic discoveries of
planetoids was made by Dr. Max Wolf in 1892, and his ob-
servatory at Heidelberg subsequently became a headquar-
ters for discoveries of this kind. Planetoids are now found
wholesale in this manner by photography."

In the early days of photography nebulae were considered
the most unpromising subject for the photographic plate
to deal with. Most of these objects appeared so faint that
but little encouragement in that direction was offered the
celestial photographer.

"One of the brightest and most promising of nebula? is
that in the sword of Orion, and this was naturally one of
the first of these objects to receive photographic attention.
In September, 1880, Dr. Henry Draper began photo-
graphing nebulae with this object, and succeeded, with 51
minutes exposure, in getting a good picture of the brighter
portions on dry plates. This was the first nebular photo-
graph. It was followed by other photographs, one of
which showed stars down to the 14.7 magnitude which
were visually beyond the reach of the same telescope.
These pictures marked a new era in the study of
nebulae. When the results were communicated to the
French Academy by Dr. Draper, Janssen took up the
work with a reflecting telescope having a silver-on-glass
mirror of very short focus, constructed in 1870 for the
total solar eclipse of 1871. With this Janssen found it
easy to photograph the brightest parts of a nebula
with comparatively short exposures. Unfortunately for
science, the death of Dr. Draper, in 1882, put a stop in
America to the work he had inaugurated, but it was at
once taken up in England by Common, who, with a 3-foot
reflector, attained rapid and immediate success. His pho-
tographs of the great nebula of Orion are still classic.
They were a great advance over the work of Draper, for
the reflector was not only a larger telescope, but was

' CELESTIAL PHOTOGRAPHY 47

also better adapted for photographic purposes, and espe-
cially for photographing nebulae. In fact, as we shall see
in a later chapter on nebulae, much of the progress in their
study has been due to photography."

The photography of nebulae was carried on with
remarkable success at Lick Observatory during the in-
cumbency of Professor James E. Keeler as Director,
Using the Crossley reflecting telescope, presented to the
Observatory by Dr. Common, he made a photographic
study of nebulae, and reached the conclusion that there
are at least 120,000 of the spiral type within the range of
this instrument. Professor Perrine, who succeeded to this
work on the death of Professor Keeler, believes that half
a million is nearer the figure, and that with more sensitive
photographic plates and longer exposures the number of
spirals would exceed a million.

Not only stellar motion, but stellar distances, can be
measured by photography. Professor Pritchard, at Ox-
ford, has used the sensitive plate to sound the celes-
tial depths. His first experiments were undertaken with
the star 61 Cygni, and by measuring 200 negatives which
had been made in 1886 he derived for that star a parallax
of 0.438", which was in satisfactory agreement with Ball's
value of 0.468". This work was subjected to detailed
scrutiny, and the Astronomer Royal was convinced that
it was more accurate than that of Bessel's results, ob-
tained with the heliometer. This was the beginning of
the method of measuring a parallax from photographic
plates. Professor Kapteyn showed in 1889 that from such
plates, exposed at desired intervals, parallaxes could be
derived wholesale. He applied his system in 1900 to a
group of 248 stars with encouraging success. In fact, it
was suggested that a photographic parallax "Durchmus-
terung" should be undertaken after the completion of the
astrographical chart of the heavens.

When used in connection with the spectroscope the
photographic plate has a field singularly suited to display

48 ASTRONOMY

its possibilities. Here it deals not alone with what can
be seen, but it enters into regions where the eye takes no
cognizance of things. For tho it is partly b'ind to the light
which affects the eye, it can readily penetrate the regions
where man, in turn, is blind. By special treatment of the
plate photography registers those rays invisible to the eye
and permits their accurate measurement.

The spectograph, or combination of photographic ap-
paratus with spectroscope, must be so arranged as to
show with distinctness the greatest number of lines, the
individual lines being separated; consequently there are
various types of spectrograph, depending upon the pur-
pose for which they are to be employed.

One of the combinations of the spectroscope with the
photographic apparatus is found in Professor Hale's spec-
troheliograph, which consists of a spectroscope across
whose slit the solar image moves at a uniform speed. In-
stead of the eyepiece there is a second slit which permits
light from only a single line to pass and fall on the moving
photographic plate, so that an image of the Sun, or a
sun spot in light of a single wave length, can be made to
fall upon the plate and thus be recorded.

The general effect of photography in astronomy may
be summarized in the brief statement that it has removed
the astronomer from the eyepiece of the telescope and
has substituted the more sensitive photographic plate with
its permanent record. "Hence it is that the present-day
student of astrophysics does not correspond with the
traditional idea of the astronomer," says Professor Hale.
"His work at the telescope is largely confined to such
tasks as keeping a star at the precise intersection of two
cross-hairs, or on the narrow slit of a spectrograph, in
order that stars and nebulae, or their spectra, may be
sharply recorded upon the photographic plate. His most
interesting work is done, and most of his discoveries are
made, when the plates have been developed and are sub-
jected to long study under the microscope."

CHAPTER V

THE LAW OF GRAVITATION

When the astronomers of old tried to account for the
apparent motions of the heavenly bodies by complete sys-
tems of epicycles, they must surely have asked themselves,
Why do the planets move so regularly? What makes them
move thus ? If they did, they troubled themselves but little
to answer the inquiries in anything but a perfunctory way.
For the most part they were content to regard the stars as
the playthings of divinity, and the cause of their motions,
therefore, as a mystery forever veiled to human eyes.
Still one astronomer, Anaxagoras, did have some idea of
a force which holds the planets in their orbits and which
might be the same as that which operates upon substances
at the surface of the Earth.

After his day (499 [?]-42/ [?] b.c.) the idea seems not
to have been expressed by any one until the awakening of
science in the seventeenth century. Then Kepler darkly
hinted at some attractive force, because his discovery of
the mathematical curve described by the planets seemed
to demand the existence of some constantly exerted con-
trolling force and also because he had read Gilbert's 'De
Magnete,' in which he was made acquainted with the phe-
nomena of electrical attraction.

Such a force as he had in mind would act to maintain

the motion of the planets and to drive them along in their

orbits. But this was hardly the solution of the problem,

since as Galileo found, the motion of a body of itself must

49

50 ASTRONOMY

continue indefinitely, unless there is some cause at work
to alter or stop it. This formed the first and most impor-
tant of the laws of motion which, if not independently dis-
covered by Newton, were subsequently to be stated by him
with greater force and conciseness. The laws were of
primary importance, because they afforded a new and
correct way of considering not only the underlying reasons
for the motions of the planets but of all mechanical prob-
lems involving matter and motion.

Aside from the three great laws of planetary motion
established by Kepler as the result of many observations,
the most important lesson taught by him, and one that was
readily learned by Newton, was that the motions of the
planets were not to be attributed to the influence of mere
geometrical points, such as the centers of the old epicycles,
but to the actual presence of other bodies. Kepler sug-
gested, in particular, that the planets might be considered
as connected with the Sun and therefore as sharing to
some extent the Sun's motion of revolution. From the
Sun emanated that special kind of influence which he as-
sumed. Yet, while Kepler considered the Sun as the
source of this hypothetical force, he believed in a more
general gravity or attraction between bodies.

He was unfortunate enough, however, to conceive of it
diminishing simply in proportion to the distance between
the two bodies, a mathematical impossibility, as was dem-
onstrated by Newton. This is the more surprising as he
had demonstrated that the intensity of light was recipro-
cally proportional to the surface over which it was spread
and that it varied inversely as the square of the distance
from the luminous body. It was also unfortunate that,
while Kepler's ideas of the nature of gravity were sound
and accurate in many respects, they bore no particular logi-
cal connection either one with another or with his theory of
planetary motion. They are, however, worthy of comment
as indicating the situation before Newton took these and
other speciulative ideas and the three isolated laws of

THE LAW OF GRAVITATION 51

planetary motion and bound them together into one beauti-
ful doctrine which must underlie all astronomical science.
Kepler in his work, 'Commentaries on the Motions of
Mars,' definitely states that gravity is a corporal affection,
reciprocal between two bodies of the same kind, which
tends, like the action of a magnet, to bring them together.
When the Earth attracts a stone, the stone at the same
time attracts the Earth, but by a force feebler in propor-
tion as it contains a smaller quantity of matter. He then
proceeds to state that if the Moon and the Earth were not
retained in their respective orbits by an animal or other
equipollent force, the Earth would mount toward the Moon
one-fifty-fourth part of the interval which separates the
two and the Moon would descend the fifty-three remaining
parts, supposing it to have the same density. This idea of
gravity, according to Kepler, was indeed general and
served to explain the cause of the tides, as is clearly indi-
cated in the following passage:

"If the Earth ceased to attract its waters, the whole
sea would mount up and unite itself with the Moon.
The sphere of the attracting force of the Moon ex-
tends even to the Earth and draws the waters toward
the torrid zone, so that they rise to the point which
is the Moon in the zenith."
After Kepler had promulgated his famous laws of plane-
tary motion many minds independently conceived a force
to account for the remarkable uniformity of that motion.
Thus the idea occurred to Robert Hooke, to Christopher
Wren and perhaps to Edmund Halley, who was Newton's
most intimate friend and who probably did more than any
other man of his time to popularize the idea of universal
gravitation. It remained for the towering genius of Sir
Isaac Newton (1643-1727) to formulate into a mathemati-
cal law of gravitation the effect of that universal force
with which every schoolboy is now acquainted. The honor
of having anticipated Newton was claimed by Hooke, and
fche two entered into an acrimonious controversy. Hooke

52 ASTRONOMY

never brought forward convincing proof of his claims.
So far as Newton is concerned, the great merit of his work
lay not so much in conceiving the law of gravitation as in
his brilliant demonstration of its truth.

Starting with Kepler's laws of planetary motion, he
showed not only that they were true, which was hardly a
task of merit after Kepler had considered the observations
of Tycho Brahe and all other astronomers whose recorded
observations would throw any light on the subject, but
why these laws were true, and why no other laws could
have accounted for the conditions actually observed in the
motion of the planets. And, furthermore, underlying these
famous planetary laws he discovered must be the attraction
of gravitation. By a mathematical analysis unrivaled in
the history of astronomy he proved his theorem com-
pletely. Not only did he suggest, as did Kepler, that the
power of attraction resided in the Sun, but he proved
mathematically that as a necessary consequence of that
attraction every planet must revolve in an elliptical orbit
around the Sun, having the Sun as one focus; that the
radius of the planet's orbit must sweep over equal areas
in equal times and that in comparing the movements of
two planets it is necessary that the squares of the periodic
times be proportional to the cubes of the mean distances.
These facts were discovered by Kepler; they were ex-
plained by Newton, with the aid of the powerful and cele-
brated mathematical reasoning which he had created. The
explanation was the law of gravitation.

It occurred to Newton that if a diagram of the path of
the Moon for any given period, say one minute, be made, it
would be found that the Moon departs from a straight line
during that period by a measurable distance. In other
words, the Moon has been virtually pulled toward the
Earth by an amount that is represented by the difference
between its actual position at the end of the minute and the
position it would occupy had it moved in a straight line,
which according to Galileo's law of motion, it should fol-

THE LAW OF GRAVITATION 53

low unless some external force deflected it. By measuring
the amount of deflection, he had a basis for determining
the amount of the deflecting force. This deflection New-
ton found by his calculation to be thirteen feet. Obviously
the force that acted on the Moon made it fall toward the
Earth a distance of thirteen feet during the first minute of
its fall.

Galileo had shown that the rapidity of a body's fall to
the Earth increased at a uniform rate — what is now termed
the acceleration of gravity. In other words, the higher
the starting point of the fall, the greater will be the final
velocity. Hence the amount of the attracting force is in
some way related to the distance between the two bodies, a
relation which Newton expressed by stating that the falling
body is attracted to the Earth by a force which varies in-
versely as the square of the distance between them.

If the attracting force then varies inversely as the square
of the distance, would the Moon drop toward the Earth at
the calculated rate of thirteen feet in the first minute?
That was the problem which presented itself to Newton.
The mathematical solution was simple, based as it was on
a comparison of the Moon's distance with the length of the
Earth's radius. Unfortunately there were no accurate di-
mensions of the Earth available when Newton made his
fir&t calculation in 1666. Hence he found, on the basis of
the erroneous data at his disposal, that the Moon fell to-
ward the Earth fifteen instead of thirteen feet during the
first minute, a discrepancy so great that he dismissed the
matter from his mind.

When in 1682 his attention was called to a new and ap-
parently accurate measurement of a degree of the Earth's
meridian made by the French astronomer Picard, he at-
tacked the problem anew. As he proceeded with his com-
putation it became more and more certain that this time
the result harmonized with the observed facts. So com-
pletely was he overwhelmed that he was forced to ask a
friend to complete the simple calculation. When the com-

54 ASTRONOMY

putation was ended it was known that the force which
causes bodies to fall to the Earth extends outward to the
Moon, and that by reason of this force the Moon circles
around the Earth.

It required but a slight stretch of the imagination to as-
sume that a force which can span the distance between the
Earth and the Moon may also span the distance from the
Sun to the Earth and the other members of the solar sys-
tem. That such is really the case, Newton proved by a
mathematical calculation of the orbital motions of Jupiter's
satellites and of the various planets.

These discoveries and fundamental principles enunciated
by Newton were elaborated with great exactness in his
'Principia,' and the section which discusses the motions
of the Moon, confessedly one of the most difficult
problems in celestial mechanics, has been termed by
Sir George Airy the greatest chapter on physical science
ever written. That it has stood the test of time is demon-
strated by the fact that Newton's results have scarcely been
extended in the centuries which have elapsed since their
publication. The entire work is a marvel of exact mathe-
matical reasoning by "the greatest genius the world has
ever produced," according to Lagrange's estimate of New-
ton's intellectual powers.

Galileo had experimentally shown before Newton that
the rate at which two bodies fall to the ground from equal
heights is independent of their weights. A mass of gold
and a mass of lead, altho of unequal weight, reach the
Earth at the same time if dropped simultaneously from the
same height. Newton repeated the experiment very ex-
actly. He realized as a result that weight (gravitation) is
constant. But because a pound of lead weighs less than
two pounds of lead (in other words, is attracted with one-
half the force) merely for the reason that it contains less
matter, he was forced to the conclusion that gravitation is
dependent upon quantity of matter as well as distance.
Thus he introduced the very difficult conception of mass as

THE LAW OF GRAVITATION 55

distinguished from weight, or the force of attraction ex-
erted on it by the Earth. The former, of course, is abso-
lute and constant, but the latter varies with the position of
the material in question on the Earth's surface or else-
where in the universe.

If the mass of Venus is seven times that of Mars, then the
force with which the Sun attracts Venus is seven times as
great as that with which it would attract Mars if placed at
the same distance ; and therefore also the force with which
Venus attracts the Sun is seven times as great as that with
which Mars would attract the Sun if at an equal distance
from it. Hence, in all cases of attraction, the force is pro-
portional not only to the mass of the attracted body, but
also to that of the attracting body as well as being inversely
proportional to the square of the distance. Gravitation
thus appears no longer as a property peculiar to the central
body of a revolving system, but as belonging to a planet
in just the same way as to the Sun, and to the Moon, or to
a stone in just the same way as to the Earth.

Moreover, the fact that separate bodies on the surface
of the Earth are attracted by the Earth and therefore in
turn attract it, suggests that this power of attracting other
bodies, which the celestial bodies are shown to possess, does
not belong to each celestial body as a whole, but to the
separate particles of which it is composed; so that, for
example, the force with which Jupiter and the Sun attract
each other is the result of compounding the forces with
which the separate particles making up Jupiter attract the
separate particles making up the Sun. Thus is suggested
finally the law of gravitation in its most general form:
'Every particle of matter attracts every other particle with
a force proportional to the mass of each and inversely pro-
portional to the square of the distance between them.'

When Newton completed his Trincipia' astronomy be-
came in the fullest sense an exact science. Given the posi-
tions, velocities and motions of the Sun, Earth, Moon and
other planets, then the manner in which they interact on

56 ASTRONOMY

one another can be learned and even their form and dimen-
sions determined. In short, astronomy, from a more or less
mystical science became in earnest a mathematical science.
When the motions and orbits of heavenly bodies were once
observed, the positions of these bodies could be computed
for future epochs.

In his Trincipia' Newton confines himself to the demon-
stration of the laws of gravitation. He says nothing about
the means by which bodies are made to gravitate toward
each other. His mind did not rest at this point. He felt
that gravitation itself must be capable of being explained.
It is known that he even suggested an explanation de-
pending on the action of an ethereal medium pervading
space. But with that wise moderation which is character-
istic of all his investigations, he distinguished such specu-
lations from what he had established by observation and
demonstration and excluded from his 'Principia' all men-
tion of the cause of gravitation, reserving his thoughts on
this subject for the 'Queries' printed at the end of his
'Opticks.' The attempts which have been made since the
time of Newton to solve this difficult question are few in
number and have not yet led to any well-established result.

CHAPTER VI

THE SOLAR SYSTEM PLANETARY DISTANCES

To the ancients as well as to the moderns the Sun and
the Moon appeared not only the largest but the most im-
portant of all the celestial bodies. With the Sun and
Moon five other conspicuous spheres eventually were
linked, spheres distinguished by reason of their regular
motions. These orbs, Mercury, Venus, Mars, Jupiter and
Saturn, were named "planets" or "wanderers" to distin-
guish them from the "fixed" stars.

Venus, familiar as the evening star or the morning star,
was discovered — it is claimed — by Pythagoras in the sixth
century B.C., but even in the poems of Homer there are
references to both stars without any indication of their
identity. Jupiter, Venus, Mars and Saturn, ranking with
the brightest of the stars, and Mercury, occasionally seen
near the horizon just after sunset or before sunrise, all
were known to the ancients. A study of their movements
naturally led to the obvious conclusion that all these mov-
ing stars or planets were related in some way and that
the motion of one was more or less dependent on the
motions of the others. Hence it may be asserted that the
ancient history of astronomy begins with the system of
planets that revolve around the Sun.

What is the nature of these planets? Obviously they

are not all alike in size or distance. Even to the naked

eye their appearance seems to reveal conditions that need

explanation. Early observation and study revealed the

57

58 ASTRONOMY

fact that the planets occupied a section of the heavens
-where there were no so-called "fixed" stars. But later ob-
servation also revealed that, associated with the planets,
are a number of smaller bodies of much the same nature
known as "planetoids," or "asteroids," which, with a
single exception, occupy the zone of the heavens between
Mars and Jupiter. Lastly there are a large number of
temporary visitors to this solar system known as the
"'comets." They plunge in from space, sweep around the
sun and drift away by various paths or orbits, most of
them never to return.

Planets, satellites, planetoids and comets comprise the
Solar System. Vast and marvelously complete as that
system is, it must be admitted that it is but a part of the
great universe. It may be, as there is some reason to
suppose, that this Solar System is but one of many similar
systems scattered throughout the universe and that each of
these — including that in which the Earth is situate — is in
turn wheeling about some central orb inexpressibly dis-
tant. The Solar System to which the Earth belongs is
merely a type and not a unique example of planetary order.

The intellectual rise m Astronomy is nowhere more
clearly revealed than in the history of man's conception
of the Solar System. Perhaps the first inquiry that must
have flashed across the mind of a thinking Chaldean or
Greek concerned itself with the distances of the heav-
enly bodies. How far away are the planets? How is
their distance measured? The second question concerned
itself with their motions, Whither do they drift and why?
Around these questions cluster a group of vague guesses,
fruitless speculations and poetic fancies, from which at
last a scientific method was evolved for measuring plane-
tary distances and accounting for planetary movements.
It was not until comparatively late in astronomical history
that means were devised for ascertaining the physical con-
dition of each planet.

The distances of the planets, small as they seem in com-

PLANETARY DISTANCES 59

parison with sidereal measurements, are felt to be immense.
Using only round numbers, which are sufficiently accurate
for the present purpose, the planet Neptune, the outermost
known member of our system, is 2,800,000,000 miles from
the Sun. In a cord twenty-eight feet long each single foot
will represent a hundred million miles. On such a scale
a map of the United States could not be seen without the
aid of a microscope. Suppose a bead were placed at each
end of this line, one representing the Sun, the other Nep-
tune. Between the two, other beads will represent the other
planets. One nearly four inches from that representing
the Sun will be Mercury; another, at about seven inches.,
Venus; a third, at eleven inches, the Earth; a fourth, at

\
%

\

/' m, ^oEarth

.•Venus*'/ Mercury

/ / / \ \ \

i . \ \ \

\ / / /

i

I

!

j

v. ,...y / 1 f

S8davs / / /

.♦♦***' /*" •'*

225 days .>'

\ 365"days

687 days
Fig. 7 — The Orbits of the Four Interior Planets.

6o ASTRONOMY

seventeen inches, Mars ; a fifth, at about five feet, Jupiter ;
a sixth, at nine feet, Saturn; a seventh, at eighteen feet,
Uranus, and an eighth, Neptune, at the end.

The mean distances of the planets from the Sun are as
follows :

MILES.

Mercury 36,000,000

Venus 67,200,000

Earth 92,900,000

Mars 141,500,000

Jupiter 483,300,000

Saturn 886,000,000

Uranus 1,781,900,000

Neptune 2,791,600,000

Attempts to measure some of these distances approxi-
mately are found in early times. The idea that some of
the planets must be nearer the Earth than others must
have been suggested by eclipses and occultations — i.e.,
passage of the Moon over the Sun and over a planet or
fixed star. No direct means being available for determin-
ing the distance, rapidity of motion anciently was em-
ployed as a test of probable nearness. The stars being
seen above, it was but natural to think of the most distant
celestial bodies as the highest, and accordingly Saturn,
Jupiter and Mars, being beyond the Sun, were called
"superior planets" as distinguished from the two "inferior
planets," Venus and Mercury. Uranus and Neptune are
modern additions to the solar system and could not have
been included in the hypothesis. Aristotle (384-322 B.C.),
for example, arrived at the conclusion that the planets are
farther off than the Sun and Moon as the result of an oc-
cupation of Mars by the Moon and as the result of similar
observations made in the case of other planets by the
Egyptians and Babylonians. Ptolemy (second century
a.d.), altho far more original and daring in his astronomi-

PLANETARY DISTANCES

61

cal conceptions than Aristotle, was able to add but little
toward a solution of the problem. He expressly states
that he had no means of estimating numerically the dis-
tances of the planets or even of knowing the order of the
distance of the several planets. He followed tradition in
conjecturally accepting rapidity of motion as a test of

QeniTOfNEPTti^

Fig. 8 — The Orbits of the Five Exterior Planets.

nearness and placed Mars, Jupiter, Saturn (which per-
form the circuit of the celestial sphere in about 2, 12 and
29 years, respectively) beyond the Sun in that order. As
Venus and Mercury accompany the Sun, and may there-
fore be regarded as on the average performing their revo-
lutions in a year, the test to some extent failed in their

62 ASTRONOMY

case, but Ptolemy again accepted the opinion of the "an-
cient mathematicians" — probably the Chaldeans — that
Mercury and Venus lie between the Sun and Moon, Mer-
cury being the nearer to the Earth.

Copernicus gave the first glimpse of the truth. To quote
Berry in his "Short History of Astronomy": "From the
fact that Venus and Mercury were never seen very far
from the Sun, it could be inferred that their paths were
nearer to the Sun than that of the Earth, Mercury being
the nearer to the Sun of the two, because never seen so
far from it in the sky as Venus. The other three planets,
being seen at times in a direction opposite to that of the
Sun, must necessarily revolve round the Sun in orbits
larger than that of the Earth, a view confirmed by the
fact that they were brightest when opposite the Sun (in
which positions they would be nearest to us). The order
of their respective distances from the Sun could be at
once inferred from the disturbing effects produced on their
apparent motions by the motion of the Earth. Saturn
being least affected, must on the whole be farthest from
the Earth, Jupiter next and Mars next. The Earth thus
became one of six planets revolving round the Sun, the
order of distance — Mercury, Venus, Earth, Mars, Jupiter,
Saturn — being also in accordance with the rates of motion
round the Sun, Mercury performing its revolution most
rapidly (in about 88 days), Saturn most slowly (in about
30 years)."

It was not until John Kepler (1571-1630) published his
"Epitome of the Copernican Astronomy," his "Harmony
of the World" and a treatise on "Comets" that astrono-
mers were given a definite formula which enabled them
to determine planetary distances with any exactitude.
Kepler's speculative and mystic temperament led him con-
stantly to search for relations between the various numeri-
cal quantities occurring in the Solar System. By a happy
Inspiration he tried to discover a relation between the

PLANETARY DISTANCES 63

sizes of the orbits of the various planets and their times of
revolution round the Sun. After a number of unsuccess-
ful attempts he discovered a simple and important rela-
tion commonly known as Kepler's third law:

"The squares of the times of revolution of any two
planets (including the Earth) about the Sun are pro-
portional to the cubes of their mean distances from
the Sun."

In other words, given the periods, there is need only to
find the interval between any two of them in order to
infer at once the distance separating them all from one
another and from the Sun. Here was the plan. What was
next to be discovered was the scale upon which the plan
was to be drawn. There must be first a trustworthy meas-
ure of the distance of a single planet from the Sun, the
Earth, for example, and the problem would be solved.

How is this measure to be obtained? Sir Robert Ball
in his "Story of the Heavens," gives this simple example
for partial explanation: "Stand near a window where
you can look at buildings . . . or at any distant object.
Place on the glass a thin strip of paper vertically in the
middle of one of the panes. Close the right eye and note
with the left eye the position of the strip of paper rela-
tively to the objects in the background. Then, while still
remaining in the same position, close the left eye and
again observe the position of the strip of paper with the
right eye. You will find that the position of the paper on
the background has changed.

"Move closer to the window and repeat the observation,
and you find that the apparent displacement of the strip
increases. Move away from the window and the displace-
ment decreases. Move to the other side of the room, the
displacement is much less, tho probably still visible. We
thus see that the change in the apparent place of the strip
of paper, as viewed with the right eye or the left eye,
varies in amount as the distance changes; but it varies in

64 ASTRONOMY

the opposite way to the distance, for as either becomes
greater the other becomes less. We can thus associate
with each particular distance a "corresponding particular
displacement. From this it will be easy to infer that, if
we have the means of measuring the amount of displace-
ment, then we have the means of calculating the distance
from the observer to the window. It is this principle
applied on a gigantic scale which enables us to measure
the distances of the heavenly bodies.

"Look, for instance, at the planet Venus ; let this corre-
spond to the strip of paper and let the Sun, on which
Venus is seen in the act of transit, be the background.
Instead of the two eyes of the observer, we now place
two observatories in distant regions of the Earth ; we
look at Venus from one observatory, we also look at it
from the other; we measure the amount of displacement
and from that we calculate the distance of the planet. All
depends, then, on the means which we have of measuring
the displacement of Venus as viewed from the two differ-
ent stations."

Two observers standing upon the Earth must be some
thousands of miles apart in order to see the position of the
Moon altered with regard to the starry background to
obtain the necessary data upon which to ground their
calculations. The change of position thus offered by one
side of the Earth's surface at a time is not sufficient, how-
ever, to displace any but the nearest celestial bodies.
When there is occasion to go farther afield, a greater
change of place must be sought. This can be obtained as
a consequence of the Earth's movement around the Sun.
Observations, taken several days apart, will show the
effect of the Earth's change of place during the interval
upon the positions of the other bodies of our system. But
when the depths of space beyond are to be sounded and
an effort is made to reach out for the purpose of measur-
ing the distance of the nearest star the utmost change of
place is necessitated. This results from the long journey

PLANETARY DISTANCES 65

of many millions of miles which the Earth performs
around the Sun during the course of each year. Still, even
this last change of place, great as it seems in comparison
with terrestrial measurements, is insufficient to show any-
thing more than the tiniest displacements in a paltry forty-
three out of the entire host of stars.

It is thus readily realized with what an enormous dis-
advantage the ancients coped. The measuring instruments
at their command were utterly inadequate to detect such
small displacements. It was reserved for the telescope to
reveal them, and even then it required the great telescopes
of recent times to show the slight changes in the position
of the nearer stars which were caused by the Earth's being
at one time at one end of its orbit and some six months
later at the other end — stations separated by a gulf of
about 186,000,000 miles.

It was from an opposition of Mars observed in 1672 by
John Richer ( ?-i6g6) at Cayenne in concert with Giovanni
Domenico Cassini at Paris that the first scientific estimate
of the Sun's distance was derived. The Sun appeared to be
nearly 87,000,000 miles away. John Flamsteed (1646-
1720), the first Astronomer Royal of England, deduced
81,700,000 from his independent observations of the same
occurrence. Jean Picard's (1620-1682) later result was
just one-half Flamsteed's (41,000,000). Philippe De
Lahire thought that the Earth must be separated from the
Sun by at least 136,000,000 miles. The transits of Venus
in 1761 and 1769 were employed, after other attempts had
been made, to measure the Sun's distance.

The transit of 1769 is of particular interest, not only for
a fairly good determination of the Sun's distance, but
also for the reason that the celebrated Captain Cook was
commissioned to sail to Otaheite for the purpose of wit-
nessing the transit of Venus. At Otaheite, on June 3d,
the phenomenon was carefully observed and meas-
ured. Simultaneously with these observations others were
obtained in Europe and elsewhere. From a combination of

66 ASTRONOMY

all the observations, an approximate knowledge of the
Sun's distance was gained. The most complete discussion
of these observations did not, however, take place for some
time. It was not until the year 1824 that the illustrious
Johann Franz Encke computed the distance of the Sun and
gave as the definite result 95,000,000 miles. Later Urbain
Jean Joseph Le Verrier (1870) reduced the estimate to
91,320,000 miles, which held good until Prof. Simon New-
comb in 1882 gave the figure 92,475,000 miles. In 1900
nearly all the observatories of the world under the direc-
tion of Maurice Loewy and the French Academy of Sci-
ence began a new computation which will lead to more
exact results. The old problem of measuring a planet's
distance from the Sun its not yet completely solved. If
Sir David Gill's plan of basing a new set of calculations
on the opposition of Eros in 193 1 is carried into execution,
the Sun's distance will be ascertained to within 10,000
miles. Present knowledge declares the distance of the
planets from the Sun with an error not exceeding one-
fiftieth of one per cent.

CHAPTER VII

THE SOLAR SYSTEM — THE MOTIONS OF THE PLANETS

The motions of the planets also formed the basis for
archaic theorizing. That the planets move, the ancients
were fully aware, for the very word "planet" means "wan-
derer." The strip of the celestial sphere through which
move the Sun, the Moon and the five planets known
to the ancients (Mercury, Venus, Mars, Jupiter and Saturn)
was called the Zodiac, because the constellations in it were
named after living things, with one exception. The
Zodiac was divided into twelve equal parts, the "signs
of the Zodiac," through one of which the Sun passed every
month, so that its position could be roughly given by stat-
ing in what sign it was. The stars in each sign were
formed into a constellation, the sign and the constellation
each receiving the same name. The relative movements
of the planets as the ancients conceived them are thus
summarized by Berry: "In Pythagoras occurs perhaps
for the first time an idea which had an extremely impor-
tant influence on ancient and medieval astronomy. Not
only were the stars supposed to be attached to a crystal
sphere, which revolved daily on an axis through the Earth,
but each of the seven planets (the Sun and Moon being
included) moved on a sphere of its own. The distances
of these spheres from the Earth were fixed in accordance
with certain speculative notions of Pythagoras as to num-
bers and music; hence the spheres as they revolved pro-
duced harmonious sounds which specially gifted persons
67

68 ASTRONOMY

might at times hear. This is the origin of the idea of the
music of the spheres which recurs continually in medieval
speculation and is found occasionally in modern literature.
At a later stage these spheres of Pythagoras were devel-
oped into a scientific representation of the motions of the
celestial bodies, which remained the basis of astronomy
till the time of Kepler."

Philolaus, the Pythagorean, who lived about a century
after his master, introduced for the first time the idea of
the motion of the Earth. He appears to have regarded
the Earth, as well as the Sun, Moon and five planets, as
revolving round some central fire, the Earth rotating on
its own axis as it revolved, apparently in order to insure
that the central fire should always remain invisible to the
inhabitants of the known part of the Earth. Altho pure
fancy, the idea of Philolaus was a valuable contribution
to astronomical thought.

Despite the immense influence of the Pythagoreans,
most Greeks shared Plato's idea that any careful study of
celestial motions was degrading rather than elevating,
for the whole subject smacked too much of the unesthetic
section of geometry. Still, Plato (429-347 B.C.) did give
a short account of the celestial bodies, according to which
the Sun, Moon, planets and fixed stars revolve on eight
concentric and closely fitting wheels or circles around an
axis passing through the Earth.

This idea of Plato's was more or less followed by later
philosophers. Thus Eudoxus of Cnidus (409-356 B.C.) at-
tempted to explain the more obvious peculiarities of plane-
tary motion by means of a combination of uniform circu-
lar motions. The celestial motions were to some extent
explained by means of a system of 27 spheres, 1 for the
stars, 6 for the Sun and Moon, 20 for the planets. There
is no clear evidence that Eudoxus made any serious at-
tempt to arrange either the size or the time of revolution
of the spheres so as to produce a precise agreement with
the observed motion of the celestial bodies, tho he knew

PLANETARY MOTIONS 69

with considerable accuracy the time required by each
planet to return to the same position with respect to the
Sun; in other words, his scheme represented the celestial
motions qualitatively but not quantitatively.

Aristotle adopted this scheme of Eudoxus, but need-
lessly complicated it by treating the spheres as material
bodies and added 22 more spheres, thus making 56 in all.
He argued against the possibility of the Earth's revolving
around the Sun on the ground that there ought to be a
corresponding apparent motion of the stars, an objection
finally disposed of only during the nineteenth century,
when it was discovered that this motion can be seen only
in a few cases because of the unutterably great distance of
the stars.

No substantial advance can be noted until Hipparchus
(160-125 B.C.) made an extensive series of observations
with all the accuracy that his instruments would permit
and critically made use of old observations for comparison
with later ones so as to discover astronomical changes too
slow to be detected in a single lifetime — an essentially
modern method. He systematically employed a geometri-
cal scheme (that of eccentrics and epicycles) for the repre-
sentations of the motions of the Sun and the Moon, a
mode suggested in substance by Apollonius of Perga, who
flourished in the third century b.c.

The great services rendered to astronomy by Hipparchus
can hardly be better expressed than in the words of the
great French historian of astronomy, Delambre, who is in
general no lenient critic : "When we consider all that Hip-
parchus invented or perfected and reflect upon the number
of his works and the mass of calculations which they
imply, we must regard him as one of the most astonishing
men of antiquity and as the greatest of all in the sciences
which are not purely speculative and which require a com-
bination of geometrical knowledge with a knowledge of
phenomena to be observed only by diligent attention and
refined instruments."

70 ASTRONOMY

The last great name encountered in tracing the record
of changing conceptions of planetary motions is that of
Ptolemy (100-170 a.d.), whose reputation rests on his
"Almagest," which may be regarded as the astronomical
gospel of the Middle Ages. Hipparchus, as we have seen,
found the current representations of the planetary motions
inaccurate and collected a number of new observations.
These, with fresh observations of his own, Ptolemy em-
ployed in order to construct an improved planetary system.
Following the idea of Hipparchus, Ptolemy thought that
the Sun and Moon moved in circular orbits around the
Earth as a center. Ptolemy's chief work was to expand
the system of epicycles so that it could explain discrepan-
cies between theory and observation, discrepancies over-
looked or ignored by Hipparchus. The deviations of the
planets from the ecliptic, for example, were accounted for
by tilting up the planes of the epicycles. Thus with the
aid of the system of Hipparchus, supplemented with his
own idea of tilting epicycles, he worked out with great
care and labor the motions of the planets. Altho the Hip-
parchian-Ptolemaic doctrine was framed on an extravagant
"estimate of the importance of the Earth in the scheme of
the heavens, yet it must be admitted that the apparent
movements of the celestial bodies were thus accounted for
with considerable accuracy. For fourteen centuries the
Almagest was regarded as the final authority on all ques-
tions of astronomy and it may be considered as the loftiest
piece of calculation appertaining to the Ancient World.

CHAPTER VIII

THE SOLAR SYSTEM — MODERN INVESTIGATION

The Ptolemaic system of astronomy was discredited
only at an epoch nearly simultaneous with that of the dis-
covery of the New World by Columbus. The true ar-
rangement of the solar system was then expounded by
Nicholas Copernicus (1473-1543) in the great work, "De
Revolutionibus," to which he devoted his life. The first
principle established by these labors showed the diurnal
movement of the heavens to be due to the rotation of the
Earth on its axis. Copernicus pointed out the fundamental
difference between real motions and apparent motions.
He proved that the appearances presented in the daily
rising and setting of the Sun and the stars could be ac-
counted for by the supposition that the Earth rotated,
just as satisfactorily as by the more cumbrous supposition
of Hipparchus and Ptolemy. He showed, moreover, that
if the ancient supposition were true, the stars must have
an almost infinite velocity and declared that the rotation
of the entire universe around the Earth was clearly pre-
posterous.

The second great principle, which has conferred immor-
tal glory upon Copernicus, assigned to the Earth its true
position in the universe. Copernicus transferred the cen-
ter, about which all the planets revolve, from the Earth to
the Sun, and he established the somewhat crushing truth
that the Earth is merely a planet, pursuing a track be-
tween the paths of Venus and of Mars and subordinated

72 ASTRONOMY

like all the other planets to the supreme sway of the Sun.
This great revolution swept from Astronomy those dis-
torted views of the Earth's importance which arose, per-
haps not unnaturally, from the fact that the observers
chanced to live on this particular planet. Whether the
actual services rendered by Copernicus are commensurate
with his fame may be doubted. He labored under the
weight of an ecclesiastical tradition that could not be
abandoned without some risk. He was a bold man indeed
who dared to overthrow or even to question orthodoxy
and to diminish the Earth's overshadowing importance in
the Solar System.

The Copernican system was not flawless either in theory
or logic and many objections could be made to it, particu-
larly by an astronomer who had observed and studied the
movements of the heavenly bodies. After the example of
the ancients, Copernicus assumed as an axiom the uni-
form, circular motion of the planets, and, as the only
motions which are observed are in a state of incessant
variation, he was obliged, in order to explain the inequali-
ties to suppose a different center for each of the orbits.
The Sun was placed within the orbit of each of the plan-
ets, but not in the center of any of them. In other words,
he still adhered to a system of epicycles. Consequently
the Sun performed no other office than to distribute light
and heat. Excluded from any influence on the system,
the Sun became a stranger to all the motions. The "fixed"
stars were alleged to be stationary, and it was necessary
to suppose that they were almost infinitely distant, inas-
much as they always seemed to preserve the same position
when viewed from the opposite sides of the Earth's orbit.

While various astronomers showed some disposition to
accept the Copernican teaching, most of them were bitterly
opposed to it on ecclesiastical, traditionary and scientific
grounds. Tycho Brahe (i 546-1601) was the most distin-
guished of these opponents. Being an indefatigable ob-
server and practically the first to realize the value of con-

THE SOLAR SYSTEM 73

tinuous observation, he enriched astronomy by a star
catalogue and studies of the movements of the other
heavenly bodies. Tycho accepted the Copernican concep-
tion of a central Sun, but rejected the idea that the Earth

rtt*£!&^&J*to>£

Fig. 9 — The Solar System According to Copernicus. (From
the De Revolutionibus.)

moved. Thus he sought to effect a compromise between
the Ptolemaic and Copernican systems. It was the study
of a comet in 1577 that led Tycho to formulate his ideas
of the solar system. He believed that the comet (X) as
shown in the accompanying diagram was revolving around

74

ASTRONOMY

the Sun at a distance greater than that of Venus and as-
sumed that both the Sun (C) and the Earth (A) were
centers of revolving systems, the five planets revolving
around the Sun and the entire system in turn moving
around the Earth. This incorrect proposition, which

-Tycho's System of the World.
the Comet of I577-)

(From his book on

was one of the least of Tycho Brahe's contributions to
astronomical science, is significant, showing as it does how
difficult it was for the principles of Copernicus firmly to
establish themselves and planetary motion to be explained
satisfactorily.

Whatever Tycho may have thought of the Copernican

THE SOLAR SYSTEM 75

system, his contemporary, Galileo (1564-1642), was will-
ing to accept it. It has been shown how Galileo with the
telescope of his invention was able to extend astronomical
science and to introduce new methods of observation,
which came naturally to one who was a leader in the ex-
perimental science of his time. But even before his work
with the telescope Galileo had adopted the astronomical
views of Copernicus and collected arguments for their
support. He was able in 1604 to confirm the discovery of
Tycho Brahe that changes take place in the heavens be-
yond the planets and that there was an important region
beyond the Earth and its immediate surroundings. As
was but natural, the use of the telescope broadened Gali-
leo's horizon, and, true scientist that he was, he imme-
diately brought to bear his new discoveries on the funda-
mental conceptions.

Thus his discovery of the satellites revolving around
Jupiter as the planets themselves revolved around the
Sun not only rendered necessary the explanation of these
new bodies, but dealt a serious blow at the infallibility of
Aristotle and Ptolemy, neither of whom had any idea of
the existence of these satellites. Further support was
given to the Copernican theory by the ocular demonstra-
tion of these satellites revolving around Jupiter and not
dropping behind, just as the Moon was required to move
around the Earth, a mechanical difficulty brought forward
by the opponents of the Copernican idea. As Galileo de-
veloped his astronomical ideas and discoveries he naturally
came into conflict with ecclesiastical authority and there
began the unfortunate controversy as to the relative valid-
ity in scientific matters of observation and reasoning on
the one hand and the authority of the Church and Bible
on the other.

Controversies such as this were conspicuous in the
latter part of Galileo's life. They culminated in his
famous trial and formal abjuration of his alleged errors
and in his conviction "of believing and holding the doctrines

76 ASTRONOMY

— false and contrary to the Holy and Divine Scriptures —
that the Sun is the center of the world and that it does
not move from east to west, and that the Earth does move
and is not the center of the world; also that an opinion
can be held and supported as probable after it has been
declared and decreed contrary to the Holy Scriptures."
Despite Galileo's abjuration, his general attitude toward
the Church and the Bible is contained in his approval of
the saying of Cardinal Baronius, "That the intention of
the Holy Ghost is to teach us not how the heavens go,
but how to go to heaven." His attempts to explain and
demonstrate the Copernican system in his great astronomi-
cal treatise, "Dialog on the Two Chief Systems of the
World, the Ptolemaic and Copernican," led to his trial and
conviction before the Inquisition.

Kepler, another of Galileo's contemporaries, did more
even than the great Italian to bring about a proper con-
ception of the solar system and the motions of the planets.
A student under Tycho, it was but natural that Kepler
should have imbibed from his master a respect for syste-
matic observation, regardless of the correctness or incor-
rectness of Copernican views. As a result Kepler early
adopted the Copernican doctrine, opposed tho it was by
his master. His observations led him to the conclusion,
however, that even Copernicus had not revealed all the
mysteries of planetary motion and that the hypothetical
circles in which the planets revolved around the Sun,
according to Copernicus, did not agree with the paths
observed. Under the instruction of Tycho, Kepler ad-
dressed himself to the problems involved in the planet
Mars, whose positions as seen in the sky were a combined
result of its own motion and that of the Earth, as both
move around the Sun. Actual observation of the planet
and the consideration of various geometrical theories that
suggested themselves eventually led to the conception that
the path of the planet must be some form of an oval.

THE SOLAR SYSTEM

77

Finally Kepler reached the conclusion that instead of
being circular, the planet's motion must lie in the simple
curve known as an ellipse and formed by taking an oblique
section of a cone. While the circle has but a single cen-
ter, the ellipse depends for its form upon two fixed points,
each of which is termed a focus. It can be drawn by

Fig. ii — Drawing an Ellipse.

using two pins stuck in a sheet of paper and by inserting
a pencil within a loop of string that also includes the two
pins. The curve may be traced by moving the pencil,
while the string is kept taut. It will be found that if the
two points are kept close together the curve approaches
in form a circle, while if they are separated the figure
becomes elongated and possesses what the mathematicians
term greater eccentricity. At any rate, every point on the
curve is such that the sum of its distance from the two
foci is always the same. Kepler found that the Sun was

78 ASTRONOMY

at one focus. When the planet was near that focus, it
moved with greater velocity than when at the opposite
part of its orbit. The speed of motion, however, was al-
ways proportional to the areas swept out by a straight
line from the Sun in equal intervals of time. In other
words, there were formulated the now famous first and
second laws of Kepler as follows :

i. The planet describes an ellipse, the Sun being in one
focus.

A

Fig. 12 — Equal Areas in Equal Times.

2. The straight line joining the planet to the Sun sweeps
out equal areas in any two equal intervals of time.

Kepler not only established these laws for Mars, but
immediately applied his principle to the Earth and then
claimed (without proof, however) in his "Epitome of the
Copernican Astronomy" that these two fundamental laws
applied also to all the planets and to the motions of the
Moon. Accompanying these two laws was the third,
already discussed, in which it is stated that the squares of
the times of revolution of any two planets (including the
Earth) about the Sun are proportional to the cubes of
their mean distances from the Sun.

It was the disclosure of these wonderfully simple rela-

THE SOLAR SYSTEM 79

tions that laid the foundation for the Newtonian law of
gravitation. Contemporary judgment, of course, could not
anticipate the culmination of a later generation. What it
could understand was that the first law of Kepler attacked
one of the most time-honored of metaphysical conceptions
— the Aristotelian idea that the circle is the perfect figure
and that planetary motions consequently must be circular.
Not even Copernicus had doubted the validity of this as-
sumption.

Kepler was too great a genius to rest content with the
mere observation that the planets move in ellipses. Next
he desired to determine why they do so move. It remained
for Isaac Newton (1643-1727) to answer the question
satisfactorily ; yet Kepler had a curious premonition of the
law of gravitation. "Whereas the Ptolemaic system," com-
ments Berry, "required a number of motions round mere
geometrical points, centers of epicycles or eccentrics,
equants, etc., unoccupied by any real body, and many such
motions were still required by Copernicus, Kepler's scheme
of the solar system placed a real body, the Sun, at the
most important point connected with the path of each
planet and dealt similarly with the Moon's motion round
the Earth and with that of the four satellites round Jupi-
ter. Motions of revolution came in fact to be associated
not with some 'central point' but with some 'central
body/ and it became therefore an inquiry of interest to
ascertain if there were any connection between the motion
and the central body. The property possessed by a magnet
of attracting a piece of iron at some little distance from
it suggested a possible analogy to Kepler, who had read
with care and was evidently impressed by the treatise 'On
the Magnet' ('De Magnete'), published in 1600 by Will-
iam Gilbert of Colchester (1 540-1603). He suggested that
the planets might thus be regarded as sharing to some
extent the Sun's own motion of revolution. In other
words, a certain 'carrying virtue' spread out from the
Sun, with or like the rays of light and heat, and tried to

80 ASTRONOMY

carry the planets round with the Sun." Kepler says him-
self in his "Epitome" :

"There is, therefore, a conflict between the carrying
power of the Sun and the impotence or material slug-
gishness (inertia) of the planet; each enjoys some
measure of victory, for the former moves the planet
from its position and the latter frees the planet's body
to some extent from the bonds in which it is thus
held, . . . but only to be captured again by an-
other portion of this rotatory virtue."

Mean Velocity

\

Mean VelocW
Fig. 13 — Varying Velocity of Elliptic Motion.

Thus is faintly indicated the great theory of gravitation
which, as developed by Newton, was to supply a satisfac-
tory explanation of planetary motion and which is the
underlying basis of all modern astronomy.

Newton had become convinced that the attracting power
of the Earth extended even to the Moon, and that the ac-
celeration produced in any body — whether it be as distant
as the Moon or close to the Earth — was inversely propor-
tional to the square of the distance from the Earth's
center and also proportional to the mass of the body.

THE SOLAR SYSTEM 81

Then he found that the motions of the planets could be
explained by an attraction toward the Sun, which pro-
duced an acceleration inversely proportional to the square
of the distance from the center of the Sun, not only in
the same planet in different parts of its path, but also in
different planets.

Again it follows from this that the Sun attracts any
planet with a force inversely proportional to the square
of the distance of the planet from the Sun's center and
also proportional to the mass of the planet. Accordingly,
if the Earth or Sun attracts a body, the body must exert
a similar force on the Earth or the Sun, and gravitation
is not only a property of the central body of a revolving
system, but belongs to every planet in just the same way
as to the Sun and to a Moon or to a stone just as to the
Earth.

After Newton had established provisionally the law of
gravitation and the laws of motion, it remained for him
to prove that the observed motions of the planets agreed
with those calculated. A situation of the greatest com-
plexity, however, was relieved by the fact that the mass
of even the largest planets is so very much less than that
of the Sun that the motion of any single planet is affected
but slightly by the others, and it may be assumed to be
moving very nearly as it would if the other planets did not
exist, due allowance being made subsequently for minor
disturbances or perturbations produced in its path. One
by one the various irregularities observed were explained,
and the motion of the Moon and its various eccentricities
were computed with accurate numerical results.

For many years the solar system remained in its fancied
integrity. There was no change in the original five planets
and the number of satellites first discovered by Galileo
and added to by subsequent observers had reached an
apparent culmination when G. D. Cassini (1625-1712)
had detected the second pair of satellites of Saturn in
1684. Accordingly, when Herschel, following his custom

82 ASTRONOMY

of making a review of the heavens with each new tele-
scope that he constructed, found March 13, 1781, with a
Newtonian telescope, seven feet in length, a small star
which appeared so much larger than its companions and
of such uncommon appearance, he suspected it to be a
comet. Further study revealed that it was more than a
comet and of far greater interest. When needfully ob-
served and its path calculated, it was found that no ordi-
nary cometary orbit would in any way fit its motion.

Anders Johann Lexell (1740-1784) first recognised that
Herschel's body was not a comet but a new planet revolv-
ing around the Sun in a nearly circular path and at a dis-
tance about nineteen times that of the Earth and nearly
double that of Saturn. A vain attempt by Herschel to
name the new planet after his royal patron, George III.,
"Georgium Sidus," finally resulted in the proposal and ac-
ceptance by British and continental astronomers alike of
the name Uranus, which harmonized with the names
of the other planets.

This discovery was of especial interest, inasmuch as
Johann Daniel Titius ( ?-i796), a professor at Wittenberg,
had pointed out the remarkable symmetry in the disposi-
tion of the planets. In a note, published in 1772, he showed
that the distance of the six known planets from the Sun
could be represented with a close approach to accuracy by
a certain series of numbers increasing in the regular pro-
gression, o, 3, 6, 12, 24, 48, etc. Adding 4 to each number,
the results would give the relative distances of the six
known planets from the Sun. In applying this law (which
does not hold good in the case of Neptune) it was
found that the term of the series following that which
corresponded with the orbit of Mars was not represented
in the list of planets. Accordingly Johann Elbert Bode
(1747-1826), a German astronomer, assumed a hypotheti-
cal planet to take this place. When Uranus was discov-
ered its distance was found to fall but slightly short of
perfect conformity with the law of Titius, and it stimu-

THE SOLAR SYSTEM 83

Fig. 14 — Comparative Sizes of the Planets.

lated the search for a new body, which, as will be seen in
the chapter on the planetoids, proved to be the small planet
Ceres.

The study of Uranus after its discovery by Herschel
furnished many difficulties to astronomers. Despite the
most careful calculations of the movements of the planet
through more than a century's observations, the conclusion

84 ASTRONOMY

was reached that considerable errors existed or that the
planet was wandering from its course. In fact, these dis-
turbances had aroused the interest of several mathemati-
cians and astronomers, and a young English student, John
Couch Adams (1819-1892), soon after his graduation from
Cambridge, communicated in 1845 t0 tne Astronomer
Royal numerical estimates of the elements and orbits of
the unknown planet which he assumed was acting on
Uranus and was the cause of the disturbances. In fact,
he indicated the actual place in the heavens of the hypo-
thetical planet. At practically the same time a French
astronomer, Urbain Leverrier (1811-1877), who had made
a careful study of the solar system in response to a request
from Dominique F. J. Arago, the head of the French Ob-
servatory, prepared an elaborate memoir in which he
demonstrated that only an exterior body could produce the
disturbances observed and that such a body must occupy
a certain and determinate position in the zodiac. He also
assigned the orbit of the disturbing body, indicating that it
would be as visible and bright as a star of the eighth mag-
nitude. In fact, he supplied data to Professor Galle, of the
Berlin Observatory, which enabled that astronomer to find
in the heavens on September 23, 1846, within less than a
degree of the spot indicated an object with a measurable
disk. A reasonably complete map of this portion of the
sky, in which all the stars were noted, proved beyond
question that the object was not a star, while its move-
ment as predicted was ample confirmation of its planetary
nature. Adams' work, which antedated that of Leverrier,
had not received attention originally in Great Britain at
the hands of the Astronomer Royal, but as the matter as-
sumed importance the Cambridge Observatory also par-
ticipated in the search and on September 29, 1846, the
planet was seen again. Thus Neptune was discovered.
To show the rapidity of astronomical research in the nine-
teenth century it may be remarked that it required but

THE SOLAR SYSTEM 85

seventeen days for the discovery of a satellite by Lassell
(1799-1880) with a two-foot reflecting telescope.

Astronomers have suspected the existence of still other
planets, and the belief has been expressed that such a
body exists nearer to the Sun than Mercury, which, as has
been seen, enjoys the reputation of being the closest of
all the planets to the central luminary. The average dis-
tance of Mercury from the Sun is about 36,000,000 miles,

X

<?>-

Eartl4 iJMsp* ■( SUN j f

v / /

/

Fig. 15 — The Transit of "Vulcan/' the Supposed Intramer-
curial Planet.

so that there would be space enough for such a planet.
Its peculiar position in close proximity to the Sun, how-
ever, would act against its being observed. A small lumi-
nous point in this position would be altogether invisible,
even with the best modern telescope, while its setting
and rising, simultaneous almost with those of the Sun,
make it invisible at these times even under the most favor-
able conditions. If this planet should pass across the
Sun's disk just as do Mercury and Venus, it would be

86 ASTRONOMY

seen. While from its size it would be much less of a spec-
tacle than the two planets mentioned, it might be detected.
Claims have been advanced by astronomers that they have
seen such a transit of a small spot.

The first suggestion of an intra-Mercurial planet came
from the distinguished French astronomer, Leverrier, who
in 1859 advanced such a hypothesis in an attempt to
explain the movements of the planet Mercury. His theory
involved a body of about the size of Mercury revolving at
somewhat less than half its mean distance from the Sun,
or at a greater distance if of less mass and vice versa,
whose motion in great part would explain the irregulari-
ties observed. In the same year Dr. Lescarbault at Or-
geres maintained that he had observed such a body cross-
ing the Sun's disk, and the name of Vulcan was bestowed
upon it. Several astronomers claimed to have seen the new
planet. Their observations were not well authenticated,
and on the dates fixed for the probable transits no trace
of the planet could be found. The strongest test was the
examination of the sky at the time of a solar eclipse, for
then the light of the Sun was cut off and a strange body
could be readily identified. Despite a careful watch at
subsequent eclipses and an examination of photographic
plates, only negative results have been obtained. To-day
the belief that there is any body of considerable size
within the orbit of Mercury is held only by a few astrono-
mers and very guardedly stated.

If the search for an intra-Mercurial planet was unsuc-
cessful, it has in no way deterred astronomers from en-
deavoring to find other unrecognised members of the
solar system. Much interest has been aroused by a
hypothetical ultra-Neptunian planet, which of course
would be the furthest from the Sun of all the members of
the solar system. The basis for such a hypothesis is the
reduction of observations made of the positions and
motions of Uranus and Neptune. Neptune has been under
observation for only a small part of a revolution, so that

THE SOLAR SYSTEM 87

data thus far obtained seem to many astronomers insuffi-
cient for the purpose. Yet a number of astronomers have
sought by calculation to prove the existence of such a
planet. While their results are discordant, yet they indi-
cate very closely the regions of the sky where search for
the hypothetical body might be rewarded with success.

Professor W. H. Pickering, of Harvard, in 1909 evolved
a method for the discovery of the distant planet, a method
which he first tested by application of the data available
to Adams and Leverrier for the discovery of Neptune,
and found that the method would succeed. Proceeding
then with his hypothetical planet, which he termed "O,"
he found that it was 51.9 times as far distant from the
Sun as the Earth, tho its mass was but twice that of this
planet, and that it had a period of revolution of 373.5
years. The problem presented by Uranus, Neptune and
"O," according to Professor Pickering, is quite the same
as that of Mercury, Venus and the Earth, which has been
thoroly studied, so that the relative motions are well under-
stood.

But in investigating the effect that such a hypothetical
planet would have on the motion of Uranus, Professor A.
Gaillot recently arrived at the conclusion that there were
indications pointing to the possibility of still another and
more distant planet also exercising a perturbing influence.
The results of his calculations and studies therefore indi-
cate the possible existence of two ultra-Neptunian planets,
one at a distance from the Sun equal to 44 times the
Earth's mean distance and having a mass about stitcrf
the mass of the Sun, the other having a distance 66 times
that of the Earth and a mass about TH5W that of the Sun.
While these figures disagree with those of Professor Pick-
ering, yet the position calculated for the second planet
agrees quite closely with that of the Harvard astronomer.
The problem is by no means solved. It is mentioned to
show that a plausible case has been made out for at least
one ultra-Neptunian planet.

88 ASTRONOMY

After astronomers had definitely decided how far the
planets and the Sun are situated -from the Earth and how
they move with respect to one another, they began to won-
der if perhaps the whole solar system did not in turn re-
volve around some central orb. The possibility first
occurred to Tobias Mayer (1726), John Michel (1767) and
Joseph Jerome Le Francois Lalande (1776), but the prob-
lem was not attacked with any degree of success until Sir
William Herschel in 1782 began to draw conclusions from
his study of the Milky Way and decided that the entire solar
system was drifting toward the constellation Hercules. But
Herschel's theory did not meet with general acceptance for
many years. Other astronomers suggested various stars
as possible central suns controlling the movement of our
Solar System. Thus Madler not only proposed that Al-
cyone, the principal star in the Pleiades, should be the
central sun, because of its situation at a point of neutrali-
zation of opposing tendencies and consequently at rest, but
even went so far beyond the limits of astronomy as to
declare that "Here was the seat of the Almighty, the
Mansion of the Eternal." -It is hard even for science to
quell the imagination and to confine an observer to facts.
Madler's theory was short-lived. Further study of the
stars demonstrated the soundness of Herschel's views.

When a modern telescope is turned toward the Milky
Way this white girdle of the celestial sphere is resolved
into a vast number of stars of which more than 140,000,000
already have been counted on photographs. Each of these
stars is a sun like that which governs the Earth, probably
surrounded by planets like the Earth, and all these solar
systems also are moving, many of them more swiftly than
ours.

It was inferred from Herschel's measurements of stellar
positions, distances and motions that the solar system was
situated comparatively near the center of a universe
shaped like a thin, double-convex lens. This universe was

THE SOLAR SYSTEM 89

supposed to rotate as a unit about its center, with the
result that the Sun (comparatively near that center, but
absolutely at an immense distance from it) moved in a
circle of dimensions so vast that since the discovery of its
motion it had not deviated appreciably from a straight line,
but had steadily directed its course toward the constella-
tion Hercules.
This simple scheme must now be abandoned, for it has

• . • -^ .• • • *%% ,-' •. * «... •.,

***• » * • • • • •->.'*,.* * \ . • . !»..♦'

**«?*• ♦ %\*'z *."••**» • '•*•***

<^:.:x ^^ --<^\

Fig. 16 — The Place and Destination in the Universe of the
Solar System.

been discovered by Professor J. C. Kapteyn, a Dutch
astronomer, that the visible universe consists of two dis-
tant parts.. The scientific imagination is compelled to
picture two processions of stars moving in paths which
make an angle of 115 degrees with each other. One of
these stellar streams moves three times as fast as the
other. The Sun forms a part of one of the streams and
is at present at their intersection.

Altho it is known in a general way that the entire solar
system is moving through space at the rate of 12 miles a

90 ASTRONOMY

second, the shape and size of the Sun's orbit are utterly-
unknown. The changes of envifonment, accordingly, that
will accompany the description of it defy conjecture. The
actual course of the solar system is inclined at a small angle
to the plane of the Milky Way. Presumably it will become
deflected, but perhaps not sufficiently to keep the system
clear of entanglement with the galactic star-throngs. In
the present ignorance of their composition, no forecast of
the results can be attempted; they are uncertain and ex-
orbitantly remote. Hence, in a sense, the world knows
where it is and in what direction it is moving. But what
is the goal, when shall it be reached and what will happen
then ? Or, in this crossing of the congested thoroughfares
of the heavens, will the world be shattered by a collision
and resolved into a glowing nebula? This has been the
fate of many stars, of several within the period of human
history and of one — Nova Persei — within a few years and
witnessed in all its destructive detail by astronomers still
living.

CHAPTER IX

THE SUN 1 : ECLIPSES ; SUN-SPOTS AND AURORA J THE PHO-
TOSPHERE

The preeminence of the Sun among the celestial bodies
and its obvious importance to the life of the world has
given to it a unique place, not only in astronomy but in
philosophy and religion. The rising of the Sun and the
light that it casts over the Earth are distinctly symbolical
of the conquest of light over darkness or the triumph of
truth over error, so that in many schemes of mythology the
Sun god assumes the highest rank and rules over the other
elements, such as the Moon, the rains and floods and the
stars. Of this there are abundant instances in all primitive
religions. Among the Oriental nations, to whom we owe
the idea of a flood from which the world emerged, the Sun
god reigned supreme. In Egypt, among the Chaldeans
and Babylonians and later among the Greeks and Romans
the Sun held like sway. In Persia, Sun worship was
developed into a more formal religion, which survived for
many years.

After the mind of man had developed to a point where
it was concerned with philosophical speculation rather
than with the mysticism handed down by the priesthood,
there came a desire to understand the relation of the Sun
to the Earth and to the universe. Its motion across the
sky was obvious as well as its change of position from
time to time. On this, as has been seen, various theories
were founded. It was early realized that the Sun de-
91

92 ASTRONOMY

scribed an annual path on the celestial sphere, which path
is a great circle, and this great" circle, known as the eclip-
tic because eclipses take place only when the Moon is in or
near it, is at an angle to the equator of the sphere. This
angle is termed the obliquity of the ecliptic and was meas-
ured by the Chinese, it is claimed, as early as noo B.C. with
remarkable accuracy. Later the same feat was claimed for
Pythagoras or Anaximander in the sixth century B.C.,
both of whom probably derived their information from the
Chaldeans or Egyptians.

When the Sun crosses the equator the day is equal to
the night, which occurs twice a year, at the vernal equinox
about March 21 and the autumnal equinox about Septem-
ber 23. When the Sun is at its greatest distance from the
equator on the north side the time is known as the summer
solstice, and when at its greatest distance on the south side
it is termed the winter solstice. The positions of these
points in the heavens were also known to the early Chinese
astronomers with considerable accuracy, while Anaximan-
der, who supposed the Sun to be of equal magnitude with
the Earth, used a gnomon or vertical pillar casting a
shadow to observe the solstices and equinoxes.

Anaximenes is said to have believed that the Sun was a
mass of red-hot iron or of heated stone somewhat larger
than the Peloponnesus. He looked upon the heavens as a
vault of stones, prevented from falling by the rapidity of
its circular motion, while the Sun could not proceed be-
yond the tropics On account of a thick and dense atmos-
phere which compelled it to retrace its course. Later
Philolaus of Crotona, who was a disciple of Pythagoras
and followed his master's teaching that the Earth revolved
about the Sun, assumed that the Sun was a disk of glass
which reflected the light of the universe. Eudoxus of
Cnidus, about 370 B.C., stated that the diameter of the
Sun was nine times greater than that of the Moon, which
marked a triumph over the illusions of sense. About the
time of Alexander, the Great Pytheas, using the gnomon,

THE SUN 93

determined the length of the shadows cast at the summer
solstice in various countries. His observations are the
most ancient of those preserved after those made in China.

The study of the Sun was undertaken very systemati-
cally by Hipparchus, who discovered that the solar orbit
was eccentric and that the Sun moved at different speeds
at different parts of its journey. With his observational
data Hipparchus produced the first tables of the Sun which
are mentioned in astronomy. He was enabled to determine
the difference between the solar day or time as shown by
the Sun and the time indicated b)r some such measuring
device as the clepsydra or water-clock.

The motion of the Sun was also studied by Ptolemy.
He compiled solar tables more extensive than those of Hip-
parchus, which were employed until Albategni (b. 815)
made a new compilation of greater accuracy, and which
served as a connecting link between the astronomers of
Alexandria and those of modern Europe. The obliquity of
the ecliptic was constantly being studied. Ulugh Begh, a
Tartar prince and grandson of the great Tamerlane, at
Samarkand, using a gnomon 100 feet in height, determined
the obliquity of the ecliptic or precession of the equinoxes,
and secured data for the construction of astronomical
tables which were of considerable accuracy.

The apparent motions of the Sun furnished many diffi-
cult problems for the astronomer, since the observational
data lacked accuracy on account of the absence of satis-
factory instruments. Measuring angles by the shadow
cast and positions in the sky by crude forms of angular
measurement were not adequate for exact work. Not
until the advent of the quadrant and the telescope with its
various adjuncts was scientific measurement possible. But
there were from time to time solar eclipses, of which a
careful record was maintained and which the ancient
priests noted in connection with their calendar observa-
tions. These eclipses played a most important part in
ancient astronomy.

94 ASTRONOMY

An eclipse of the Sun occurs when Earth, Moon and
Sun are in direct line at the tkne of new Moon. As the
latter lies between the Earth and the Sun its dark body
will pass across the Sun's disk, cutting off the direct illumi-
nation. If the Earth cuts off the sunlight from the Moon
there is a lunar eclipse. Solar eclipses are of three kinds,
partial, annular and total. In the first the Moon, instead
of passing directly between the Earth and the Sun, slips
past on one side and cuts off from sight only a portion of
the Sun's surface. In the annular eclipse the Moon is cen-
trally interposed between the Sun and the Earth, but falls
short of the apparent size required to conceal the solar
disk entirely. Consequently at the height of obscuration a
bright ring is visible around the Moon. In a total eclipse,
however, the Sun completely disappears behind the dark
body of the Moon. The difference between a total and an
annular eclipse depends upon the fact that the apparent
diameters of the Sun and the Moon are so nearly equal as
to preponderate alternately one over the other through
the slight periodical changes in their respective distances
from the Earth.

It is the total eclipse that particularly arouses the atten-
tion of astronomers, for it cuts off entirely the light of the
Sun, and in addition to enabling the observation of the
various parts of its surface as it passes across its disk to be
made, also makes it possible for an observer to see the
stars and planets in the daytime even if they are very near
the Sun. A total eclipse of the Sun is not visible on the
entire Earth, but only along a comparatively narrow band,
lying roughly from west to east and measuring about 165
miles in width. A partial eclipse is seen for about 2,000
miles on either side of this band, but otherwise the phe-
nomenon is not visible on the Earth's surface.

Chinese records going back over 4,000 years describe a
solar eclipse which occurred during the twenty-second cen-
tury b.c. That eclipse carried with it a distinct moral lesson.
Two bibulous state astronomers, Ho and Hi, unfortunately

THE SUN 95

happened to be drunk on the day of its occurrence and
hence incapable of supervising the performance of the
required rites, which consisted in beating drums, shooting
arrows and other ceremonies intended to frighten away
the mighty dragon who was about to swallow up the Lord
of Day. Altho the eclipse was only partial, nevertheless
great confusion resulted, and Ho and Hi were put to
death as a lesson to later astronomers.

Another early record of a total eclipse comes from Baby-
lon, 1063 b.c. Several centuries later Assyrian tablets
record solar eclipses. Herodotus, Plutarch and the Bible
all refer to the phenomenon. Thus one is enabled to
determine with accuracy the time of historical events.
Likewise in the Anglo-Saxon chronicles several notable
eclipses are recorded.

The Sun appears as a brilliant white body. Just as
the Earth is enveloped by an atmosphere, so the Sun is
surrounded by several layers of gaseous and vaporous
matter, with the result that so far as an observer on the
Earth is concerned its nucleus is quite invisible. These
layers are more or less transparent, just as the atmosphere
of the Earth is transparent, so that the bright white body
of the Sun is visible only through these various envelopes.
This bright white portion is called the photosphere and is
the source of the light and heat which is radiated to the
Earth. Here are found the sun-spots distinctly seen with
the telescope and even by the naked eye. Under the pho-
tosphere it may be that the more solid portions of the Sun
are situated, but it is obvious that its surface consists of
highly incandescent vapors above which is a smoke-like
haze. Upon this rests the reversing layer, which is com-
posed of glowing gases, but is cooler than the photosphere
beneath. It has a thickness of between 500 and 1,000 miles
and contains, as the spectroscope shows, many of the ele-
ments of which the Earth is composed, in the form of
vapor. Above the reversing layer comes the chromosphere.
The chromosphere is between 5,000 and 10,000 miles in

96

ASTRONOMY

thickness and is composed of glowing gases, chief among
which is hydrogen. The chromosphere is a brilliant
scarlet in hue, but the redness is entirely overpowered by
the intense white light of the photosphere, which shows
through from behind. The most interesting features of the
chromosphere are the red prominences, which are the
marks of violent agitation in its upper portions and which

Photosphere

Fig. 17 — The Solar Structure.

are such a notable feature in total solar eclipses. After
the chromosphere comes the corona, which is the outer
envelope of the Sun and consists of a halo of pearly white
light of irregular outline which fades away into the sur-
rounding sky and extends outward for many millions of
miles. The corona suffers so much from the brilliancy of
the photosphere that it is only on the occasion of a total
solar eclipse that it can be seen in all its remarkable
beauty.

THE SUN 97

As the photosphere, the reversing layer and the chromo-
sphere are all sources of light, the solar spectrum observed
in spectroscopes is composed of the three separate spectra
combined. For this reason eclipses afford welcome oppor-
tunities for studying the Sun's surface. When the Moon
completely covers the photosphere its brilliant light is cut
off and the other features of the Sun can be examined
visually, and, what is more important, spectroscopically
and photographically. Thus when the spectroscope is di-
rected to the reversing layer during a total eclipse the
dark lines of the solar spectrum change into bright lines
or are reversed. But this reverse spectrum is a phe-
nomenon of a moment only, and as the Moon progresses
an altered spectrum is obtained. That of the chromo-
sphere is of sufficiently long duration to permit an esti-
mate of its depth and nature, and finally, when this is cov-
ered up, there is the corona, which has a distinct spectrum
of its own, containing a strange line, the distinguishing
green of which has not yet been identified with any
element known upon the Earth.

Modern conceptions of the Sun are due very largely to
the use of the telescope, the spectroscope and the spec-
troheliograph, especially the last two instruments. With
the telescope, when the intense light of the Sun is properly
reduced, it is possible to examine its surface and obtain a
certain amount of information as to its nature or to obtain
photographs of that surface by very short exposures. But
on the spectroscope and the spectroheliograph the astrono-
mer depends for his knowledge of the constitution and
composition of the great center of the solar system. The
development and nature of the spectroscope, as used in
solar research, have been already discussed, but it is ap-
propriate to add here a brief explanation of the spectro-
heliograph, for to .its use is due not only a large part of
present-day information of the prominences, but more
recently an explanation of the sun-spots themselves and
the study of various features of the photosphere.

98 ASTRONOMY

The spectroheliograph was first devised in successful
working form by Prof. George E. Hale at Kenwood Ob-
servatory, Chicago, in 1889. 'The principle of this instru-
ment is very simple," writes Professor Hale. "Its object
is to build up on a photographic plate a picture of the solar
flames by recording side by side images of the bright spec-
tral lines which characterize the luminous gases. In the
first place, an image of the Sun is formed by a telescope on
the slit of a spectroscope. The light of the Sun after
transmission through the spectroscope is spread out into a
long band of color, crossed by lines representing the vari-
ous elements. At points where the slit of the spectroscope
happens to intersect a gaseous prominence the bright lines
of hydrogen and helium may be seen extending from the
base of the prominence to its outer boundary. If a series
of such lines corresponding to different positions of the
slit on the image of the prominence were registered side
by side on a photographic plate, it is obvious that they
would give a representation of the form of the prominence
itself.

"To accomplish this result it is necessary to cause the
solar image to move at a uniform rate across the first slit
of the spectroscope, and, with the aid of a second slit
(which occupies the place of the ordinary eye-piece of the
spectroscope), to isolate one of the lines, permitting the
light from this line and from no other portion of the spec-
trum to pass through the second slit to a photographic
plate. If the plate be moved at the same speed with which
the solar image passes across the first slit, an image of the
prominence will be recorded upon it." The same method
answers for the study of the sun-spots and other features
of the Sun's surface. As the result of telescopic, spectro-
scopic and spectroheliographic observation it is now known
that the Sun's principal features are its sun-spots, its pho-
tosphere, its chromosphere and its corona.

A sun-spot when examined through a telescope consists
of a dark central region called the umbra into which long,

THE SUN 99

narrow filaments reach. The space occupied by these fila-
ments is termed the penumbra. The darkness of the umbra
is not absolute, but is relative to the more brilliant surface
of the photosphere, and if observed alone would be far
more brilliant than the most powerful arc light. These
dark spots on the Sun were familiar objects in the days of
pretelescopic observation. But their importance to as-
tronomy dates with their discovery in 1610 by Galileo with
his telescope. The great Italian astronomer did not an-
nounce his discovery until May, 1612, by which time sun-
spots had been seen independently by Thomas Harriott
(1560-1621), John Fabricius (1587-1615), who published
his observations in June, 161 1, or before Galileo, and the
Jesuit Father Christopher Scheiner (1575-1650), all of
them pioneer observers with the telescope.

Before sun-spots were clearly observed with the tele-
scope it was assumed that they were due to the transit of
Mercury. Even Father Scheiner, after his telescopic stud-
ies, suggested that the spots might be small planets revolv-
ing around the Sun and appearing as dark objects when-
ever they passed between the Sun and the observer. It
was recognised, however, that the spots appeared to move
across the face of the Sun from the eastern to the western
side, or roughly from left to right. Father Scheiner's view
was also held by Jean Tarde, Canon of Sarlat; by Father
Malpertius, a Belgian Jesuit, and later by William Gas-
coigne, the inventor of the micrometer. Galileo, however,
advanced a cloud theory, while Simon Marius, "astronomer
and physician" to the brother Margraves of Brandenburg,
proposed the ingenious "slag theory," according to which
the dark spots were the cindery refuse of a great solar
conflagration and occasionally expelled in the form of
comets, which afterward blazed up with renewed vigor.
Galileo in a controversy with Father Scheiner proved him
wrong in his planetary theory, while the occurrence of
comets in 1618 won supporters for the theory of Marius.
Galileo also ascertained the rotation of the Sun in a period

ioo ASTRONOMY

of between 25 and 26 days, as well as the general zone of
the sun-spots.

The next important contribution to sun-spot theory
came from Derham, whose observations were made during
the years 1703-1711. He believed that the spots on the
Sun were caused by the action of some new volcano, whose
smoke and other "opacous matter" produced the spots. As
they decayed they became half shadows encircling the
darker portions and finally became bright spots. Lalande,
the celebrated French astronomer, believed that the spots
were rocky elevations, about which the penumbra repre-
sented shoals or sandbanks, while around flowed enormous
oceans. Lalande's explanation, as well as that of Derham,
was clearly based upon terrestrial analogies, which were
employed with considerable frequency by early astrono-
mers.

In 1769 Alexander Wilson, of Glasgow (1714-1786), ex-
amining the large sun-spot visible that year, noted that, as
the Sun's rotation carries a spot across its disk, there was
a change in its appearance, and that the same effect of
perspective was produced as if it were a saucer-shaped
depression, the bottom forming the umbra or central black
spot and the sloping sides the penumbra or surrounding
portion of half shadow. The penumbra appeared narrow-
est on the side nearest the center of the Sun and widest on
the part nearest the edge. Hence Wilson assumed "that
the great and stupendous body of the Sun is made up of
two kinds of matter, very different in their qualities, that
by far their greater part is solid and dark, and that this
immense and dark globe is encompassed by a thin cover-
ing of that resplendent substance from which the Sun
would seem to derive the whole of his vivifying heat and
energy." Wilson went on to explain that the excavation
of spots might be occasioned "by the working of some sort
of elastic vapor which is generated within the dark globe,"
and that the luminous material, which was more or less

THE SUN 101

fluid, was acted upon by and tended to throw down and
cover the nucleus.

Sir William Herschel devoted considerable attention to
the Sun, and observing the variation in the sun-spots,
reached the conclusion in 1801 that it indicated a certain
variability in the total amount of solar radiation, which he
assumed might have some connection with terrestrial phe-
nomena, especially the weather. He endeavored first in
1801 to trace connection between the price of wheat, natu-
rally influenced by the effect of weather on the crops, and
the occurrence of sun-spots, claiming that when the latter
were scarce there was a diminished solar activity, which
caused colder weather, with obvious results. Ingenious as
this theory was, there were not sufficient substantiating
meteorological data. It finds, however, a counterpart in
modern studies of the Sun's heat, not necessarily connected
exclusively with sun-spots, however, whereby it is hoped to
establish some useful knowledge of the relation between the
amount of heat radiated from the Sun and weather con-
ditions on our Earth.

Herschel's observations of the Sun and sun-spots were
continued by his son, Sir John Herschel, at the Cape of
Good Hope (1836-1837). John Herschel assumed that
their motion was due to fluid circulations similar to those
producing the trade and anti-trade winds on the Earth.
"The spots, in this view of the subject," he said, "would
come to be assimilated to those regions on the Earth's
surface where for the moment hurricanes and tornadoes
prevail, the upper stratum being temporarily carried down-
ward, displacing by its impetus the two strata of luminous
matter beneath, the upper of course to a greater extent
than the lower, and thus wholly or partially denuding the
opaque surface of the Sun below."

Such observation of sun-spots, made with consider-
able thoroness by the astronomers mentioned, as well as
numerous others, did not establish any regularity in
their appearance or effacement. It remained for Heinrich

102 ASTRONOMY

Schwabe (1790-1875) at Dessau to announce in 1843 that
the sun-spot phenomena reached a maximum probably in a
decennial period. This announcement, altho coming as it
did after a patient study of the Sun, attracted no particular
attention until a series of sun-spot statistics were pub-
lished in Humboldt's "Kosmos." Then the correctness of
Schwabe's observations and deductions was apparent to
all. When compared by Dr. John Lamont and Sir Edward
Sabine with various periodical magnetic disturbances, it
was found that the two cycles of changes agreed with ex-
traordinary exactness. It was a remarkable coincidence
that the observations of a number of investigators were in
complete harmony. A study of sun-spot records estab-
lished the decennial period more correctly at 11. 11 years.
Thus commenced a recognition that magnetic disturbances
on the Earth were related in some way to sun-spot phe-
nomena. For many years no direct connection could be
established, altho various theories were forthcoming. Like-
wise further attempts were made to identify the variations
of sun-spots with meteorological phenomena, but without
success, until Wolf in 1859 by an examination of the
Zurich chronicles (1000-1800 a.d.) found data which en-
abled occurrences of the Aurora Borealis to be correlated
with a disturbed condition of the Sun.

From this time on the influence of the Sun on terrestrial
conditions assumed new importance. The beautiful phe-
nomenon of the aurora, which consists of a glow in the
sky about the north and south poles, had been observed for
ages, but the first scientific connection of importance re-
corded was in 1716, when Halley stated that the Northern
Lights were due to magnetic "effluvia." In 1^41 Hiorter at
Upsala observed that they produced an agitation of the
magnetic needle. This connection was further demon-
strated by Arago (1819), so that by the middle of the
nineteenth century the connection of the Aurora with the
Sun and in turn with terrestrial magnetism was as evident
as it was insufficiently explained.

THE SUN 103

The first result of the modern study of the sun-spots
was to put an end to the old notion that there was a dark
and cold interior of the Sun and that the sun-spots were
merely rents in the brilliant cloud covering through which
the interior portion could be seen. The late Prof. S. P.
Langley, one of the most active of the modern students of
the Sun and its surface, thought that the filaments which,
taken together, constitute the penumbra, were everywhere
present on the surface. Professor Hale states: "He re-
garded them as resembling the stalks of a wheat-field, seen
on end in the undisturbed photosphere, and revealing more
of their true characteristics in the penumbra, where they
are bent over and drawn out toward the central part of the
spot. Langley believed that we are observing clouds of
luminous vapors rising from the Sun's interior, the seats
of convection currents which bring to the surface the im-
mense supplies of heat radiated by the Sun into space.
Separating these luminous columns are darker regions,
characterized by a lower degree of radiation.

"The minute details can be recorded only with the great-
est difficulty. Under ordinary atmospheric conditions the
solar image is not seen as a sharp and well-defined object,
but its details are continually blurred by the effect of
irregularly heated currents in our atmosphere. Even under
the best conditions the moments of very sharp definition
are few, and the greatest patience and perseverance are
required on the part of an observer who would record his
impressions of the solar structure. At the best, drawings
based upon visual observations must be unsatisfactory,
since even the skilled hands of Langley could not secure
the perfect precision which is so desirable. It accordingly
might be hoped that here, as in other departments of solar
research, photography would afford the necessary means
of securing results unattainable by the eye. Unfortunately,
however, this hope has been only partially realized."

The influence of sun-spots is not confined to magnetic
and electrical phenomena. The researches of Koppen,

104 ASTRONOMY

which have been confirmed by Newcomb, show that the
average temperature of the Earth determined by the com-
bination of a great number of thermometric observations
made at several stations indicated a fluctuation of 0.30 to
0.70 C. during the 11-year sun-spot period. In other words,
the temperature of the Earth's atmosphere indicates small
fluctuations which correspond with the sun-spot period,
thus indicating that the solar heat radiation varies with the
number of the sun-spots. The mean temperature of the
Earth is greatest at the time of minimum sun-spots and
lowest at time of maximum sun-spots. Hence the determi-
nation of the amount of heat radiated by the Sun at vari-
ous times, especially at sun-spot maxima and minima, is
a matter of considerable terrestrial importance.

The study of the sun-spots carried on by Professor Hale
with the spectroheliograph and other apparatus, including
special red-sensitive plates of considerable speed, reached
an interesting stage in 1908, when it was demonstrated
that sun-spots are centers of attraction which draw toward
them the hydrogen of the solar atmosphere. Subsequently
it was found that these spots are the seats of great cyclones,
in which cool hydrogen gas is set whirling and is sucked
down in the great maelstrom of the Sun, rushing into the
center of the spot at a rate of about 60 miles a second.
Consequently the spots are the center of great solar dis-
turbances which are of an electro-magnetic nature. Ac-
cording to the modern electronic theory of matter and
electricity, ''electrons," or minute particles of matter, in
their terrific cyclonic velocity produce magnetic lines of
force. It was found by Professor Zeeman that, when light
is passed through a strong magnetic field, the lines of the
spectrum are subdivided and appear double. This Zeeman
effect Professor Hale has found in the spectrum of the
sun-spots. If one looks at the center of a spot the light
travels in the direction of the axis of the whirl or cyclone,
while in viewing a spot at the edge of the Sun the direc-
tion is at right angles to the axis and is manifested accord-

THE SUN 105

ingly in the spectrum. This theory has been thoroly con-
firmed, so that to-day it is known that sun-spots are mag-
netic fields of great intensity. The important discoveries
made by Professor Hale and his associates may thus be
summarized as follows : First, that the spots are cooler
than the surrounding region ; second, that they are centers
of violent cyclones, and, third, that they are magnetic fields
of great intensity.

In addition to the sun-spots the photosphere includes
other interesting features, notable among which are the
'faculae,' "little torches," so named by Father Scheiner.
These bright, globular objects, besides the sun-spots, are
the only other phenomena of the Sun's surface visible by
direct observation. Schroter showed that the facube are
heaped-up ridges of the disturbed photospheric matter.
Secchi and Young assumed that the faculse are the result of
violent eruptive action of the sun-spots, but it remained for
the spectroheliograph to give a clear idea of their nature.
Professor Hale states that "they are usually most numer-
ous in the vicinity of sun-spots, and near the Sun's limb
they are sometimes very conspicuous brilliant objects cov-
ering large areas. Near the center of the Sun, however,
they are practically invisible, tho faint traces of them can
sometimes be made out on photographs taken with a suit-
able exposure. This increase of brightness toward the
Sun's limb is assumed to be due to the elevation of the
faculse above the photospheric level and their escape from
a considerable part of that absorption which so materially
reduces the brightness of the photosphere. Rising above
the denser part of the absorbing veil and thus suffering
but little diminution of light, they appear near the Sun's
limb as bright objects on a less luminous background. The
chief difference of the faculae from the rest of the photo-
sphere lies in their greater altitude, as photographs have
shown that they may be resolved into granular elements
similar to those constituting the photosphere. But they are
the regions from which immense masses of vapors rise to

io6 ASTRONOMY

the solar surface and for that reason are important in the
solar mechanism.

"Near the edge of the Sun their summits lie above the
lower and denser part of that absorbing atmosphere which
so greatly reduces the Sun's light near the limb, and in this
region the faculse may be seen visually. At times they may
be traced to considerable distances from the limb, but as a
rule they are inconspicuous or wholly invisible toward the
central part of the solar disk. The Kenwood experiments
had shown that the calcium vapor coincides closely in form
and position with the faculae, and hence the calcium clouds
were long spoken of under this name. In the new work at
the Yerkes Observatory the differences between the cal-
cium clouds and the underlying faculse became so marked
that a distinctive name for the vaporous clouds appeared
necessary. They were therefore designated flocculi, a
name chosen without reference to their particular nature,
but suggested by the flocculent appearance of the photo-
graphs."

"With the spectroheliograph," Professor Hale relates,
"it was at once found possible to record the forms, not only
of the brilliant clouds of calcium vapor associated with the
faculse and occurring in the vicinity of the sun-spots, but
also of a reticulated structure extending over the entire
surface of the Sun. . . . From a systematic study of
spectroheliograph negatives, in the course of which the
heliographic latitude and longitude of the calcium clouds,
or flocculi, in many parts of the Sun's disk were measured
from day to day (by Fox), a new determination of the rate
of the solar rotation in various latitudes has been made.
This shows that the calcium flocculi, like the sun-spots,
complete a rotation in much shorter time at the solar equa-
tor than at points nearer the poles. In other words, the
Sun does not rotate as a solid body would do, but rather
like a ball of vapor, subject to laws which are not yet
understood."

CHAPTER X

THE SUN — II : THE REVERSING LAYER; THE CHROMOSPHERE
AND CORONA ; RADIATION PRESSURE AND SOLAR ENERGY

The so-called "reversing layer" was discovered by the
late Professor Charles A. Young during the eclipse of
December 22, 1870, on which occasion he placed the spec-
troscope with its slit tangential to the Sun's limb, so that
it ran along a shallow bed of incandescent vapors. When
the Moon reduced the size of the crescent of the Sun the
dark lines of the spectrum and the spectrum itself gradu-
ally faded away until all at once, as suddenly as a bursting
rocket shoots out its stars, the whole field of view was
covered by bright lines more numerous than one could
count. This phenomenon lasted for about two seconds and
gave the impression of a distinct reversal of the Fraun-
hofer spectrum, showing bright lines for dark in every
case. That such a reversing layer should exist was de-
manded by KirchofY's theory of the production of the
Fraunhof er lines, and implied a stratum of mixed vapors at
a lower temperature than that of the surface of the Sun.
It was by such a stratum that the missing rays of the solar
spectrum were stopped. The spectrum from this portion
alone should supply bright lines if the overpowering bril-
liancy of the solar background could be cut off, which can
occur only at the time of an eclipse. This observation of
Professor Young's, with its important bearing on the the-
ory of Kirchoff, was not confirmed until 1896, when photo-
107

108 ASTRONOMY

graphic evidence was forthcoming. During the eclipses
of 1898 and 1900 abundant corroborative material was ob-
tained, and the reversing layer'as a reality was conclusively
demonstrated. The total depth of this reversing layer has
been placed at from 500 to 600 miles. It continues in a
normal state of tranquillity, for little change is produced in
the aspect of the dark lines.

The chromosphere or envelope of glowing gases which
covers the Sun completely was detected by observers of
eclipses in the eighteenth and nineteenth centuries. There
is a record in a letter from Captain Stannyan to Flamsteed,
the British Astronomer Royal, describing an eclipse wit-
nessed at Berne on May 1 (O. S.), 1706, in which he states
that the Sun's "getting out of the eclipse was preceded by
a blood-red streak of light from its left limb." Halley and
De Louville in 1715 noted a similar phenomena, and it was
also observed during annular eclipses of 1737 and 1748, but
with the ruby brilliancy toned down to "brown" or "dusky
red" by the surviving sunlight. During the eclipses of
1820, 1836 and 1838 similar observations were made, but it
was not until the eclipse of the 8th of July, 1842, that the
virtual discovery of the chromosphere as a solar appendage
may be said to have been made. The eclipse of 1868, which
was observed spectroscopically and photographically as
well as with the telescope, served to make clear the nature
of the chromosphere and to reveal that it is a continuous
envelope of hydrogen and other incandescent gases, some
thousands of miles in thickness and of the same eruptive
nature as the prominences which are shot out from it. In
other words, it seems to be a collection of minute flames
set close together and giving it the appearance of a large
conflagration. The summits of the flames of fire, which
incline when the Sun's activity is greatest, are erect during
its phase of tranquillity.

The chromosphere is marked by an irregular distribu-
tion over the Sun's surface, which in no way partakes of
the character of an atmosphere. Professor Hale in 1897

SOLAR ENERGY 109

discovered a low stratum of carbon vapor. Such rare
metals as gallium and scandium have been discovered with
the spectroscope. The vapors of magnesium, iron and
several other substances are conspicuously represented in
the spectrum of the chromosphere, and with the Yerkes
telescope the fine bright lines due to the vapor of carbon
also may be seen.

The solar prominences are conspicuous eruptive or flame-
like emanations from the chromosphere which are seen
at total eclipses of the Sun. They project like red flames
beyond the dark edge of the Moon, and were first de-
scribed by Lector Vassenius, a Swedish professor of
Gothenburg, who observed the total eclipse of May 2
(O.S.), 1733. One of these reddish clouds outside of the
solar disk was so large that it could be detected with
the naked eye. The phenomenon excited his admiration
and wonder. The prominences were also observed in 1778
by the Spanish Admiral Don Antonio Ulloa, who was
convinced of their connection with the Sun on account
of their color and magnitude. By some observers the
solar prominences were regarded as the illuminated sum-
mits of lunar mountains, and by Arago they were de-
scribed as solar clouds shining by reflected light. Abbe
Peytal, in 1842, spoke of them as self-luminous and as
a third or outer solar envelope composed of the glowing
substance of the bright rose tint which produced moun-
tains, just as clouds were piled above the Earth's surface.
In 185 1 Hind, an English astronomer, noted on the south
limb of the Moon "a long range of rose-colored flames,"
which Dawes spoke of as a low ridge of red prominences
resembling in outline the tops of a very irregular range
of hills. Airy also noted this rugged line of projections,
and spoke of its brilliancy and "nearly scarlet" color.
But the truly solar origin of the phenomena was not con-
clusively demonstrated until i860, when the prominences
were photographed by Secchi and De la Rue, and shown
to be independent of the motion of the Moon. In 1868,

no ASTRONOMY

with the growth of spectroscopy and solar chemistry, the
gaseous nature of the prominences and their connectioa
with the Sun was made evident. They were found to
consist of immense masses of hydrogen and helium gas
rising from the chromosphere and reaching an altitude of
hundreds of thousands of miles.

The Corona. — The corona is a beautiful lustrous solar
wrapping, which can be observed only during a total
eclipse. The winged circle, the winged disk, or the ring with
wings, as it is variously called, found upon Assyrian and
Egyptian monuments, may be reproductions of the phe-
nomenon. The first definite mention of a solar corona
is to be found in Plutarch, in connection with
the eclipse which probably took place in 71 a.d. He
writes that the obscuration caused by the Moon "has no
time to last and no extensiveness, but some light shows
itself around the Sun's circumference which does not
allow the darkness to become deep and complete." Kepler
mentions a ray of light seen around the eclipsed Sun in
1567, and ascribes it to some sort of luminous atmosphere
around the Sun. In 1706 Cassini, observing an eclipse of
the Sun in France, saw the "crown of pale light" around
the lunar disk, and stated that it was caused by the illumi-
nation of the zodiacal light. Halley, observing an eclipse
in London in 171 5, describes minutely the phenomena of
the luminous ring rising around the Moon to a great
height, and showing considerable brilliancy. The eclipse
of 1842 was the first to indicate the corona's great im-
portance to astronomers, and from that date it received
careful attention and earnest study.

In 1869 Professor Harkness and Professor Young dis-
covered a bright line of unknown origin in the coronal
spectrum, showing that it consists in large part of glowing
gases. With the advent of astronomical photography, and
with the development of the spectroscope, more attention
than ever was given to the careful study of the corona

SOLAR ENERGY in

in the limited time available on the occasion of a total
eclipse. The corona is described by Professor Hale as
a "faintly luminous veil of light extending outward in
long streamers from the surface of the photosphere to dis-
tances of several millions of miles, and exceeded in bril-
liancy, even in its brightest parts, by the full Moon. In
many ways its streamers resemble those of the Aurora
Borealis, and it is indeed possible that their origin may
be ascribed to some similar electrical cause. During the
few minutes of a total eclipse they are not seen to undergo
change of form, but the outline of the corona does vary
greatly from year to year, in sympathy with the general
variation of the solar activity.

"Spectroscopic observations have shown that the corona
consists mainly of gases unknown to the chemist. That is
to say, the lines in its spectrum do not coincide in position
with the lines of any terrestrial element. Whether these
gases, which are probably very light, will ultimately be
found on the Earth cannot be predicted. Like helium,
first known in the Sun, they may eventually be encoun-
tered in minute quantities in some mineral where they
have hitherto escaped the chemist's analysis. The fact
that the lower part of the corona gives a continuous spec-
trum, with a feebler solar spectrum superposed upon it,
indicates that minute incandescent particles are present,
which are hot enough to radiate white light, and which
scatter enough sunlight to account for the presence of
the solar spectrum. " The strange line in the green por-
tion of the spectrum does not correspond with that of
any element with which we are acquainted upon the Earth,
and accordingly a hypothetical element has been assumed,
to which the name of "coronium" has been applied.

Chemical Composition. — Anaximenes* idea that the Sun
was a glowing ball of incandescent iron came almost as
near the truth as subsequent speculations by philosophers
and astronomers, until about the middle of the nineteenth

ii2 ASTRONOMY

century. In 1859 KirchofFs great discovery of the ex-
planation of the Fraunhofer lines in the solar spectrum
made it possible to ascertain the chemical compositon of
the Sun. Thus, as has been shown, the bright-colored
band formed of light from a small hole in Newton's shut-
ter passing through a prism made that prism the forerun-
ner of an instrument able to teach the nature and constitu-
tion of bodies far distant in the heavens. Thomas Mel-
vill had examined with a prism various flames in which
different substances had been introduced, and had reached
the conclusion by the middle of the eighteenth century
that certain vapors, notably sodium, contained light which
had a definite place in the spectrum. Melvill's deep yel-
low ray became the sodium line in the spectroscope of
Fraunhofer.

When Kirchoff noted the identity of certain lines char-
acteristic of terrestrial elements, and then assumed their
presence in the Sun, he laid the foundation of the modern
science of solar chemistry. Wherever light can be ob-
tained from a heavenly body it is now possible to resolve
it into its spectral elements and thus to identify the sub-
stances. In fact, as substances such as helium were found
in the Sun by Lockyer, which had no terrestrial counter-
part, hypothetical elements were assumed. The spectro-
scope has shown in the Sun the presence not only of gases
such as hydrogen and helium, but iron, sodium, mag-
nesium, calcium, and many other substances. Hence the
chemical composition of the Earth and the Sun are much
the same, altho there is evidence of the existence in
the Sun of substances not yet found in the Earth. When
the spectroscope was applied to the analysis of the chro-
mosphere and its prominences it was found that they are
composed of the vapor of calcium and of the light gases
helium and hydrogen. Sun-spots, too, have been found to
have a characteristic chemical composition, while the
corona emits rays which probably indicate the presence
in it of very light and tenuous gases.

Corona of the Sun, Taken During Total Eclipse.
Observatory.)

(Yerkes

SOLAR ENERGY 113

With the light given off by the Sun there is to be con-
sidered a phenomenon which only recently has been dem-
onstrated by experiment, namely, that light exerts a pres-
sure which can act on minute particles quite as effectively
as gravitation. Gravitation, however, attracts entire
masses, but pressure acts only on surfaces, so that a force
such as the pressure of light, to be effective, must deal
with very minute masses. If you subdivide a mass into a
large number of minute particles the effect of gravitation
is not changed, but a point will be reached in the subdivi-
sion where particles may be obtained having much surface
and very little weight. If such a particle has a diameter
of Vkmood °f an *ncn it will be exactly baianced in space,
pulled by gravitation (weight) on the one hand, pushed
by light on the other. If the particle is even smaller than
Viooow °f an in°h *n diameter the pressure of light upon it
pushes it away with terrific force. It is the radiation
of the Sun acting on the minute particles that produces
the phenomenon of comets' tails, and it is this pressure
which may be responsible for the brilliant phenomena of
the corona, visible only during the vanishing moments of
a total eclipse. No one has ever satisfactorily explained
how the highly attenuated matter composing both the
prominences and the corona is supported without falling
back into the Sun under the pull of solar gravitation.
Now that Arrhenius has cosmically applied the effects of
light pressure a solution is presented.

How difficult it is to account for such delicate stream-
ers as the "prominences" on the Sun is better compre-
hended when we fully understand how relentlessly power-
ful is the grip of solar gravitation. The Sun admittedly
projects vapors into space, vapors which must condense
into drops when they encounter the cold of outer space.
If the drops are larger than the critical size which de-
termines whether light-pressure or gravitation shall pre-
vail they will be snatched back by the Sun's gravitational
attraction and give rise to the curved prominences that

ii4 ASTRONOMY

are often observed. If they have approximately the criti-
cal diameter they will float above the Sun in the form
of beautiful carmine clouds, balanced in space by the
equal and oppositely acting forces of gravitation and
radiation pressure. These clouds have hitherto been par-
ticularly puzzling, for in the absence of a dense solar
atmosphere their existence seemed a celestial paradox.
If the condensed drops are smaller than the critical diame-
ter they will be projected by the pressure of light far
beyond the Sun, to form the beautiful pearly corona.

From the fact that comets have passed through the
corona without any very apparent retardation, some idea
of its tenuity may be gained. Assuming that the corona
consists of particles of such size that the radiation pres-
sure on. each exactly equals its weight, Arrhenius finds
that the entire corona weighs no more than 12,000,000
long tons, which is equivalent to four hundred large trans-
atlantic steamers, and is not more than the amount of
coal burned on the Earth every week.

Compared with the infinity of the space in which it
is poised, the Earth is smaller than a vanishingly small
speck on a sheet of paper having an area of many square
miles. So far as the Earth is concerned, the Sun is
very much in the position of a man who throws away
all but a single cent of a fortune consisting of twenty-
three million dollars; for only V2800000000 °f his radiated
energy reaches this globe. What, then, becomes of the
huge number of corpuscles which are shot from the Sun
and which never strike the Earth? It is conceived by
Arrhenius and his followers that many of them must col-
lide with corpuscles discharged by suns other than that
of this solar system — suns ineffably distant, so that their
light reaches the Earth only after the lapse of countless
centuries, and so that they are seen not as they gleam
now, but as they gleamed when Egypt was young and
Greece was a wilderness inhabited by savages. Such col-
lisions must result in the formation of larger masses up

SOLAR ENERGY 115

to a limit determined by the electrical charges carried by
the corpuscles.

Solar Energy. — The Earth depends upon the Sun for its
supply of heat and light. The Sun is transmitting heat
to the Earth, and unless the supply of energy is being re-
plenished in some way it is obvious that it must be losing
in heat and temperature. From its effect on the Earth an
estimate can be had of the amount of heat radiated by
the Sun annually. If it be assumed that the Sun has the
same heat capacity as water, and hydrogen is the only
substance with a greater heat capacity, it would fall in
temperature about 40 F. annually. If, therefore, the great
luminary were simply a hot body, cooling off, its present
rate of radiation could not be maintained for more than
3,000 years. That the Sun has been radiating a much
longer period than this is obvious from geological and
biological evidence, so that some other cause must be
sought.

Up to the nineteenth century the doctrine of infinite
durability was generally held by astronomers and geolo-
gists. For that reason no particular attention was paid
to the nature of the heat of the Sun and its source; but
with the formulation of the doctrine of the conservation
of energy it was realized that the energy of the Earth
must proceed from the Sun for the greater part, and con-
sequently it became necessary in turn to question the
source of solar heat. Robert Mayer, to whom is due the
earliest conception of the conservation of energy, asked
himself: If the Sun is hot, why does it not cool off?
In 1848 Mayer published some answers to his own ques-
tion in a paper which failed to receive the approval of
the French Academy of Sciences. His conclusions were
as follows: The Sun cannot be a glowing mass sending
out radiation without compensation. Solar heat cannot be
due entirely to chemical changes, nor can it be due to
solar rotation. In his opinion it was the result of meteors

n6 ASTRONOMY

falling into the Sun. He did not overlook the fact that
the resulting increase in the mass of the Sun would in-
crease its attraction for the planets and would shorten
the sidereal year. This was contrary to the facts of ob-
servation, and Mayer was forced to an incorrect concep-
tion of the undulatory theory of light to explain the situa-
tion. Six years later William Thomson, subsequently
Lord Kelvin, reached independently almost the same con-
clusion as Mayer, but he was able to explain the increase
in the Sun's mass resulting from meteoric showers, for
according to the gravitation theory the "added matter is
drawn from space, where it acts on the planets with very
nearly the same force as when incorporated in the Sun."

Lord Kelvin then ventured an estimate of the age of
the Sun, which was the first attempt in this direction made
by a physicist. He assumed that the solar energy of rota-
tion was derived from the fallen meteors, which, allowing
for the constant loss of solar energy by radiation, could
be acquired in 32,000 years. Taking into consideration
the limited amount of meteoric matter available near the
Sun, he concluded that "sunlight cannot last as at present
for three hundred thousand years." This theory attracted
little attention when promulgated by him in 1854, and
was abandoned by him later.

But in the same year Helmholtz, working along the
lines of the nebular hypothesis of Kant and Laplace, de-
rived the heat of the Sun from the contraction of the
nebula from which the Sun and planets were formed.
He asserted also a further contraction of the Sun, now
assumed to be in progress, by which the kinetic energy
obtained was converted into heat, and compensated for
the loss of solar heat by radiation. Accordingly, if the
Sun contracts one ten-thousandth part of its radius each
year, enough heat would be generated to supply radia-
tion for 2,100 years. Helmholtz' computation gives
twenty-two million years as the probable age of the Sun,
based on a uniform radiation and homogeneous density

SOLAR ENERGY 117

of that body. Later, S. P. Langley, with experimental
data derived from the direct radiation of the Sun, reduced
this age to eighteen million years. This theory immedi-
ately supplanted that of the falling meteors, which more
serious reflection demonstrated could account only for
the slight increase in the solar heat as compared with
the energy of shrinkage.

'• Lord Kelvin, in 1862, returned to the subject, favoring
a theory like that of Helmholtz', concluding that "we
may accept as the lowest estimate for the Sun's initial
heat 10,000,000 times a year's supply at the present rate,
but 50,000,000 or 100,000,000 as possible in consequence
of the Sun's greater density in his central parts." "As
for the future, . . . inhabitants of the Earth cannot con-
tinue to enjoy the light and heat essential to their life
for many million years longer unless sources now un-
known to us are prepared in the great storehouse of crea-
tion." Studies of the Sun's heat were continued by
Lord Kelvin, and in his theory he incorporated a dis-
covery made in 1870 by J. Homer Lane, an American,
which paradoxically demonstrated that within certain
limits the more heat a gaseous body loses by radiation the
hotter it will become.

This theory of Helmholtz', as modified by Kelvin, en-
countered a serious rival in 1882 when Sir William Sie-
mens proposed that a rotating Sun hurled by centrifugal
action at the equator enormous quantities of gas into
space, which returned to it again at the poles, somewhat
after the manner of a regenerative furnace. Helmholtz'
theory was modified in 1899 by Professor T. J. J. See,
who abandoned the German scientist's hypothesis of homo-
geneous density for the Sun and applied Lane's law, in-
vestigating minutely the more complex case of central
condensation. As the result of his study the probable
age of the Sun was extended from twenty-two million to
about thirty-two million years.

All of these researches came before the discovery of

Ii8 ASTRONOMY

radium, and when the extraordinary properties of this
new substance were known it was stated that a bare frac-
tion of a per cent, of radium present in the Sun would
account for and make good the heat that is annually lost
by radiation. Should this hypothesis hold good, an en-
tirely new aspect is given to the problem of solar heat
and to the heat of the Earth. This innovation, advanced
by members of the younger school of physicists, all of
whom had prosecuted with vigor researches in radioactiv-
ity, did not appeal to Lord Kelvin, who maintained in
1906 that the gravitation theory was still sufficient to
account for the Sun's heat. No evidence has been pro-
duced of the presence of radium in the Sun, altho helium
has been found there. As helium is obtained from radium,
the existence of radium in the Sun is quite probable.

Sir George H. Darwin, in discussing the effect of these
recent discoveries on solar age, says: "Knowing, as we
now do, that an atom of matter is capable of containing
an enormous store of energy in itself, I think we have no
right to assume that the Sun is incapable of liberating
atomic energy to a degree at least comparable with that
which it would do if made of radium. Accordingly, I see
no reason for doubting the possibility of augmenting the
estimate of solar heat, as derived from the theory of
gravitation, by some such factor as ten or twenty."

It is obvious, therefore, that while the contraction the-
ory explains the origin of a vast amount of the Sun's
heat, yet there are other sources of internal energy
which recent discoveries plainly indicate are of great
importance, so that the scientist at this stage is unable
to declare positively the age of the Sun or to make any
accurate estimate as to the probable duration of the time
through which it will afford light and heat to the Earth
and the other planets. Nevertheless, it seems assured that
millions of years hence, how many cannot even roughly be
determined, the Sun will be reduced from a ball of glowing
vapor to a gigantic black cinder rushing through space.

SOLAR ENERGY 119

Unwarmed by any central luminary, its crust will be
washed by oceans of air liquefied by a cold too intense for
any living creature to endure.

The light of the Sun is obviously more intense than
any other luminant known to man. If compared with
' the full Moon, 600,000 times as much light is received
from the Sun. Expressed in another way, the Sun gives
over 60,000 times as much illumination as a standard
candle at a distance of one yard. But not all of the light
and heat which is radiated from the Sun comes to the
Earth. Professor Langley, in his experiments at Mt.
Whitney in 1881, found that a clear atmosphere would
cut off 40 per cent, of the rays coming perpendicularly
to the Earth's surface. Gases in the atmosphere, such
as carbon dioxide, cut off even a greater amount, and the
general absorption is greater at the violet end than at
the red. It is for this reason that high altitudes and a
clear atmosphere are essential for solar investigation;
and for this reason, too, that the setting Sun appears red,
the bluish rays being absorbed in traversing a greater
amount of terrestrial atmosphere than when the Sun is
high in the sky.

The temperature of the Sun can be estimated from its
brilliancy and from spectroscopic and bolometric studies.
It is a common experience that a filament of an incan-
descent lamp emits more light and glows more brightly
when the amount of current is increased. At first the
filament is red, but as more current is permitted to flow
it becomes yellow, and finally a brilliant white. This is
marked by an increase in temperature; and secondly, the
temperature depends upon the brilliancy of the glow. The
same analogy holds good in the case of the Sun and the
stars. If the wave length of the radiation is known,
and the color which emits the greatest amount of heat in
the spectrum — and this can be measured by the bolometer
by a simple law — it is possible to calculate the absolute
temperature of a star. Then by deducting 2700 from the

120 ASTRONOMY

result its temperature in the ordinary Centigrade degrees
is obtained. Thus in the case- of the Sun the maximum
heat radiation occurs in the greenish-yellow light, which
gives a temperature for the rotating disk of the Sun of
about 5,ooo0 C, or 9,000° F. The atmospheric absorption
already referred to serves to cut down the intensity of the
radiation, so that, taking this and other amounts into con-
sideration, the temperature of the Sun's disk can be esti-
mated at about 6,200° C.

Similar investigation in the case of Sirius and Vega,
which are white, or younger stars, give a temperature
about 1,000° C. higher than that of the Sun, while the
red star, Betelgeux, which is a declining star, older than
the Sun, has a temperature some 2,500° C. less. The
temperature of the Sun furnishes different results, de-
pending on the manner in which the problem is attacked.
Arrhenius, in his work on "Worlds in the Making," sum-
marizes recent work, and states that: "From the inten-
sity of the radiation, Christiansen, and afterward War-
burg, calculated a temperature of about 6,000° C. Wilson
and Gray found for the center of the Sun 6,200°, which
they afterward corrected into 8,000°." Owing to the
absorption of light by the terrestrial and the solar at-
mospheres, too low values are always found. That ap-
plies to a still greater extent to any estimate based upon
the determination of that wave length for which the heat
emission from the solar spectrum is maximum. Le Cha-
telier compared the intensity of sunlight filtered through
red glass with the intensities of light from several ter-
restrial sources of fairly well-known temperatures treated
in the same way. These estimates yielded to him a solar
temperature of 7,600° C. Most scientists accept an abso-
lute temperature of 6,500°, corresponding with about
6,200 C. That is what is known as the "effective tempera-
ture" of the Sun.

If the solar rays were not partially absorbed this tem-
perature would correspond with that of the clouds of the

SOLAR ENERGY 121

photosphere. Since red light is little absorbed, compara-
tively, Le Chatelier's value of 7,600° C, and the almost
equal value of Wilson and Gray of 8,ooo° C, should repre-
sent approximately the average temperature of the outer
portions of the clouds of the photosphere. The higher tem-
perature of the faculae is evident from their greater light
intensity, which, however, may be partly due to their
greater height. Carrington and Hodgson saw, on Sep-
tember 1, 1859, two faculae break out from the edge of
a sun-spot. Their splendor was five or six times greater
than that of the surrounding parts of the photosphere.
That would correspond with a temperature of about 10,000
or 12,000° C. The deeper parts of the Sun which broke
out on these occasions evidently have a higher tempera-
ture, and this is not unnatural, since the Sun is losing
heat by radiation from its outer portions.

At all events, the Earth receives a large amount of
energy in the form of light and heat which amounts to
three horsepower for every square yard of space perpen-
dicular to the Sun's rays ; and while this really is not avail-
able for mechanical purposes, yet solar engines have been
constructed in which this energy has been transformed
into power. While the energy of the Sun is not, gener-
ally speaking, available for mechanical purposes, yet indi-
rectly the heat received by the Earth has made possible
plant and animal life, on which depends the source of
all energy.

The transmission of the Sun's heat to the Earth is one
of the important problems of present-day physics and
meteorology, inasmuch as the amount of radiation or
heat emitted by the Sun, spoken of by physicists as the
"solar constant" and the relation of this radiation to ter-
restrial temperature, as well as the study of the radiation
of different parts of the Sun's disk, are all topics of fun-
damental importance. The solar constant is measured
outside of the Earth's atmosphere at mean solar distance,
and the intensity of radiation is employed for a unit,

122 ASTRONOMY

which, when fully absorbed for one minute over a square
centimeter of area placed at right angles to the ray, would
produce sufficient heat to raise the temperature of a
gram of water i° C.

If once the original quantity and kind of heat emitted
by the Sun be known, its effect on the constituents of
the atmosphere on its journey to the Earth, how much
of it reaches the soil, how through the change of the
atmosphere it maintains the surface temperature of our
globe, and finally, how in diminished quantity, or altered
kind, it is returned to outer space, it would be possible
to predict nearly all of the phenomena of the weather.
Thus it has been known that when there is a small de-
crease in the solar radiation there follows a marked and
general decline in temperature. So that, knowing the
variation of radiation, it should be possible to predict
changes in climate.

These data are secured by measuring the total intensity
of the radiation, as it arrives at the Earth's surface, with
the pyrheliometer, or thermometer with blackened bulb,
carefully protected from all other influences except the
direct rays of the Sun; and in the second place by meas-
uring the heat or energy in different parts of the solar
spectrum with the spectro-bolometer. The absorption of
the atmosphere nearer the Earth for different areas re-
quires measurements to be made at several stations, for at
Washington, near the sea-level, the intensity of radiation
actually observed is only about three-fourths as great as
that observed in the clear atmosphere of Mount Wilson,
at a height of 6,000 feet. Recent observations made at
Mount Wilson and Washington by Mr. Abbot, of the
Astrophysical Laboratory of the Smithsonian Institution,
indicate that heat sent out to the Earth from the Sun
in the course of a year is capable of melting an ice shell
114 feet thick over the whole surface of the Earth. The
solar radiation is not a constant quantity, but varies with
the decrease in solar distance, the changes occurring from

SOLAR ENERGY 123

month to month and from year to year. The variation is
due to the changes in the source of radiation rather than
to the effects of our atmosphere or external causes.

The distance and position of the Sun as regards the
other members of the Solar System has been considered in
a previous chapter. It is known that its apparent diameter
is 32' 4", which corresponds with 866,500 miles, or 109^2
times the diameter of the Earth. This would give it an
area 11,950 times that of the Earth and a volume
1,306,500 times greater. The actual mass of the Sun is
332,000 times that of the Earth, but its average density is
only about a quarter as great, so that the Sun, which has
a density of 1.41 as compared with water, is four times as
large as it would have to be if its density were the same
as that of the Earth. Taking into consideration this light-
ness as well as the high temperatures prevailing in the Suny
one is forced to the conclusion that the body of the Sun
must be in a gaseous state. The conditions under which
the gases are found must be quite different from those
with which we are acquainted on the Earth. Gravity at
the surface of the Sun exceeds by more than twenty-seven
times gravity on the Earth.

The motion of the Sun as regards the other stars in the
heavens has been treated elsewhere, but it is proper here to
refer to its rotation on its axis from west to east, which
takes place in a period of about 25 days and is apparent
from the motion of the sun-spots, tho it can also be de-
tected by directing a spectroscope toward the edges of the
limb and noting by Doppler's principle how one side is
approaching the observer and the other is retreating. The
time of rotation is not the same for all parts of the disk,
but depends upon the position of the spots selected. Those
nearest the equator show the most rapid rotation.

CHAPTER XI

Ancient records make no mention of the discovery of
Mercury, yet the existence of the planet was surely known
even in the days of Nineveh, when a chief astronomer
directly refers to the planet in a report which he made to
King Assurbanipal of Assyria. The planet is occasionally
mentioned in early and medieval astronomical literature.
It is stated that Copernicus regretted that he had never
been able to observe it properly in the high altitude of
Frauenburg.

That the planet should have been overlooked by the
ancients is not strange when it is considered that it is
never visible in the higher altitudes except occasionally
near the horizon, just after sunset or before sunrise. In
the clear sky over an eastern desert the primeval astrono-
mer doubtless saw the bright star in that part of the hori-
zon where the setting Sun was still shedding its beams.
Its luster diminishes as the planet draws near the horizon
at sunset, until finally it sets so soon after the Sun that
it is invisible. Years may elapse before a similar oppor-
tunity is afforded.

If a similar phenomenon took place at sunrise, the primi-
tive astronomer might have inferred that Venus and Mer-
cury were identical, especially as a long series of observa-
tions would establish the fact that one of these bodies was
never seen until the other had disappeared. Accordingly
some of the ancient astronomers assumed that there was
124

MERCURY 125

but one morning and evening star. But as records accu-
mulated it was recognised that there were two bodies
which might serve as morning and evening stars. A cer-
tain regularity in the recurrence of each planet was noted,
and it was possible to make predictions of accuracy as to
the time when either could be seen after sunset or before
sunrise.

'While by the time of Plato it was known that Venus and
Mercury performed their revolution in approximately the
same time, it was recognised that Mercury's period was
different and more rapid than that of other planets. An
older tradition, attributed to the Egyptians, stated that
both planets revolved around the Sun. Ptolemy states
that they could be regarded as oscillating to and fro on
each side of the Sun. That the ancient astronomers might
well have been confused by the appearance of Mercury
as morning and evening star follows from a considera-
tion of the planets' position and motion relatively to the
Sun and the Earth. The consideration, moreover, ap-
plies to Venus as well.

Quoting C. G. Dolmage's "Astronomy of To-day,"
"when furthest from us Mercury is at the other side
of the Sun, and cannot then be seen owing to the
blaze of light. As it continues its journey it passes
to the left of the Sun, and is then sufficiently away from
the glare to be plainly seen. It next draws in again to-
ward the Sun, and is once more lost to view in the blaze
at the time of its passing nearest to us. Then it gradually
comes out to view on the right hand, separates from the
Sun up to a certain distance as before, and again recedes
beyond the Sun, and is for the time being once more lost
to view.

"To these various positions technical names are given.
When the inferior planet is on the far side of the Sun
from us it is said to be in 'Superior Conjunction.' When
it has drawn as far as it can to the left hand, and is then
as east as possible of the Sun, it is said to be at its 'Great-

126 ASTRONOMY

#

Greatest ,

Western ( ■ <?

0.

it

66Ct)#(i(i66

Fig. 1 8 — Orbit and Phases of an Inferior Planet.
Corresponding views of the same situations of an inferior planet
as seen from the earth, showing consequent phases and alter-
ations in apparent size.

MERCURY 127

est Eastern Elongation.' Again when it is passing nearest
to us it is said to be in 'Inferior Conjunction'; and finally,
when it has drawn as far as it can to the right hand, it is
spoken of as being at its 'Greatest Western Elongation.' "

The continual variation in the distance of the inferior
planets, Venus and Mercury, from the Earth, during their
revolution around the Sun, will of course be productive of
great alterations in apparent size, which no doubt also
had its effect in bewildering the ancients. At superior
conjunction, being then farthest away, Mercury ought to
show the smallest disk; while at inferior conjunction, be-
ing the nearest, it should appear much larger. When at
greatest elongation, whether eastern or western, it should
naturally present an appearance midway in size between
the two.

From these considerations it would seem that the best
time for studying the surface of Venus or Mercury is at
inferior conjunction, or when the planet is nearest to the
Earth. But that this is not the case will at once appear
if it is considered that the Sun's light is then falling upon
the side most distant, leaving the proximate side unil-
lumined. In superior conjunction, on the other hand,
the light falls full upon the side of the planet facing the
Earth ; but the disk is then so small and the view besides
is so dazzled by the proximity of the Sun that observa-
tions are of little avail. In the elongations, however, the
sunlight comes from the side, and so we see one-half of
the planet lit up; the right half at eastern elongation and
the left half at western elongation. Piecing together the
results given at these more favorable views, it is possible,
bit by bit, to gather some small knowledge concerning the
surface of Mercury or Venus.

Consequently it will be seen that the inferior
planets show various phases comparable with the wax-
ing and waning of the Moon in its monthly round.
Superior conjunction is, in fact, similar to full Moon,
and inferior conjunction to new Moon; while the eastern

128 ASTRONOMY

and western elongations may be compared respectively
with the Moon's first and last quarters. When these
phases were first seen by the early telescopic observers,
the Copernican theory was felt to be immeasurably
strengthened; for it had been pointed out that if this sys-
tem were correct, the planets Venus and Mercury,
were it possible to see them more distinctly, would of ne-
cessity present phases when viewed from the Earth.

The apparent swing of an inferior planet from side to
side of the Sun, at one time on the east side, then passing
into and lost in the Sun's rays, to appear once more on the
west side, is the explanation of what is meant when one
speaks of an "evening star" or a "morning star." Mercury
of Venus is called an "evening star" when it is at its east-
ern elongation — that is to say, on the left hand of the Sun ;
for, being then on the eastern side, it will set after the Sun
sets, as both sink in their turn below the western horizon
at the close of day. Similarly, when either planet is at its
western elongation — that is to say, to the right hand of the
Sun — it will precede him, and so will rise above the eastern
horizon before the Sun, receiving therefore the designa-
tion of "morning star.

Mercury's motions were early studied. With the planet
was associated the first of the remarkable astronomical
predictions that now have become almost commonplace,
so carefully are they worked out by astronomers. Kep-
ler, from his studies of the motions of the planet, was the
first to realize that if the orbit of Mercury, which, as has
been seen, lies within that of the Earth and thus nearer
the Sun, were exactly circular and in the plane of the
ecliptic, a transit of Mercury across the Sun's disk would
occur once in each synodical revolution, or period between
two successive conjunctions of the planet with the Sun as
seen from the Earth, the epoch of the transit could be cal-
culated very easily. But Mercury's orbit is inclined 7
degrees to the ecliptic and is very eccentric, for which
reason his calculations were only approximately correct.

MERCURY 129

It was in 1627 that Kepler predicted transits of both
Mercury and Venus across the Sun, and assigned the date
of November 7th for the former. Gassendi, to whom
was entrusted the task of proving Kepler's prophecy, be-
gan his observation on the 5th of November, watching
the image of the Sun formed by a lens on a white screen
from light admitted through a hole in a dark room. But
Kepler was not in error by as much as that. Only five
hours after the time assigned did the transit actually begin.
Thus was commenced a series of observations of the
time when Mercury crosses the great central luminary of
this system. These transits are by no means rare, for
thirteen of them were observed in the nineteenth century.
They afford opportunity for observation which enables
the movements of the planet to be calculated with accu-
racy.

After the transits of 1661 and 1677 La Hire constructed
new tables of Mercury and predicted a transit on May 5,
1707. The calculation was in error by nearly a day.
After the transit of May, 1753, had been observed several
hours later than La Hire's and as much earlier than
Halley's prediction, Lalande constructed new tables, and
predicted the transit of 1786, which actually took place
fifty-three minutes later than the time announced by
Lalande and about as much earlier than the time com-
puted by Halley's tables. Lalande then corrected his ta-
bles and predicted the transits of 1789, 1799 and 1802 with
a fair approach to accuracy. The tables compiled by Lin-
denau in 1813 still left much to be desired.

The problem of Mercury's motion was in this condition
when it was attacked by Leverrier, who was under no
illusion in regard to its difficulty. Leverrier did not suc-
ceed in overcoming the difficulty until 1859. As a result
of his work, supplemented by that of later investigators,
astronomers are now in possession of at least fairly ac-
curate information on the subject of the planet's orbital
phenomena. It is known, for example, that Mercury is the

130 ASTRONOMY

swiftest in its movements of all the planets. With the ex-
ception of some of the satellites,"it has an orbit that departs
most from the circular form — in other words, an orbit
marked not only by the greatest eccentricity, but also by the
greatest inclination of all planetary orbits to the ecliptic.
This eccentricity of the orbit is such that the Sun is
seven and one-half million miles out of its center, while
the actual distance of the planet from the Sun ranges from
about twenty-nine million miles to forty-three million, or
a mean distance of about thirty-six millions of miles. The
velocity varies from thirty-five miles a second, when the
planet is at perihelion or nearest the Sun at the pointed
end of its oval course, to about twenty-three miles a sec-
ond at aphelion.

To explain the perturbations in Mercury's motion, Abbe
Moreux has recently advanced the following hypothesis:
"The Sun is unquestionably surrounded by matter which
gives rise to the observed phenomena of the corona. Pho-
tographs made during the eclipse of 1905 showed an outer
corona extending to a hitherto unsuspected distance from
the Sun. This outer corona is ellipsoidal and its major
axis is very nearly coincident with that of the zodiacal
light. This region, then, is filled with a resisting medium,
composed of matter which is gradually falling in toward
the Sun. The mechanism of this fall, or contraction, is
very complex, and as we have no idea of the density of the
matter, it is difficult to calculate its changes in form.

"The resistance which the medium opposes to a mov-
ing body is likewise difficult to estimate, but we know
from the observed acceleration of comets that this re-
sistance is by no means negligible. At the epochs of max-
imum coronal development this resisting medium may
easily extend to the orbit of Mercury. Hence that planet
in its revolution around the Sun must traverse masses of
rarefied gas and swarms of minute particles by which its
course is modified. We are far from knowing the quan-
titative effect of the resisting medium, and much study of

MERCURY 131

the corona will be required to complete the solution of this
interesting problem."

Because of the lack of surface markings the rotation
periods are very uncertain. Recently Professor McHarg,
in France, has deduced a rotation of 24 h. 8 m. G. Schia-
parelli has put forward the view that both Venus and
Mercury rotate in a time equal to the individual period
of revolution around the Sun, and thus always turn the
same face toward the Sun. Such a motion, which is
analogous to that of the Moon around the Earth, could
be easily explained as the result of tidal action at some
past time when the planets were, to a great extent, fluid.
This may be true, as Mercury is one of the planets
which has no Moon and consequently will be influenced
directly and solely by the Sun in so far as tidal phenomena
are concerned. These tidal effects must be especially se-
vere on account of the proximity of Mercury to the Sun.

Because o^ this lack of a satellite the mass of the planet
is very difficult to determine. At various times Laplace,
Encke and Leverrier have given values from 1/9 to 1/30
of the Earth's mass. The first convenient opportunity
of placing this planet in a gravitational balance and of de-
termining its mass was presented in August, 1835, when
Encke's comet in the course of its eccentric path through
the solar system penetrated within the orbit of Mercury
at its perihelion. The attractive power of the planet
produced only a slight deviation from the regular course
of the comet, sufficient, however, to show that the value
previously assumed for Mercury's mass was nearly twice
too large. At the same time the calculations made from
these data have not removed the uncertainty that still
prevails upon this point; Simon Newcomb decided on a
value of about 1/21 that of the Earth, while William
Harkness made it 1/25.

Beyond its motion comparatively little is known of
Mercury. The spectroscope seems to indicate that the
atmosphere of Mercury contains water vapor just as does

132 ASTRONOMY

the Earth. But the studies of. Professor Lowell at Flag-
staff, Arizona, do not show any signs of clouds or obscu-
rations, and there are no indications of any atmospheric
envelope. In fact, the surface of Mercury has been termed
colorless, "a geography in black and white."

The first student of the surface of Mercury was Johann
Hieronymus Schroter (1745-1816), who, as will be
seen, was a pioneer in the study of the topography of the
Moon. In April, 1792, Schroter concluded from the grad-
ual degradation of light on its brightly illuminated disk
that Mercury possessed a tolerably dense atmosphere.
During the transit of May 7, 1799, he was struck with the
appearance of a ring of softened luminosity circling the
planet to a height of 3" and about a quarter of its own
diameter.

Referring to this ring of softened luminosity, Agnes
Clerke writes in her 'History of Astronomy': "Altho a
'mere thought' in texture, it remained persistently vis-
ible both with the seven-foot and the thirteen-foot re-
flectors, armed with powers up to 288. A similar ap-
pendage had been noticed by De Plantade at Montpellier,
November 11, 1736, and again in 1786 and 1789 by Pros-
perin and Flaugergues; but Herschel, on November 9,
1802, saw the preceding limb of the planet projected on
the Sun cut the luminous solar clouds with the most per-
fect sharpness. The presence, however, of a 'halo' was
unmistakable in 1832, when Professor Moll, of Utrecht,
described it as a 'nebulous ring of a darker tinge approach-
ing to the violet color.' Again to Sir William Huggins
and Stone, on November 5, 1868, it showed as lucid and
most distinct.

"No change in the color of the glasses used, or the pow-
ers applied, could get rid of it, and it lasted throughout
the transit. It was next seen by Christie and Dunkin at
Greenwich, May 6, 1878, and with much precision of de-
tail by Trouvelot at Cambridge, Mass. Professor Holden,
on the other hand, noted at Hastings-on-Hudson the total

MERCURY 133

absence of all anomalous appearances. Nor could any
vestige of them be seen by Barnard at Lick on November
10, 1894. Various effects of irradiation and diffraction
were, however, observed by Lowell and W. H. Pickering
at Flagstaff; and Davidson was favored at San Francisco
with glimpses of the historic aureola, as well as of a cen-
tral whitish spot which often accompanies it. That both
are somehow of optical production can scarcely be
doubted."

The planet's physical condition presents many problems
and the spectroscope affords little information as to its
constitution. As it shines only by reflected light from the
Sun, its spectrum is but a fine echo of the Fraunhofer
lines. An atmosphere like that of the Earth was sus-
pected, however, by H. C. Vogel in 1871 from his spec-
troscopic studies, tho on the very slightest grounds.

Later observations made at the Potsdam Observatory by
Miiller confirm this conclusion and demonstrate that Mer-
cury has a rough rind of dusky rock, which absorbs all
but 17 per cent, of the direct light radiated upon it by
the near-by Sun. This would seem to show the utter
absence of any appreciable Mercurian atmosphere. The
probable lack of atmosphere is also shown by the circum-
stance that when Mercury is just about to transit the face
of the Sun, no ring of diffused light is seen to encircle its
disk, as would be the case if it possessed an atmosphere.

According to the Belgian astronomer, Stroobant, the
linear diameter of Mercury expressed as a fraction of the
Earth's is 0.350 instead of the previously accepted 0.373,
the radius 2,232 kilometers (1,386.7 miles), and the vol-
ume, compared with that of the Earth, 0.042.

CHAPTER XII

For all time Venus has been known as evening and
morning star. By the ancients it was supposed to be two
separate bodies which were named Phosphorus (Lucifer)
or Morning Star and Hesperus (Vesper) or Evening
Star. For Pythagoras in the sixth century B.C. is claimed
the honor of discovering the identity of the two stars, but
it was probable that he restated the views of Eastern
astronomers.

As the most brilliant star of the heavens, Venus is cer-
tainly the one that has always been most observed. Glit-
tering in the sky like a clear diamond, its pure white light,
which, on a night when there is no Moon, is strong enough
to cast a shadow, naturally would impress all whose lives
are spent out of doors. Hence Venus has always figured
in literature as the "Shepherds' Star." Homer speaks of
the planet as Kailistos — the "Beautiful," and as a type of
beauty its worship figures in all mythologies. At a time
when the planets were personified as gods and goddesses
it is easy to understand why this star was selected to typ-
ify Love.

Not only on account of its splendor in the evening sky,
but for other phenomena, Venus is familiar in history and
literature. The historian Varro (116-28 b.c.) states that
"iEneas on his voyage from Troy to Italy saw this planet
constantly above the horizon," and the same historian is
quoted by St. Augustine as speaking of a change in the
134

VENUS 135

color and brilliancy of Venus. In 1716 the visibility of
Venus in full daylight was hailed as a marvel by the peo-
ple of London, and in 1750 its appearance at noon aroused
general astonishment in Paris. Again, in 1797, when
Napoleon returned to Paris from his conquest of Italy he
found the attention of the populace divided between his
reappearance and a similar striking midday phenomenon.
In fact, Bonaparte always associated this star with his
fortunes, and one evening while engaged in viewing the
starry heavens, pointing to the planet, said to Prince Tal-
leyrand, "Do you see? That is my star! So long as it
shines I will have no doubt of success."

In more recent times many brilliant appearances of the
planet have been observed, the interval between periods
of greatest visibility amounting to eight years. At one
of these, in April, 1905, at Cherbourg, Venus appeared
as a bright meteor of appreciable size and full form, an
effect which was due partly to the formation of a halo.

Acording to the Ptolemaic theory, Venus was always
between the Earth and the supposed orbit of the Sun.
Hence it was possible that, at the most, but half of its
illuminated surface could be visible to the Earth. When
Galileo, with his telescope, in 1610, made the striking
discovery that Venus appeared in various phases just as
the Moon, exhibiting the gibbous as well as the crescent
phase, it was a strong and almost the last argument neces-
sary to establish beyond question the Copernican theory
that the planets revolve around the Sun. Were Venus
as large as Jupiter, for example, the phases would read-
ily be discerned by the unaided eye. Indeed, Sir Robert
Ball has wondered what would have been the effect on the
history of astronomy had Venus been of the size of Jupi-
ter so that its crescent form could have been seen without
a telescope. Then the elementary truth would have been
apparent that Venus was a dark body revolving around the
Sun. The analogy between it and our Earth would have

136 ASTRONOMY

been at once perceived and the theory of Copernicus long
since might have been established.

The mean distance of Venus from the Sun is 67,200,000
miles. With the exception of the Moon and an occasional
comet no other heavenly body comes so near the Earth at
any time. The orbit is marked by the smallest eccentricity
in the planetary system, and is therefore more nearly cir-
cular than that of any other planet. The planet's great-

Fig. 19 — The Four Principal Phases of Venus.

est and least distances from the Sun vary from the mean
only by about 470,000 miles each way.

Venus is the nearest planet to the Earth, as there is
only 26,000,000 miles between the orbits of the two at
inferior conjunction or their nearest approach, yet Venus
is not as well known as Mars, since when Venus passes
nearest the Earth it is then between it and the Sun, so
that the hemisphere which is illuminated is not visible to
the Earth.

The appearance of Venus in the sky as the evening and
the morning star was no less impressive to the ancients

VENUS 137.

than the beautiful character of the star itself. It is as
familiar now as it was to the shepherds of old that when
Venus disengages itself in the evening from the rays of
the setting Sun it departs from the Sun a little more
every night, increasing its brilliancy until a certain dis-
tance in the east is reached, appearing, like the Moon, to
travel toward the left of the observer. At the end of a
few months it has removed itself from the Sun to an
angular distance that may amount to as much as 480, at
which time the planet sets more than three hours after
the Sun. After shining for some months, little by little the
planet begins to return toward the Sun, receding more
and more from the Earth, then passing behind the central
luminary and thus ceasing to be the evening star.

After an interval a new star is seen in the early morn-
ing to precede the rising of the Sun, advancing by imper-
ceptible degrees every day, and eclipsing likewise all the
bodies of the heavens by its dazzling light. At this time it
proceeds toward the west, that is, toward the observer's
right hand, and we have now the "morning star." After
having preceded the rising of the Sun by three hours,
Venus resumes its course anew toward the Sun and
again is lost in the glare of day. It is then pass-
ing between the Sun and the Earth and is at its greatest
proximity to the Earth. Sometimes it passes just in front
of the Sun, as it did in 1874 and 1882, which phenomenon
is known as a transit. As it happens but twice in a cen-
tury a transit is an occurrence of considerable importance
to astronomers.

These transits are valuable for the easy determination
of the position of the planet, for the investigation of its
atmosphere and for the determination of the solar paral-
lax by comparing the amount of apparent displacement
in the planet's path across the solar disk when the transit
is observed at widely separated stations on the Earth's
surface. These transits occur in June and December, taking
place in cycles whose intervals are 8, 105.5, 8, 121.5 years.

138

ASTRONOMY

They have occurred on the following dates: Dec. 7, 1631;
Dec. 24, 1639; June 5, 1761; June 3, 1769; Dec. 9, 1874;
Dec. 6, 1882; and will occur again on June 8, 2004, and
June 6, 2012.

The first observed transit, namely, that of 1639, was
watched in England by two persons, Jeremiah Horrocks
and a friend, William Crabtree, whom Horrocks had fore-
warned of its occurrence. That the transit was observed
at all was due entirely to the remarkable ability of Hor-
rocks. According to the calculations of Kepler, no transit

T » E

e u n

VeJggS

Fig. 20 — The Transit of Venus. Path Across the Sun in
the Transits of 1874 and 1882.

VENUS 139

could take place that year (1639), as the planet would just
pass clear of the lower edge of the Sun. Horrocks, how-
ever, worked the question out for himself, and came to the
conclusion that the planet would actually traverse the
lower portion of the Sun's disk. The event proved him to
be right. Horrocks, who was said to have been a veritable
prodigy of astronomical skill, unfortunately died about
two years after this celebrated transit, in his twenty-
second year.

The transits of Venus next observed in 1761 and 1769
were taken advantage of by Edmund Halley to suggest a
means of ascertaining the distance of the Sun. The idea
had originated in rather vague form with Kepler, but was
suggested more definitely by James Gregory in 1663. After
Halley had observed the transits of Mercury in 1677 he
realized the advantages of the method and published sev-
eral papers urging preparations for observing the transit.

"He pointed out," says Berry, treating of this point in
his "Short History of Astronomy," "that the desired
result could be deduced from a comparison of the
durations of the transit of Venus as seen from different
stations of the Earth, i.e., of the intervals between the first
appearance of Venus on the Sun's disk and the final dis-
appearance, as seen at two or more different stations. He
estimated, moreover, that this interval of time, which
would be several hours in length, could be measured with
an error of only about two seconds, and that in conse-t
quence the method might be relied upon to give the dis-
tance of the Sun to about 1/500 part of its true value.:
As the current estimates of the Sun's distance differed
among one another by 20 or 30 per cent., the new method,
expounded with Halley 's customary lucidity and enthusi-
asm, not unnaturally stimulated astronomers to take great
trouble to carry out Halley's recommendations. The re-
sults, however, were by no means equal to Halley's ex-
pectations."

Immense trouble was taken by governments, academies

140 ASTRONOMY

and private persons in. arranging for the observation of
the transits of 1761 and 1769. For the former, observing
parties were sent as far as to Tobolsk, St. Helena, the
Cape of Good Hope and India, while observations were
also made by astronomers at Greenwich, Paris, Vienna,
Upsala and elsewhere in Europe. The next following
transit was observed on an even larger scale, the stations
selected ranging from Siberia to California, from the
Varanger Fjord to Otaheite (where no less famous a per-
son than Captain Cook was placed), and from Hudson's
Bay to Madras.

The expeditions organized on this occasion by the
American Philosophical Society may be regarded as the
first of the contributions made by America to the science
which has since owed so much to her; while the Empress
Catherine of Russia bore witness to the newly acquired
civilization of her country by establishing a number of
observing stations on the soil of her Empire.

A variety of causes prevented the moments of contact
between the discs of Venus and the Sun from being ob-
served with the precision that had been hoped. The values
of the parallax of the Sun deduced from the earlier of
the two transits ranged between 8" and 10"; while those
obtained in 1769, tho much more consistent, still varied be-
tween 8" and 9", corresponding with a variation of about
10,000,000 miles in the distance of the Sun. The whole set
of observations was subsequently very elaborately dis-
cussed in 1822-4 and again in 1835 by Johann Franz
Encke, who deduced a parallax of 8"\57i, corresponding
with a distance of 95,370,000 miles, a number which long
remained classical. The uncertainty of the data is shown
by the fact that other equally competent astronomers have
deduced from the observations of 1769 parallaxes of 8".8
and 8".9.

The transits of Venus in 1874 and 1882 were memorable
as the first in which photographic methods and the heliom-
eter were applied, in addition to the older methods of time

VENUS 141

observation to measure the occasion of contact. The older
methods left unavailable the remainder of the transit, but
the heliometer made it possible to ascertain the planet's
apparent position upon the Sun's disk at any time with
great precision. In the transit of 1874 the most elaborate
photographic measures ever undertaken until this time,
were carried out in the United States where conditions
were most favorable.

A continuous succession of photographs was made as
the planet traveled across the Sun's disk, and various sys-
tems of reference enabled its position to be identified with
great accuracy at any part of the transit. Yet results
were hardly more valuable than those obtained by the
other methods. Hence in 1882 the American observers
modified their apparatus considerably from previous prac-
tice by employing the photographic plate and using the
heliostat to reflect the Sun's rays through a telescope lens
5 inches in diameter and of 40-foot focal length. Doubt-
less better results could be obtained to-day with modern
solar photographic equipment and dry plates. No great
advances were recorded as the result of the numerous ex-
peditions during these two transits.

Elaborate expeditions were equipped, particularly for
the second of the two transits, and the observations, which
were voluminous, were thoroly and systematically dis-
cussed. The results were a somewhat larger value of the
solar parallax, amounting to 8".857, or rather less than
93,000,000 miles, as obtained by Newcomb after combining
the heliometer with photographic measurements. As all
measures of this kind seem to be affected by some constant
systematic error, it is the concensus of opinion among
astronomers that other methods are more to be trusted
in determining the solar parallax than those based on the
transits of Venus.

The great brilliancy of Venus is due to its relatively
small distance from the Sun and the Earth and to its great
reflecting power. The albedo of Venus, or the ratio of re-

142 ASTRONOMY

fleeted to incident light, is 0.76. In other words, the planet
reflects three-fourths of the light that falls upon it from
the Sun, a proportion which is little exceeded by freshly-
fallen snow. This indicates that Venus is surrounded by a
dense and cloudy atmosphere. Similar indications are
given by the spectroscope, pointing to the existence of an
atmosphere containing water vapor and generally similar
to our own, but denser.

This atmosphere of Venus, owing to its great refractive
power, sometimes appears as a luminous ring in transits
of the planet. From observations made at the transit
of 1882 it has been computed that the density of the
atmosphere of Venus is 1.8 that of the Earth's at-
mosphere, which produces a refractive displacement of
33". According to Maedler's calculations, the atmosphere
of Venus is 1.7 times as dense as that of the Earth.

Many astronomers believe therefore that Venus is sur-
rounded by a dense atmosphere into which the Sun's rays
do not penetrate to a depth sufficient to cause much loss
by absorption — probably because they are reflected by
opaque clouds. From this it would follow that the mark-
ings which have been observed on the disc of Venus
through the telescope for centuries may be purely atmos-
pheric. As the displacement of the surface markings of
a planet when viewed through a telescope furnish the.
basis for estimates of the period of rotation, it may be that
calculations so made are without justification.

These markings of Venus are altogether different from
those of any other planet. They are faint, indistinct and
apparently variable, for the drawings of Venus made by
different observers before the advent of photography are
very unlike. The first astronomer to observe any mark-
ings on Venus was Domenico Cassini, who discovered a
bright spot in 1666. In the following year he discovered a
second spot, from which he deduced a period of rotation
of about 24 hours, an estimate which was reduced to 23
hours and 22 minutes in Jacques Cassini's revision of his

VENUS 143

father's work. On the other hand, Bianchini, who studied
the planet in 1726- 1727, deduced from his observations a
period of rotation equal to 24 days and 8 hours.

Thus began the long and violent controversy in regard
to the rotation of Venus. From more than 10,000 obser-
vations made between 1830 and 1841 De Vico computed a
mean value of 23 hours, 21 minutes and 23.93 seconds for
the period of rotation of Venus. This value, furthermore,
agrees very closely with the period of 23 hours and 21
minutes deduced by Schroter from observations of the
deformation of the horns of Venus (1 788-1 793), and it
has consequently been adopted in most treatises on astron-
omy. But in 1890, however, belief in the short period was
shaken by Schiaparelli's discoveries. Before this Schia-
parelli had made the surprising announcement that the
period of rotation of Mercury was equal to the period of
its revolution about the Sun, and he now asserted that the
same law governed the motions of Venus, a statement
based on observations made in the winter of 1877-78,
which showed that the bright spots then visible never
varied their positions with respect to the terminator, or
boundary line between the planet and its shadow. Obser-
vations made in 1895 gave additional support to the view
that Venus rotates on her axis in the period of her revolu-
tion about the Sun (225 days), and consequently always
turns the same hemisphere toward the Sun, just as the
Moon always presents the same face to the Earth.

This theory, which on its announcement rested solely
on Schiaparelli's authority, soon obtained strong inde-
pendent support. It has apparently been completely con-
firmed by the observations made by Lowell at Flagstaff,
Arizona, in 1895-6. Lowell saw markings that have been
seen by no other astronomer, before or since, but the most
surprising thing about his discoveries is that he saw no
spots, but only bands or lines bearing a superficial resem-
blance to the canals of Mars. The whole configuration re-
mained unchanged for hours, which could not be the case

144 ASTRONOMY

if the period of rotation were only 24 hours. Further-
more, the markings remained fixed relatively to the ter-
minator. Lowell denies the existence of clouds on Venus,
altho he finds in certain phenomena of twilight evidence of
the presence of a very dense atmosphere.

Many astronomers have refused to accept the conclu-
sions of Schiaparelli, Perrotin and Lowell, and have ad-
hered to the theory of a short period of rotation. Schiapa-
relli's first announcement was vigorously disputed, soon
after its publication, by the French astronomer, Trouvelot.
During his residence at Cambridge, Mass., from 1875 to
1882, Trouvelot pointed out seventeen changes in the
markings which Schiaparelli regarded as fixed. Trouve-
lot regarded the bright spots on the limb of the planet
as very high mountains, rising above the atmosphere.

The observed changes in the appearance of the horns
of the crescent planet have also been ascribed to moun-
tains. In the eighteenth century Schroter announced the
discovery of several mountains on Venus. In 1789 the
southern horn of the crescent appeared blunted and near
it on the dark disc of the planet shone a bright peak, the
height of which was estimated by Schroter as 117,000
feet, or 22 miles. Trouvelot deduced from his own obser-
vations a period of rotation of about 24 hours. It is very
remarkable that two such experienced and trustworthy ob-
servers as Schiaparelli and Trouvelot could be led to such
widely differing results from their frequent observations
of the same object during the same period. It may be
regarded as certain that the problem of the rotation of
Venus cannot be solved by observing markings, which is
not surprising if they are purely atmospheric phenomena.

Sir William Herschel, who has justly been called the
greatest observer of all time, doubted the reality of
the markings of Venus and regarded them as at-
mospheric phenomena, but this view was not shared by
any of his contemporaries. Beer and Maedler, who were
also most excellent observers, studied Venus repeatedly

VENUS 145

in 1833 and later years, but they rarely, if ever, saw any-
thing that deserved the name of a marking.

Attempts have been made to solve the interesting and
difficult problem of the rotation of Venus with the aid of
the spectroscope, but these attempts have also failed to
give a positive result. The effort was first made by the
Russian spectroscopist, Bjelopolsky, at the Pulkowa ob-
servatory. The spectrophotographic measurements which
were begun in 1900 indicated a short period of rotation.
Although Bjelopolsky did not regard the correctness of
the result as assured, his authority led to the belief that
the spectroscope had decided the question in favor of the
short period. Then Lowell, who had deduced a period of
225 days from telescopic observations, sought confirmation
for this value by the spectroscopic method. He first
tested his new and delicate apparatus and method by ap-
plying them to Mars, and obtained for that planet a period
of rotation of 25 hours and 35 minutes, which agrees
fairly well with the known period, 24 hours and 37
minutes. For Venus, Lowell's spectrophotographs give a
rotational velocity of from 5 to 8 meters (16.4 to 26.2
ft.) per second, which corresponds with a long period of
rotation, tho one much shorter than 225 days, for which
the equatorial velocity would be about 2 meters (6.56 ft.)
per second.

Neither of the results obtained at Pulkowa and at Flag-
stafl has yet been confirmed elsewhere. The spectroscopic
method is too difficult to be attempted except by a particu-
larly well equipped and favorably situated observatory.
The theory of slow rotation receives powerful support
from certain arguments advanced by the distinguished
English astronomer, Sir George H. Darwin. The ro-
tational velocity of every planet must be gradually
diminished by tidal friction among its parts. In this way
the period of the Moon's rotation has been made equal to
the period of her rotation about the Earth, and the rota-
tional periods of Mercury and Venus may have been simi-

146 ASTRONOMY

larly lengthened to equality with their periods of revolu-
tion about the Sun, which exerts a far greater tidal action
upon these inferior planets than upon the Earth.

If Venus has already reached this stage, all the water
on the planet must be accumulated in the form of ice on
the dark and intensely cold hemisphere. Hence, as An-
toniadi has pointed out, the presence of clouds and water
vapor on the illuminated hemisphere casts grave doubt
on the correctness of Schiaparelli's theory. The presence
of clouds in the atmosphere of Venus is denied only by
Lowell, who saw the peculiar canal-like markings dis-
tinctly whenever terrestrial atmospheric conditions per-
mitted.

No trace of markings, by the way, was seen by Hansky
and Stefanik, who studied the planet in the summer of
1907 at the Mont Blanc observatory, which is more
elevated than Flagstaff, under very favorable atmospheric
conditions, and deduced a period of rotation slightly less
than 24 hours. So the problem of the rotation of Venus is
still unsolved, altho a majority of astronomers appear to
share the view of Schiaparelli and Lowell. The hope that
it can be solved by observing markings has proven illu-
sory, but there is good reason to believe that the spectro-
scope will ultimately furnish the solution.

Venus in size is almost identical with the Earth. Its
diameter expressed in miles is 7,800, as compared with
7,920 for the Earth. Its circumference consequently
amounts to 23,400 miles, as compared with 25,000 miles,
that of the Earth. The period of revolution around the
Sun is 225 days.

For many years observers have been looking for satel-
lites of Venus. Fontana (1645), Cassini (1672 and 1686)
and Montaigne (1761), among others, imagined that they
had made such a discovery, but these have for many
years been considered optical illusions.

CHAPTER XIII

THE EARTH

The conception of the Earth formulated by the ancients
assumed it to be a flat disk floating on water, which idea
persisted in some form or other despite various phe-
nomena clearly impossible of explanation upon such
a hypothesis. After some more or less vague spec-
ulation on the shape of the Earth by Thales (64o[?]-
546 b.c.[?]), a clear and rational idea was advanced by
Pythagoras, who flourished in the sixth century B.C.
and taught that the Earth, in common with the heavenly
bodies, was a sphere and that it received no sup-
port at the center of the universe. The idea of the
sphericity of the Earth was doubtless derived by analogy
from the appearance of the Moon. The theory became an
established part of Greek science and philosophy and con-
sequently has continued to the present day, anticipating
by some 2,000 years the acceptance of the belief in the
Earth's rotation. In fact, the spherical form of the Earth
figured prominently in Greek astronomy, and the division
of the Earth into zones and the idea of poles was
promulgated by the Alexandrian philosophers and even
before their time. Indeed, not only did they realize the
shape of the Earth, but also its size. Eratosthenes (276-
195 or 196 b.c), whom we have already mentioned
in our chapter on pretelescopic work, made one of the
first scientific measurements of the Earth, obtaining a re-
sult which must be considered the first good geodetic
147

i48 ASTRONOMY

measurement. He found from the application of simple
geometry that the angular distance of the Earth's surface
between Alexandria and Syene must be l/B0 of the circum-
ference of the Earth. Posidonius, who was born about the
end of the life of Hipparchus, made a new measurement of
the circumference of the Earth, much after the fashion of
that of Erathosthenes, but reaching a value too small.
One of the crucial arguments for the sphericity of the
Earth is that when a ship sails away the hull first dis-
appears from view while the masts are visible. This
was first advanced by Pliny (23-79 a.d.).

From the Greeks and Romans to the Arabs may be a far
journey, but little was done in the study of the Earth until
the Arab astronomers at Bagdad, under the patronage of
Caliph Al Mamun, made a series of measurements of a
meridian of the Earth which agreed with those of Ptolemy.
These measurements virtually sufficed until Willebrod
Snell ( 1 591-1626), a Dutch mathematician who had dis-
covered the law of the refraction of light, made a series
of measurements of the Earth's surface from which he
computed the length of a degree of a meridian to be about
67 miles, an estimate subsequently corrected to about 69
miles by one of his pupils and differing but a few hundred
feet from the value now accepted. A measurement by
Richard Norwood (i59o[ ?]-i675) of the distance from
London to York, made in 1636, enabled a degree of latitude
to be obtained with an error of less than half a mile, and
finally Picard, in 1671, made a measurement of this quan-
tity, which we have seen was sufficiently accurate to be
used by Newton in computing the mass and motion of the
Earth and in demonstrating his law of gravitation.

This outline shows that the spherical shape of the Earth
was constantly held in mind by scientists for about 2,500
years. Other views of the Earth's shape also obtained.
The idea of a flat Earth had to be vigorously opposed
by arguments founded on experiment, observation and
discovery. After the phenomenon of a disappearing ship,

THE EARTH 149

one of the most important of the arguments against the
flatness of the Earth's surface was based on the different
elevations of the Pole Star in the sky, depending upon the
position of the observer. Now the Pole Star and the sur-
rounding stars, tho they may be at different heights in
the sky, form the same pattern wherever on the Earth's
surface they are seen, thus showing that they are im-
mensely distant and that lines from all points of the
Earth's surface directed to the Pole Star would be parallel
or have the same direction. As one travels north it is seen
that the Pole Star sinks lower and lower in the heavens
or has a greater angular distance from our zenith. If the
Earth were flat this would not be the case, for an angular
distance of 7^2 degrees difference in the position of two
stations 520 miles apart in a north-and-south line would
mean that the star is only 3,450 miles distant, which is
manifestly impossible. Furthermore, it is a well-known
experience that in crossing the Atlantic from Eng-
land to the United States the Sun, by a watch, rises an
hour later for every 600 miles we travel west. If the
Earth were flat the Sun would rise at the same instant
over its entire surface. To answer both these conditions —
namely, the sinking in the heavens of the Pole Star and
the equal delays in the time of sunrise in traveling equal
distances — a round globe is required.

Still the globular form of the Earth was not accepted
generally until after the historic voyage of Christopher
Columbus and that epoch of exploration and discovery
which resulted in the circumnavigation of the Earth
in 1519. After the demonstration of the sphericity
of the Earth came the determination of its exact
figure by the systematic measuring of arcs of meridians.
When the results were mathematically examined the con-
clusion was reached that the Earth was not a perfect
sphere, but flattened at the ends, or in other words that
it was an oblate spheroid. Newton in his Principia, pub-
lished in 1687, showed from theoretical considerations that

ISO ASTRONOMY

the Earth bulged out at the equatorial regions. On the
other hand, in the following century (in 1745) the Cas-
sinis, who were leading astronomers of France and deeply
interested in the measurement of the Earth's surface, main-
tained that the Earth was contracted at the equator and
bulged at the poles, or in other words was prolate, the
difference in the two ideas being concisely expressed by
Professor Moulton, who likens Newton's idea of the
Earth's figure to an orange and that of the Cassinis to a
lemon.

It is interesting that Newton's proof of the oblateness
of the Earth anticipated the direct observations of the
astronomer and geodesist. To-day it can be demonstrated
in simple form by means of observations of the time of
vibration of a pendulum, such as are carried on systemati-
cally by such scientific agencies as the United States Coast
and Geodetic Survey.

It is possible to supply various mathematical proofs of
the oblateness of the Earth in connection with its motions
and with its relations to the Moon and to the solar system.
But these hardly concern us in the present pages, and we
may dismiss the matter with the assertion that the ellip-
ticity of the Earth as determined by Harkness in 1891
amounts to V300, as compared with a value of x/295 estab-
lished in 1866 by Colonel Clarke, of the English Ordnance
Survey, and accepted for many years as a standard, and
*/„„, as was given by Newton for the ellipticity of a me-
ridian section of this oblate spheroid. Clarke's spheroid
of 1866, which is generally adopted in geodetic and other
computations, would give a radius at the equator of
3,963.307 miles and 3,949.871 miles at the poles, these
figures having been slightly corrected in 1878 in the sec-
ond place of decimals. The radius of the Earth is in many
ways a fundamental measure, especially in astronomy, and
for that reason its determination is a matter of no small
importance. Thus observations made with a view of find-
ing the distance of the Moon, when discussed and reduced,

THE EARTH 151

simply give us this distance in terms of the equatorial
radius of the Earth, so that to determine the distance in
miles we must know the number of miles in the Earth's
radius.

As a consequence of the spheroidal shape of the Earth
a degree of latitude, as well as of longitude, varies with
the point at which it is measured. Thus at the equator one
degree of latitude is equal to 68.704 miles, but at the pole
a similar degree is equal to 69.407 miles. The difference in
a degree measured on the parallels of latitude, which are
small circles, is of course obvious, and while at the equator
a degree of longitude amounts to 69.652 miles, at latitude
400, or approximately that of New York (400 43') , it
would equal 53.431 miles and at the north pole it would
decrease, of course, to zero.

Newton in his researches on gravitation, by comparing
the attraction exerted by the Earth with that of the Sun
and other bodies, was able to connect its mass with that of
the planets and Sun. The problem of determining the
mass of the whole Earth in terms of a given terrestrial
body — that is, in tons, pounds or kilograms — was indeed a
serious one, and it was first attacked in Great Britain by
Nevil Maskelyne (1732-1811), who was Astronomer Royal.
If we know the volume of the Earth and its density, re-
ferred to some standard as water, air or iron, it is of course
easy to determine its mass. But the determination of its
density is no less a problem than the determination of its
mass directly. It is possible, of course, to compute the
relative amount of water, but this gives only a small
amount of the mere surface of the Earth. We can de-
scend in mine shafts a mile or more and investigate the
nature of the material, but this again affords no clue to
the thousands of feet of materials below, so that an esti-
mate of the average density based on such data cannot be
considered at all trustworthy. Newton, taking into con-
sideration the various elements of the Earth, estimated its
density at between five and six times that of water. Mas-

152 ASTRONOMY

kelyne, however, bearing in mind the phenomenon of the
deflection of the plumb-line observed by Bouguer and
La Condamine in their expedition to measure an arc of
latitude in Peru (a phenomenon caused by the attraction
of the mountain Chimborazo), selected the narrow ridge
of Schehallien in Perthshire and found that the attraction
of the mountain caused a deflection of about 12" on each
side of the ridge. This attraction of the mountain as
compared with the attraction of the Earth on the plumb-
bob enabled a value for the density of the Earth of about
4y2 times that of water to be obtained. It remained for
the famous Cavendish experiment, carried out by Henry
Cavendish (1731-1810), to substitute for the mountain a
pair of heavy balls. This gave a value for the density of
the Earth of 5^ times that of water, a value confirmed by
numerous repetitions of Cavendish's classical experiment.
Expressed in pounds, the mass of the Earth is a little
more than 13 billion billion, a figure that the human mind
utterly fails to grasp.

There must have been a time when the Earth contained
much more heat than at present. The Sun is not supply-
ing heat enough to make good the losses sustained by
the glebe. These losses must have been taking place
over a period of countless years. One is forced to believe
that not only once was our Earth much hotter than at pres-
ent, but that its temperature was a red heat. Going back
still further, the now solid globe must have been actually
a molten mass. Being a molten mass, it is not difficult to
realize that it readily assumed a globular form on its jour-
ney through space. A falling drop of rain is a globe; a
drop of oil suspended in a liquid with which it does not
mix has a spherical shape. Therefore if a mass of material
as large as the Earth were so soft that its individual par-
ticles obeyed the forces of attraction exerted by each part
of the mass on all the other parts, it is quite obvious that
it would assume a globular shape, especially under the
influence of motion. If this great sphere were caused to

THE EARTH 153

rotate around an axis, like a ball of clay on a potter's
wheel, any lack of true sphericity would not only be over-
come, but there would be a tendency for the molten body
to bulge at the equator and flatten down at the poles, form-
ing an oblate spheroid such as by actual measurement
the Earth is found to be. Indeed, not only is the Earth
an oblate spheroid, but most of the planets which can be
measured have become flattened at the poles for the same
reason.

In connection with the spheroidal shape of the Earth,
mention should be made of a recent theory which seeks to
demonstrate that the shape of the Earth is being trans-
formed into a polyhedral form, or more exactly, that there
is a tendency in the direction of its assuming the figure of
a regular tetrahedron, or four-sided figure, each of whose
faces is triangular. This is manifested by the lithosphere
or solid portion of the Earth. The water is massed on the
faces of such a tetrahedron, the force of gravity acting
most powerfully at the center of each of the four surfaces.
The tetrahedron hypothesis assumes that a more or less
solid crust was formed when the Earth was still perhaps
in an approximately spheroidal form. As contraction oc-
curred in the interior the external shell proved too large.
Hence a different form has to be assumed, which, it is
asserted, would be the tetrahedral. On each of the faces
of such a tetrahedron one of the four oceans would lie, the
Arctic Ocean being assumed as the fourth for the region
about the North Pole. The continents would correspond
with North and South America, Euro-Africa, Asia-Aus-
tralia and Antarctica. This hypothesis is borne out in many
respects by geological and other considerations. First ad-
vanced in the 70's of the nineteenth century by Lowthian
Green, the hypothesis has been put forward in recent years
with further developments and attempts at its proof, so
that it is considered as a tenable and possible explanation,
tho its absolute soundness has not been demonstrated.

Recognising and proving the spherical shape of the

154 ASTRONOMY

Earth by no means explains the apparent motion of the
Sun, Moon and stars as regards the planet. As the nature
of the solar system was developed by Copernicus and
Newton, people were compelled to recognise that the Earth
revolved around the Sun, and, furthermore, that it ro-
tated daily about its axis.

These motions are necessary to explain various phe-
nomena. For example, when a body moves in a circle a
force is needed to act on it, pulling toward the center pro-
portionally to the square of the rate of spin and propor-
tionally to the distance. Every pound of mass in the
Moon would need a pull equal to about the weight of 3
ounces to keep it in a circle which it traveled around once
in 24 hours. Every pound in the Sun would need a pull
about three-quarters of a hundredweight. The nearest
fixed star would need a pull on every pound, of about 9,000
tons. One cannot imagine that the Earth could exert such
forces and so. are obliged to think that the Earth is revolv-
ing and not the sky.

Final proof of the rotation of the Earth was fur-
nished in the middle of the last century by Foucault.
His experiments are constantly repeated in physical labora-
tories and experimental lectures. From the dome of the
Pantheon in Paris he suspended a heavy ball by a
fine cord and caused it to swing in a single plane. The
path of the ball across the floor could be traced, and it was
found that after it had been in vibration for some time it
was swinging in a different direction from that in which it
had started. The reason was that the floor had revolved
under the ball with the rotation of the Earth.

The daily rotation of the Earth on its axis and its yearly
rotation about the Sun supply us with our means of meas-
uring time. According to Professor Poynting, "The Earth
is a clock, the line to the Sun is the finger and the sky is
the face. But the Sun is not a regular timekeeper. Our
twenty-four-hour day is only the average between suc-
cessive noons or times when the Sun is due south. If

THE EARTH 155

compared with a good clock, the Sun is in parts of the
year too soon and in other parts too late, sometimes as
much as a quarter of an hour. This cannot be due to
change in the Earth's rate of spin, for to change a spin
there must be either a force acting at one side of the cen-
ter of gravity or a change of shape. The forces on the
Earth exerted by the Sun and Moon act almost exactly
through the center of gravity and so affect the rate of spin
hardly at all. The Earth does not change its shape suffi-
ciently to account for the variation in the solar day.

"The variation in the solar day is due partly to the in-
clination of the Earth's axis to the plane in which it moves
around the Sun, partly to variation of the Earth's motion
round the Sun at different times of the year.

"The fixed stars keep good time, getting round in about
4 minutes less than 24 hours. By them clocks are rated.
Their day is the true time of one revolution of the Earth.'*

Among the movements of the Earth there is one that is
so minute as to be of peculiar interest to the astronomer.
It is a small displacement or wobbling of the axis of the
Earth or theoretical pole. This pole shifts its place
through a circle of some few yards in diameter in the
course of a period of somewhat over a year, and as a result
there is a variation of latitude on the Earth's surface.
This was discovered first in 1884 and 1885 by Dr. Kiistner
at Berlin, and after observations made at various stations
by members of the International Geodetic Association in
1888, it was found that this synchronous variation was oc-
curring in a periodic manner at a number of stations. The
general nature of the occurrence was denned by Prof.
S. C. Chandler in 1891, who stated that the pole of the
Earth might be supposed to describe a circle with a radius
of 30 feet in a period of fourteen months. One reason as-
signed for this phenomenon is a change in the distribution
of mass, due to the temporary occurrence of heavy falls
of snow or rain limited to one continent, or to the transpor-
tation of great bodies of air and water from place to place

156 ASTRONOMY

by atmospheric or ocean currents, so that the globe is made
slightly lopsided and temporarily to forsake its normal
axis. Also it has been supposed that the Earth is not abso-
lutely rigid. As a quasi-plastic mass it would yield to cer-
tain strains which tend to protract the time of circulation
of the displaced pole. In fact, in all consideration of the
various phenomena of gravitation and tides we have to
recognise that in practice the astronomer of to-day is not
dealing with the theoretically rigid body of Newton and
the earlier mathematicians.

One of the important conditions of the rotation of the
Earth is that it moves around an axis which is not vertical
but which is inclined toward the plane of its orbit. Dol-
mage in his volume, "Astronomy of To-day," tells us that
"If the axis of the Earth stood 'straight up,' so to speak,
while the Earth revolved in its orbit, the Sun would plainly
keep always on a level with the equator. This is equiva-
lent to stating that, in such circumstances, a person at the
equator would see it rise each morning exactly in the east,
pass through the zenith — that is, the point directly over-
head of him at midday — and set in the evening due in the
west. As this would go on unchangingly at the equator
every day throughout the year, it should be clear that, at
any particular place upon the Earth, the Sun would in
these conditions always be seen to move in an unvarying
manner across the sky at a certain altitude, depending
upon the latitude of the place. Thus the more north one
went upon the Earth's surface the more southerly in the
sky would the Sun's path lie, while at the north pole
itself the Sun would always run round and round the
horizon. Similarly the more south one went from the
equator the more northerly would the path of the Sun lie,
while at the south pole it would be seen to skirt the hori-
zone in the same manner as at the north pole. The result
of such an arrangement would be that each place upon the
Earth would always have one unvarying climate, in which

THE EARTH 157

case there would not exist any of those beneficial changes
of season to which we owe so much.

"The changes of season which we fortunately experi-
ence are due, however, to the fact that the Sun does not
appear to move across the sky each day at one unvarying
altitude, but is continually altering the position of its path,
so that at one period of the year it passes across the sky
low down, while at another it moves high up across the
heavens and is above the horizon for a much longer time.
Actually the Sun seems little by little to creep up the sky
during one-half of the year, namely, from mid-winter
to mid-summer, and then just as gradually to slip down it
again during the other half, namely, from mid-summer to
mid-winter. It will therefore be clear that every region of
the Earth is much more thoroly warmed during one por-
tion of the year than during another — i.e., when the Sun's
path is high in the heavens than when it is low down.

"Once more we find appearances exactly the contrary
from the truth. The Earth is in this case the real cause
of the deception, just as it was in the other cases. The
Sun does not actually creep slowly up the sky and then
slowly dip down it again, but owing to the Earth's axis
being set aslant, different regions of the Earth's surface
are presented to the Sun at different times. Thus in one
portion of its orbit the northerly regions of the Earth are
presented to the Sun and in the other portion the southerly.
It follows, of course, from this that when it is summer in
the northern hemisphere it is winter in the southern and
vice versa.

"The fact that in consequence of this slant of the Earth's
axis the Sun is for part of the year on the north side of
the equator and part of the year on the south side leads to
a very peculiar result. The path of the Moon around the
Earth is nearly on the same plane with the Earth's path
around the Sun. The Moon, therefore, always keeps to
the same regions of the sky as the Sun. The slant of the
Earth's axis thus regularly displaces the position of both

158 ASTRONOMY

the Sun and the Moon to the north and south sides of the
equator respectively in the manner we have been describ-
ing. Were the Earth, however, a perfect sphere such
change of position would not produce any effect.

"At present the north pole of the heavens is quite close
to a bright star in the tail of the constellation of the Little
Bear, which is consequently known as the pole star, but
in early Greek times it was at least ten times as far away
from this star as it is now. After some 12,000 years the
pole will point to the constellation of Lyra, and Vega, the
most brilliant star in that constellation, will then be con-
sidered as the pole star. This slow twisting of the Earth
is technically known as precession, or the precession of the
equinoxes.

"We have seen that the orbit of the Earth is an ellipse
and that the Sun is situated at what is called the focus, a
point not in the middle of the ellipse, but rather toward
one of its ends. Therefore during the course of the year
the distance of the Earth from the Sun varies. The Sun, in
consequence of this, is about 3,000,000 miles nearer to us
in our northern winter than it is in our northern summer,
a statement which sounds somewhat paradoxical. This
variation in distance, large as it appears in figures, can,
however, not be productive of much alteration in the
amount of solar heat which we receive, for during the first
week in January, when the distance is least, the Sun only
looks about one-eighteenth broader than at the commence-
ment of July, when the distance is greatest. The great
disparity in temperature between winter and summer de-
pends, as we have seen, upon causes of quite another kind,
and varies between such wide limits that the effects of this
slight alteration in the distance of the Sun from the Earth
may be neglected for practical purposes."

The most striking effect of gravitation in its universal
aspect is seen on the Earth in the case of the tides, which
are due to the attraction of the Moon on the Earth and
especially on the waters of its oceans. While the ancients

THE EARTH 159

could not explain the cause of the daily change in the
tides, the phenomenon was obvious to all mariners or those
dwelling by the sea or even on great rivers. The
connection of the Moon with this phenomenon, it is
claimed, was understood as early as iooo B.C. by the
Chinese, for at full and new moon exceptionally high
tides were observed, and the name of spring tides
was applied as contrasted with the minimum high
water which was observed at the neap tides at the second
and fourth quarters of the Moon. Not only the Chinese but
the Greeks and Romans noticed this same phenomenon,,
and Julius Caesar in his "Commentaries" tells us that when
he was embarking his troops for Britain the tide was high
because the Moon was full, while both Pliny and Aristotle
connect the time of high water with the age of the Moon.
The connection between the Moon and the tide was thus
early established and practically applied by navigators,
who were quick to realize the nature of the phenomenon,
even tho they and the astronomers were unable to present
any satisfactory explanation.

A general explanation of the tides as due to the disturb-
ing action of the Moon and Sun, especially the former,
was put forward by Newton. Newton's explanation, as
described by Berry in his 'Short History of Astronomy/
was as follows: "If the Earth be regarded as made of a
solid spherical nucleus, covered by the ocean, then the
Moon attracts different parts unequally, and in particular
the attraction, measured by the acceleration produced on
the water nearest to the Moon, is greater than that on the
solid Earth and that on the water farthest from the Moon
is less. Consequently the water moves on the surface of
the Earth, the general character of the motion being the
same as if the portion of the ocean on the side toward the
Moon were attracted and that on the opposite side repelled.
Owing to the rotation of the Earth and the Moon's motion,
the Moon returns to nearly the same position with respect
to any place on the Earth in a period which exceeds a day

160 ASTRONOMY

by (on the average) about 50 minutes, and consequently
Newton's argument showed that low tides (or high tides),
due to the Moon, would follow one another at any given
place at intervals equal to about half this period; or, in
other words, that two tides would in general occur daily,
but that on each day any particular phase of the tides
would occur on the average about 50 minutes later than
on the preceding day, a result agreeing with observation.
Similar but smaller tides were shown by the same argu-
ment to arise from the action of the Sun and the actual
tide to be due to the combination of the two. It was shown
that at new and full moon the lunar and solar tides would
be added together, whereas at the half moon they would
tend to counteract one another, so that the observed fact
of greater tides every fortnight received an explanation.
A number of other peculiarities of the tides were also
shown to result from the same principles."

From Newton's time to the present the tides have sup-
plied material for astronomical and mathematical research,
so that through the efforts of many astronomers and other
scientists their theory and occurrence are now well under-
stood, and the various governments make ample provision
through hydrographic offices and otherwise to provide such
information for mariners. In the theoretical discussion
in modern times one name stands out prominently, that of
Sir George H. Darwin, of England, whose researches on
the tides of the Earth have developed to a point where they
have a most important bearing on cosmical theory.

It is desirable here to outline the chief features of tides
in their everyday occurrence. The term "flood-tide" is
applied to the rising water, which reaches "high-water"
mark at the moment when the tide is highest, while in fall-
ing the tide is said to "ebb" until low water, or the lowest
mark, is reached. Near the times of new and full moon
occur the "spring tides," which are the highest tides of the
month as distinguished from the "neap-tides," which are
the smallest, occurring when the Moon is in quadrature,

THE EARTH 161

the relative heights of spring and neap-tides being about as
7 to 4. At the time of "spring-tides" the interval between
the corresponding tides of successive days is less than the
average value of 24 hours and 51 minutes and the tides
are said to "prime," the interval amounting to about 24
hours and 38 minutes, while at "neap-tides" the interval
increases to 25 hours and 6 minutes, and then the tides
are said to "lag." The highest tides of all occur when the
new or full moon occurs at the time the Moon is in peri-
gee, especially as this takes place about January 1st, when
the Earth is nearest to the Sun.

Not only are the tides interesting for their relation to
navigation, but on account of their intimate connection
with the motion of the Earth. The daily movement of the
tides is a drag on the energy of our planet and is acting
to cut down its speed of rotation. If the speed of rotation
of the Earth diminishes it is obvious that our day is being
lengthened, that to-day is longer than yesterday and that
to-morrow will be longer than to-day, even tho these in-
creases are all but inappreciable in the time that the human
mind can fathom. Sir Robert Ball in his "Story of the
Heavens" summarizes the variation wrought by the tides
in the following paragraph: "Let us take a glance back
into the profound depths of times past and see what the
tides have to tell us. If the present order of things has
lasted, the day must have been shorter and shorter the
farther we look back into the dim past. The day is now
twenty-four hours; it was once twenty hours, once ten
hours, and once six hours. How much farther can we
go? When the six hours it past, we begin to approach a
limit which must at some point bound our retrospect. The
shorter the day the more is the Earth bulged at the equa-
tor ; the more the Earth is bulged at the equator the greater
is the strain put upon the materials of the Earth by the
centrifugal force of its rotation. If the Earth were to go
too fast it would be unable to cohere together; it would
separate into pieces, just as a grindstone driven too rap-

162 ASTRONOMY

idly is rent asunder with violence. Here, therefore, we
discern in the remote past a barrier which stops the pres-
ent argument. There is a certain critical velocity which is
the greatest that the Earth could bear without risk of rup-
ture, but the exact amount of that velocity is a question
not very easy to answer. It depends on the nature of the
materials of the Earth; it depends upon the temperature;
it depends upon the effect of pressure and on other details
not accurately known to us. An estimate of the critical
velocity has, however, been made, and it has been shown
mathematically that the shortest period of rotation that
the Earth could have, without flying into pieces, is about
three or four hours. The doctrine of tidal evolution has
thus conducted us to the conclusion that at some incon-
ceivably remote epoch the Earth was spinning around its
axis in a period approximating to three or four hours."

The existence of tides in the solid Earth, as well as in
the oceans, was first indicated by Lord Kelvin in 1867.
The first numerical estimate of the amount of the tidal
oscillations of the solid Earth was made by Sir George
H. Darwin in 1882 and was based on an indirect method
and resulted from observation of the tides of the ocean.
Further indirect methods were successfully applied later,
with more complete data.

Sir George H. Darwin has expressed the opinion that we
may now feel confident that the Earth yields to the tidal
forces to the same degree as would a steel globe. The
amazing accuracy of Dr. Hecker's recent observations
(1908) are marked by certain collateral considerations,
among which may be mentioned the flexure of coast lines,
due to the varying pressure of oceanic tides, and the ef-
fects of high and low barometric pressure in bending the
Earth's surface. It is impossible to give an accurate
measurement of the amount of the vertical motion of the
Earth's surface, but it is probable that at spring tides
in this latitude the surface of the solid Earth moves up
and down through about six inches.

CHAPTER XIV

THE MOON

That the Moon is the nearest the Earth of all the heav-
enly bodies is one of the most obvious facts which con-
front the star-gazer. Its regular motion must have been
early appreciated by primitive man, after he had realized
that the rising and setting of the Sun marks regularly
recurrent intervals of time. As he was able to reflect
upon his observations and properly to coordinate them, he
doubtless noted the connection between the form of the
Moon and its position in the sky with respect to the Sun.
For by keeping count with pebbles or rude notches cut in a
stick he would learn that the interval of time between re-
curring full moons was always the same, and that the
series of changes he could observe followed in regular
order. Thus when the Moon appeared after sunset near
the place where the Sun had disappeared he saw a thin
crescent, the hollow side of which was turned away from
the Sun. A little later the Moon set.

The next night he observed the Moon further from the
Sun, with a thicker crescent, and noted also that it set
later, an effect that gradually increased until the semi-
circular disk, with the flat side turned away from the Sun,
was seen in rather less than a week after the first appear-
ance of the crescent. In another week the semi-circle en-
larged to a complete disk and the Moon rose about sunset
and set about sunrise, being in a direction nearly opposite
to that of the Sun. From this time on the size again
163

164 ASTRONOMY

Direction from which the Sun's rays are coming.

I I X J !

g

fearttt)

fir

Various positions and illuminations of the Moon by the Sun
during her revolution around the Earth.

^ JL J£^ JL JL o f c ft a

f#jf)0003<»«

Af0wJ Ot.wnt First Quarter Gibbous full Gibbous Last Quart** Crestvnt Akw

Fig. 21 — Orbit and Phases of the Moon.
The corresponding positions as viewed from the Earth, showing
the consequent phases.

diminished; the semi-circular form was seen once more
with the flat side still turned away from the Sun and to-
ward the west instead of the east as the Moon approached
the Sun on the other side, rising before it and setting in the
daytime. Again he saw the crescent and marked that the
time of rising approached that of sunrise, until the Moon
became altogether invisible. Two or three nights inter-
vened and the new Moon reappeared, whereupon the whole

THE MOON 165

series of changes was repeated. In other words, this primi-
tive man must have formulated a lunar month.

All ancient records recognise this lunar month of twenty-
nine and a half days, and that interval must have been
adopted long before the year. By the time of Chaldean
and Egyptian astronomy, however, the year was known, so
that the first conception of the lunar month is lost in the
mists of antiquity. The Chaldeans studied the motions of
the Sun and the Moon. Their calendar records, which they
seem to have maintained with considerable care, enabled
them to discover that eclipses occurred after a period
known as the Saros, consisting of 6,585 days.

...jNew

fielow the Plane

-a

Fig. 22 — The Moon's Path Around the Sun.

The nature of the Moon was, of course, to them a
mystery. It was known to move around the sky. The
Babylonians supposed that, having a bright and a dark
side, the different phases were caused by the bright side's
coming more and more into view during its movement
around the sky. In the seventh century Pythagoras
taught more correctly that the Moon, like other heavenly
bodies, was spherical, and that it was bright because it
received the light of the Sun. The phases, he rightly
judged, were due to a greater or less amount of the
illuminated half turned toward us, and the curve forming
the boundary between the bright and dark portions of the
Moon was to him conclusive evidence of a spherical shape.
He was later supported in his theory by Aristotle, who

166 ASTRONOMY

made a similar clear and definite statement of the reason
for the phases of the Moon.

Perhaps the first systematic study of the Moon was
that of Aristarchus, a famous member of the Alexandrine
school, who flourished in the first half of the third cen-
tury and wrote a treatise "On the Magnitudes and Dis-
tances of the Sun and Moon," which still survives. Tak-
ing the Moon when it was half full, so that a line drawn to
it from the Sun made a right angle with a line from the
Moon to the Earth, by measuring the angle between the
Moon and the Sun he was able to determine the ratio of
the sides of this triangle and the relative distance of the
Moon and the Sun from the Earth. While the method of
Aristarchus was ingenious, yet the result he obtained,
that the Sun was 18 to 20 times as far distant as the
Moon, was sadly in error, for the actual distance is nearly
four hundred times. Still the difficulties in the way of
accurate observation were enormous for lack of proper
instruments. Aristarchus also took advantage of the
solar eclipse to ascertain the distances of these two bodies,
and reasoned correctly that when the Moon sometimes
rather more than hides the surface of the Sun and some-
times does not quite cover it, their diameters must vary
as their real distances. He even obtained by eclipse ob-
servations a value for the diameter of the Moon in terms
of that of the Earth. What would have been an excellent
approximation was marred, however, by an incorrect esti-
mate of the apparent size of the Moon, which he took as
two degrees instead of one-half a degree. Nevertheless
this work was an important advance in practical as-
tronomy. It paved the way for Hipparchus, who discussed
the motions of the Moon, motions which, long before his
time, were known to be irregular and much more com-
plicated than those of the Sun. The path of the Moon is
always changing and its motions are subject to variation.

Hipparchus notes that the part of the Moon's path
in which the motion is most rapid is not always in the

THE MOON" 167

same position on the celestial sphere, but moves continu-
ously. He was able to realize the different motions of the
Moon, to distinguish the various months based upon them
and to employ the Chaldean and other early eclipse ob-
servations for determining the position of the Moon at
various earlier epochs. Hipparchus really evolved a most
complicated set of motions. He was aware of the short-
comings of his theory, but was unable to reconstruct it in
a satisfactory manner. Following out the eclipse method
of Aristarchus, Hipparchus made a determination of the
relation between the distances of the Sun and Moon,
measuring their angular diameters, and found that the
distance of the Moon is nearly 59 times the radius of the
Earth. Combining this figure for the distance of the
Sun with that of Aristarchus, a value of 1,200 times the
radius of the Earth was obtained, which value was em-
ployed for many centuries.

Ptolemy, in the fourth book of the "Almagest," discusses
the length of the month with the theory of the Moon and
makes the important announcement of a further inequal-
ity in the Moon's motion, which Hipparchus had only
suspected and which was due in large part to its position
with respect to the Sun. This was later termed "evection"
and involved a still further complication of the mathemati-
cal computations of Hipparchus because it meant the use
of an epicycle and a deferent, which was itself a moving
eccentric circle, the center of which revolved around the
Earth. Ptolemy's mathematics and ingenuity were able
to fit his theory to observations. Altho his work showed
many inconsistencies which, great as he was, he was
unable to control, nevertheless it represented a notable de-
velopment in astronomy. To Ptolemy is due the parallax
method for obtaining the distance of the Moon by observ-
ing its direction from two points on the Earth's surface
and by finding the distance between these two points in
terms of the Earth's radius. This distance of the Moon
he estimated was 59 times the radius of the Earth. With

168 ASTRONOMY

this value, according to the method of Hipparchus just
mentioned, he computed the distance from the Earth to
the Sun. He found that the distance was 1,210 times the
radius of the Earth, which value was equally in error as
compared with the modern figure. The actual distance is
about twenty times this amount.

So readily were the motions of the Moon observed and
so carefully were the records maintained that even at an
early date much was known about its motions. These data
enabled much calculation of the Moon's motion to be car-
ried on, so that early mathematical astronomy dealt with
the Moon in no small degree. Various irregularities of
its motion and appearance were discussed, and practically
all of the notable astronomers made some contributions
to our knowledge of the satellite. Even an outline of
these discussions would lead the reader far afield into the
depths of mathematics, for which reason it is possible to
mention only a few of the important researches and to
indicate merely their general nature. The study of the
Moon had a practical importance, however, as lunar posi-
tions were early used to determine longitude in navigation
and lunar tables were calculated by government and other
astronomers.

Nowhere was there more interest manifested in the
problem of the motion of the Moon than at Greenwich
Observatory, where the matter had always been a spe-
cialty of that institution. "It is a curious fact," states
the late Professor Newcomb in his "Reminiscences of an
Astronomer," "that while that observatory supplied all
the observations of the Moon, the investigations based
upon these observations were made almost entirely by
foreigners, who also constructed the tables by which the
Moon's motion was mapped out in advance. The most
perfect tables made were those of Hansen, the greatest
master of mathematical astronomy during the middle of
the century, whose tables of the Moon were published by
the British Government in 1857. They were based on a

THE MOON 169

few of the Greenwich observations from 1750 to 1850.
The period began with 1750, because that was the earliest
at which observations of any exactness were made. Only
a few observations were used, because Hansen, with the
limited computing force at his command — only a single
assistant, I believe — was not able to utilize a great number
of the observations. The rapid motion of the Moon, a
circuit being completed in less than a month, made numer-
ous observations necessary, while the very large devia-
tions in the motion produced by the attraction of the Sun
made the problem of the mathematical theory of that
motion the most complicated in astronomy. Thus it hap-
pened that, when I commenced work at the Naval Ob-
servatory in 1861, the question whether the Moon exactly
followed the course laid out for her by Hansen's tables
was becoming of great importance.

"For a year or two our observations showed that the
Moon seemed to be falling a little behind her predicted
motion. But this soon ceased, and she gradually forged
ahead in a much more remarkable way. In five or six
years it was evident that this was becoming permanent;
she was a little farther ahead every year. What could it
mean? To consider this question, I may add a word to
what I have already said on the subject.

"In comparing the observed and predicted motion of
the Moon, mathematicians and astronomers, beginning
with Laplace, have been perplexed by what are called 'in-
equalities of long period.' For a number of years, perhaps
half a century, the Moon would seem to be running ahead
and then she would gradually relax her speed and fall
behind. Laplace suggested possible causes, but could not
prove them. Hansen, it was supposed, had straightened
out the tangle by showing that the action of Venus pro-
duced a swinging of this sort in the Moon; for one hun-
dred and thirty years she would be running ahead and
then for one hundred and thirty years more falling back
again, like a pendulum. Two motions of this sort were

170 ASTRONOMY

combined together. They were claimed to explain the
whole difficulty. The Moon, having followed Hansen's
theory for one hundred years, would not be likely to de-
viate from it. Now it was deviating. What could it
mean?

"Taking it for granted, on Hansen's authority, that his
tables represented the motions of the Moon perfectly
since 1750, was there no possibility of learning anything
from observations before that date? As I have already
said, the published observations with the usual instruments
were not of that refined character which would decide a
question like this." But observations of stars which might
be available, and which had been made in many instances,
would have an important bearing on this work. For if
an occultation or passage of the Moon between a star and
the Earth occurred and its time registered, the path of the
Moon in the heavens and the time at which it arrives
at each point of the path would be determined if the time
of the occultation was known within one or two seconds.
Professor Newcomb continues: "It was not until after
the middle of the century (seventeenth), when the tele-
scope had been made part of astronomical instruments for
finding the altitude of a heavenly body, and after the
pendulum clock had been invented by Huygens, that the
time of an occultation could be fixed with the required
exactness. Thus it happens that from 1640 to 1670 some-
what coarse observations of the kind are available, and
after the latter epoch those made by the French astrono-
mers become almost equal to the modern ones in precision.

"The question that occurred to me was, Is it not pos-
sible that such observations were made by astronomers
long before 1750? Searching the published memoirs of
the French Academy of Sciences and the Philosophical
Transactions, I found that a few such observations were
actually made between 1660 and 1700. I computed and re-
duced a few of them, finding with surprise that Hansen's
tables were evidently much in error at that time. But

THE MOON 171

neither the cause, amount or nature of the error could be
well determined without more observations than these.
Was it not possible that these astronomers had made more
than they had published? The hope that material of this
sort existed was encouraged by the discovery at the Pul-
kowa Observatory of an old\ manuscript by the French
astronomer Delisle, containing some observations of this
kind. I therefore planned a thoro search of the old rec-
ords in Europe to see what could be learned."

By good fortune suitable observations were found at
the Paris Observatory and their relations and the method
of making were studied and everything necessary was
copied. This work took some six weeks, but Professor
Newcomb says, "The material I carried away proved the
greatest find I ever made. Three or four years were spent
in making all the calculations I have described. Then it
was found that seventy-five years were added, at a single
step, to the period during which the history of the Moon's
motion could be written. Previously this history was
supposed to commence with the observations of Bradley
at Greenwich, about 1750; now it was extended back to
1675, and with a less degree of accuracy thirty years far-
ther still. Hansen's tables were found to deviate from
the truth in 1675 and subsequent years to a surprising
extent, but the cause of the deviation is not entirely un-
folded even now.

"One curious result of this work is that the longitude
of the Moon may now be said to be known with greater
accuracy through the last quarter of the seventeenth cen-
tury than during the ninety years from 1750 to 1840. The
reason is that, for this more modern period, no effective
comparison has been made between observations and Han-
sen's tables."

As the Moon rotates around the Earth while the Earth
is passing in its orbit about the Sun, it will be obvious
that twice in its journey the Sun must come into a line of
intersection of the Moon's orbit with that of the Earth.

i;2 ASTRONOMY

When this happens at the time of full moon the Earth will
lie directly between the Moon and the Sun, so that the
light from the Sun is intercepted and a shadow is formed
on the Moon's surface. Sometimes the Moon only par-
tially enters the Earth's shadow and then the eclipse, as
this phenomenon is termed, is partial. If, however, the
Sun is situated on the line of intersection at the time of
new moon, then the Sun will be eclipsed, and a solar
eclipse so rich in astronomical significance occurs. Solar
eclipses have been discussed elsewhere and their im-
portance explained. Lunar eclipses, besides enabling us

Fig. 23 — Form of the Earth's Shadow, Showing the Penum-
bra, or Partially Shaded Region. Within the Penumbra
the Moon Is Visible; in the Shadow It Is Nearly Invis-
ible.

to check the motions of the Moon and furnishing an
interesting spectacle, afford little scientific information.

When the black shadow on the Moon is first detected in
the case of a total lunar eclipse, it is interesting to watch
its encroachment until the entire surface of the satellite is
covered. Even the Moon, cm which no direct sunlight can
fall, is often visible, glowing with a copper-colored hue,
sufficiently bright to enable several of the markings on the
surface to be seen. This is due to refraction of the
atmosphere, which bends the sunbeams that have just
grazed the Earth and permits them to fall within the
shadow. In their journey through the denser atmosphere
they have become rich in red rays, which gives to the disk
a ruddy or copper-like hue analogous to that of the Sun
at sunrise or sunset. Whether the eclipse of the Moon is
total or partial depends on the extent to which it passes

THE MOON 173

into the shadow of the Earth, as the accompanying dia-
gram will indicate clearly. Lunar eclipses are useful to
the astronomer for determining the length of the synodic
month and also for determining the temperature to which
the Moon has been raised, for when it enters the shadow
all direct light from the Sun is cut off and the Moon
becomes cold very rapidly. Furthermore, the position
of the Moon with respect to the stars can be determined
on such occasions with great accuracy. Like solar
eclipses, eclipses of the Moon can be predicted with high
precision, and they are regularly announced in almanacs
and ephemerides.

The Moon always presents the same face to the Earth, a
phenomenon discovered by Galileo. It must follow that
the Moon rotates on its axis once in the same number of
seconds that it requires for a revolution around our planet.
This is explained by the fact that tides on the Moon, as in
the case of the Earth, have lengthened the period of rota-
tion by their braking action. At a time when the Moon
was still a hot, semi-molten mass, the attraction of the
Earth produced great tides, not tides of water, but tides
of molten rock. These tides on the Moon checked its
rotational velocity and eventually constrained the Moon to
rotate on its axis in precisely the same period as that
which it requires to revolve around the Earth. All this
happened eons ago. There is no longer evidence of any
tidal action, because the Moon is frozen. Altho there can
hardly be tides on the Moon, yet there may be tides in
the Moon.

"It may be that the interior of the Moon is still hot
enough to retain an appreciable degree of fluidity," writes
Sir Robert Ball, "and if so, the tidal control would still
retain the Moon in its grip; but the time will probably
come, if it have not come already, when the Moon will be
cold to the center — cold as the temperature of space. If
the materials of the Moon were what a mathematician
would call absolutely rigid, there can be no doubt that the

174 ASTRONOMY

tides could no longer exist, and the Moon would be eman-
cipated from tidal control. It seems impossible to predi-
cate how far the Moon can ever conform to the circum-
stances of a rigid body, but it may be conceivable that at
some future time the tidal control shall have practically
ceased. There would then be no longer any necessary
identity between the period of rotation and that of revolu-
tion. A gleam of hope is thus projected over the astron-
omy of the distant future. We know that the time of
revolution of the Moon is increasing, and so long as the
tidal governor could act, the time of rotation must increase
sympathetically. There will then be nothing to prevent
the rotation remaining as at present, while the period of
revolution is increasing. The privilege of seeing the
other side of the Moon, which has been withheld from all
previous astronomers, may thus in the distant future be
granted to their successors."

While study of the Moon and its motions continued, a
beginning was made in the Renaissance to examine the
surface of this satellite. Leonardo da Vinci (1452-1519)
was the first to explain correctly the dim illumination seen
over the rest of the surface of the Moon when the bright
part was only a thin crescent. This he maintained is due
to the earthshine or slight illumination of the Moon by
light reflected from the Earth, just as moonshine is able
to illuminate the Earth.

Galileo's lunar observations through his telescope were
epoch-making. Not only was he able to disprove many
common conceptions of the nature of the satellite and its
surface, but also to present a mass of evidence of a positive
character. In spite of the familiar dark markings, the
Moon was really supposed to be a smooth sphere. After
the introduction of the telescope, however, it was recog-
nised by Galileo that the surface of the Earth's satellite
was dotted with various inequalities, which he assumed to
be mountains, valleys and seas. Thus he correctly ac-
counted, in part at least, for the unevenness of the surface.

THE MOON 175

He was not content with mere observation of the features
of the Moon's surface, but measured the height of some
of the more conspicuous lunar mountains and obtained
for them an estimated elevation of four miles, a figure
which agrees fairly well with modern estimates. Having
seen and measured mountains on the Moon's surface, it
seemed natural that there should be water. The large
dark spots he erroneously regarded as seas, altho he was
not responsible for the corresponding names applied to
these supposed expanses of water by some of his suc-
cessors, and still preserved in lunar maps. The chief
marks of astronomical progress as revealed by Galileo's
observation of the Moon were that it was a body in many
respects similar to the Earth, that it was not a perfect
sphere, and that there is no fundamental difference be-
tween celestial bodies and our own Earth, either in their
motions or in their general nature, which was important
in the final establishment of the Copernican theory.

One other discovery of Galileo's in connection with the
Moon is of great importance. It had been known for
many years that as the Moon revolved around the Earth
the same markings were constantly seen. With the tele-
scope these markings could be studied so much more dis-
tinctly that it occurred to Galileo to ascertain whether
there was any change in the Moon's disk, or whether its
appearance was always exactly the same. He found that
as the Moon moves in its orbit around the Earth that
slight changes are seen in its appearance. In other words,
small portions of the hemisphere alternately on its north-
ern and southern half are exposed. The simplest of the
motions of the Moon in this way subsequently came to be
known as "librations."

Kepler in his Epitome of the Copernican astronomy
demonstrated that his planetary laws applied to the motion
of the Moon around the Earth, despite irregularities
which introduced enormous complications. In this work,
however, he devotes much attention to the theory of the

176 ASTRONOMY

Moon, explaining in considerable detail both evection and
variation.

Galileo established the fact that the Moon was similar
to the Earth in many respects. The analogy was carried
somewhat further by certain of the pioneer workers with
large telescopes. Even Herschel held that because the
Moon closely resembled the Earth, it might be a suitable
habitat for human beings. The dark spots once taken for
seas and bearing that name on lunar maps are, in reality,
lava, while the craters which dot the surface of the
satellite, with one or two possible exceptions, belong to
volcanoes long since extinct. The dark lines once known
as "rills," which it was assumed were rivers, are plainly
without water. If there is a lunar atmosphere its density
must be very small, in fact less than that of the atmosphere
far above the Earth. That there is a very rare lunar
atmosphere seems to be probable. In fact, the assump-
tion of an atmosphere is necessary for the explanation of
certain phenomena.

After Galileo's lunar studies the next important work
was that of John Hevel, of Danzig (1611-1687), who pub-
lished in 1647 ms Selenographia, in which not only the
text but the plates were prepared by him. Here he sys-
tematically describes and names the chief features of the
Moon, the immense craters and seas, employing many
names taken from the Earth, such as Apennines, Alps,
and Mare Serenitatis for the Pacific Ocean, all designa-
tions still found on modern lunar maps. Not all of his
names have survived. John Baptist Riccioli (1598-1671),
in a treatise on astronomy called The New Almagest,
1651, gave to various mountains and craters the names
of distinguished men of science and philosophers. Hence
the names of Plato, Archimedes, Tycho and Copernicus
are found on lunar maps. These names have survived in
considerable number. More modern map-makers, such as
Beer and Maedler, whose map was published in 1837, and
Schmidt of Athens, who published a map of the Moon

THE MOON 177

seven feet in diameter in 1878, have carried out this idea.
Modern astronomers are likewise honored with the names
of various points represented on the maps.

For the origin of the Moon the mind must be forced to
look back millions and millions of years to a time when
our Earth existed in a very different form. Then it was
not a solid mass, but a globe of molten material on which
floated a crust perhaps some thirty-five miles thick. It
rotated not in a period of 24 hours, the present day's
length, but at a terrific velocity which may have made the
day some three hours in length. Such a speed of revolu-
tion naturally produced a most powerful centrifugal force.
One day a cataclysm occurred. Five thousand cubic mil-
lions of miles of matter were thrown off into space. Thus
the Moon was born. A great scar was left on the surface
of the Earth, a scar which in the opinion of Professor
William H. Pickering is the basin of the Pacific Ocean.
After this rending of the Earth the remaining parts of
the crust, afloat on the liquid interior, were split along
irregular lines into two pieces, which drifted apart and
were filled by the waters of the Atlantic and Pacific
Oceans. This theory of Pickering's is diametrically op-
posed to that of Professor T. J. J. See, who claims that
the Moon is in reality a planet, captured by the Earth in
its wanderings through space, and that all satellites have
been thus captured.

Whatever the origin of the Moon, it is the largest of
all the planetary satellites, yet smaller in mass than the
Earth, from which it is separated by a distance that varies
between 222,000 and 253,000 miles. Its gravity is equal
to about one-sixth that of the Earth, for which reason
the same amount of energy acting, for example, in a vol-
canic upheaval, produced mountains higher than those on
the Earth. This mass of the Moon is about l/n that of
the Earth, or 73,000,000,000,000 tons.

The accompanying diagram shows the comparative size
of the Earth and the Moon. The diameter of our planet

178 ASTRONOMY

is 7,914 miles, while that of the Moon is 2,160 miles, so
that the diameters stand very nearly in the relation of
4 to 1, while the superficial area of the Moon is equal to
about yi3 part of the surface of the Earth. The average
distance of the Moon from the Earth is also fairly con-
stant, and the average fluctuations do not exceed more
than about 13,000 miles on either side of its mean value
of 239,000 miles.

•Fig. 24 — Comparative Sizes of the Earth and the Moon.

The Moon is essentially a dead planet in the eyes of
most astronomers. Its fires long since have been ex-
tinguished. It is a great globe of chilled slag. Its craters
have no counterpart on the Earth. The lunar crater is
a great circular plane, 50 or even 100 miles in diameter,
around which rises a precipice perhaps five or ten thou-
sand feet, while in the center there may be a hill or two
about half as high'. Thousands of these volcanoes are
visible in the telescope. How these craters were formed is
a puzzle. Some astronomers hold that they mark the im-
pact of countless meteorites. Others assume them to be

THE MOON 179

the product of gigantic bubbles in a once molten mass that
burst. Again it is claimed that they are volcanoes resem-
bling those of the Earth.

Most astronomers tell us that the craters have long
been dead, that the Moon has had for centuries no atmos-
phere and therefore cannot have water or support plant
or animal life. Prof. William H. Pickering, however,
tells us of an exceedingly rare lunar atmosphere and main-
tains that the Moon's craters are not all extinct. He even
claims that certain great white expanses are snow and
ice, and, furthermore, that there is evidence of the growth
and decay of vegetation. To support his views of vanish-
ingly feeble volcanic activity he calls attention to the little
crater named Linne, after the famous naturalist, Linnaeus.
In modern times this crater has unquestionably undergone
changes. Only a few centuries ago it was described on the
old maps as "a crater of moderate size" and later as "a
very small round brilliant spot." A dead volcano cannot
alter its shape.

If there are still a few intermittently active volcanoes
they must expel water and carbonic acid gas, judging
by earthly volcanoes. But water cannot be found on the
Moon as a liquid, for the temperature of its surface is
probably not far from 4600 F. below zero. Many of the
elevated peaks are capped with a silver glow, which also
characterizes the lining of the "craterlets." In Pickering's
eyes that white sheen is snow. He believes also in ac-
cumulations of snow and ice at the poles. If carbonic acid
is expelled by lunar volcanoes it must cling to the Moon
with great tenacity because of its weight. Since it is the
food of plant life, it may possibly support vegetation on
the Moon. Professor Pickering sees in variable dark
spots on the planet organic life resembling vegetation. He
figures that since certain lichens grow in certain regions
of the Earth where the temperature never rises above the
freezing point, there is no reason why that vegetation may
not flourish on the Moon's surface.

CHAPTER XV

Mars is another instance of a celestial body whose dis-
covery long antedates written history, for which reason it
is natural to assume that its appearance in the heavens and
its motions have always been known to mankind. Its
ruddy light naturally associated it in the Greek mind with
the God of War, just as the soft, warm glow of Venus
was considered appropriate to the Goddess of Love. To
the Chaldean and to other ancient star-gazers the mo-
tions of the planet were indeed of interest. No planet
has received greater attention from astronomers or has
been used more for the solution of important astronomical
and cosmical problems.

Thus it was from an observation of the passage of the
Moon in front of Mars, or what is termed by astronomers
an occultation of the planet, that Aristotle concluded that
the planets are more distant than the Sun and Moon.
What reason he had for including the Sun it is difficult to
conceive, as an occultation by that body would be quite
impossible. Later it was from the study of the motions of
the planet made for Tycho Brahe that Kepler was able to
derive his great laws of planetary motion. By observing
the great "Hour-glass Sea," or Syrtis Major, on Mars,
Huygens in 1659 noted the rotation of the planet on its
axis, and from a measurement of the planet made at its
opposition Richer was able to obtain a reasonably accurate
estimate of the distance of the Sun in 1673.
180

MARS 181

In 1783 Herschel had written: "The analogy between
Mars and the Earth is perhaps by far the greatest in the
whole solar system." For over a century the development
of this analogy has been one of the most important fea-
tures of astronomy, especially in popular estimation. To-
day, with a wealth of scientific and observational data
acquired during the last score of years, the question, altho
more fully and intelligently discussed, is as far from solu-
tion as ever.

Mars revolves around the Sun in an orbit whose mean
distance is 141,500,000 miles. But this orbit is so eccen-
tric, amounting to 0.093, that the distance varies about
thirteen million miles. As this distance from the Sun is
about one-half as far again as that of the Earth, the light
and heat that Mars receives are only about 4/9 of the light
and heat received by the Earth, an important consideration
in discussing the possibility of life on its surface.

The real diameter of Mars is about 4,200 miles, a figure
correct within 20 miles, so that its surface is 23/ioo and its
volume 0.147 or */? that of the Earth. The mass of the
planet, derived from observations of its satellites, is about
y».4 that of the Earth, so that its density is 0.73 and the
attraction of gravity on its surface 0.38 that of the Earth.
Mars reflects 0.26 of the light that it receives, which is
just double that of Mercury.

In Mars may be seen a striking difference between the
superior and inferior planets, for the crescent phases of
the latter are conspicuously absent. This is because the
orbits of Mars and the other superior planets lie outside
of the Earth. Consequently such a planet never comes
between the Earth and the Sun. At quadrature a certain
amount of the unilluminated portion is turned toward the
Earth so that the planet appears like a Moon three or four
days from full, with a distinctly gibbous appearance.

Kepler suspected the existence of satellites from a some-
what cryptic utterance of Galileo's in regard to the triple
Saturn. No particularly sound reasons were advanced for

182 ASTRONOMY

the existence of such satellites.- Still that they might be
found is forcibly reflected in eighteenth-century literature.
In "Gulliver's Travels" (1726) is the striking statement
that an astronomer on the floating island of Laputa had
discovered two tiny satellites of Mars. Dean Swift even
advanced the extraordinary statement that one of these
moons revolved around the planet in 10 hours, the ap-
proximate correctness of which will presently be appar-
ent. Furthermore, Voltaire in his "Micromegas," published
in 1752 and obviously modeled on "Gulliver's Travels,"
also speaks of these small bodies. Altho literature and
imagination had preceded observation, astronomers could
not detect any companions to the ruddy planet until August
II, 1877, when Professor Asaph Hall, using the 26-inch
refractor of the U. S. Naval Observatory at Washington,
which at the time of its construction (1873) was the lar-
gest telescope in the world, studied the vicinity of Mars
with unusual care. In 1877 it was at conjunction, so
that conditions were favorable for an exhaustive inquiry.
On August 11 Professor Hall detected a minute speck
of light near the planet, and a few days later, August
16, he ascertained positively that it was a satellite. On
the following evening a second body was discovered, be-
wilderingly rapid in its motion. Since that time the two
satellites have been frequently observed at opposition even
with much smaller instruments. On July 18, 1888, the
large Lick telescope was able to reveal them when their
brightness was only about one-eighth that at the time of
their discovery.

The names appropriately assigned to the new satellites,
Deimos and Phobos (Fear and Panic), were taken from
the Iliad and represent the companions in battle of the
God of War. The satellites are very small, having diame-
ters respectively of six and seven miles, as photometrically
measured at Harvard University soon after their discovery
by Professor Pickering. Phobos, the inner, is the larger
and traverses an elliptical orbit in 7 h. 39 m. 22 s. at a dis-

MARS 183

tance of only 3,760 miles. Deimos, its companion, moves
in a nearly circular orbit in 30 h. 18 m. at a distance from
the planet of 12,500 miles.

It should be remembered in all discussions of the nature
of the surface of the planet Mars and in developing theo-
ries based thereon that astronomers must deal with a little
disk which once in fifteen years has a maximum of about
Vuooo the area of the full Moon. When nearest the Earth
Mars is at a distance of 35,500,000 miles and when ob-
served with a telescope magnifying 1,000 times it appears
only as large as the Moon with an ordinary field glass,
enlarging six or seven times. In other words, Mars is
brought to a distance of 35,500 miles. Hence it is difficult
to distinguish minute details. Consequently visual and
photographic observation of Mars must be of the most
refined character, for especially in the former there is
opportunity for psychological or subjective phenomena to
occur in connection with the observation. While it is
neither rational nor scientific to deny what credible observ-
ers claim they have seen through the telescope in examin-
ing the planet, yet it must be recognised that visual obser-
vations are attended by necessary limitations and should be
received in a spirit of caution rather than of enthusiasm.

The mapping of Mars is no recent matter, for even in
1659 a rough sketch of the surface of the planet was made
by Huygens in which the V-shaped marking at the equator
pointing to the north can be identified as the Syrtis Major.
This was followed by rough sketches from time to time
down to 1840, when Maedler first began a systematic
charting of the planet. His map was followed in 1864 by
Kaiser's, by Flammarion's in 1876 and Green's in 1877.
Drawings of various parts of the planet were made during
these intervals, but were not combined into good charts.

Observations made by Professor Lowell and his staff
at the observatory at Flagstaff, Arizona, have had the
study of this planet especially in view. The Lowell ob-
servatory itself is situated 7,300 feet above the sea and

184 ASTRONOMY

contains an excellent 24-inch telescope with which it is
claimed that the faintest stars shown on the chart made
at the Lick Observatory with a 36-inch instrument are
perfectly visible. The atmosphere is well adapted for
telescopic work, so that Professor Lowell and his col-
leagues possess singular advantages over other observers.
On the other hand, in many important respects there has
been a lack of corroboration of their observations.

The surface of Mars, as seen in the telescope, is com-
posed of two white polar caps, which wane with the ap-
proach of summer; orange areas, which are supposed by
Lowell and his followers to be deserts, and blue-green
areas, which change their hue to orange during the Mar-
tian autumn and winter and reassume their verdant tint
in spring. The planet is covered with a network of fine
lines, first discovered by Schiaparelli in 1877 and called by
him "canals," a designation by which they are still known.
These canals connect the polar caps with the temperate
and equatorial zones. According to Professor Lowell,
they may be regarded as planetary irrigation ditches which
serve the purpose of leading the melting water of the
poles to those desert regions which would still blossom if
properly watered. The canals disappear with the approach
of winter and creep down from the poles toward the equa-
tor in summer, a phenomenon which long puzzled astrono-
mers until Pickering ingeniously suggested that we see
not the canals themselves (for they are much too narrow),
but the vegetation which fringes their banks, which with-
ers as the cold of winter descends and which flourishes
with the melting of the snows.

It was Schiaparelli who also first announced the curious
doubling of the canals in certain instances, an announce-
ment which was at first received with derision but which
has since been confirmed by his disciple, Lowell, and
the astronomers of the great observatories. This curi-
ous gemmation has not been satisfactorily explained,
altho Professor Lowell attributes it in part to vegetal

MARS 185

causes. By opponents of his theory the doubling is re-
garded as an optical illusion, which can hardly be the
case, because all and not certain of the canals would ex-
hibit the phenomenon. Furthermore, there is some photo-
graphic evidence offered by Mr. Lampland, of Professor
Lowell's staff, that both the canals and their doubling are
real.

The visual observations of Professor Lowell have been
undertaken with great thoroness and have brought to light
much material which he has employed in the development
of his theories. To do justice to his work in a single
chapter is manifestly impossible. It will be most sat-
isfactory to the reader to recapitulate in Professor Low-
ell's own words, derived from his work on "Mars and
Its Canals," the fundamental facts which he has recorded
and on which he bases his conclusions. They are as
follows :

"(1) Mars turns on its axis in 24 h. 37 m. 22.65 s. with
reference to the stars and in 24 h. 39 m. 35 s. (as a
mean) with regard to the Sun. Its day therefore is only
about forty minutes longer than ours.

"(2) Its axis is tilted to the plane of its orbit by about
23° 59' (most recent determination, 1905). This gives the
planet seasons almost the counterpart of our own in char-
acter, but in length nearly double ours, for

"(3) Its year consists of 687 of our days, 669 of its own.

"(4) Polar caps are plainly visible which melt in the
Martian summer to form again in the Martian winter, thus
implying the presence of a substance deposited by cold.

"(5) As the polar caps melt they are bordered by a blue
belt, which retreats with them. This excludes the possibil-
ity of their being formed of carbon dioxide and shows that
of all the substances we know the material composing
them must be water.

"(6) In the case of the southern cap, the blue belt has
widenings in it in places. These occur where the blue-
green areas bordering upon the polar cap are largest

186 ASTRONOMY

"(7) The extensive shrinkage x>i the polar snows shows
their quality to be inconsiderable and points to scanty
deposition due to dearth of water.

"(8) The melting takes place locally after the same gen-
eral order and method, Martian year after year, both in
the south cap

"(9) And in the north one. This is evidenced by the
recurrence of rifts in the same places annually in each.
The water thus let loose can, therefore, be locally
counted on.

"(10) That the south polar cap is given to greater ex-
tremes than the north one implies again, in view of the
eccentricity of the orbit and the tilt of the axis, that depo-
sition in both caps is light.

"(11) The polar seas at the edges of the caps being
temporary affairs, the water from them must be fresh.

"(12) The melting of the caps on the one hand and their
re-forming on the other affirm the presence of water vapor
in the Martian atmosphere, of whatever else that air
consist.

"(13) Since water vapor is present, of which the molec-
ular weight is 18, it follows from the kinetic theory of
gases that nitrogen, oxygen and carbonic acid, of molecular
weights 28, 32 and 38 respectively, are probably there, too,
owing to being heavier.

"(14) The limb-light bears testimony to this atmosphere.

"(15) The planet's low albedo points to a density for
the atmosphere very much less than our own.

"(16) The apparent evidence of a twilight goes to affirm
this.

"(17) Permanent markings show upon the disk, proving
that the surface itself is visible.

"(18) Outside of the polar cap the surface is divided
into red-ocher and blue-green regions. The red-ocher
stretches have the same appearance as our deserts seen
from afar,

MARS 187

"(19) And behave as such, being but little affected by-
change.

"(20) The blue-green areas were once thought to be
seas. But they cannot be such because they change in tint,
according to the Martian season, and the area and the
amount of the lightening is not offset at the time by corre-
sponding darkening elsewhere

"(21) Nor by any augmentation of the other polar cap
or precipitation into cloud. It cannot, therefore, be due
to shift of substance.

"(22) Furthermore, they are all seamed by lines and
spots darker than themselves, which are permanent in
place, so that there can be no bodies of water on the planet.

"(23) On the other hand, their color, blue-green, is
that of vegetation. This regularly fades out, as vegetation
would, to ocher for the most part, but in places changes
to a chocolate-brown.

"(24) The change that comes over them is seasonal in
period, as that of vegetation would be.

"(25) "Each hemisphere undergoes this metamorphosis
in turn.

"(26) That it is recurrent is again proof positive of an
atmosphere.

"(27) The changes are metabolic, since those in one
direction are later reversed to a restoration of the original
status. Anabolic as well as katabolic processes thus go on
there — that is, growth as well as decay takes place. This
proves them of vegetal origin.

"(28) The existence of vegetation shows that carbonic
acid, oxygen and undoubtedly nitrogen are present in the
Martian atmosphere, since plants give out oxygen and take
in carbonic acid.

"(29) The changes in the dark areas follow upon the
melting of the polar caps, not occurring until after that
melting is under way,

"(30) And not immediately then, but only after the lapse
of a certain time.

188 ASTRONOMY

"(31) Tho not seas now, from their look the dark areas
suggest old sea bottoms, and when on the terminator ap-
pear as depressions (whether because really at a lower
level or because of less illumination is not certain).

"(32) That they are now the parts of the planet to sup-
port vegetation hints the same past office, as water would
naturally drain into them. That such a metamorphosis
should occur with planetary aging is in keeping with the
kinetic theory of gases.

"(33) Terminator observations prove conclusively that
there are no mountains on Mars, but that the surface is
surprisingly flat.

"(34) But they do reveal clouds which are usually rare
and are often, if not always, dust-storms.

"(35) White spots are occasionally visible, lasting un-
changed for weeks in the tropic and temperate regions,
showing that the climate is apparently cold,

"(36) But at the same time proving that most of the
surface has a temperature above the freezing-point.

"(37) In winter the temperate zones are more or less
covered by a whitish veil, which may be hoar-frost or may
be cloud.

"(38) A spring haze surrounds the north polar cap dur-
ing the weeks that follow its most extensive melting.

"(39) Otherwise the Martian sky is perfectly clear, like
that of a dry and desert land."

These facts, according to Professor Lowell, make rea-
sonably evident on Mars the presence of —

"(1) Days and seasons substantially like our own.

"(2) An atmosphere containing water vapor, carbonic
acid and oxygen.

"(3) Water in great scarcity.

"(4) A temperature colder than ours, but above the
Fahrenheit freezing-point, except in winter and in the ex-
treme polar regions.

"(5) Vegetation."

While all of Professor Lowell's observations and results

MARS 189

are not accepted by all astronomers, and there is consider-
able opposition to his conclusions, nevertheless they are
of interest and worth stating as representing this aspect
of the matter in his own words. Comparative studies of
lunar and Martian spectra made on the summit of Mt.
Whitney in September, 1909, by Campbell of the Lick
Observatory seem to preclude the possibility of much
water on Mars. Campbell's photographs show that the
Martian atmosphere is no richer in water than the Moon's,
which if true summarily disposes of Martian life. Slipher
of Lowell's staff claims to have obtained ample spectro-
scopic evidence of water. The following paragraphs, taken
from the concluding chapter of Lowell's book on "Mars
and Its Canals," may be said fairly to sum up his views
on Martian life:

"That Mars is inhabited by beings of some sort or other
we may consider as certain as it is uncertain what those
beings may be. The theory of the existence of intelligent
life on Mars may be likened to the atomic theory in chemis-
try, in that in both we are led to the belief in units which
we are alike unable to define. Both theories explain the
facts in their respective fields and are the only theories
that do, while as to what an atom may resemble we know
as little as what a Martian may be like. But the behavior
of chemic compounds points to the existence of atoms too
small for us to see, and in the same way the aspect and
behavior of the Martian markings implies the action of
agents too far away to be made out.

"One of the things that makes Mars of such transcen-
dent interest to man is the foresight it affords of the course
earthly revolution is to pursue. On our own world we are
able to study only our present and our past; in Mars we
are able to glimpse, in some sort, our future. Different as
the course of life on the two planets undoubtedly has been,
the one helps, however imperfectly, to better understanding
of the other."

The views expressed by Professor Lowell in the work

190 ASTRONOMY

just quoted were further developed by him in the course
of a few years succeeding its publication, and in "Mars as
the Abode of Life," published in 1908, he expresses him-
self as even more firmly convinced that Mars is inhabited
by a race of intelligent beings. Additional study of
Martian phenomena, according to Professor Lowell, in-
dicates that the canals and oases as he sees them are proof
that life of no mean order of intelligence prevails on the
planet. He suggests :

'Tart and parcel of this information is the order of in-
telligence involved in the beings thus disclosed. Peculiarly
impressive is the thought that life on another world should
thus have made its presence known by its exercise of mind.
That intelligence should thus mutely communicate its ex-
istence to us across the far stretches of space, itself re-
maining hid, appeals to all that is highest and most far-
reaching in man himself. More satisfactory than strange
this, for in no other way could the habitation of the planet
have been revealed. It simply shows again the supremacy
of mind. Men live after they are dead by what they have
written while they are alive, and the inhabitants of a planet
tell of themselves across space as do individuals athwart
time by the same imprinting of their mind."

To this he adds the statement that conditions for life
on the planet are approaching an end, as it is rapidly dry-
ing up, and that the energies of the inhabitants are being
the slowly diminishing supply of water. "The drying up
of the planet is certain to proceed until its surface can
support no life at all." So that, as compared with the
Earth, Mars presents a distinctly later period of evolu-
tion. No such danger at present confronts our own
planet. But assuming that these explanations are correct,
it is not improbable that the cooperative action of all
the nations of the world may be required at some future
date to deal with a similar problem.

In opposing the idea of canals on the surface of Mars,
rmich stress has been laid on the alleged fact that the

MARS 191

finer markings and some of the apparent doublings are
based upon optical illusions or psychological phenomena.
Thus, to prove that instinctively the eye would arrange
in the form of straight lines vague suggestions of mark-
ings, E. W. Maunder, of the Greenwich Observatory,
England, and J. E. Evans, of the Royal Hospital School
at Greenwich, in 1902 performed an experiment with a
number of schoolboys. A circular disk was exhibited
five or six inches in diameter, on which was represented
with some accuracy the shaded area of the planet as it
might appear in the telescope. Instead of canals, a few
faint, wavy lines and a larger number of faint dots were
inserted promiscuously. The boys were told to fill in with
pencil on small circles on sheets of paper the details of the
object exhibited to them, exercising as much care as pos-
sible. The object of the experiment was not communicated
to the boys. None of them had any idea of the nature of
the planet's surface. The result was that boys at the
greatest distance from the object, where the faint lines
and dots were just beyond the limits of separate visibility,
drew canals strikingly like those noted by the telescopic
observers of the planet. Hence it was supposed that
the eye in some way integrated such faint stimuli as
irregular scattered dots and faint, wavy lines into straight
lines which have no objective existence. Much has
been made of this test, but it may hardly be called con-
clusive, for Flammarion when repeating the experiment
with French boys was unable to secure in their sketches
lines resembling canals. Moreover, photographs taken
during the opposition of 1905 at Flagstaff bring out a
large number of the more prominent canals as straight
lines.

It is possible that psychology plays an important part,
and Professor Andrew Ellicott Douglass, of the Uni-
versity of Arizona, who has had opportunities to observe
the planet at several favorable oppositions, believes that

192 ASTRONOMY

the subjective phenomena have much to do with Martian
observations. He says:

"One may confidently say that such realities do exist.
But with the very faint canals whose numbers reach occa-
sionally well into the hundreds, discordance reigns su-
preme, and it is frequently found that different drawings
by the same artist antagonize each other across the page.

"Considerations along these lines led me to study seri-
ously the origin of these inconsistent faint canals by the
methods of experimental psychology, and the application
of these methods has resulted in a new optical illusion and
new adaptations of old and well-known phenomena." The
most important of these phenomena was the halo effect-
where secondary images are produced under unusual con-
ditions, which images affect the minute details of the sur-
face. Professor Douglass also found that the irregular
refraction of the eye produced apparent rays as from a
small spot, which could be obviated in part by changing
the position of the observer's head. "The ray illusion," he
says, "is to me a very satisfactory explanation of many
faint canals radiating from those small spots on Mars
called 'lakes' or 'oases.' The only objective reality in
such cases is the spot from which they start."

Referring again to the halo he reaches the conclusion
that—

"The halo with its light area and secondary image ac-
counts for details which have no objective reality, such as
bright limbs of definite width, canals paralleling the limb
or dark areas, numerous light margins along dark areas
and light areas in the midst of dark, abundantly exempli-
fied in Schiaparelli's map of 1881-82."

Professor Lowell, it must be said, has the unique ad-
vantage, or misfortune it may be, to see canal-like mark-
ings that are not visible to other astronomers. Thus on
Venus he saw bands or lines which he considered bore a
superficial resemblance to the canals of Mars and were ap-
parently permanent and not due to clouds. Again he claims

MARS 193

to have seen lines resembling canals on the third satellite
of Jupiter, which others have failed to recognise. Hence
many astronomers believe that he is predisposed to see
such phenomena. Furthermore, canal-like appearances have
been noted on the surface of the Moon by Professor W. H.
Pickering as radiating from the central peaks of the north-
western slopes of the central mountain range of the crater
of Eratosthenes. Pie observed two canals which in a small
telescope appear straight, yet, when seen with a large
glass, present considerable irregularity of structtfre. Other
and new branches or canals were also seen. In various
parts of the same crater, but especially in the southeastern
and northern portions, numerous small canals and lakes
present themselves. These markings are practically identi-
cal in appearance with those seen upon the planet Mars.
They are too small to be well shown on photographs and
seem to be of much more regular structure than the larger
markings, which are also called canals. It is possible that
this difference is due merely to the fact that the larger
markings are better seen. There can be no free water
on the Moon's surface; hence any canals with flowing
water are quite out of the question. Yet the appearance is
vouched for as remarkably similar to that on Mars by an
observer who knows the surfaces of both bodies.

CHAPTER XVI

JUPITER

As Jupiter comes next to Venus in point of brilliancy
of the heavenly bodies, it is but natural that it should
have been known to the ancients from a remote antiquity
and that its discovery or early observation should be lost
in a far distant past. The brightness of Jupiter varies
with its position, and the relative brightness of the planet
at an average conjunction at the nearest and most remote
oppositions is respectively as the numbers 10, 27, and 18.

The orbit of the planet is but slightly inclined to the
ecliptic, or i° 19', and the planet itself moves with an or-
bital velocity of about eight miles a second in the sidereal
period of 11.86 years, which is the time of its revolution
around the Sun from a star to the same star again, as
seen from the Sun. The mean distance of the orbit of the
planet from the Sun is 483,000,000 miles. The eccen-
tricity is nearly one-twentieth, the greatest and least dis-
tances from the Sun being 504,000,000 and 462,000,000
miles respectively. The average distance of the planet
from the Earth at opposition is 390,000,000 miles, while at
conjunction it is 576,000,000 miles. The minimum opposi-
tion distance, occurring about October 6, amounts to 369,-
000,000 miles, while at aphelion in April the distance is
greater by about 42,000,000 miles.

Jupiter is larger than all the rest of the planets in the
solar system, whether its bulk or its mass be taken into
consideration. Its surface is 119 times and its volume
194

JUPITER 195

1,300 times that of the Earth. The mean diameter is 86,500
miles, or about eleven times that of the Earth. But if the
relative masses and volumes of the two planets be com-
pared, it will be found that Jupiter has a density less than
one-quarter that of the Earth, or .24, and almost the same
as that of the Sun. A body on Jupiter would weigh 2^$
times as much as upon the surface of the Earth, because
the mean superficial gravity of the planet is 2.64 times as
great. Owing to its rapid rotations and its elliptical shape
the difference between the force of gravity at the equator
and the pole is much greater and amounts to }i of the
equatorial gravity, where on the Earth it is as 1/iw>.

The planet is brightest, as is also Saturn, in the center
of the disk, which it will be recalled is the case with the
Sun, but not with Mars, Venus and Mercury. On account
of this resemblance to the ■ Sun, the idea has been sug-
gested that the planet may be, to some extent, self-
luminous.

The planet receives too small an amount of heat from
the Sun to account for the rapid changes which beyond
question are taking place on its visible surface. Conse-
quently, to produce these changes the heat must come
from the planet itself. Probably the body is at a temper-
ature little below that of incandescence, not having solid-
ified to any appreciable extent.

The most striking feature of Jupiter is its system of
oright satellites, four of which were the first fruits of
astronomical discovery as it is now understood, and were
revealed to Galileo when he directed his small telescope
toward the planet. In fact, this historic observation o£
January 7, 1610, meant much to astronomy. When the
great Italian scientist determined the periods of these
strange bodies, or "Medicean stars," as he termed them, a
new era was opened in astronomy. The four satellites, in
addition to being numbered in the order of their distance
from the planet, are also known by the mythological names
of Io, Europa, Ganymede and Callisto, and revolve in

196 ASTRONOMY

sidereal periods, ranging from i.day, iSy2 hours to 16 days,
i6x/4 hours at relative distances of between 262,000 and
1,169,000 miles. From the small telescope of Galileo in
the opening years of the seventeenth century to the Lick
refractor at the close of the nineteenth is indeed a far
cry, but the four satellites of Jupiter remained alone until
a fifth was added to their number by Professor E. E. Bar-
nard at Mt. Hamilton in September, 1892. This dis-
covery was as much a triumph for the modern telescope
as the original detection of the four moons was an
achievement for the "Optick Tube," for the satellite is
visible only with telescopes having a greater aperture
than 18 or 20 inches. It has a period of 11 hours,
57.4 minutes, and its nearness to the planet, 112,500 miles,
makes it additionally difficult to see.

But this was by no means the end, for where the eye
failed the photographic plate was available. In Decem-
ber, 1904, and January, 1905, Professor C. D. Perrine of
Lick Observatory added, by photography, two new moons
to Jupiter's system. Both of these bodies revolve at a
greater distance than the older known satellites. Still
more recently P. Melotte of Greenwich Observatory,
while examining a photograph made there on February
28, 1908, found a faint object which proved to be an
eighth satellite of Jupiter, photographed several times at
Greenwich, at Heidelberg and at Lick Observatory. The
movement is retrograde, which anomaly is of vast cosmi-
cal importance.

The discovery of the satellites of Jupiter by Galileo was
still another point which brought him in conflict with the
Church. In 161 1 there was published a tract in which
it was mentioned that the satellites of Jupiter were un-
scriptural. This, apparently, was a minor issue with the
ecclesiastical authorities, for the evidence of the telescope
was incontrovertible.

Galileo believed that it was possible to determine the
longitude at sea by means of the satellites of Jupiter,

JUPITER 197

and corresponded with the Spanish Court in reference
to a method which he had devised. He held that if the
movements of Jupiter's satellites and, in particular, the
eclipses which constantly occur when the satellites pass
into Jupiter's shadow, could be predicted, then a table
could be prepared giving the dates at some standard
place, say Rome, at which the eclipses would occur.
The local time of the eclipse could be readily pbserved
and referred to the local time — that is, its noon or
when the Sun is highest in the sky, with no great amount
of error, and the difference in time between the two
places would naturally give the difference in longitude.

In Galileo's day astronomers and navigators had no ac-
curate means of keeping time, such as the modern chro-
nometer, which, carried on a ship, can be kept at Green-
wich or some other standard time and give the difference
between that and local time immediately. Galileo knew
nothing of the pendulum clock of Huygens, or, more,
especially, the chronometer of John Harrison (1693-
1776), which has made possible the accurate determina-
tion of longitude at sea. The motions of the satellites
continued to arouse general interest, and Kepler, taking
the movements of the four satellites around the parent
planet, as recorded by Galileo and Simon Marius, found
that his laws of planetary motion applied to the satellites
as well as to the planets themselves.

But perhaps the most striking of the discoveries made
with Jupiter, after the actual detection of its satellites,
was that of the Danish astronomer, Olaus Roemer (1644-
1710), in 1675, when engaged in the study of the motion
of Jupiter's satellites. He ascertained that the intervals
between successive eclipses of a moon, which were caused
"by its passage into Jupiter's shadow, were regularly less
when Jupiter and the Earth were approaching each other
than when they were receding. Accordingly he made the in-
genious assumption that light travels through space, not in-
stantaneously, but at a certain definite tho very great speed.

198 ASTRONOMY

Accordingly, if Jupiter is approaching the Earth, the
time which the light from one of his moons takes to reach
this planet must be gradually decreasing, and consequently
there is less interval between successive eclipses as seen
from the Earth than when the great planet is departing^
from it. Now the difference of the intervals thus observed,
together with the known rates of motion of Jupiter and of
the Earth, which, of course, could be calculated, made it
possible to form a rough estimate of the speed at which
light travels. Roemer had not sufficient observations at
his command to investigate this problem very thoroly,
but he was able to compute the apparent retardation of the
eclipses between opposition and conjunction and thus to
obtain a value for twice the time required for light to
come from the Sun to the Earth, which time was very
nearly 500 seconds, or eight minutes and twenty seconds.
This was the first work on the so-called "equation of
light." It was many years before astronomers accepted
Roemer's really wonderful method for obtaining the dis-
tance from the Sun. To-day the process is reversed. By
elaborate physical experiments made on the Earth's sur-
face it is possible to obtain an accurate value for the
velocity of light and then by means of the light equation
to deduce the distance from the Sun.

James Bradley (1693-1762), the third Astronomer
Royal of Great Britain, also devoted himself to the study
of the satellites of Jupiter. With Cassini's observations,
which he used as the basis for some tables, as well as
many of his own dealing with the eclipses of the satel-
lites, he noted a large number of discrepancies between the
observations and the tables, and found even more pecu-
liarities in their motions than did the early observers.
Using Roemer's suggestion of the finite time consumed by
light in traveling from Jupiter to the Earth, which Cas-
sini and other astronomers of his time had rejected, Brad-
ley was able to make a series of new and valuable tables
of Jupiter's satellites, which were printed in 1719 in Hal-

JUPITER 199

ley's "Planetary and Lunar Tables." Bradley's knowledge
of the satellites of the planet was applied to the method of
the determination of longitude suggested by Galileo, and
with great accuracy he found the longitudes of Lisbon
and of New York.

Jupiter rotates on its axis, which is inclined about 30 to
the orbit, once in about 9 hours and 55 minutes,xa time
which is difficult to obtain more than approximately, for
when different spots are observed different results are
obtained. These spots were first observed in 1665 by Gio-
vanni Domenico Cassini (1625-1712), who was also the
first to study the so-called belts. He was able to report
the discovery of the rotation of the planet by watching the
movement of the spots when observed through the tele-
scope. One of these spots is the famous "great red spot"
first observed in modern times by Professor C. W.
Pritchett in Glasgow, Missouri, in July, 1878.

This is a rosy cloud attached to the whitish zone beneath
the dark southern equatorial band. Of enormous size,
measuring some 30,000 miles in longitude and somewhat
less than 7,000 miles in latitude, it was seen by several}
observers in Europe in the year of its discovery, and in
the following year was observed by almost every astrono-
mer possessed of a telescope. For three years the red
spot was conspicuous. Then it began to fade. When the
planet returned to opposition in 1882 and 1883 Rica's
observations of it at Palermo, May 31, 1883, were expected
to be the last. It began to recover, however, toward the
end of the year, and at the beginning of 1886, according to
W. F. Denning, an English observer, had much the same
aspect as in October, 1882.

Before the "great red spot" astronomers had noticed
various markings on the" planet, one of which, as we have
seen, was recorded by Cassini in 1665 as having a rotation
period of 9 hours and 56 minutes. This spot reappeared
and vanished eight times within the next forty-three years
and was last seen by Maralda in 1713. It was, however,

200 ASTRONOMY

very much smaller than the recent object and showed no
unusual color. Agnes M. Clerke, from whose 'History of
Astronomy' is abstracted this brief description of the
"great red spot," further discusses the phenomenon as fol-
lows: "The assiduous observations made on the 'Great
Red Spot' by Mr. Denning at Bristol and by Professor
Hough at Chicago afforded ground only for negative con-
clusions as to its nature. It certainly did not represent the
outpourings of a Jovian volcano; it was in no sense at-
tached to the Jovian soil — if the phrase have any applica-
tion to that planet ; it was not a mere disclosure of a glow-
ing mass elsewhere seethed over by rolling vapors. It
was, indeed, certainly not self-luminous, a satellite pro-
jected upon it in transit having been seen to show as
bright as upon the dusky equatorial bands.

"A fundamental objection to all three hypotheses is that
the rotation of the spot was variable. It did not then ride
at anchor, but floated free. Some held that its surface
was depressed below the average cloud level and that the
cavity was filled with vapors. Professor Wilson, on the
other hand, observing with the 1 6-inch equatorial of the
Goodsell Observatory in Minnesota, received a persistent
impression of the object's 'being at a higher level than the
other markings.'

"A crucial experiment on this point was proposed by Mr.
Stanley Williams in 1890. A dark spot moving faster
along the same parallel was timed to overtake the red spot
toward the end of July. An unique opportunity hence ap-
peared to be at hand for determining the relative vertical
depths of the two formations, one of which must inevit-
ably, it was thought, pass above the other. No forecast
included a third alternative, which was nevertheless
adopted by the dark spot. It evaded the obstacle in its
path by skirting around the southern edge.

"Nothing, then, was gained by the conjunction beyond
an additional proof of the singular repellent influence
exerted by the red spot over the markings in its vicinity.

JUPITER 201

It has, for example, gradually carved out a deep bay for
its accommodation in the gray belt just north of it. The
effect was not at first steadily present. A premonitory
excavation was drawn by Schwabe at Dessau, September
5, 183 1, and again by Trouvelot, Barnard and Elvins in
1879; yet there was no sign of it in the following year.

4Tts development can be traced in Dr. Beddicker's beau-
tiful delineations of Jupiter, made with the Parsonstown
3-foot reflector, from 1881 to 1886. They record the belt
as straight in 188 1, but as strongly indented from January,
1883 ; and the cavity now promises to outlast the spot. So
long as it survives, however, the forces at work in the
spot can have lost little of their activity. For it must be
remembered that the belt has a shorter rotation-period!
than the red spot, which, accordingly (as Mr. Elvins of
Toronto has pointed out), breasts and diverts, by its in-
terior energy, a current of flowing matter, ever ready to
fill up its natural bed and override the barrier of obstruc-
tion."

The object is now always inconspicuous and often prac-
tically invisible, and may be said to float passively in the
environing medium. Yet there are sparks beneath the
ashes. A rosy tinge faintly suffused it in April, 1900, and
its absolute end may still be remote.

Besides the spots, Jupiter exhibits curious belts or
bands. Herschel, that observer 'par excellence,' frequently
turned his telescope to Jupiter as to other planets, and
became greatly interested in its bright bands. In 1793
he was the first to interpret these as bands of clouds. In
fact, telescopic examination of Jupiter during the nine-
teenth century established the fact that the visible surface
of the planet appears as layers of clouds, and its low den-
sity, 1.3, as compared with water, 1, and the Earth, 5.5,
together with the rapid changes, indicates that the planet
is, to a great extent, in a fluid condition, and that there
is a high temperature at a very moderate distance below
the visible surface.

CHAPTER XVII

The planet Saturn was considered by the ancients to
be the most distant of the moving heavenly bodies, a posi-
tion it retained even after the triumph of the Copernican
ideas and the establishment of the modern conception of
the solar system. The reason for this was that the period
of its oscillatory motion to and fro in the heavens was
longest of all of the planets. The ancients noted that
it took 293^ years for Saturn to return to the same place
among the stars, as compared with 12 years for Jupiter,,
and correspondingly less for the other planets down to
88 days for Mercury. Accordingly they considered that
Saturn was the most distant.

Eudoxus (409 b.c.[?]-356 b.c.[?]), as has been shown,
believed that the motion of Saturn, as in the case of Mars
and Jupiter, could be represented roughly by supposing
that each planet oscillated to and fro on each side of a
fictitious planet which moved uniformly around the celes-
tial sphere in or near the ecliptic. The slow period of
Saturn also made it the most distant of the planets in the
system as devised by Copernicus, who computed its dis-
tance from the Sun as nine times that of the Earth, which
may be compared to his credit with the modern figure of
gT/2 times.

After the development of observational astronomy
Tycho Brahe in 1563 made his first recorded observation
at the University of Leipzig, noting the close approach of

SATURN 203

Jupiter and Saturn, which he found was quite a month in
error in the prediction of the Alfonsine Tables, published
in Spain in 1252 and in general use by astronomers'
throughout Europe. The next important observation of
Saturn was indeed of epoch-making significance in astron-
omy. With his new telescope it was but natural that
Galileo should examine Saturn as he did the other planets.
Turning his telescope toward Saturn he observed that
that planet, too, was not single and complete, but appar-
ently consisted of three parts, or, as it appeared in a draw-
ing made at the time by him, of a central body and two
satellites in close proximity, which naturally seemed to
resemble those of Jupiter. At subsequent observations he
failed to see more than the central and larger portion,
and, consequently, completely baffled, he left the problem
as a legacy to his successors.

The two appendages were seen and described under
varying conditions by a number of astronomers, but the
true solution was first furnished by Huygens (1629-1695),
when he studied with one of his powerful telescopes the
appearance of the planet. Huygens announced in 1655
that he had discovered a single satellite, which he named
Titan. With a still more powerful instrument he found
that the effect of two component bodies observed by Gali-
leo was due to the fine ring which surrounded the
planet and was inclined at a considerable angle to the
plane of the ecliptic and consequently to the plane in which
Saturn proceeds around the Sun. As the ring was ex-
tremely fine it became invisible, either when its area was
directly opposite to the observer or when it was directed
toward the Sun, as in that case it received no light for
reflection. Near this opposition or invisibility the ring ap-
pears to be foreshortened and presents the appearance of
two arms projecting from the body of the planet. The
ring, of course, gradually widens from its opposition or
invisibility and the opening becomes visible, a period of

204 ASTRONOMY

seven years e-asping between such a state and when the
ring is seen at its widest.

With the observations of Huygens the reasons for Gali-
leo's varied observations were furnished. To make the
matter more conclusive Huygens collected and published
a series of early drawings by various observers, which
drawings he compared with his own observations. Thus
what Galileo conceived as two satellites was really the

Fig. 25 — Relative Sizes of Saturn and the Earth.

ring when seen with its greatest breadth. The disap-
pearance of these satellites occurred when the edge of
the ring was presented to his view, the revolution of the
planet giving to an observer on the Earth a series of
phases in which the appearance of the planet is remark-
ably different.

What Huygens saw is now familiar to every one who
has observed Saturn through a telescope. Surrounding
the central body are rings parallel to the planet's equator,
but inclined about 27 degrees to the plane of its orbit and

SATURN 205

28 degrees to the ecliptic, their nodes being at longitude
1 68° in Aquarius and at longitude 3480 in Leo. The plane
of the rings remains sensibly parallel to itself for a very-
long time. For fifteen years, or half a revolution of Sa-
turn, their northern face is seen and during the remaining
half of the revolution their southern face. When the
Earth passes over the plane of the rings at the time of
transition their edge is presented so that the ring virtu-
ally disappears from view, as occurred, for example, in
1908, and in 1612 had occasioned Galileo's perplexity.
The thickness of the rings is less than 100 miles; conse-
quently their edge was quite invisible through his feeble
telescope. The disappearance of the rings recurs at in-
tervals of about fifteen years.

In 1675 Giovanni Domenico Cassini (1625-1712) no-
ticed a dark marking in the ring which later was found
to mark the division of the ring into two distinct schemes,
a narrow and outer ring, to which the name of "Cassini's
Division" was given. As was natural, the peculiar con-
struction of the rings of this planet and the accompany-
ing satellites was the subject of deep and earnest inquiry,
both mathematical and telescopic, tho but little substantial
progress was made in explaining the formation and oc-
currence. In his analysis of the nature and motions of the
planets and their satellites the mechanical problem of the
stability of Saturn's rings was left even by Laplace in a
very unsatisfactory condition; for he made no attempt
to determine the kind or amount of irregularity in the dis-
tribution of their weight, which he assumed was necessary
in any considerations of them as rotating solid bodies.

In 1849 W. C. Bond at Cambridge, Mass., found that
Saturn was accompanied by a third comparatively dark
ring, lying immediately within the bright rings, and to
this the name "Crape Ring" has been applied. Professor
Bond, who devoted much attention to the study of Saturn,
at first denied the solidity of the planet's structure and
asserted that the fluctuations in its aspect were entirely

206 ASTRONOMY

at variance with any such hypothesis. He and other as-
tronomers had frequently detected in both of the bright
rings fine dark lines of division, and as these frequently
lapsed into imperceptibility the condition was due in his
opinion to the real mobility of their particles and indicated
a fluid formation. The known solidity of the rings was
then demonstrated on abstract grounds by Professor Ben-
jamin Peirce of Harvard University, who maintained that
they were formed by streams of some fluid denser than
water. In 1857, in England, James Clerk Maxwell, the
famous mathematician and physicist, presented a mathe-
matical discussion of the subject, in which he stated that
neither solid nor fluid rings could exist and that the sys-
tem could be composed only of a great multitude of uncol-
lected particles which revolved independently in a period
corresponding with their distance from the planet. This
idea of a satellite formation, remarkably enough, had been
several times entertained and lost sight of, so that when
advanced by Maxwell it was a virtual novelty. The hy-
pothesis met the test of telescopic observation.

The mathematical theory of the ring system found an
analogy in the assemblage of planetoids, both visible and
invisible, which are known to be revolving around the Sun
with orbits situated between Mars and Jupiter. If seen from
a considerable distance such a swarm of these small
particles would give the impression of a continuous solid
body, so that on its physical basis this theory did not
seem improbable. It was pointed out by Kirkwood in
1867 tnat tne division between the two main orbits, first
made by Cassini, could be explained by the perturba-
tions due to certain of the satellites, just as the corre-
sponding gaps of the minor planets are explained by
the action of Jupiter. But in modern astronomy the
probability of a mathematical theory is not sufficient, and
its acceptance must depend upon direct and conclusive evi-
dence from telescope, spectroscope or photographic plate.

This was supplied most effectively by Professor James E.

SATURN 207

Keeler (1857-1900), at Allegheny Observatory, Pittsburg-,
in 1895. He pointed his spectroscope to the planet and
found by examining the light waves from opposite sides
that the main body was in rotation. The light from one
side was approaching the Earth and from the other
it was receding. This rotation of the planet, of course,
had been realized for some years, but the axial rotation
of the rings had never before been demonstrated, and
this Keeler proved, revealing the strange fact that the in-
terior part of the rings rotated faster than the exterior,
which of course would not hold true in the case of a solid
body. The motion slowed off outward in agreement with
the diminishing speed of particles traveling freely, each in
its own orbit.

The visibility of the rings when the Sun and the Earth
are on opposite sides of their plane is explained by Pro-
fessor Barnard as due to the filtration of sunlight through
a cloud of cosmical dust. He regards the knots as the
result of the radiation of parts of the clouds which are
denser, but not necessarily thicker than the rest under the
illumination of sunlight, which gives to them their adja-
cent portions of less density. Percival Lowell, however,
believes that the rings of Saturn are not flat and of uni-
form thickness, but rather resemble a concentric series of
tores or anchor rings, and that the knots represent their
fixed portions.

Professor Sir G. H. Darwin explains the rings of Sa-
turn by considering them an abortive satellite, scattered by
tidal action into annular form, for they lie closer to the
planet than is consistent with the integrity of a revolving
body of reciprocal bulk. This interesting appendage, ac-
cording to Professor Darwin, will eventually disappear, as
the constituent particles will be dispersed inward in part
and will be gathered to the surface of the planet, while in
part they will scatter outward where they may coalesce
unhindered by the strain of unequal attraction. Then
one modest planet revolving within Mimas would be all

208 ASTRONOMY

that would remain of appurtenances which lend char-
acter to the planet.

The dimensions of these wonderful rings of Saturn
doubtless will arouse the reader's curiosity. The planet
itself has an equatorial diameter of 75,000 miles. Out-
side of this first comes the crape ring at a distance of
from nine to ten thousand miles of clear space, and
somewhat less than 10,000 miles in width. The crape
ring is joined to the second ring, which is the most
brilliant and is about 16,500 miles in width. Then comes
a gap of about 1,600 miles, and there lies the outer ring,
10,000 miles in width, with an exterior diameter of about
168,000 miles. Hence the entire ring system has a width
of between 36,000 and 37,000 miles. A model of the outer
ring, constructed on the scale of 10,000 miles to the inch,
could be made with an approximation to accuracy from a
sheet of writing paper nearly seventeen inches in diameter.

Cassini's discovery of the dark markings in Saturn's
ring was one result of a series of telescopic observations
which he made of the planet, in which he discovered four
new satellites — Japetus in 1671, Rhea in 1672, and Dione
and Thetis in 1684. This list of satellites was increased by
two more in 1789, when Herschel, using his 40-foot tele-
scope for the first time, August 28th, detected a sixth
satellite of Saturn, Enceladus, and on September 17th dis-
covered a fainter satellite, Mimas. Both of these were
nearer to the planet than any of the five previously ob-
served. In September, 1848, W. C. Bond, of Cambridge,
Mass., discovered Hyperion, and two days later this same
satellite was also observed at Liverpool by William Las-
sell.

A ninth satellite of Saturn, Phcebe, was discovered by
Professor W. H. Pickering on photographic plates taken
at the Harvard Observatory at Arequipa, Peru, and was
announced in July, 1904, the satellite being seen on a num-
ber of recent photographs. This satellite is much smaller
than any of the existing moons; so much so, in fact, that

SATURN 209

at is beyond the visibility of the human eye with any
existing telescope. It revolves around Saturn at a dis-
tance of many millions of miles, far beyond the orbit of
Japetus and with a period correspondingly longer, and,
strange to say, in an opposite direction from its fellows.

Professor Pickering also discovered in 1905 a tenth sa-
tellite of Saturn, Themis, which revolves much closer to'
the planet. It is said to be the faintest object in the solar
system, and is a striking illustration of what astronomical
photography can accomplish in the way of discovery.

Saturn moves in an orbit which is somewhat more ec-
centric than that of Jupiter, but which is at a mean dis-
tance from the Sun of 886,000,000 miles. The equa-
torial diameter is about 75,000 miles and the polar
diameter about 68,000, giving a mean diameter of 73,000
miles, or a little more than nine times that of the Earth
and a volume greater by 760 times. Yet Saturn is a very
light body, having a mass only 95 times that of the Earth
and a density of one-eighth, which would give it a specific
gravity of five-sevenths that of our own planet.

The same arguments advanced in favor of a high
temperature for Jupiter can be used with increased force
in the case of Saturn. It may be assumed that a large
proportion of this bulky globe is composed of heated vapors
which are vigorously circulated by the process of cooling.

Professor Asaph Hall, of Washington, in 1876 made ob-
servations of the white spot which was visible on the sur-
face of the planet for some weeks. He established a
period of rotation of 10 hours, 14 min. and 24 sec, which
agrees closely with 10 hours, 16 min. determined by Her-
schel in 1794. Hall's value has been confirmed by other
observers and is generally accepted.

Saturn shows belts similar to those of Jupiter, with a
brilliant zone at the equator. The edges of the disk are
not so brilliant as the central portion, for the pole of the
planet is at times marked with a darkish cup of greenish
color.

CHAPTER XVIII

URANUS AND NEPTUNE

The discovery of Uranus distinctly represents one of
the most important results of modern methods in astron-
omy. The other planets considered were known from pre-
historic times. Even the least conspicuous of them could
be observed with the naked eye under favorable conditions.
Just as the satellites of Jupiter were the first fruits of
telescopic discovery in the heavens, so Uranus was the
first planet to be telescopically added to the list which
through centuries had been known and studied.

Its discovery was made on March 13, 1781, by Sir
William Herschel while engaged in the systematic exami-
nation of every stellar body visible with his 7-foot reflect-
ing telescope. Herschel states that: "In examining the
small stars in the neighborhood of 'H. Geminorum' I per-
ceived one that appeared visibly larger than the rest;
being struck with its uncommon appearance I compared it
to *H. Geminorum' and the small star in the quartile be-
tween 'Auriga' and 'Gemini,' and finding it so much larger
than either of them I suspected it to be a comet."

Though it appeared as a star of the sixth magnitude,
its difference from the other stars was at once appre-i
ciated by Herschel and is evidence of that keenness of
sight which was so characteristic of him. Observations
of the new body and study of its orbit failed to establish it
as a comet. Within three or four months of its discovery
the conclusion was reached first by Anders Johann Lexell

URANUS AND NEPTUNE 211

(1740-1784) that it was a new planet which revolved
around the Sun in an orbit nearly circular at a distance
of about nineteen times that of the Earth and nearly-
double that of Saturn.

Herschel's discovery at once won for him a national
reputation and royal honors, which he attempted to re-
ciprocate by conferring the name of his royal patron,
George III., on the new planet, calling it Georgium Sidus.
But the name never gained currency outside of England.
After a vain attempt to apply Herschel's name to it, the
old mythological nomenclature was observed and the new
planet became permanently known as Uranus, at the sug-
gestion of Bode, after the father of Saturn and the grand-
father of Jupiter. The name means Heaven itself, be-
yond which it was supposed nothing further could be
found.

Following its discovery by Herschel, with a reflecting
telescope 40 feet in length and of 4-foot aperture, came
the detection on January 11, 1787, of two moons or satel-
lites of Uranus, to which the names of Oberon and Titania
were subsequently given. Herschel discovered that these
moons moved almost at right angles to the ecliptic in a
direction contrary to that of all previously known mem-
t>e»s of the solar kingdom except the comets. He sus-
pected the existence of four more such satellites, but he
was not able to assure himself positively of their exist-
ence. In fact, it was only his large telescope and his keen
eye that enabled the first two moons to be observed. But
with the progress of astronomy and the improvement of
instruments other discoveries were bound to come.

Within the paths of Oberon and Titania, Ariel and Um-
briel were found, October 24, 1851, by William Lassell, a
wealthy brewer, who during his life devoted himself
assiduously and with great success to astronomy, espe-
cially telescopic observation. These satellites, altho not
easily visible in a telescope on account of their distance,
are much larger than the satellites of Mars or in fact

212 ASTRONOMY

many of the planetoids. It is estimated that their diame-
ters are between 500 and 1,000 miles. Oberon, which is dis-
tant from the planet 365,000 miles, has a period of rota-
tion of 13^ days; Titania, the largest and brightest, dis-
tant 273,000 miles, has a period of 8.7 days ; Umbriel, dis-
tant 167,000 miles, has a period of 4.1 days; and Ariel, dis-
tant 120,000 miles, has a period of 2.5 days. These satel-
lites all move in the same plane, which is inclined about
980 to the plane of the planet's orbit. The satellites re-
volve in a retrograde direction.

The surface markings observed on Uranus by many
astronomers have been vague and transitory, so that any
determination of the period of rotation is but approxi-
mate. Nevertheless, a period of 10 or 12 hours has been
indicated. It is stated that the plane of the equator is
inclined something like 10 to 30 degrees to the plane of the
orbit of the satellite. The disk of the planet shows a flat-
tening at the poles, so that it has an elliptical section.

The appearance of Uranus to the naked eye is that of a
small star of about the sixth magnitude. It was on this
account that a high power telescope was required to dif-
ferentiate it from the myriad other stars of this size to
establish it as a planet. It is so far away that there is but
little change in its position whether it is in opposition or
quadrature. Measuring the disc, which appears in the
telescope to be of a sea-green color, the diameter of the
planet is found to be about 32,000 miles, or four times as
great as that of the Earth, which would give it a volume
64 times greater. But, like the other distant planets,
Uranus is composed of lighter materials, so that while 64
times as large its mass is but 15 times that of the Earth,
or, in other words, Uranus would compare with the Earth
in about the same proportion as regards volumes as does
the Moon with the Earth.

The elliptical orbit in which Uranus moves at a mean
distance from the Sun of nearly 1,800,000,000 miles re-
quires 84 years for its passage, and the diameter of this

URANUS AND NEPTUNE 213

orbit is 3,600,000,000 miles. The orbit is slightly less ec-
centric than that of Jupiter and amounts to 83,000,000
miles, while the periodic time of the planet is 84 years and
its synodic period 369 days and 16 hours. It moves with
an orbital velocity of 4^ miles per second. In the first half
century after its discovery Uranus gave astronomers con-
siderable trouble, as observations showed that it was not
following exactly its computed path and that it deviated
by a substantial amount. About 1830 Bessel suggested
that the discrepancies in the observed and calculated orbits
might be due to an unknown planet, then more distant
from the Sun than Uranus, and such was found to be the

As Uranus was the triumph of telescopic discovery, so
Neptune represents one of the greatest achievements of
mathematical astronomy. In fact, when the French as-
tronomer, Leverrier, at Paris, wrote to Galle, at Berlin,
substantially as follows, "Direct your telescope to a point
on the ecliptic in the constellation of Aquarius in longi-
ture 3260 and you will find within a degree of that place
a new planet looking like a star of about the ninth mag-
nitude, and having a perceptible disk," the German as-
tronomer, within thirty minutes after he had begun his
search, on the night of September 2^, 1846, was able to
find the new planet but 52' distant from the point indi-
cated by Leverrier.

The discovery of Neptune came, as has been suggested,
from discrepancies observed in the path of Uranus, which
oftentimes were almost so marked as to be observed with-
out the aid of the telescope. As an explanation of the dis-
turbance, an unknown exterior body was suggested, which
was not only plausible but so obvious that several astrono-
mers were devising mathematical plans of campaign for
its discovery.

A young graduate of Cambridge University, John Couch
Adams (1819-1892), who had distinguished himself in

214 ASTRONOMY

mathematical work, assiduously addressed himself to the
problem, and in 1845 sent to the Astronomer Royal at
Greenwich numerical estimates of the elements and mass
of the unknown planet, together with an indication of its
actual place in the heavens. Unfortunately, Adams' work,
for various reasons, was not taken up by the government
astronomers, and in the meantime Urbain Jean Joseph
Leverrier (1811-1877), as a result of the study of the
stability of the solar system, and especially of the Uranian
difficulty, to which his attention had been directed in 1845
by Arago, announced before the French Academy that
only an exterior planet could produce the observed effects.

Such an announcement aroused astronomers to a point
of expectancy, and in fact, as Sir John Herschel declared
to the British Association regarding the hypothetical new
planet, "We see it as Columbus saw America from the
coast of Spain." In less than two weeks from the time
of this utterance the message quoted was sent to Galle at
Berlin. Within a week Neptune was also observed in
England, where delays lost the honor of priority for
Adams.

Once the existence of the new planet was established,
a few weeks' observation made possible the computation
of its orbit and its identification with what had been con-
sidered a fixed star.

Neptune supplied the exception which proved the rule
in the case of Bode's law, discussed in the chapter on
planetoids, for its mean distance from the Sun was 2,800,-
000,000 miles instead of 3,600,000,000 as would be required
under the terms of the law. Furthermore, Neptune has
an orbit with an eccentricity of only 9/1000, so that its path
is more nearly circular than that of any other member of
the solar system except Mercury. But so large is its orbit
that the small eccentricity makes a variation of over
50,000,000 miles in the distance of the planet from the Sun
at different parts of its orbit. Moving with an orbital

URANUS AND NEPTUNE 215

velocity of about 3^ miles a second, it requires 164 years
for its journey around the Sun.

Neptune has but one satellite, which Lassell found in
1846, within a month of the original discovery of the
planet. Its distance is about 221,500 miles and its period
of revolution is 5 days and 21 hours. It is a small body,
about 2,000 miles in diameter, or about the size of the
Earth's Moon, and it moves backward just as the satel-
lites of Uranus, in an orbit that is inclined 1450 to that
of the planet.

Like Uranus, Neptune varies little, as its distance is
so great that any variation by change in the position of
the planet would not affect its appearance on the Earth.
Its diameter is estimated at about 35,000 miles, which
would give a volume 85 times that of the Earth (or ac-
cording to Professor T. J. J. See, U. S. N., 27,190 miles) ;
but, as also the case with Uranus, it is much lighter than
the Earth. As its density is probably about 0.20, its
mass, which astronomers compute from the motion of its
satellite, is about seventeen times that of the Earth. On
account of its great distance it receives from the Sun
about V906 the amount of heat that falls upon the Earth.
If its capacity for absorbing and retaining heat are the
same, the theoretical temperature would be about 3600 F.
(2200 C), or between the temperature of liquid air and
the boiling point of hydrogen, a temperature so cold that
on the Earth complex methods to produce it have to be
adopted in a physical laboratory. Nevertheless, the amount
of light received from the Sun is not insignificant, and
the noonday illumination of the planet would be some 700
times that of brightest moonlight. If the Sun were placed
at Neptune, its light would equal that of 687 full moons.

No surface markings have yet been seen on Neptune.
Consequently astronomers are unable to determine its rate
of rotation by direct observation; but from various in-
tricate processes Neptune is believed to have a slower
rotation than Jupiter or Saturn.

CHAPTER XIX

THE PLANETOIDS

In the chapter on the Solar System it has been shown
how Titius in 1772 pointed out that if 4 should be added
to the following series (irregular in that the first number
should be Yz instead of o) : o, 3, 6, 12, 24, 48, a sequence
of numbers would be obtained that indicated the relative
distances of the six planets, with the striking exception
that next after Mars there was no representative. Bode
filled this gap with a hypothetical planet, and his name is
often associated with that of Titius in the statement of
this law.

After the discovery of Uranus by Herschel in 1781 the
law of Titius or Bode, with which this planet conformed,
gained considerable respect from astronomers, altho no
mathematical or other reason was known to uphold the
singular relation between the distances of the planets
from the Sun. At any rate, the conviction that there
must be an undiscovered planet between the orbits of
Mars and Jupiter, as was even suspected by Kepler in his
"Mysterium Cosmographicum" of 1596, was strengthened,
and accordingly it was proposed by a voluntary association
of astronomers in 1789 to undertake a systematic search
for the missing planet. An organization was arranged on
a fairly methodical basis by Baron Von Zach; but before
these "celestial police/' as the astronomers enlisted for
this purpose were humorously termed by their organizer,
had secured results from their plan of cooperation the
216

THE PLANETOIDS 217

missing planet was found under interesting and somewhat
extraordinary circumstances.

At an observatory at Palermo, Sicily, Giuseppe Piazzi
(1746-1826) had been at work for some nine years on the
preparation of a catalogue of the stars. On January 1,
1801, he noted an eighth magnitude star, which on subse-
quent evenings shifted its position and induced him to be-
lieve that he had discovered a new kind of comet without
tail or coma, and so he described it in a letter to Bode at
Berlin. A fortnight later the wandering body changed its
retrograde for direct motion, and before the observations
could be repeated by other European astronomers Piazzi's
moving star approached too near the Sun to be visible any
longer. For its rediscovery some accurate knowledge of
its path was essential.

Never before had there been such meager data on which
to calculate the motion of a celestial body, for even in the
case of Uranus observations for almost a century had been
made and recorded on the assumption that that planet was
a fixed star. That the new star was a planet, however,
lay outside the realm of consideration. It soon was found
that the calculation of its orbit could not be adjusted, for
the observations did not harmonize with any sort of para-
bolic cometary orbit. Then the supposition arose that it
was a planet and that it had an elliptical orbit. But how
was such a path to be calculated? The case had never
yet arisen where a planet which had a relatively rapid
motion had been under observation for so short a time
and the orbital velocity of which was unknown. Men of
the hour were not wanting, and a brilliant young German
mathematician, Carl Friedrich Gauss (1777-1855), of
Gottingen, now entered the ranks of astronomy. He com-
puted the orbit of the new body with remarkable skill, and
in November of the same year presented his conclusions
to the observing astronomers. It was only on the last
night of the year that the sky was sufficiently clear for
good seeing. At the Gotha Observatory, almost in the

218

ASTRONOMY

very position assigned by Ganss to the runaway planet, a
strange star was discovered, which filled the gap in the
series of Titius, and in the following year (March, 1802),
at Bremen, Dr. Heinrich Olbers (1758-1840) also observed
a similar star. The first of these, at the request of Piazzi,

Fig. 26 — Paths of Minor Planets.

was named Ceres, after the protecting divinity of Sicily,
while the second, which furnished an additional problem
for Gauss, was named Pallas. Harding, the assistant of
Schroter at Lilienthal, discovered (1804) Juno as the
third, and Olbers (1807) Vesta, the brightest of all the
planetoids, as the fourth — all lying between the orbits of

THE PLANETOIDS 219

Mars and Jupiter. There were thus, in contradiction to the
law of Titius, instead of one planet, four present in the
zone between Mars and Jupiter. They were designated
"asteroids" by Sir William Herschel, tho the more modern
term "planetoids" is to be preferred.

Thus was inaugurated the discovery of minor planets,
which continued during the nineteenth century and is still
in progress. Where one new planet might be anticipated
according to older doctrines, many were found. Various
theories have been advanced to explain their existence and
occurrence. That Ceres and Pallas were parts of an ex-
ploded planet was an ingenious theory which received for a
time considerable acceptance, but which was disproved by
Professor Newcomb. The recognition of the minor
planets became of the highest importance in astronomical
science, and led to the construction of star maps and
to further interest in the observations of the heavens.

The discovery of the fifth planetoid did not occur for
a number of years and then fell to the lot of an amateur
astronomer, Postmaster Hencke, at Driessen, who was in-
dustriously searching with his telescope the quarter of the
sky lying opposite the Sun. He made a practice of map-
ping all the little stars and of comparing them again on
the following nights. At last (1845) ne found a new
planetoid, to which he gave the name Astraea, and in 1847
a second, Hebe. Since then no year has passed without
the discovery of one or more new planets. This may be
attributed not only to the example of Hencke, with his
systematic industry, but also to the perfecting of the
telescope and to the publication of printed special star
charts upon the basis of the localizations of numerous
fixed stars. Thus by the year 1868 the number of little
planets had increased to 100; in 1879, 200; 1890, 300; 1895,
400; 1903, 500; 1906, 600; 1907, 700; 1908, 800. Conse-
quently, instead of one looked-for planet, suspected by
Kepler, a complete stream is present between Mars and
Jupiter, as indicated in Fig. 26.

220 ASTRONOMY

The zeal of astronomers was directed toward the ob-
servation of the numerous newly-discovered heavenly-
bodies, and the most fortunate discoverers achieving suc-
cess in this field were Hind (at London), Goldschmidt, an
amateur (at Paris) ; Gasparis (at Naples), Robert Luther
(at Bilk near Dtisseldorf), Chacornac and the brothers

THE MOON;

Fig. 27 — Comparative Sizes of the Moon and Three Minor
Planets.

Paul and Prosper Henry, who discovered at Paris, during
the construction of stellar charts of the zodiac, a number
of planets among the numerous charted small fixed stars ;
and from the United States, Watson at Ann Arbor, and
C. H. F. Peters at Clinton, who astonished the world by
their numerous discoveries. Johann Palisa should be
named above all. While director of the observatory at
Pola he discovered 83 new planets, the majority with a

THE PLANETOIDS 221

small telescope. Later he moved to Vienna, where large
refractors were available. The number of small stars
now becoming visible changed the accustomed groupings,
and thus hindered rather than favored the further
discovery of new planets by him. In Vienna, however,
he made himself especially serviceable by following up
the faint planets already discovered.

Certain orbits of the minor planets are shown in Fig. 26,
all of which are described in a standard or west-to-east
direction. At the Lick Observatory (Mt. Hamilton, Cal.)
in 1894 and 1895 Professor E. E. Barnard made a series
of direct measurements of the three largest of the minor
planets, obtaining for the diameter of Ceres and Vesta
405 miles for each, and for Pallas 322 miles. The size
of these minor planets compared with the Moon is shown
in Fig 27. The planetoids in the aggregate do not bulk
very large, for we know from the calculations of the
distinguished Leverrier, who studied the perturbations
of Mars, that the total mass of all known or un-
known bodies between Mars and Jupiter cannot exceed
a fourth that of the Earth. Later knowledge derived
from light observations would place the total mass of
those already known at many hundred times less than
this limit. If the combined mass were as great as Vioo
the mass of the Earth, it would produce perturbations in
Mars of which there is no evidence. B. M. Roszel in
1895 estimated that the terrestrial globe contains 3,240
times the aggregate mass of the first 311 minor planets.

In December, 1891, Max Wolff at Heidelberg began that
photographic observation and discovery of the little
planets, which has resulted in rich gains. Repeated and
long exposures of the photographic plate are made, an
equatorial telescope fitted with a camera being directed
at some point in the heavens, and kept there by means of
clockwork. Consequently the fixed stars register them-
selves upon the plate as points, and the moving planets
as short faint streaks, which are discovered by means of

222 ASTRONOMY

the microscope after development and fixing. In this way-
more planets have been discovered in Heidelberg since
1892 than by all previous observers together. Charlois
at Nice emulated Professor Wolff in applying the photo-
graphic method, so that there has developed since 1892,
with great success, a new field of astronomical photog-
raphy.

An American astronomer, the Rev. Joel Metcalf, at
Taunton, Mass., has improved the technique of observation.
In 1905 he found two and in 1906 twelve new planetoids.
Since 1892 newly discovered planets have received pro-
visional designations of the letters A, B to Z ; AA, AB to
AZ; BA, etc., to ZZ, and with the number of the year
preceding. If it has once been established that they can
be observed sufficiently long to guarantee finding them
again, and that they are identical with none of the earlier
planets, they receive the next numeral and name, and are
thus included in the system. In this manner an unneces-
sary increase in the lost planets of the system is avoided.
In 1907 the crucial moment came in which the planet ZZ
was discovered, and the series of provisional designations
began again.

Up to 1896 all the 432 planetoids moved in the zone
between the solar orbits of Mars and Jupiter; and, in-
deed, all with the same right-handed motion. Consequently
no little attention was attracted when on August 13, 1898,
Witt, at the Urania at Berlin, photographically discovered
a planet with an unusually rapid motion. The computa-
tion of its orbit showed great proximity to the Earth, and
a mean solar distance of 1.46 times the distance of the
Earth; hence less than that of Mars (1.52). Was this
celestial body to be counted in with the stream of planet-
oids between Mars and Jupiter? Since the possibility
existed that still other planets might be discovered which
were not confined to the zone between the orbits of Mars
and Jupiter, it was determined to include it with the

THE PLANETOIDS 225

planetoids with the numeral 433, but to distinguish it by
the masculine name Eros.

The orbit of this planetoid was found to lie in that
forbidden territory within the path of Mars and be-
tween it and the Earth. With the single exception of
the Moon it is the nearest large heavenly body to the
Earth. This little planet, hardly more than twenty miles
in diameter, in addition to its anomalous position, pos-
sesses remarkable characteristics, among them an orbit
of great eccentricity, which causes it to be at aphelion
some 24,000,000 miles beyond the mean distance of Mars.
An examination of Fig. 28 will reveal how very interesting
its orbit is, for on the rare occasions on which the planet-
oid comes nearest to the Earth it is closer to the Earth
than Mars or Venus can ever be.

Furthermore, this little planet not only is variable but
varies in its variability, which has given rise to the as-
sumption that either rapid changes are taking place on it,
or its light reflecting power varies with its position as re-
gards the Earth and Sun, as might be the case if the
planetoid were really double, with two components re-
volving around a center of gravity, or if the body itself
were unequally reflective in its various parts. The mean
distance of Eros from the Sun is 135,000,000 miles, but at
aphelion it is 165,000,000 miles away. Its perihelion dis-
tance is only 105,300,000 miles, or about 12,400,000 miles
greater than the mean distance of the Earth. On account
of the large inclination of the plane of its orbit to the plane
of the ecliptic Eros never approaches the Earth nearer
than about 13,500,000 miles.

These near approaches occur only when Eros is in op-
position at its perihelion, which unfortunately happens
rarely. Its sidereal period is 1.76 years, from which it
follows that its synodic period is 2.32 years. If an oppo-
sition occurs at perihelion, then in 37 years another will
occur very nearly at perihelion, for 37 is almost evenly
divisible by 1.76 and 2.32. The next most favorable op-

224 ASTRONOMY

position will occur in 1931, and will furnish an unexcelled
opportunity for obtaining the solar parallax.

Because of its eccentric orbit Eros acquires a highly
practical significance in respect of the determination of
the true size of the entire planetary system. From the laws
of the orbits is found, in fact, very accurately (to six
decimal places) the relations of the planetary distances.
On the other hand, these distances themselves are very
little known. They are inversely proportional to the solar
parallax (8.80 sec), of which one can be confident of but

Fig. 28 — The Orbit of Eros Referred to Those op the Earth
and Mars.

two decimal places. We have already seen that by means
of observation of the transits of Venus, observation of the
nearer planetoids and of Mars, determinations of the ve-
locity of light and of the parallactic lunar equation, great
pains were taken in the last century to ascertain with more
exactness the solar parallax, and thus the extent of our
entire planetary system. It appears that the observation
of Eros yields this quantity with three times the exactness
of the earlier methods, and that nothing else than the ob-
servation of the lunar orbit for the determination of the
parallactic equation can compete with it. Thus the dis-
covery of Eros performed a useful service for astronomy.
Two months after the discovery of Eros, Wolf, still at

THE PLANETOIDS 225

Heidelberg, discovered the planet Hungaria (No. 434).
This, next to Eros, is, with its solar distance of 1.94 units,
the innermost planet of the stream. Its orbit lies, how-
ever, entirely in the zone beyond the orbit of Mars.

Just as Eros goes beyond the zone between the orbits of
Mars and Jupiter toward the inside, so three lately-dis-
covered planets do so toward the outside; tho, to be sure,
to only a small degree. These have also received mas-
culine names.

Highly theoretical investigations have been carried out
by Hansen, Glyden, Poincare and Sir George Darwin,
which have gained from orbital computation continually
new points of view and have led to beautiful mathematical
ideas. Here belongs the question of the simple relations
of the times of revolution of a planet and of Jupiter as
the most important disturbing planet, or the so-called li-
bration of the orbit, as it comes to light in some measure
with Hecuba (No. 108) and with planets of the Hecuba
type. The value of the little planets is thus not to be
sought upon the practical but rather upon the theoretical
side.

But the trouble and time which hundreds of planets un-
interruptedly required for their rediscovery and the cor-
rection of their orbits was so considerable that the ques-
tion arose whether the gain was worth all the trouble.
So, in the eighties of the nineteenth century, a number of
astronomers demanded that a choice be made of the plan-
ets, and that only a part be followed up with accuracy,
and that the rest (since indeed they apparently showed
merely the same phenomena) be abandoned without at-
tention. This would hardly have lessened the labor. For,
since the discovery of planets could not be prohibited, the
majority of the planets would have been lost and would
be continually discovered over again. Professor Tietjen at
Berlin found a middle course. Since 1891 he abandoned
the yearly ephemerides (tables which give the course of
the planets during the entire year) and stated merely in

226 ASTRONOMY

one line, for each planet, time and place of opposition and
the daily variation in its position. For selected, important
and interesting little planets opposition ephemerides were
printed, which gave the position of the planets daily with
exactness for six weeks of their best visibility.

The majority of the planetoids appear as mere points
and are immeasurably small. However, Weiss estimates,
on the basis of brilliancy, the largest of them at 342 kilome-
ters (212 miles) and the smallest at 16.14 and 10 kilome-
ters (10 and 6.2 miles) in diameter.

There is no doubt that many small bodies are in motion
among them whose diameters are of 1 kilometer (3,281
feet), of perhaps 1 meter (39.37 inches) and even of 1
millimeter (0.039 inch), and that the majority on account
of their faintness are not visible to Earth. The planet-
oids are of no immediate use to mankind. Their orbital
calculations can be used neither for the determination
of time nor for the guidance of mariners. But the
ideal beauty of these planets is that their reappear-
ance is continually a test of the correctness of mathemati-
cal assumptions and that they exhibit over and over again
the validity of Newton's law of the universal attraction of
all bodies.

CHAPTER XX

COMETS, METEORS AND METEORITES

The term "comet," derived from the Latin, coma, or
hair, applied to celestial bodies which appear to have a
hairy appendage, goes back to the time of the Romans.
A similar word, cometa, or cometes, was used by Cicero,
Tibullus and other ancient writers. The modern as-
tronomer speaks of the coma of a comet as distinct from
the tail, and applies the term to the misty hazy light sur-
rounding on every side a small central bright spot, which
he calls the nucleus of the comet.

While the ancients distinguished between comets and
meteors, or shooting stars, yet they believed them to be of
the same nature, and to be found in the Earth's atmos-
phere not far above the clouds, or at all events much
lower than the Moon. The earlier and Pythagorean
view, however, was much more correct, according to
modern doctrine; for it held that comets were bodies
with long periods of revolution, which idea, like others
attributed to Pythagoras, probably came from eastern
philosophers of unknown nationality. Apollonius, the
Myndian, believed that the Chaldeans were responsible for
this notion of the comets ; for they spoke of them as trav-
elers that penetrated far into the upper or most distant
celestial space. Similar views, typical of the imitative
faculty of the Romans, says Humboldt, were held by
Seneca and Pliny. The Greek philosophers in many cases
preferred to disregard observations. Hence the fanciful
227

228 ASTRONOMY

theories of Aristotle as. regards comets, as well as other
astronomical matters, were prevalent for many centuries.
Aristotle even believed that the Milky Way was a vast
comet which continually reproduced itself.

The comet, with its brilliant head, flaming tail and
uncertain appearance, could not be regarded otherwise
than as a divine omen to announce some remarkable
event or to forebode evil, particularly pestilence and war.
Indeed, for many years the death of monarchs was be-
lieved, especially by those to whom the wish was father of
the deed, to be announced by these brilliant messengers in
the sky. In some cases comets were associated with mis-
fortunes, not merely as anticipating or announcing them,
but as the actual causes. Seneca's statement that "This
comet was anxiously observed by every one, because of
some great catastrophe which it produced as soon as it
appeared, the submersion of Bura and Helice" referred to
a very brilliant comet which appeared in 371 b.c. about the
same time that these two towns of Achaia were swallowed
up by the sea during an earthquake. A comet which ap-
peared in 43 b.c. was generally believed to be the soul of
Caesar on its way to heaven. Josephus informs us that the
destruction of Jerusalem was announced by several prod-
igies in 69 a.d., among them a sword-shaped comet which
is said to have hovered over the city for the space of a
year ! Another classical instance may be quoted from
Pliny (23-79 A-D0 m ms "Natural History" where he
says: "A comet is ordinarily a very fearful star; it an-
nounces no small effusion of blood. We have seen an
example of this during the civil commotion of Octavius."
When the comet of 79 a.d. appeared, the Roman Emperor
Vespasian refused to be intimidated by the frightening in-
terpretation placed upon it. 'This hairy star does not
concern me," he is reported to have said; "it menaces
rather the King of the Parthians, for he is hairy and I
am bald." Not long after the appearance of the comet he

COMETS, METEORS AND METEORITES 229

died. No doubt the prophecies of the imperial sooth-
sayer were more highly regarded thereafter.

The ancient Greeks and Romans were not the only ones
who took these heavenly apparitions seriously. In France
the great eclipse of 840 was said to have hastened the end
of Louis le Debonaire, and it was firmly believed that
the comet which appeared a year or two previously
presaged this occurrence. Much of the mysticism attach-
ing to figures found expression in the superstition that the
Christian era could not possibly run into four figures.
Hence the end of the world was looked for by many of the
inhabitants of Europe when the year 1000 approached. So
widespread was this belief that husbandry and toil were
neglected. When a comet appeared the feeling was
strengthened. Nothing remarkable occurred, however, be-
yond the natural consequences of such wholesale neglect
of the proper care of the soil. Famine and pestilence in
succeeding years were the result.

The comet, which, we shall see, was the famous comet
of Halley that blazed forth in April, 1066, was believed
to presage the success of the Norman Conquest, and the
invasion of England by the Normans "guided by a comet"
was a familiar topic for the chroniclers of the time. The
abdication of Emperor Charles V was reported to have
been influenced by the comet of 1556, but the event had
already taken place before the comet made its appearance.
Gian Galeazzo, the Visconti Duke of Milan, viewed the
comet of 1402 as a celestial sign of his approching death.

A striking example of the manner in which the comet
was regarded is contained in the contemporaneous descrip-
tion by Ambroise Pare, the father of French surgery
(1517-1590), in which he speaks of the fear inspired by
the comet of 1528. "This comet," said he, "was so hor-
rible, so frightful, and it produced such great terror in the
vulgar, that some died of fear and others fell sick. It
appeared to be of excessive length, and was of the color
of blood. At the summit of it was seen the figure of a

230 ASTRONOMY

bent arm, holding in its hand a great sword, as if about
to strike. At the end of the point there were three stars.
On both sides of the rays of this comet were seen a great
number of axes, knives, blood-colored swords, among
which were a great number of hideous human faces, with
beards and bristling hair."

The comet of 1472, apparently, was the first comet to
receive scientific study and not be regarded solely as a
cause of superstitious terror. A series of observations
were made in Franconia by Johann Miiller, of Konigsberg,
known as Regiomontanus.

By the time of Tycho Brahe, while comets were not
satisfactorily explained, yet they were being considered on
a more rational basis, and the correctness of the Aristote-
lian doctrine was, as in other matters, being questioned.
It was believed before this that comets were generated in
the higher regions of the atmosphere. But in 1507, on the
appearance of a brilliant comet, Tycho, in an elaborate
series of observations, satisfied himself that the strange
body was at least three times as far off as the Moon and
also that it was revolving around the Sun in a circular
orbit at a distance greater than that of Venus. Comets
subsequently were observed by Tycho and his pupils. His
observations in this field led to those ideas of the solar
system of his which we have discussed. It was but natural
that Kepler, as a follower of Tycho, should have paid
especial attention to comets. In 1607 he observed the
comet now known as Halley's comet. Kepler believed
that comets were celestial bodies which move in straight
lines and after having passed the Earth recede indefi-
nitely into space. Assuming that these strangers in the
heavens would never reappear, he did not consider that
their paths required serious study, for which reason he
made no observations to ascertain their movements and
test his theory. Before Kepler, Jerome Fracastor (1483-
1543) and Peter Apian (1495-1552) had observed that a
comet's tail always points away from the Sun, no matter

COMETS, METEORS AND METEORITES 231

in what direction it may be traveling, and with this
observation Kepler agreed, adding as an explanation the
supposition that the tail was formed by rays of the Sun
penetrating the body of the comet and carrying away with
them some portion of its substance. This theory, after
due allowance has been made for the change in our con-
ception of the nature of light, is of interest as an anticipa-
tion of the modern theory of comets' tails. Kepler found
himself compelled in his "Treatise on Comets," 1619, in
which the foregoing observations were published, to refer
to the meaning of the appearance of a comet and its in-
fluence on human affairs. At this time there were striking
events enough in the affairs of Europe to prove any theory
of the influence of comets on human life. He realized, how-
ever, that comets are very numerous, for he states, "There
are as many arguments to prove the motion of the Earth
around the Sun as there are comets in the heavens."

The motion of the comets was also studied by Galileo in
1623 as a part of the motion of the Earth in the Coperni-
can theory of the solar system. But the first and most
important contribution to the true explanation came from .
Dorfel of Saxony, who proved from the comet of 1681 that
the orbits of comets are either very elongated ovals or
parabolas and that the Sun occupied a focus of the curve.
Newton, discussing this subject in his Principia, reached
independently the same conclusion a few years later and
established it as a universal law by incontrovertible mathe-
matical proof.

By the seventeenth century a considerable number of
comets had been recorded. John Hevel, of Danzig (1611-
1687), published two large books on comets, Prodromus
Cometicus (1654) and Cometographia (1668), which
contained the first systematic account of all recorded
comets.

It was a brilliant thought of Newton's that led him to
consider whether gravitation toward the Sun could not
explain a comet's motion just as well as that of the planet,

232 ASTRONOMY

and if so, as he took pains to prove in the beginning of
the Principia, such a body must move either along an
ellipse or in one of two other allied curves, the parabola
and hyperbola.

Edmund Halley (1656-1742), who had been a friend and
active associate of Newton's and had assisted him for sev-
eral years in the preparation of the Principia, followed

Fig. 29 — An Elongated Ellipse and a Parabola.

Newton's principles in the observation of comets. He com-
puted the paths of the comets of 1680 and 1682, and
especially one of 1 531 whose appearance was recorded by
Appian. His studies contributed much to the material
dealing with this subject in the Principia, especially the
later editions. In 1705 he published a Synopsis of Comet-
ary Astronomy, in which he calculated 24 cometary orbits.
Discussing in detail a number of these, he was struck with
the resemblance between the paths described by the comets
of 1 53 1, 1607 and 1682, and the approximate equality in
the intervals between their respective appearances and

COMETS, METEORS AND METEORITES 233

that of a fourth comet observed in 1456. Moreover, there
was historical record of a comet in 1380, as well as in
1305. He at once concluded that all four comets were
really different appearances of the same comet, which
moved around the Sun in an elongated ellipse in a period
of about 75 or 76 years, and he accounted for the small
differences in the different intervals between the appear-
ances of the comet by perturbations caused by planets in
whose neighborhood the comet passed. He then made the

V

Fig. 30 — Halley's Comet During Cycle 1835-1910.
Note : The places given are for January 1 of the dates indicated.

first prediction of the probable reappearance of the comet,
assigning the date, 1758, when next it would be seen after
a 76-year interval. Halley, no less than modern astron-
omers, was aware of the disturbance that the presence of
the planets might work in a comet and its orbit, and how
its time might be altered. He confidently announced the
actual appearance of the comet, but left shortly before his
death, at the age of 85 years, the following quaintly
worded statement in regard to the comet : "Wherefore, if
according to what we have already said, it should return
again about the year 1758, candid posterity will not refuse
to acknowledge that this was first discovered by an Eng-

234 ASTRONOMY

lishman." When the time arrived the comet was looked
for by astronomers; the French savant, Alexis Claude
Clairaut (1713-1765), computed the various perturba-
tions which might have affected its journey. As
its path lay through the orbits of Jupiter and Saturn
and as it passed close to both of these great planets,
his calculations showed that there might be expected a
retardation of 100 days on Saturn's account and 518 days
for Jupiter. On Christmas Day, 1758, a month and a day
before the date assigned by Clairaut, and in the year an-
nounced a half century before by Halley, the comet was
actually discovered by George Palitzsch (1723-1788), of
Saxony, and the great astronomical prophecy was thus
fulfilled. A new member was added to the solar system;
the wandering and fear-inspiring comet was thus brought
into harmony with the other members and made subject
to the fundamental calculations of the astronomer. What-
ever superstition had attached to these wonderful appa-
ritions had now all but passed, and comets were found to
present problems no less interesting than other celestial
"bodies when their fundamental motions were known.

In 1835 Halley's comet duly reappeared and passed
through its perihelion within a few days of the time set
for it by astronomers. It was observed, among others,
by Sir John Herschel at the Cape of Good Hope. In
1910 this comet returns again along the orbit shown in
Fig. 30. It was first discovered by Wolf, of Heidelberg,
on September 11, 1909.

In the study of Halley's comet in connection with its
appearance, much attention has been devoted by astron-
omers to its earlier history, particularly that recorded in
Chinese and European annals. Messrs. Cowell and Crom-
melin, at the Greenwich Observatory, have carefully ex-
amined the previous work of Hind and have found it in
the main correct. Halley's comet unquestionably must be
identified with one that occurred in 1066, in the year of
the Norman Conquest, a representation of which is now

COMETS, METEORS AND METEORITES 235

extant in the Bayeux tapestry supposed to have been
worked by Queen Matilda and her ladies.

In more modern times comets have been associated
with some important development of scientific theory
rather than with historical events. The comet of 181 1,
visible from March 26th of that year until August 17th of
the following year, received the attention of Sir William

Fig. 31 — The Comet of 1066 as Represented in the Bayeux
Tapestry. (From the World of Comets.)

Herschel, who discovered that it shone partly by its own
light, which developed as it approached the Sun. This
comet had a tail at one time 100 million miles in length
and 15 million miles in diameter. Dr. Olbers suggested
that electrical repulsion was responsible for the formation
of the tail. A comet famous for the fact that it was the
first of the family of Jupiter's comets to be discovered was
that named for Johann Franz Encke, for many years

236

ASTRONOMY

director of the Berlin Observatory. It was discovered by
Pons of Marseilles, November 26, 1818, but in the calcu-
lation of its orbit and other elements Encke found that
it revolved about the Sun in a period of 3^ years, which
is considerably shorter than that of any other known

Fig. 32 — The Orbit of Encke's Comet.

comet. Furthermore, he established its identity with
comets seen by Mechain in 1786, by Caroline Herschel in
1795, and by Pons, Huth and Bouvard in 1805. Encke's
calculations, after establishing its periodicity, assigned the
date of May 24, 1822, for its next return to perihelion,
and tho on account of the position of the Earth at

Morehouse Comet, October ±, iyuo.
The stars are shown as streaks owing to camera having followed
comet's path. (Yerkes Observatory.)

COMETS, METEORS AND METEORITES 237

that time it was invisible in the northern hemisphere,
it was detected at Sir Thomas Brisbane's observatory at
Paramatta by Riimker very nearly in the position indi-
cated by Encke. This was only the second instance of
the recognised return of a comet, so that Encke's work
as an astronomical achievement should be considered with
that of Halley.

Biela's comet, discovered by an Austrian officer of that
name at Josephstadt, in Bohemia, February 27, 1826, pre-
sents as interesting features as that of Encke. It was seen
ten days later at Marseilles by the French astronomer
Gambart. Both observers announced its discovery and
the computation of its orbit in the same issue of the
Astronomische Nachrichten. Tho the comet was iden-
tified with similar appearances in 1772 and 1805, it was
not visible after the latter date with the naked eye. In
1832 Sir John Herschel observed it as a conspicuous
nebula without a tail. While the day of active superstition
in regard to the appearance of comets had passed with
the demonstration of Halley's theory, nevertheless Biela's
comet occasioned widespread popular excitement, founded,
moreover, on the statements of scientific men that a col-
lision with the Earth might occur. The possibility is one
of the remotest of cosmical happenings.

In 1846 Biela's comet reappeared, and when first seen,
November 28, presented no unusual appearance. Grad-
ually it became distorted, however, and elongated. Within
two months it divided into two separate bodies, which
were visible until April 16th of the following year. This
striking phenomenon of a double comet was noted by
many astronomers at different observatories, and thus
established what Seneca had reproved Ephorus for sup-
posing to have taken place in 373 B.C. and what Kepler had
noted in 161 8, but without convincing astronomers at
large of the correctness of his impression. These two
Biela comets contained a small amount of matter and
performed their revolutions around the Sun independently

238 ASTRONOMY

without experiencing any appreciable mutual disturbance,
which indicated that at an interval of only 157,250 miles
their attractive power was virtually inoperative. Various
interesting phenomena showing internal agitation were
observed and variations of brilliancy and form were dis-
tinctly evident. In 1852 Biela's comet again appeared in
its double form, but since that time has not been observed.
The disruption occasioned by its proximity to Jupiter in
1841 is believed to have been the beginning of the dis-
integrating process which resisted in its disappearance.

The greatest comet of the nineteenth century was that
of Donati, which was seen by him at Florence, June 2,
1858. By the end of September, when the comet had
reached its perihelion, the tail had attained full develop-
ment, and on October 10 it stretched in a maximum curve
over more than a third of the visible hemisphere, repre-
senting a length of 54,000,000 miles. For the 112 days
during which it was visible to the naked eye the fullest
observations were made. The comet stands by itself, as it
is not possible to identify it with any other. At aphelion
its orbit extended out into space to $J/> times the distance
from the Sun to Neptune, and for its circuit, which is
effected in a retrograde direction, requires more than
2,000 years, so that its next return should be about the
year 4000 a.d.. It was computed by M. Faye that the
volume of this comet was about 500 times that of the Sun.
On the other hand, he calculated that the quantity of mat-
ter it contained was only a fraction of the Earth's mass.
This shows how almost inconceivably tenuous the material
forming the comet must have been — much more rarefied
indeed than the most perfect vacuum which can be pro-
duced in an air pump. This tenuity is shown by the fact
that stars were seen through the tail "as if the tail did
not exist." A mist of a few hundred yards in thickness
is sufficient to hide the stars from our view, while a thick-
ness of thousands of miles of cometary matter does not
suffice even to dim their brilliancy.

COMETS, METEORS AND METEORITES 239

Dr. Barnard states that our knowledge of the extremely
rapid transformations in the tails of comets dates from
the photographs of Swift's comet of 1892, taken at the
Lick Observatory, and similar ones taken of the same
object by Professor Pickering at Arequipa. While only
an insignificant affair visually, and but fairly visible tc*
the naked eye, Swift's comet showed upon the photo-
graphic plates the most extraordinary ai.d rapid trans-
formations. One day its tail would be separated into at
least a dozen individual streams and the next present only
two broad streamers, which a day later had again sep-
arated into numerous strands, with a great mass, ap-
parently a secondary comet, appearing some distance back
of the head in the main tail, with a system of tails of its
own.

The photographs of Brook's comet of 1893 showed such
an extraordinary condition of change and distortion in
the tail as to suggest some outside influence, such as the
probable collision of the tail with a resisting medium, pos-
sibly a stream of meteors. The long series of photographs
obtained of this comet showed great masses of cometary
matter drifting away into space, probably to become meteor
swarms. Had it not been for photography, the comet,
instead of proving to be one of the most remarkable on
record, would have passed without special notice. The*
these phenomena were so conspicuously shown, scarcely
any trace of the disturbance was visible with the tele-
scope. On account of the apparent insignificance of the
comet visually, no photographs were made of it elsewhere
during its active period. The application of photography
to cometary studies has been an important feature of the
investigation of later comets, none of which since 1882
have been marked by great brilliancy.

The general nature and appearance of a comet is thus
clearly described by the late Professor R. A. Proctor;
"When first seen in a telescope, a comet usually presents
a small round disk of hazy light, somewhat brighter near

240 ASTRONOMY

the center. As the comet approaches the Sun the disk
lengthens, and if the comet is to be a tailed one, traces
begin to be seen of a streakiness in the comet's light.
Gradually a tail is formed, which is turned always from
the Sun. The tail grows brighter and longer, and the
head becomes developed into a coma surrounding a dis-
tinctly marked nucleus. Presently the comet is lost to
view through its near approach to the Sun. But after a
while it is again seen, sometimes wonderfully changed in
aspect through the effects of solar heat. Some comets are
brighter and more striking after passing their point of
nearest approach to the Sun (or perihelion) than before;
others are quite shorn of their splendor when they reap-
pear. On the other hand, the comet of 1861 burst upon
us in its full splendor after perihelion passage.

"As a comet approaches the Sun a change takes place
in the appearance of the coma and nucleus, and in some
instances a tail is generated. The process actually ob-
served is generally this: In the forward part of the
nucleus a turbulent action is seen to be in progress, lead-
ing to the propulsion toward the Sun of jets or streams
of misty-looking matter. Sometimes a regular cap or en-
velope is seen to be projected in this manner toward the
Sun, or even a set of envelopes one within the other. The
matter thus thrown off is not suffered to pass very far
from the nucleus toward the Sun, but is swept away, as
fast as formed, in the contrary direction. If the funnel
of a steam engine were directed forward, instead of up-
ward, then the appearance presented by the emitted steam
as the engine rushed on (against a hurricane, suppose, to
make the illustration more perfect ^ would exemplify the
process which seems to be taking place around the front
of the nucleus and far behind it as the matter formed is
continually swept away from the Sun. The same Sun
which attracts the nucleus seems to repulse the emitted
matter with inconceivable energy.
. "When we see the tail of a comet occupying a volume

COMETS, METEORS AND METEORITES 241

thousands of times greater than that of the Sun itself,
the question naturally suggests itself: 'How does it hap-
pen that so vast a body can sweep through the solar
system without deranging the motion of every planet?'
Conceding even an extreme tenuity to the substance com-
posing so vast a volume, one would still expect its mass to
be tremendous. For instance, if we supposed the whole
mass of the tail of the comet of 1843 to consist of hydro-
gen gas (the lightest substance known to us), yet even
then the mass of the tail would have largely exceeded

\
\

\

\\

^*

Fig- 33 — The Tail of a Comet Directed from the Sun.

that of the Sun. Every planet would have been dragged
from its orbit by so vast a mass passing so near. We
know, on the contrary, that no such effects were produced.
The length of our year did not change by a single second,
showing that our Earth had been neither hastened nor
retarded in its steady motion round the Sun. Thus we
are forced to admit that the actual substance of the comet
was inconceivably rare. A jarful of air would probably
have outweighed hundreds of cubic miles of that vast
appendage which blazed across our skies to the terror of
the ignorant and superstitious.

"The dread of the possible evils which might accrue if

242 ASTRONOMY

the Earth encountered a comet will possibly be diminished
by the consideration of the extreme tenuity of these ob-
jects. But the feeling may still remain that influences
other than those due to mere weight or mass might be
exerted upon terrestrial races in the course of such an
encounter. On account of their enormous volumes, it is
not so utterly improbable that we should encounter them
as that we should meet the comparatively minute nuclei.
In fact, the Earth actually did pass through the tail of
the comet of 1861. At about the hour when it was cal-
culated that the encounter should have taken place a
strange auroral glare was seen in the atmosphere, but
beyond this no effect was perceptible."

In distinction to the comets moving in regular orbits
around the Sun, the possible portions of one much larger
cometary body which became dispersed by gravitational
action or through violent encounter with the suns sur-
rounding must be mentioned. These comets, which appar-
ently have been seized by the gravitative attraction of
planets, are compelled to revolve in short ellipses around
the Sun well within the limits of the solar system. These
comets are spoken of as "captures," and while Jupiter,
Saturn, Uranus and Neptune each possess families of this
kind, it is the first named which are the most important, as
they number about 30, and in this family may be included
not only the bodies that Jupiter has attracted, but those
that have been robbed from other planets. Comet
families are not found in the case of the terrestrial
planets, because the gravitative power of the Sun in their
vicinity is so much greater than any attractive force
which they could manifest. In addition, when a comet
enters the inner portion of the solar system, it has such
velocity that the gravitational attraction of the planets
within these regions is not powerful enough to cause any
appreciable deflection. If a captured comet is acted upon
by further disturbing causes, its new orbit may be disar-

COMETS, METEORS AND METEORITES 243

ranged and it may be again diverted into celestial space.
The nature of these orbits is shown in Fig. 34.

The facts, learned from modern cometary study are
summed up by Miss Clerke as follows : "First, comets
may be met with pursuing each other, after intervals of
many years, in the same, or nearly the same, track; so

Fig- 34 — Jupiter's Family of Comets.

The dotted portions represent the parts of the orbits below, s. e.
to the south of the ecliptic.

that identity of orbit can no longer be regarded as a sure
test of individual identity. Secondly, at least the outer
corona may be traversed by such bodies with perfect
apparent immunity. Finally, their chemical constitution
is highly complex, and they posssess, in some cases at

244

ASTRONOMY

least, a metallic core resembling the meteoric masses which
occasionally reach the Earth from planetary space."

The first serious study of the physical nature of comets'
tails was undertaken in 1811 by Dr. Heinrich Olbers, the
astronomer of Bremen. He assumed that the formation
of a tail was due to expelled vapors on which two forces,
solar and cometary, acted and balanced each other. In

Fig. 35 — Bredichin's Theory of Comets' Tails.

other words, he believed that the tails were emanations,
not appendages, and consisted of rapid outflows of highly
rarefied matter which, in great part, had become perma-
nently detached from the nucleus.

This theory is especially interesting in the light of
modern investigation. It served for many years until
that of Bredichin, who in an examination of various
comets' tails found that the curvilinear shapes of the out-

COMETS, METEORS AND METEORITES 245

line fall into one or another of three special types as
indicated in the accompanying illustration. Type 1, or the
straightest, is most probably due to the element hydrogen.
In Type 2 a number of hydrocarbons are present in the
body of the comet, while in the third type iron or some
element with high atomic weight was assumed. Some
comets may have tails of more than one of these forms, as,
for example, in the case of Donati's comet, which had a
straight as well as a curved tail. Of Type 2, comets with
a number of tails have been recorded, such as the one
of the year 1744. Bredichin calculated that a repulsive
force adequate to produce the straight tail of Type 1
need only be about 19 times as much as the attraction of
gravitation, while tails of the second type a repulsive
force about equal to 3.2 to 1.5 times that gravitation
would suffice, while those of the third type would require
a repulsive force about 1.3 to 1 times that of gravitation.
These parts are nearly inversely proportional to the
atomic weights of hydrogen, hydrocarbon gas and iron
vapor, which ratio suggested to Bredichin the composition
of the various types of tail. While he was unable to
demonstrate that the tails were the result of electrical ac-
tion, yet he assumed some hypothetical repulsive force
which electrical action seemed to explain better than
any other.

Professor E. E. Barnard in 1905 stated that a repellent
influence of some sort must come from the Sun, and with
it he included an ejecting force proceeding from the
comet itself and a resistant force of some kind. The
repellent force from the Sun may be found in the pres-
sure of light, which Professor J. Clerk Maxwell assumed
must be exerted by light rays according to mathematical
reasoning. "Radiation pressure," as it is termed, was not
experimentally proven for many years, but in 1900-1901
it was established as a scientific fact by the Russian phys-
icist Lebedev and in America by Nichols and Hull. This
grinciple thus demonstrated, Professor Svante Arrhenius

246 ASTRONOMY

applied cosmically and held responsible for the gener-
ation of streams of matter flowing from the comet's head.
As the comet approaches the Sun this pressure exceeds
the force of gravity and acts upon the cometary substance
so as to drive out multitudes of the minute particles in a
direction away from the Sun. Such a swarm of particles
receiving the light from the Sun would appear as the
familiar luminous streamer recognised in the comets' tails.

When examined with the spectroscope, a comet's tail
shows a faint continuous spectrum, produced doubtless by
the sunlight reflected by the small particles, in addition to
spectral bands due to gaseous hydrocarbons and cyanogen.
Cyanogen is due to electric discharges, for such discharges
are observed in comets whose distance from the Sun is so
great that they cannot appear luminous owing to their own
high temperatures. In other words, the composition of a
comet is not unlike the blue flame of a gas stove, which is a
combination of hydrogen and carbon. As the comet
dashes toward the Sun and its temperature consequently
rises, the spectroscope reveals the presence of iron, mag-
nesium, and other metals in the nucleus. With a closer
approach to the Sun the hydrocarbons split up into hydro-
gen gas and hydrocarbons of a higher boiling-point.
Finally, a time comes when these more refractory hydro-
carbons in turn decompose into free carbon in the form
of soot. Because interstellar space is airless the soot
cannot burn, but must accompany the comet in the form
of a very fine dust. This dust, propelled away from
the Sun by radiation pressure, constitutes the tail of many
a comet. Some of the soot particles may be larger
than the critical size. They will be jerked forward toward
the Sun in advance of the comet to form what is known
as the comet's "beard," a rather rare phenomenon.

This phenomenon of the pressure of light is able to
explain the fact that the minute particles ejected from
the nucleus of a comet can pass over great distances in
small intervals of time, which was one of the hardest

COMETS, METEORS, AND METEORITES 247

points to overcome in explaining the rapid change of posi-
tion of the comet's tail passing around the Sun.

In addition to the light-pressure of the Sun, the electri-
cal energy of the Sun must be called upon to explain the
occurrence of tails which are ejected from the nucleus
with a force that may be as much as 40 times more power-
ful than gravitation.

Meteors, often called shooting stars, which is some in-
stances, at least, must be the remains of comets, are small
solid bodies which revolve around the Sun, generally in
great numbers, following approximately the same orbit,
and are encountered by the Earth in its annual revolution.
Then they graze the Earth or even fall toward it, but, for-
tunately for its inhabitants, they seldom reach its solid
surface, because they are raised to incandescence and dis-
sipated in vapor by the heat generated by friction in their
swift rush through the atmosphere. At certain seasons of
the year the Earth traverses comparatively dense swarms
of meteors and is subjected to a veritable bombardment.
The effect to the eye of these flashing meteors is most
striking and brilliant, particularly if the point of the
swarm intersected contains a large aggregation of
meteors. The apparent radiation of a meteoric shower
from a common point or radiant is an effect of perspec-
tive, as the meteors of a swarm in reality pursue parallel
paths. Three of these meteor swarms are of particular
interest, as under certain conditions they give rise to fine
displays. These are known as the Bielids, the Leonids,
and the Perseids, the first two occurring in November
and the last named in August.

The Bielids, deriving their name from the connection of
their orbit with that of the comet Biela, are also known
as Andromedids on account of their apparent source in
the constellation of Andromeda. M. Egenitis, Director
of the Observatory at Athens, traced back the Androm-
edid shower to the times of the Emperor Justinian.
Theophanes, the chronicler of that epoch, writing of

248 ASTRONOMY

the famous revolt of Nika . in the year 532 a.d.,
says: "During the same year a great fall of stars came
from the evening till the dawn." M. Egenitis notes an-
other early reference to these meteors in 752 a.d., during
the reign of the eastern Emperor Constantine Coprony-
mous. Writing of that year, Nicephorus, a Patriarch of
Constantinople, states: "All the stars appeared to be de-
tached from the sky and to fall upon the Earth." But

OriifeJitLMsfaz*

Fig. 36 — The Orbit of a Shoal of Meteors.

it was not until the nineteenth century that Bielids aroused
much attention, and then it was in great part due to the
fact that apparently the same orbit was occupied by them
as by Biela's comet, which we have seen was not observed
after its appearance in 1852. The Bielid shower, however,
since that time has shown increased activity, which was
especially true in the years in which the comet, were it in
existence, would have been scheduled to pass near the
Earth.

In the case of the Leonids, records of their occurrence
go back as far as 902 a.d., which is called "year of stars,"
because on the night of October 12, while the Moorish
Ibrahin Ben Ahmed was dying before Cosenza in Calabria,
"a multitude of falling stars scattered themselves across the

COMETS, METEORS AND METEORITES 249

sky like rain," and naturally aroused great excitement
among those who beheld the phenomenon, which they con-
sidered a celestial portent of unusual significance. In
1698 modern history of the Leonids began. A maximum
Leonid shower has occurred with considerable regularity
at periods of about 33 years from that date. In 1799, on

Fig- 37 — The History of the Leonids.

the nth of November, Humboldt and Bonplandt witnessed
a notable display in South America. On November 12,
1833, meteors were said to have fallen as thickly as snow-
flakes; in seven hours 240,000 were estimated to have
appeared. The radiant from which the meteors seemed to
come was found to be situated in the head of the constel-

250 ASTRONOMY

lation of Leo, from which circumstance the name
Leonids results. Professor Dennison Olmstead, of Yale
University, assigned to this cloud of cosmical particles the
path of a narrow ellipse in an orbit around the Sun and
intersecting that of the Earth. This marked the beginning
of an important department of astronomy. In 1837 Olbers
established the periodicity of the maximum shower which
indicated a regular distribution of the meteoric supply. In
1866, as conjectured by Olbers, another time of maximum
occurrence took place, which seemed to demonstrate that
while the Earth cut through the orbit each year about
the same date, at the 33-year period the swarm was at a
point of maximum density in the orbit. In 1899, however,
much disappointment was caused by the failure of the
Leonid shower to take place on the scheduled date, a
failure which was explained as due to the attraction of
one of the larger planets which had diverted the orbit
from its old position so that the Earth failed to pass
through the swarm. The cometary connection in the case
of the Leonids is shown by the fact that they seem to
travel in the orbit of Tempers comet of 1866.

The Perseids date back to the year 811 a.dv and derive
their name from the constellatio of Perseus, where their
radiant point is situated. They are seen on the 10th of
August in continental Europe. As this is the day of
Saint Lawrence, they are known as the "tears of Saint
Lawrence/' But this date is not the only one on which
meteors from this swarm are to be observed, for they fall
in greater or less numbers from about July 8th to August
22d. They are very rapid in their motion and the trails
often persist for a minute or two before they are dissemi-
nated. The Perseids have an easterly motion, shifting each
night by a small amount. Their orbit cuts the orbit of the
Earth almost perpendicularly, and they are supposed to
be the debris of an ancient comet which traveled the
same path. Various comets, especially that of Tuttle in

COMETS, METEORS AND METEORITES 251

1862, seem to have had the same orbit, and the meteors
are quite evenly distributed along the path.

There are other swarms of meteors, such as the Lyrids,
the Orionids, etc., and Mr. W. F. Denning, of England,
who has made a specialty of this field, has accounted for
3,000 other less conspicuous showers. Astronomers have
shown that the various meteor swarms and comets move
in the same orbits. Accordingly the theory has been
proposed that when a comet is captured by a
planet the material of the tail is driven off into space
and the remaining material, disintegrated by the various
forces at work, is distributed along the orbit. Conse-
quently the phenomenon of a meteoric shower occurs
when the orbit of a swarm and that of the Earth intersect
and when the Earth and the meteors arrive at this inter-
section at the same time. Leverrier showed that the
Leonids resulted from the capture of a parent comet in
126 a.d. at the time of a near approach, and that the
disintegration, not entirely completed, is already far ad-
vanced. He claimed that the Perseids were of much older
formation.

Meteors have been observed from the earliest days, but
are of such minor importance as compared with comets
that they attracted no particular attention. In 1719
Brandes and Benzenberg, at Gottingen, by making simul-
taneous observations of the beginning and end of the
path of a meteor from different stations a few miles apart
were able to determine not only its position, but its veloc-
ity, and subsequent observations similarly made indicate
that meteors appear at altitudes of 60 to 100 miles and
that they move over paths of 40 or 50 miles, traveling at
a rate of 10 to 40 miles a second.

When a meteor enters the Earth's atmosphere from
interplanetary space, the friction of the atmosphere, caused
by its high velocity, develops heat and causes it to shed
a brilliant light. The temperature of a meteor rises to
many thousand degrees Centigrade, and for that reason

252 ASTRONOMY

it is usually consumed before it reaches the Earth's
surface. The products of oxidation and disintegration con-
sist simply of dust, which falls on the Earth's surface or is
distributed throughout space. In the main, meteors simply
contribute dust to the Earth.

The energy of the meteor as well as its mass can readily
be ascertained if its distance, the duration of its luminosity,
and its brightness are known, for the total amount of light
radiated can be calculated on the basis that its entire
energy is thus transformed. The mass of a meteor is not
particularly large and is usually but a small fraction of an
ounce. For the most part it is not larger than a pea or
pebble. It is the atmosphere that not only heats these
rapidly moving bodies but acts as a protection to the
Earth, for if meteors were not thus disintegrated they
would fall upon its surface in a constant bombardment. In
addition to the meteors seen with the naked eye, estimated
by the late Professor Simon Newcomb at not less than 146
billion per annum, there are doubtless ten times as many
which pass merely as streaks of light in the field of the
observer's telescope.

In addition to the extremely fine dust which settles on
the Earth as the result of the disintegration of various
celestial bodies, there are from time to time masses of
greater or less size which, rushing into the Earth's at-
mosphere with a brilliant glow due to the heat generated
by friction, fall to the Earth's surface and become more or
less embedded. The appearance is most striking, accom-
panied as it often is by a loud roar like a waterfall and
occasionally violent explosions. Thus it is stated that at
Cairo, in August, 1029, many stars passed with a great
noise and a brilliant light. These bodies, a number of
which come to the Earth's surface yearly, are termed
meteorites, siderites, uranoliths or aerolites, and appar-
ently are the connecting links between the Earth and out-
side space. Their nature is none too well known and
they present many unsolved problems. It is interesting to

COMETS, METEORS AND METEORITES 253

know that the great Mexican meteorites at the time of the
Spanish invasion were considered holy bodies by the
Indians, so that it is inferred that their fall from the
heavens was known and was regarded as a supernatural
occurrance. In the Greek and Roman records similar
attention was paid to the palladium of Troy, to the image
of Diana at Ephesus and to the sacred shield of Numa, all
of which were said to have fallen from the heavens and
were no doubt meteorites.

Meteorites are usually divided into two classes, those
composed chiefly of iron and those composed chiefly of
stone. Of the 292 actually observed meteoric falls that
took place during the last century, only twelve, or about 4
per cent, belonged to the first class, yet in our cabinets
the two classes are represented in nearly equal numbers.
The explanation of this strange anomaly lies in the fact
that unless the fall has been actually witnessed close at
hand, very few of the stony meteorites are ever found.
Of 328 in the collection of the British Museum, 305, or
93 per cent., were seen to fall. This is partly because
these bodies to ordinary inspection appear very like com-
mon stones, and therefore are not recognised as meteorites,
and partly because owing to their physical and chemical
structure they are readily decomposed by the action of the
elements.

It is the custom to associate meteorites with falling
stars, and to say therefore that they are of cometary ori-
gin. This relationship, however, is not as obvious, when
we begin to examine into the case, as at first sight appears.
A prominent difficulty is that the distribution of the
meteorites throughout the year differs very materially from
that of the falling stars and fireballs. While these last
two are about twice as numerous during the latter half of
the year as during the first half, the meteorites are more
numerous during the first half of the year. From this we
should infer that while perhaps all meteorites are fire-
balls, only comparatively few fireballs become meteorites..

254 ASTRONOMY

The dividing line between meteorites and falling stars
then lies among the fireballs-, the swiftly moving ones
being allied to the falling stars and the slowly moving ones
to the meteorites.

It is now generally accepted that the crystalline and
often conglomerate structure of these bodies proves them
to be but the fragments of much larger bodies that have
in some manner been destroyed or from which they have
otherwise become separated. Many believe that the crys-
talline structure of the iron meteorites indicates a slow
cooling, while some say that the structures of the "chon-
dres" of the stony meteorites must certainly have been
produced by a very rapid crystallization due to a sudden
exposure to a lower temperature.

It was formerly thought by some that these bodies might
have been expelled from the Sun. Altho it is quite pos-
sible that solar explosions in past ages were sufficiently
violent to project these bodies with the necessary cometary
velocities, yet we cannot believe the Sun to be the direct
source of them, since it is improbable that either solid
stone or iron should ever have existed upon its surface or
within its interior. Nor is it easy to explain how with
such an origin the meteorites should have acquired their
present orbits.

Some of the earlier cosmogonists referred their origin
to the terrestrial or lunar volcanoes. This is manifestly
impossible in the case of the Earth, since even prehistoric
volcanoes could not have expelled their products with such
force that after leaving the confines of our atmosphere
they should still retain a velocity of over seven miles per
second. Yet this is the speed required to prevent an im-
mediate return to the Earth's surface. Moreover, altho
volcanic eruptions in prehistoric times were undoubtedly
more frequent and voluminous than at present, it is by no
means certain for theoretical reasons that they were then
any more violent than they are to-day.

Meteors escaping from lunar volcanoes would not have

COMETS, METEORS AND METEORITES 255

to encounter a dense atmosphere, and, furthermore, their
required parabolic velocity would be appreciably less. But
even under the most favorable circumstances, in order to
escape both the Moon and Earth a speed of over two
miles per second would be required. That attained, they
would then be controlled by the Sun and might be picked
up at any later time by our planet in its orbit. The objec-
tion to this explanation is that no explosive volcanoes have
ever been detected upon the Moon, all the craters being of
the engulfment type. It is therefore very improbable that
such extremely violent explosions could have occurred
there.

While, as we have seen, meteorites cannot be the prod-
uct of terrestrial volcanoes, yet it is suggested by Prof.
W. H. Pickering that the stony ones were all of them
formed during the great cataclysm that occurred at the
time that the Moon separated from the Earth. When the
truly enormous pressure on the deep-lying terrestrial
strata was suddenly relieved by the departure of the upper
layers, which now form our Moon, tremendous explosive
energy must have been generated. Considerable portions
of our atmosphere must have followed the larger flying
masses, and the atmospheric resistance to the smaller ones,
swept along at the same time, would have been much di-
minished. Altho we can probably never definitely know
just what occurred at this time, it is quite possible that
considerable quantities of the smaller masses were carried
along by the blast of escaping gases and were projected
to such distances as to free themselves entirely from the
attraction of our planet. This implies a solid crust for the
Earth at the date of birth of the Moon, which previous
investigations of the place of origin of that body seem to
justify.

In support of this view of the terrestrial origin of me-
teorites we have the fact that twenty-nine terrestrial ele-
ments, including helium, have so far been recognised in
them, ten of them being non-metallic. No new elements

256 ASTRONOMY

have been found. The six which occur most frequently in
the Earth's crust, named in the order of their abundance,
are oxygen, silicon, aluminium, iron, calcium and mag-
nesium. The eight most commonly found in the stony
meteorites are these six, besides nickel and sulphur.

Nearly all the stony meteorites contain some metallic
iron, and some of them contain large quantities of it. But
,this is also true of some of our basaltic lavas. Indeed,
large masses of iron have been found in ledges upon the
Greenland coast. Some of this iron contains over 6 per
cent, of nickel, but much larger proportions have been
discovered in New Zealand, Piedmont and Oregon, where
considerable quantities of the nickel iron alloys have been
found. According to Farrington, of the twenty-one min-
erals recognised in meteorites, fourteen have been found
in our volcanic products.

It appears to Professor Pickering that the iron and stony
meteorites differ from one another in other ways besides
their composition. That some of the former are associated
with falling stars, and therefore with comets, certainly
seems plausible. That the latter are not associated with
them seems probable, and if so, whence can they have come
if not from our own Earth ?

CHAPTER XXI

THE STARS — THE CONSTELLATIONS — METHODS OF NOTING
THE STARS

The ancients assumed that the celestial sphere of the
heavens was a reality and even considered it a solid sphere
of crystal, on which they observed and marked the posi-
tions of the various stars, just as to-day the astronomer
assumes for the same purpose an ideal celestial sphere of
which the observer is the center and in which declination
corresponds with latitude on the Earth and right ascension
with longitude. They appreciated that altho the stars
moved across the sky and apparently with different rates
of motion, yet the distances between any two remained
unchanged. For this reason they imagined that the stars
were fixed on the celestial sphere. The motion of some of
the stars about a center or pole which coincided with the
Pole Star was observed, and the two poles in the heavens
were early assumed. But the most important observation
of the ancients was the relative position of the Sun and
the stars. A succession of such observations early showed
that the stars were gradually changing their position with
respect to the Sun or that the Sun was changing its posi-
tion with respect to the stars.

Thus the stars obviously vary in position and mag-
nitude, yet in ancient times little was conceived as to
their nature or possible origin. There was, of course,
the fundamental distinction between the planets or mov-
ing bodies and the idea of the fixity of the stars, which
257

258 ASTRONOMY

was early established and persisted for many centuries,
even tho Giordano Bruno (d. 1600) vaguely suggested that
the suns of space move. In 1718 Halley had announced a
shifting in the sky of Sirius, Aldebaran, Betelgeux and
Arcturus since the time of Ptolemy's catalogue, and similar
conclusions were reached by various other astronomers.
But it was only in 1838 when Bessel with the heliometer
was able to detect a motion of the star 61 Cygni that it
was clearly and conclusively demonstrated that the stars
move through space as well as other bodies in the universe.

The stars were supposed by the ancients to be situated
on the celestial sphere at a distance greater, it is true, than
the planets; yet as they were observed year after year in
essentially the same positions they were held as fixed and
immovable and a bodily rotation of the celestial sphere
itself was assumed. Just as the ancient mind had given to
the planets the names of gods and goddesses, so the wise
men of antiquity assigned to the stars similar names or
those of animals, the natural result of their vivid imagina-
tion. This they did also with groups of stars with even
greater play of the imagination.

The idea of grouping the stars into constellations dates
from the earliest times. In fact the names long ago given to
many have persisted until to-day, tho it must be confessed
that they have often proved a cause of embarrassment to the
student of astronomy. The modern mind finds it difficult to
group in imagination a series of bright points of light in
the shape of some mythological hero, bear, dog, serpent or
other animal. In most cases the choice had been made in a
most arbitrary manner, and Sir John Herschel has truly
remarked : "The constellations seem to have been purposely
named and delineated to cause as much confusion and in-
convenience as possible. Innumerable snakes twine through
long and contorted areas of the heavens where no memory
can follow them; bears, lions and fishes, large and small,
confuse all nomenclature."

The names of the constellations as we know them are

THE CONSTELLATIONS 259

doubtless of Greek origin, borrowed from Chaldean and
Egyptian astronomy. For the most part the names are
Greek. The most important are those through which the
ancients believed that the Sun passed in its annual circuit
of the celestial sphere, or, in other words, those through
which the ecliptic passes. For thousands of years these
constellations have been used to identify the position of the
Sun, especially as the Sun, Moon and five planets were al-
ways to be found within a region of the sky extending about
8 degrees on each side of the ecliptic. To this strip of the
celestial sphere the term "zodiac" was given, for with one
exception all of the constellations it contained were named
after living things. It was divided into twelve equal parts,
forming the familiar signs of the zodiac, through one of
which the Sun passes every month. These signs were
made up of a number of stars grouped into constellations.
Their names may still be seen, with but unimportant
changes, in a modern almanac just as they figured in early
Greek days. The names as given in Latin are Aries, Tau-
rus, Gemini, Cancer, Leo, Virgo, Scorpio, Sagittarius,
Capricornus, Aquarius and Pisces.

Just how the stars were originally grouped to form the
constellations by the ancients history does not record. In
an article in the Scientific American it is stated that "the
first reliable information regarding the Greek sky is ob-
tained from Eudoxus of Cnidus, an astronomer who lived
about 370 b.c. His work furnished Aratus, who lived a
hundred years later, with material for his great astronomi-
cal poem.

" Tn great numbers/ says Aratus, 'and in various courses
the stars incessantly move around the motionless skies.
The axle stands immovable. In the midst the Earth is
suspended in equilibrium, while the heavens swing around
it. The poles bound the axle on both sides. These are
encircled by the Bears, that revolve around back to back
separated by the Dragon's manifold coils.' "

Eratosthenes (about 170 B.C.) enumerates these constella-

260 ASTRONOMY

tions, and not only tells the mythological stories but indi-
cates the positions and numbers of stars in every figure,
differing from Aratus only in a few particulars.

"Ptolemy gave forty-eight constellations. The figures
were the same as the old constellations of Aratus with a
few additions. The stars, however, were marked in their
proper places and defined as to latitude, longitude and
magnitude.

"After Ptolemy a long period ensued during which the
astronomical charts were unchanged. It is to the Arabs
in the eighth century that the next advance is due. The
Caliphs of this period, among whom was Haroun al
Raschid of 'Arabian Nights' fame, were friends to science
and gathered around them men of learning, such as the
famous astronomers Ulug Bekh, Fergani, El-Batan and
Abdelrahman Sufi. To a great extent they were satisfied
with Ptolemy's work, and altho they retained a great many
of the Greek star names, they added a number derived by
tradition from the ancient Arab names. Abdelrahman
Sufi wrote a detailed and exhaustive account of the Greek
constellations, carefully following Ptolemy, and at the
same time he treated of the ancient Arabian heavens.

"So strong was their objection to the personal element
that when the Greek Zodiac was incorporated by the
Arabian astronomers they indicated the names of the
objects carried by the characters instead of the characters
themselves. Thus Virgo was called the Ears, on account
of the wheat she held in her hand ; Sagittarius was not the
Archer but the Bow, and Aquarius not the Water-bearer
but the Well Bucket.

"When the great mixture of Arabian folk-lore was
combined with the Greek sky many of the star names
were retained, but occasionally the Greek names were
changed; for instance, the beautiful red Antares in Scor-
pio was approximately called the Scorpion's Heart.

"In 1433 Ulug Bekh made at his observatory in Samar-
kand the most correct catalogue of stars up to that period.

THE CONSTELLATIONS 261

The famous astronomical tables compiled under Alphonso
X. of Castile date from 1252, and next in importance was
the great catalogue of Tycho Brahe (1 546-1 601).

"The southern hemisphere, which was uncharted by the
ancients, is of far less interest than the northern, partly
because the changes have been frequent and unimportant
and partly because the only constellation visible to us is
the Dove, introduced early in the sixteenth century. In

#«Asterope

•Taygeta

y

^AMaTa

\ Celseno

•Pfcton£

/

\

AtJas

.^-•Jflcyone... ^fcctra
^Merope

Fig. 38 —The Pleiades.

the old books it is called Columba Noae because it is
near the ship, represented sometimes at that period as
Noah's Ark. The regions around the Ship and the North
Pole have been subject to the most frequent changes since
the seventeenth century.

The most familiar constellation is the "Great Bear,"
known to Americans as "the Dipper"; three stars form
the tail of the animal and four others part of his body.
To the more prosaic American mind the analogy of the
dipper seems far more apt than that of a bear. In Cas-
siopeia, which is across the pole from the Dipper, the

262 ASTRONOMY

brighter stars form the chair .in which a lady is seated-
In many cases the position of a figure can be reproduced
with a fair degree of certainty, altho it is hard to realize
how the names were originally given.

Most of the constellations familiar by observation
or legend are those of the northern sky, because until
within modern times there were no recorded observations
of the southern heavens. Two of the southern constella-
tions, however, are noteworthy, one of which is the Cen-
taur, containing two first-magnitude stars, Alpha and
Beta Centauri, the first of which, as we shall see, is notable

THE GREAT BEAR

• « S #a

« £ * «2 ^_ THE TWINS

^ • a~~~~**"««~^.»^Castor0 A B«aut!ful Double Star

Pollux*

•Procyon (1st. Mag)
THE LrTTLE DOG

Fig. 39 — Caston and Pollux.

as being the nearest of all the stars to our Earth, while
the second constellation is the famous Southern Cross, or
Crux, having also a first-magnitude star, Alpha Crucis.
Some 5,000 years ago the Southern Cross rose above the
English horizon and was just visible in the latitude of
London, but through the centuries it has had a southerly
motion and has not been seen for many years even in the
south of Europe. Perhaps "the chambers of the South'"
in the Book of Job (ix, 9) may be this constellation, for
the Southern Cross must have been a feature in the sky
of Palestine when this book was written.

The designation of individual stars probably antedated,
the idea of constellations; this we may infer from the
allusion to the star Arcturus in Job (ix, 9). The two

THE CONSTELLATIONS 263

stars Castor and Pollux date from classical antiquity.
The names of most individual stars now used are of
Arabic origin, which fact accounts for the number of
names not Greek or Latin. Thus Aldebaran is a corrup-
tion of Al Dabaran, the follower. The modern system of
naming stars, however, consists in identifying them with
the constellation and then in giving them a separate desig-
nation by adding a letter of the Greek alphabet. Thus
the brightest star of a constellation is called Alpha, the

An Exquisite

Coloured Double

Star

Fig, 40 —The Great Square of Pegasus.

next Beta, etc. This rule, which was devised by Beyer
for his Uranometria, or star catalogue, published in 1601,
has not been followed in all cases. When the number of
stars was such as to exhaust the Greek alphabet the
Roman was employed and in some cases italics. Flam-
steed, the first Astronomer Royal of England, in his cata-
logue of stars made from observations at Greenwich
(1666-1715), introduced a system of numbering the stars.
In modern star catalogues both the Bayer letter and the
Flamsteed number are often found.

Of the individual stars, tho not the brightest, perhaps

264 ASTRONOMY

the most important to us is Polaris, the "North Star" or
"Pole Star," which is in a straight line with the two
stars marking the bottom of the Dipper, which are termed
"the pointers." It is five times as far away as the interval
between the pointers and very nearly occupies that point
of the heavens toward which the north pole of the Earth's
axis is directed. To the observer on the Earth it appears

i **»& M

t> *£ * •'.;* Poian*t77iePat*&tcu$

Fig. 41 — Polaris and Neighboring Constellations.

to mark the north. Its position, however, varies with its
latitude. It has a certain circular motion, due to the
slow shifting of the direction of the Earth's axis, known
as precession, so that in the course of some twelve thou-
sand years it will be displaced from its position as pole
star and Vega, a pale blue star of the first magnitude in
the constellation Lyra, will assume its place.

With the conception of the celestial sphere held by the
ancients it was not difficult to assume that the stars were

THE CONSTELLATIONS 265

studded about the sky at a considerable distance. Yet
beyond noting their apparent distances from one another
as expressed in angular measure, but little attention was
paid by the early astronomers to their remoteness from
the Earth. Indeed, they assumed them to be very remote,
and Aristotle remarks, quoting the opinion of "the mathe-
maticians," that the stars must be at least nine times as far
off as the Sun. Timocharis and Aristyllus, both of whom
flourished in the early part of the third century B.C., were
the first to ascertain and to record the positions of the
chief stars by means of numerical measures of their dis-
tances from fixed positions on the sky, or in other words,
to supply data for the first real star catalogue. But this
did not afford any adequate idea of the absolute or relative
distance.

After the death of Hipparchus it was recorded in an
early text-book of astronomy that the stars need not
necessarily be on the surface of the sphere, but may be at
different distances from the Earth, which, however, could
not be determined with the means at hand. In the works
of this period the conjecture is also hazarded that the Sun
and stars are so far off that the Earth would be a mere
point when seen from the Sun and quite invisible from the
stars. In the Almagest Ptalemy in his series of postulates
(Book I, chap, ii) states that the Earth is merely a point
in comparison with the distance of the fixed stars, and he
believes that it is more rational to assume that bodies like
the stars, which seem to be of the nature of fire, are more
likely to move than the solid Earth.

Copernicus believed that the stars must be at a very
great distance as compared with the size of the Earth,
because the horizon apparently divides the celestial sphere
into two equal parts, and the observer appears to be at the
center of this sphere, no matter how much he moves on
the Earth, so that the distance moved is imperceptible as
compared with the distance of the stars. He maintained
that the stars were all at the same distance from the

266 ASTRONOMY

Earth, and according to his theory some annual motion of
the fixed stars should be observed, due to the alteration of
the Earth's position in its orbit. As this was not noticed,
Copernicus assumed that the distance of the stars was too
great to cause any appreciable motion. The instruments
of those days were quite incapable of detecting such a
small amount of motion, requiring as it did greater refine-
ments of observation.

The positions of the stars were carefully noted by Tycho
Brahe on a great celestial globe 5 feet in diameter, which,
in those days, served as a star chart. With a quadrant
or quarter circle having a radius of about 19 feet, by
which the angles could be read to single minutes, he made
a number of observations of the chief star in Cassiopeia,
as interest in this constellation was aroused by the appear-
ance in it of a brilliant new star in November, 1572. As he
was unable to determine any perceptible parallax, he as-
sumed that the star must certainly be farther off than the
Moon. This discovery was important as indicating that
changes could occur in that far-distant realm of the fixed
stars which hitherto was believed to be constant both in
its appearance and constitution. This was corroborated
in 1604 by Galileo in the case of the new star in Serpen-
tarius, which he demonstrated was at least more distant
than the planets. Galileo in his "Dialogue on the Two
Chief Systems of the World" brings out the fact that the
angular magnitudes of the fixed stars, which were the
most difficult to determine, are in reality almost entirely
illusory, and indicates a method known as the double-star
or differential method of parallax by which the motion
and the distances of two stars at different distances from
the Earth can be measured. At the beginning of the
eighteenth century, tho thousands of fixed stars had been
observed and their positions noted, nothing was known of
their distance beyond the fact that it was so very great
that they exerted no sensible influence.

It was not until the time of Sir William Herschel that a

THE CONSTELLATIONS 267

successful attempt was made to determine the parallax of a
star and to ascertain the distribution of stars in the heavens.
Herschel assumed that the distances of the stars depended
upon their brightness. In the case of a star barely visible
with an 8-inch mirror and one just visible with a 4-foot
reflector he assumed that the second was six times as far
off. By making a series of measurements he estimated,
for example, that the faintest stars visible to the naked
eye were about twelve times as remote as such a bright
star as Arcturus. If Arcturus were removed to 900 times
its present distance it would just be visible in Herschel's
20-foot telescope, while more clearly seen in his 40-foot
instrument, which he assumed would penetrate about
twice as far into space. These observations of Herschel
were far more productive of results in other fields than the
determination of stellar distances.

It was by Bessel's memorable observations that the
angular motion of the star "61 Cygni" was determined in
1838. His calculations showed that the distance of this
star was about 400,000 times the distance of the Sun, or
400,000 X 93,000,000 = 37,200,000,000,000 miles. This
was the first direct solution of a problem which long
before the day of Aristotle had puzzled astronomers.

If it is difficult to realize the distances of the planets
from the Earth and the Sun in the solar system and the
extent of their orbits, what must be said as to the distance
of Earth or Sun from the stars? For this all previous
scales of measurement were deficient, and a new unit of
length, itself of bewildering magnitude, had to be devised
in order to realize distinctly the immense intervals of space
separating the stars from the Earth. Miles or kilometers
were naturally inadequate for such a task. A luminous
body emits waves which are propagated through the ether
at the rate of 186,300 miles a second, or, in round num-
bers, six billion miles a year. Accordingly the "light-
year" was taken as our unit in discussing the distances of
the stars. Thus the light from Bessel's star, 61 Cygni,

268 ASTRONOMY

would take more than six years to reach the Earth. But
61 Cygni is not our nearest star. Subsequent investigation
showed that this place belonged to Alpha Centauri, which
is four and a third light-years from the Earth and prob-
ably ten billions of miles nearer to us than any other
member of the sidereal system. Sirius is twice this dis-
tance, or eight and a half light-years; Vega is about 30
years, Capella about 32 and Arcturus about 100.

But not for every star are these distances known. They
are determined by measuring the parallax wherever pos-
sible. To quote Dr. Otto Klotz, "Parallax is the apparent
displacement a body suffers by change of place in the ob-
server. With the highly refined instruments and meth-
ods, notably the heliometric and spectrographic, defi-
nite results are being attained for a comparatively few
stars of the myriads that strew the sky and mark a mile-
stone in achievement of practical astronomy. Bold indeed
was the undertaking when man first attempted to measure
the dimensions of the Earth, but bolder was it when he
projected his measuring rod to the Sun. What shall
we say when we find him rushing off, measuring rod in
hand, at the rate of 186,000 miles a second, and, after 4%
years, reaching the nearest star, Alpha Centauri, a bright
star of the southern hemisphere? This tremendous dis-
tance, quite beyond our grasp when stated in miles, is ex-
pressed by saying that its parallax is o"76. Many of the
brightest stars are found to have no sensible parallax, while
the majority of those ascertained to be nearest to the Earth
are of fifth, sixth or even ninth magnitudes."

If the parallax and distance of a star are determined, it
is possible to make some approximation of its mass in the
case of binary stars revolving in known orbits. In other
words, if we can determine the number of seconds of arc
which separate the two members of a binary system and
translate this distance into millions of miles and then
measure the period of rotation, we can find their combined
mass in terms of that of the Sun. For example, the com-

THE CONSTELLATIONS 269

ponents of the double star Alpha Centauri, to which we
have referred, revolve around their common center of
gravity at a mean distance of nearly twenty-five times the
radius of the Earth's orbit. As they require 8 years for
a period of revolution, the attractive force of the two to-
gether must be twice that of the Sun. With single stars
such a computation cannot be made. Knowing their
parallax, however, it is possible to estimate from their
size and brilliancy some of the splendid stars in the heav-
ens, such as Canopus, Betelgeux and Rigel, and to realize
that they are thousands of times greater than the Sun and
suffer in comparison by their infinite distances. For it
must always be remembered that the brilliancy of a star
depends not only upon its intrinsic brightness but also
upon its distance from the Earth.

For five other of the visual binary stars data are avail-
able for computing their masses. Thus Sirius, which radi-
ates 32 times as much light as the Sun, is supposed to have
a combined mass for its two constituent stars of 3.7 that
of the Sun; Procyon, 0.6 that of the Sun; Cassiopeia, 1.8;
70 Ophiuchi, 1.8, and 85 Pegasi, 11.3. The average dis-
tance of the stars of a pair of those in the above list from
each other is 23 times the distance of the Earth from the
Sun, or a little greater than the mean distance of Uranus
from the Sun. It is probable, however, that most double
stars are farther apart than this. But it is evident from
the stars considered that the average mass of a pair is 3.5
times that of the Sun, while the average radiating power
is nearly six times as much.

CHAPTER XXII

THE MOTIONS AND BRIGHTNESS OF THE STARS

The term "fixed star," which has survived from ancient
times, we have already found is but relative and that all
stars have some motion. If the position of one of these
so-called fixed stars is noted by observing the time and
the height at which it crosses the south point in the sky,
and this observation is compared with accurate records
which we have, going back nearly 200 years, it will be
found that quite a number of the stars are moving slowly
across the sky. This movement is termed proper motion,
which, as seen from the Earth, is at the best an exceed-
ingly minute quantity. Thus one star, and that the most
rapid, has moved about the diameter of the Moon in the
last 300 years, or an amount which the telescope is quite
incapable of detecting. It may be said in passing, how-
ever, that the actual observation of fixed stars from the
Earth after an interval of, let us say, 100 years, would
show more difference. Yet this is due, not to motions of
the stars, but to alterations in the direction of the Earth's
axis and other causes which give apparent and common
motions.

The maximum proper motion is that of the eighth-mag-
nitude star No. 243 of the fifth hour in the Cordova zone
catalogue. The star has an apparent drift of 8"7 annually,
which would carry it round a circle of 3600 in 149,000
years if it moved uniformly at its present rate. This
amount can be appreciated when it is stated that two cen-
2 70

MOTIONS OF THE STARS 271

turies would be required for a change in position equal to
the diameter of the Moon. Arcturus, since the time of
Ptolemy, has moved more than a degree and Sirius about
half as much, the motions of these two stars having been
detected first by Halley in 1718. In fact, the late Pro-
fessor Newcomb says that if Hipparchus or Ptolemy had
made exact determinations of the positions of the stars
and to-day should rise from a long sleep, Arcturus would
be the only star in which they, or the priests of Babylon
for that matter, could detect any change in position rela-
tively to the other stars of the heavens. There is no case
in which this quantity is as large as a foot-rule seen at a
distance of 50 miles, and for comparatively few stars is
this motion certainly appreciable. Notwithstanding the
extraordinary degree of precision that has been obtained
in recent measures of parallax, for a satisfactory solu-
tion of the problem we must probably devise some new
method of using the spectroscope or some other instrument
for determining proper motions.

The study of the proper motion of stars indicates that,
on the whole, the stars are opening out from a point near
Vega and closing in to the opposite point. Professor
Kapteyn finds that the stars may be divided into two
great classes having on the average proper motions quite
different in direction and magnitude, and that these two
systems are moving through each other, one from a point
in the south of Hercules and the other from a point in the
Lynx. In the latter class the solar system doubtless be-
longs, for it seems to be traveling toward that portion of
the heavens occupied by Lyra.

The problem of determining the actual speed of travel
involves not only a knowledge of the proper motions but
also of the distance of the stars. The latter quantity, as
has been seen, is obtained by noting the change in position
or parallax and is known for about 100 stars. The motion
in the line of sight can be measured by spectroscopic meth-
ods, employing Doppler's principle. The rate of travel of

272 ASTRONOMY

stars through space has thus been ascertained to be about
10 or 20 miles a second.

How is it known that stars are moving in the line of
sight? Millions and millions of miles away a star may be
approaching the Earth, and yet through the telescope there
is no change of position and no appreciable change in
magnitude. But here use is made of an ingenious prin-
ciple, which takes its name from its discoverer, Christian
Doppler (1803-1853), a physicist of Prague, who made an
experiment with quite another object in view. Placing
some musicians on a railway car and taking his stand on
the platform, he noticed that the pitch of the sound was
raised as the moving train approached him and was low-
ered after it had passed and was receding. If this could
happen in sound, why not in light, altho the vibrations
occur infinitely faster, so that the color of a luminous body
(equivalent to pitch in sound) would be affected by its
motion? In 1868 Sir William Huggins successfully ap-
plied Doppler's principle. If a star is coming toward us or
receding there occurs a displacement in the spectra of the
manifold light waves varying from the fundamental value
of the velocity of light, 186,000 miles a second. When a
star approaches, the light waves are crowded together;
when it recedes they are drawn apart. Now as the number
of the vibrations of the light waves and their length deter-
mine the color of light or the position of waves in the
spectrum, by microscopically comparing spectrum photo-
graphs of the same star and noting the displacement of the
Fraunhofer lines it can be determined whether that star
is approaching or receding.

Doppler's law has been explained very simply by Prof.
Edgar Larkin as follows: "In the diagram (Fig. 42)
A is a ray of light from the star S, falling on the side of
the prism P, which has the property of separating any
mixture of light into separate waves. Light from the Sun
or stars is made up of a vast number of colors, all appear-
ing between the limits red and violet. Had the pencil A

MOTIONS OF THE STARS

273

not encountered the glass, it would have passed to B. But
the prism separates the white light into colors which can
be projected on any white surface. The red is invariably-
bent out of its original direction the least, the violet most,
and will respectively pass to R and V, with every other
color between. The shorter the waves the greater their
deflection from a straight course. Red waves run 33,000
to an inch and violet 64,000. An eye at E would see all
the colors between R and V direct and at H by deflection,
if a screen is allowed to receive the light from R to V.
Solar and stellar spectra are crossed at right angles by
black Fraunhofer lines. Take any one, say F, anywhere
in the spectrum and measure its position with a microme-
ter. Then the eye, either at E or H, would see a spectrum

a-

Fig. 42 — Doppler's Principle.
The pencil of light, A., coming from the star, S-, would pass to B.
if the prism, P., were removed. The glass separates the stel-
lar light into all the colors it may contain. The red rays are
bent to R. and the violet to V., forming with the colors be-
tween, the spectrum of the star.

as outlined in the upper diagram of Fig. 42, extending like
R, 1, F, 2, T, V. Let the prism move at great speed, such as
that due to the velocity of the Earth, toward the star, or
let the star move toward the Earth. Then the line F will
move to 2, or toward the violet end. But if the Earth and
stars move in opposite directions, the line F will move to 1,
or toward the red."

After the more brilliant stars of the heavens had been
identified and their positions determined with as much
accuracy as possible with the methods of observation and

274 ASTRONOMY

instruments available to the ancient astronomer, the next
development was to compare their relative brightness. In
134 B.C., at- the time of Hipparchus, a catalogue of stars
was prepared which was said to have been suggested by
the sudden appearance of a new star in the constellation
of the Scorpion. The stars were divided into six mag-
nitudes, according to their brightness. The catalogue
of Hipparchus, containing as it did 1,080 stars, has
since proved most valuable to astronomers. Next came
Ptolemy's great 'Almagest,' published in 138 a.d., which
contained a catalogue of 1,028 stars, doubtless based on
that of Hipparchus. Ptolemy used a scale of stellar mag-
nitudes, which has continued in use to the present time.
The brightest stars of the sky, such as Sirius and Arc-
turus, were regarded as of the first magnitude, and the
faintest visible to the naked eye the sixth. To-day a small
telescope will render visible stars down to the ninth mag-
nitude, of which there are over 100,000. Ptolemy em-
ployed the first six letters of the Greek alphabet for this
purpose and then subdivided the classes he made. If a star
seemed bright for its class, he added the Greek letter fi
(Mu), standing for piei^oov (Meizon), large or bright. If
the star was faint, he added e (Epsilon), standing for
eXaacroov (Elasson), small or faint. These estimates were
carefully made. If Ptolemy's original manuscript were
at hand his magnitudes would be useful to modern astrono-
mers in determining the secular variation of the bright-
ness of the stars. But the errors in the various copies and
transcripts of the Almagest which have come down to
modern times are so great that the positions, magnitudes
and identifications of about two-thirds of the stars listed
are uncertain. Indeed, the oldest manuscript of the Al-
magest dates only to the ninth century. The Persian as-
tronomer Abd-al-rahman Al-sufi (903-986) reobserved
Ptolemy's stars in 964 a.d. and noted cases where he found
a difference. This work survived in Arabic, and trans-
lated into French by Schjellerup (1874) is available for

MOTIONS OF THE STARS 275

modern astronomers. Ulugh Begh, who flourished about
1450, also published a star catalogue based on Ptolemy, but
with careful measures. In 1580 Tycho Brahe published a
star catalogue containing the records for 1,005 stars. A
supplement carrying this to the South Pole was added by
Halley, who went to St. Helena in 1677 for the purpose
of making observations of the southern heavens. In 1690
was published a catalogue by Hevel in which several new
constellations were added and which was of interest as
containing the results of telescopic observations, so that
stars invisible to earlier astronomers could be added.

No important additions to the knowledge of the bright-
ness of the stars was made until Sir William Herschel, the
greatest of modern astronomers, brought his powerful
telescopes to bear on the heavens. He found that when
two stars were nearly equal their difference could be esti-
mated very accurately. He adopted a new system for
denoting this difference, using points of punctuation — a
period denoting equality, a comma a very small interval
and a dash a larger interval. From 1796 to 1799 he pub-
lished in the 'Philosophical Transactions' four catalogues
which covered two-thirds of the portion of the sky visible
in England. Two other catalogues of his, preserved in
manuscript, have not yet been published. It is interesting
to know that these observations of Herschel's were re-
duced at the Harvard College Observatory under the
direction of Prof. E. C. Pickering. Herschel's magni-
tudes for 2,785 stars, observed over a century ago, have an
accuracy nearly comparable with the best work of to-day.
His work stood unexcelled for nearly half a century, for
no astronomer was wise enough to see how much would
be gained merely by repeating such observations. Had
observations been thus repeated every ten years and ex-
tended to the southern stars, many valuable data as to the
constancy of the light of stars would have been obtained
and our astronomical knowledge greatly increased.

In 1844 Argelander proposed to modify Herschel's

276 ASTRONOMY

method by using numbers instead of points of punctuation
to denote the intermediate brightness between the various
magnitudes, a method known by his name. His catalogue,
the great Bonn Durchmusterung (1799-1875), contains as
many as 324,198 stars visible in the northern hemisphere.
After mere judgment with the eye, it was but natural that
some more accurate means should be employed, and vari-
ous photometers, to which we have referred elsewhere,
were eventually adopted for the purpose of gaging stellar
brightness.

In 1856 Pogson showed that the scale of magnitudes of
Ptolemy, which is still in use, could be nearly represented
by assuming the unit to be the constant ratio 2.512, which
has been adopted as the basis of the standard photometric
scale. Thus an increase of four units in number would
express the magnitude corresponding with a division of
the light by one hundred, and a sixth-magnitude star would
have but one-hundredth the brightness of one of the first
magnitude.

Photometric observations have been undertaken for
many years and are now in progress on a large scale at
Harvard University, at Potsdam and at Oxford. Various
forms of photometers are employed. Simple photo-
metric work takes into consideration only the total light
of a star in so far as it affects the eye. This light may
consist of rays of many different wave lengths. In red
stars one color predominates and in the blue another.
Hence the preferred method is to compare the light of a
given wave length (color) in different stars and then to
determine the relative intensity of the rays of different
wave lengths in different stars, or at least in stars whose
spectra are of different types.

The brightness of the stars may also be measured in a
simpler but less satisfactory method by determining the
total light in a photographic image, a method open to the
same objection as eye photometry. In other words, the
rays of different colors are combined and affect the

MOTIONS OF THE STARS 277

photographic plate differently. Consequently blue stars
appear brighter than the red. Still photographic pho-
tometry is extensively used and various proportions and
corrections are employed so that satisfactory results are
obtained.

As a result of modern methods of classification the
number of stars of the first six magnitudes visible to the
naked eye is about 5,000. These are grouped in the fol-
lowing order : First magnitude, 20 ; second magnitude, 65 ;
third magnitude, 190 ; fourth magnitude, 425 ; fifth magni-
tude, 1,100; sixth magnitude, 320. It has been estimated
that over 100,000,000 stars are visible within the range of
present visual and photographic instruments.

The first-magnitude stars number only about 20 and on
account of their conspicuous brightness serve as land-
marks in the study of the heavens. Their names, con-
stellation, magnitude and color are given in the following
table :

Star. Constellation. Magnitude. Color.

Sirius a Canis Majoris * — 1.4 Bluish white

Arcturus a Bootis 0.0. ... . Orange

Vega a Lyrae 0.2 Pale blue

Capella a Aurigae 0.2 Yellowish

Rigel a Orionis 0.3 White

Canopus a Argus 0.4 Bluish

Procyon a Canis Minoris 0.5 White

Betelgeux ft Orionis 0.9 Ruddy

a Centauri 1.0 White

Achernar a Eridani 1.0 White

Altair a Aquilae 1.0 Yellowish

Aldebaran a Tauri 1.0 Red

Antares a Scorpionis 1.1 Deep red

Pollux (5 Geminorum t.i Orange

* When a star outshines a star of the first magnitude it is no longer pos-
sible to designate its brightness by i. Hence the numerical expressions o.c,
0.3 and —1.4 in the foregoing table.

278 ASTRONOMY

Star. Constellation. Magnitude. Color.

Spica a Virginis 1.2 White

fi Centauri 1.2 White

a Crucis 1.3 Bluish white

Fomalhaut a Piscis Australis 1.3 Ruddy

Regulus a Leonis 1.4 White

Deneb a Cygni 1.4 White

In connection with stellar photometry, it has probably
occurred to the reader that photographic charts would
prove very serviceable, as the brightness of the photo-
graphed image could be used. Such indeed is the case.
Astronomers interested in stellar photometry have devoted
no little attention to the study of these charts and plates.

CHAPTER XXIII

VARIABLE AND BINARY STARS

After the various stars in the heavens were classified
according to their brightness or magnitude it became ap-
parent that there were striking and important variations
in their brilliancy. When Hipparchus compared his lists
and catalogues of the brightest stars in the sky with those
of earlier observers, he was duly convinced of the occur-
rence of changes in their position and brightness. This
was strikingly emphasized during his own lifetime, when,
as we have seen, a bright star flashed up in the constella-
tion of the Scorpion and then slowly faded away. Such
changes as this in part induced Hipparchus and other
ancient astronomers to make their star catalogues, for it
was realized by them that there were from time to
time new stars and a large number of variable stars
which, while but a small part of the host of stellar
bodies, nevertheless existed in considerable number. Un-
like other astronomical phenomena, these variations in
stars cannot always be predicted. In many instances they
obey no rules, and especially in the case of new stars, or
"novae," they blaze up in sudden glory, remaining bright
and then perchance fading swiftly away in darkness. For
convenience these variable stars have been classified into
groups, all containing prominent examples and differ-
ing considerably from one another. These classes may
be summarized as follows: (i) New stars, or "novae,"
consisting of a few stars that appear suddenly like
279

280 ASTRONOMY

the star in the constellation- Scorpion discovered by
Hipparchus; (2) variable stars of long period, fluctuating
in light by large amounts during periods of several
months; (3) stars whose variations are small and irregu-
lar; (4) variable stars of short period; and (5) the so-
called Algol variables, which are usually of full bright-
ness and at regular intervals grow faint owing to the in-
terposition of a dark companion between the star and the
Earth.

Variable stars have long been known, but only about
250 were recognised by astronomers until photography and
spectroscopy were applied to their discovery. Three re-
markable discoveries, Prof. W. H. Pickering, of Harvard
College Observatory, states, are responsible for greatly in-
creasing the number. "The first was by Mrs. Fleming at
Harvard College Observatory, who, in studying the photo-
graphs of the Henry Draper memorial, found that the
stars of the third type, in which the hydrogen lines are
bright, are variables of long period. From this property
she has discovered 128 new variables and has also shown
how they may be classified from their spectra. The differ-
ences between the first, second and third types of spectra
are not so great as those between the spectra of different
variables of long period. The second discovery is that of
Professor Bailey, who found that certain globular clusters
contain large numbers of variable stars of short period.
He has discovered 509 new variables, 396 of them in four
clusters. The third discovery, made by Professor Wolf,
of Heidelberg, that variables occur in large nebula?, has
led to his disclosure of 65 variables. By similar work Miss
Leavitt, of Harvard, has found 295 new variable. The
total number of variable stars discovered by photography
during the last fifteen years is probably five times the
entire number found visually up to the present time.
Hundreds of thousands of photometric measures will be
required to determine the light-curves, periods and laws
regulating the changes these objects undergo."

VARIABLE AND BINARY STARS 281

The discovery of novae, or new, temporary stars (the
first class mentioned), continued from the time of Hip-
parchus to the invention of the telescope. Four new, or
temporary, stars were discovered in the interval between
the catalogue of this ancient astronomer and the beginning
of the sixteenth century. The Star of Bethlehem may have
been of this character. In November, 1572, a brilliant new
star appeared suddenly in the constellation of Cassiopeia,
which was observed by Tycho Brahe during the sixteen
months of its life. During this time it rivaled Venus at its
brightest and revived Tycho's interest in astronomy, which
at the time was beginning to wane. He wrote at consider-
able length a description of the star, published in 1573, and
Kepler subsequently remarked that "if that star did noth-
ing else, at least it announced and produced a great as-
tronomer." In modern times the most striking nova was a
star which was discovered in Perseus in 1901 by the Rev.
Dr. Anderson, of Edinburgh, an amateur, who in 1892
had discovered a nova in the constellation Auriga. In the
sudden appearance of Nova Persei, suggests Arrhenius,
we evidently witnessed the magnificent termination by
collision of the independent existence of two heavenly
bodies.

Typical of the second class of variable stars which ex-
hibit marked irregularities in period and in brightness
similar to those of the new stars is Eta in Argus, which
in 1677 was classed as a star of the fourth magnitude and
in 1687 and 1751 of the second and in 1827 of the first
magnitude. Then Herschel found that it fluctuated be-
tween the first and second magnitudes. In 1837 it in-
creased rapidly in brilliancy and in 1838 so far outshone
the typical first-magnitude stars that its magnitude was
denoted by 0.2. But in the following year it declined to
the first magnitude and there remained until 1843, when it
rapidly brightened until it outshone every star except
Sirius (magnitude — 1.7). Thereafter it slowly declined
to the sixth magnitude and since 1869 has fluctuated be-

282 ASTRONOMY

tween the sixth and seventh. These observed changes
point to a great collision in 1743 and a smaller one in 1838.
The smaller collision may be compared to the fall of the
Earth into the Sun, which would develop heat sufficient to
maintain solar radiation during 100 years. From the older
observations it appears probable that the star suffered at
least one earlier collision.

Another star of this type is Mira, or Omicron Ceti,
which was the first star to be recognised as a variable,
having been discovered August 13, 1596, by David Fabri-
cius and described minutely by a Dutch astronomer, Pho-
cylides Holwarda (1618-1651), in 1639. In 1667 its period
of about eleven months was fixed by Ismael Boulliau, or
Bullialdus (1605-1694), altho it was found that its fluctua-
tions varied considerably. It was described in 1780 by
Herschel, who had observed it in 1779, when it was nearly
as bright as Aldebaran. Four days later the star was
invisible even through his telescope. The maximum
brightness of this star varies from the first to the fifth
magnitude. At the minimum it falls below the sixth mag-
nitude, becoming invisible to the naked eye, and occasion-
ally below the ninth, so that at its maximum it has 1,000
times the luminosity of its minimum appearance. The
spectrum of the star indicates that it is surrounded by
three nebulous envelopes. The innermost of the surround-
ing envelopes is uniformly distributed and the others form
a ring with two points of maximum density corresponding
with the traces of two eruptive streams. This ring revolves
in 22 months and has a linear velocity or rotational period
of 14.6 miles per second. Hence it follows that the diame-
ter of the ring is 1.45 times that of the Earth's orbit and
that the mass of the central stars is slightly less than that
of the Sun. Mira Ceti is typical of most of the variable
stars in that they are red and give continuous spectra
crossed by dark bands and bright hydrogen lines.

The third class of variable stars comprise those of ir-
regular period, which differ from the so-called "new stars"

VARIABLE AND BINARY STARS 283

in that they recur at more or less regular intervals of a'
number of years. These are, for the most part, red stars,
altho there are others that fade away and are even lost
from telescopic vision, tho once seen with the unaided
eye. In some cases they have been associated with faint
nebulosities.

Typical of the fourth class of variables is the star Beta
Lyras. Stars of this class have a short period measured by
hours and days, and their variability is considered due to
eclipses by darker companions, tho both are self-luminous.
As they have a white or yellow color, dust rings in their
neighborhood are believed to play an important part in
their phenomena, tho less so than in the rays of the red
star Mira.

As typical of the fifth class, composed of variables that
change with almost absolute regularity, Algol, or Beta
Persei, may be cited. This star's variability was first noted
by Geminiano Montanari (1632-1687) in 1669, but it was
more than a century later (in 1783) when John Goodricke
(1764-1786), a deaf-and-dumb astronomer, detected the
regularity of these changes and fixed their period at
very nearly 2 d. 20 h. 49 m. Algol at its minimum
luminosity gives about one-quarter as much light as
when brightest, and the change from the first state to the
second is effected in about ten hours. The Algol type
of stars, including about 25 variables, are white in
color and are characterized by a short period, which in
most cases is less than five days. The change in intensity
of light was first accounted for by Prof. E. C. Picker-
ing as due to a second or dark star which travels about
its primary, eclipsing it at various times. As the dark
star begins to eclipse the brighter, the light diminishes
until the time of greatest obscuration is reached, after
which the normal value is attained. Pickering's theory for
Algol, which normally is a star of the second magnitude,
was demonstrated to be true by Vogel at the Potsdam Ob-

284 ASTRONOMY

servatory in 1889. Therefore jt is two stars, or a "spec-
troscopic binary."

Galileo and subsequent observers noticed that in many
instances stars which appeared to the naked eye as single
really were double. When these stars were separated
under a high magnifying power it was found that they
varied in distance, magnitude and color. Thus, if two
stars are almost in the same line of sight, they will appear
to the observer to be very closely related, altho one of the
pair may be much nearer to him than the other and their
proximity merely accidental. Such a pair of stars is
known as a "double star," or as an "optical double," and
is to be distinguished from a pair of stars at approxi-
mately the same distance from the Earth, but so affiliated
that they revolve about a common center of gravity. In
other words, their relative positions would resemble those
of the Moon and the Earth. When thus paired the combi-
nation is termed a binary system. The discovery of binary
stars was first made by Sir William Herschel. He discov-
ered that there were changes in the relative positions of the
two stars which obviously were not connected with the
motion of the Earth, but indicated an actual circling move-
ment of the bodies themselves under a mutual attraction.
He found that there was a regular progressive change in
their motion which indicated that one of the stars was
slowly describing a regular orbit around the other. Indeed,
it seemed that gravitation had its effect beyond the solar
system, as the orbit of each of such a pair of stars was
found to be an ellipse with the common center of gravity
at the focus. That is, the stars were moving in two
ellipses which were precisely similar, except that the one
described by the smaller star was larger than the other in
inverse proportion to the star's mass. While Herschel
was the first actually to see such binary stars, yet their
existence had been deemed probable by the Rev. John
Michell, who lived a short time before the great observer.

Not only are there double stars to the number of 10,000,

VARIABLE AND BINARY STARS 285

but triple and quadruple stars and even multiple stars in a
single system. The distances between these stars gener-
ally amounts to from 30" to >4", as double stars nearer
than the latter figure can be separated by only the most
powerful telescopes. But the spectroscope enables stars
much closer together to be resolved, and in fact the impor-
tant class known as spectroscopic binaries previously re-
ferred to can be studied and their physical and optical prop-
erties determined by elaborate measurements. Thus Polaris,
which in a moderate-sized telescope appears as a double
star, including one of less than the second and one of less
than the ninth magnitude, is really quite complex, for the
brighter star revealed by the spectroscope has three stars
very close together and all in circulation about one another.
Again, in the case of binaries a spectrum with two sets of
lines is seen in the spectroscope. With the telescope the
image is single, but nothing can explain the double spec-
trum except the existence of two separate bodies. Accord-
ingly, connected pairs of stars such as these are known as
spectroscopic binaries. Often they make up a system of
double stars visible in the telescope. Such an example would
be Mizar, in the handle of the Plow, which, seen with a
small telescope, appears as a double star, one of its com-
ponents being white and the other greenish. In reality these
two stars are situated so distant from each other that from
one of them the other would appear merely as an ordinary
bright star. But the telescope shows that the brighter of
these stars is again a binary system of two huge suns,
revolving around each other in a period of about 20 days.
Powerful spectroscopes now are able not only to re-
solve what appears as one point into two, but to detect
motions of the two suns around their gravitation center.
This has been simply explained by Prof. Larkin. He states
that by the application of Doppler's principle "the times of
revolution can be observed, and when the distant suns have
a sensible parallax the distance between them can be deter-
mined in miles. With time and distance known, velocities

286 ASTRONOMY

in their orbits follow at once. Then with velocity and dis-
tance the mass of both can be computed — that is, both suns
can be 'weighed' in terms of the mass of our Sun. For it
is known from the laws of gravity and motion how much
matter is required at any given distance and velocity to
set up centrifugal tendency equal and opposite to gravita-
tion.

"The Doppler principle applied to the discovery of

Fig. 43 — A Binary Star.
In this diagram S. is the place of the more massive sun, far and
away to one side of the center of the orbit. The orbit and
four portions of the revolving sun are represented by A., C.,
P., G. The two parallel lines, CE. and GF., point toward the
earth. When the moving sun is at G., coming toward the
earth, a line in the spectrum will shift toward the violet ; and
when at G., going away from the earth, the line will move
toward the red.

binaries is shown in the diagram, where S is a sun, far and
away to one side of the center of the orbit of its compan-
ion, as shown in four positions, A, C, P, G. As seen from
the Earth when at G, the flying sun will send fewer waves
per second into the spectroscope and the Fraunhofer lines
will shift toward the red. The point A is apastron and P

VARIABLE AND BINARY STARS 287

periastron. The velocity of the revolving sun at P in its
orbit is enormous, while the light and heat of S are intol-
erable to the people on any planet revolving around the
sun P. For if either sun has a retinue of worlds like
the Earth, with inhabitants, their changes of climate are
extreme. While the flying sun is moving from A through
C and around to P, the huge sun S rapidly increases its
apparent diameter and also its light and heat. After pass-
ing C in the direction of the arrows, S must look like a
blazing globe in skies of incandescent brass. In the revo-
lution of a planet around either sun it would move in be-
tween them; there would be no night; the world and its
people would be between two white-hot suns; life would
expire. But if by chance a few of the inhabitants survive
the passage through P, they would freeze when their sun
reached the distant point A." Therefore all binaries hav-
ing great eccentricity of orbits are utterly worthless for '
support of life on their planets if they have any. All
planets are invisible from space, whence it follows that
the inhabitants of stellar spaces have not heard of the
Earth.

"Let us try Kepler's third law on a binary.

"Suppose an astrophysicist on this speck of dust, the
Earth, far away in the direction of the arrows, say 100 or
200 trillion miles — it makes no difference which, provided
light from the stars reaches the Earth in quantity sufficient
to form a spectrum whose lines can be measured — wishes
to find how much matter the two suns in the diagram con-
tain. He sets the telespectroscope on the pair, night after
night, and measures with extreme accuracy the shifting
of the lines, now toward the red and then the violet. He
does not see the stars in the instrument, but their spectra
only — tiny delicate and beautiful bands made up of a few
colors and black bands. But he watches the lines with
great care when the moving sun is at C and G and meas-
ures their displacement with the micrometer. By repeated
observations he finally learns a thing of vast import, the

288 ASTRONOMY

speed of the Sun's revolution in its orbit, and this entirely
by means of the known relation between shifting of spec-
tral lines and the speeds of the flying sun at times of ap-
proach and recession. And by direct observation he notes
the time of one revolution. He multiplies velocities per
second by the number of seconds and thus finds the cir-
cumference and radius of the orbit. He at once knows
how many times farther apart the two suns are than our
Sun and Earth. Then it becomes simple arithmetic to
apply Kepler's law and find mass."

The distribution of the stars in the heavens seemed to
the ancients to be fairly even on the whole. It was noted,
however, that there was a long, relatively thin segment of
space extending across the sky in which the stars appeared
more numerous than elsewhere, and on account of its
brilliancy this was termed the "Milky Way." Aristotle
discusses it among other astronomical phenomena, and
Greek astronomers looked upon it as a great circle of the
celestial sphere. From those days to the present no satis-
factory explanation has been offered for the existence of
such a segment with the Earth apparently at its center, nor
indeed for any of its characteristic peculiarities of aspect
and its relationship to the stellar universe. Galileo, with
his telescope, found that portions of the Milky Way con-
sisted of multitudes of faint stars clustered together. It
was recognised that the stars in the heavens were more
and more closely massed as the Milky Way was ap-
proached.

With the realization that all stars were not at the same
distance, there naturally followed speculation as regards
their arrangement and distribution. The first positive con-
tribution to the various theories and the apparent distribu-
tion of the stars in space came from Thomas Wright, of
Durham (1711-1786), published in his "Theory of the
Universe" (1750). This theory is of interest, not so
much in its original form as propounded by Wright, but
for the fact that it was taken up five years later by the

VARIABLE AND BINARY STARS 289

great philosopher, Kant, and by him developed in philo-
sophically explaining the origin of the universe. Further-
more, in the hands of Sir William Herschel it became an
important astronomical theory which received serious con-
sideration for many years. Herschel's hypothesis was that
the space occupied by the stars resembled in form a thick
disk or "grindstone," close to the central part of which the
solar system was situated. When such a disk was looked
through lengthwise more stars were seen naturally than
when it was looked through breadthwise. But at the same
time there were vacant spaces or holes in the ground-
work of the Milky Way, so that we can apparently see
through the collection of stars. These holes or clefts are
difficult to explain.

The Milky Way has been described by Prof. George
C. Comstock as "a belt approximately following a great
circle of the sky, but broad and diffuse throughout one-
half of its course, while relatively narrow and well denned
on the opposite side. The broad half of the belt is cleft
in two by a dark lane running along its axis, and in addi-
tion contains numerous rifts and holes, from which the
narrow half is relatively free. The number of stars per
unit area of the sky is a maximum in the Milky Way and
diminishes progressively on either hand, while the inverse
relation is true for the nebulae, their frequency increasing
with increasing distance from the Milky Way."

But even the most superficial observer is forced to the
conclusion that the stellar system is of limited extent and
does not extend to an infinite distance. For if stars and
suns exist through an unlimited space, their luminous radi-
ations, which do not suffer in intensity in their passage
through the ether, would reach the known universe with
little diminution and the entire heavens would always blaze
with light. That such is not the case is known from
experience and from the fact that the combined illumina-
' tion furnished by all the stars is only about one-hundredth
part of that obtained from the full Moon. The theory has

29o ASTRONOMY

been advanced that the dark holes in the Milky Way, "coal
sacks" as they have been termed, consist merely of dark
stars or extinguished suns, such as the one we have seen
occasions the eclipse of Algol. But this was disputed by
the late Professor Newcomb, who says that there is no
evidence that the light from the stars in the Milky Way,
which are apparently the most distant bodies visible from
the Earth, can be intercepted by dark bodies or dark
matter, and that the stars are seen just as they are dis-
tributed in space. Furthermore, recent photographic work
indicates that there is a limit to stellar distribution and that
there is "a darkness behind the stars" which long exposure
and powerful instruments cannot pierce.

The problem is one of striking immensity. Prof. E. C.
Pickering speaks of the distribution of the stars and the
constitution of the stellar universe as perhaps the greatest
problem in astronomy. In a recent discussion he remarks :
"No one can look at the heavens and see such clusters as
the Pleiades, Hyades and Coma Berenices without being
convinced that the distribution is not due to chance. This
view is strengthened by the clusters and doubles seen in
even a small telescope. We also see at once that the stars
must be of different sizes and that the faint stars are not
necessarily the most distant. If the number of stars were
infinite and distributed according to the laws of chance
throughout infinite and empty space, the background of
the sky would be as bright as the surface of the Sun. This
is far from being the case. While we can thus draw gen-
eral conclusions, but little definite information can be
obtained without accurate quantitative measures, and this
is one of the greatest objects of stellar photometry. If we
consider two spheres, with the Sun as common center and
one having ten times the radius of the other, the volume of
the first will be one thousand times as great as that of the
second. It will, therefore, contain a thousand times as
many stars. But the most distant stars in the first sphere ■
would be ten times as far off as those in the second sphere,

VARIABLE AND BINARY STARS 291

and accordingly if equally bright would appear to have
only one-hundredth part of the apparent brightness. Ex-
pressed in stellar magnitudes, they would be five magni-
tudes fainter. In reality the total number of stars of the
fifth magnitude and brightness is about 1,500, of the tenth
magnitude 373,000, instead of 1,500,000 as we should ex-
pect. An absorbing medium in space which would dim the
light of the more distant stars is a possible explanation,
but this hypothesis does not agree with the actual figures.
An examination of the number of adjacent stars shows that
it is far in excess of what would be expected if the stars
were distributed by chance. Of the three thousand double
stars in the "Mensurse Micrometricse," the number of stars
optically double, or of those which happen to be in line,
according to the theory of probabilities, is only about forty.
This fact should be recognised in any conclusion regarding
the motions of the fixed stars, based upon measures of
their position with regard to adjacent bright stars."

CHAPTER XXIV

NEBULAE AND STAR CLUSTERS

Nebula are masses of diffused shining gas which
are scattered through space and which undoubtedly consist
of the matter out of which stars have been and are being
formed. They differ from star clusters in that the highest
powered telescopes yet constructed are unable to resolve
them into separate component stars, yet without doubt star
clusters are evolved from nebulae and their connection is
most intimate. For the first record of a nebula we go back
to Huygens (1629-1695) and find in his "Systema Satur-
nium" not only a description but a rough drawing. The
description is of the nebula Orion and is as follows:
"There is one phenomenon among the fixed stars worthy of
mention which, so far as I know, has hitherto been noticed
by no one, and, indeed, cannot be well observed except
with large telescopes. In the sword of Orion are three
stars quite close together. In 1656, as I chanced to be
viewing the middle one of these with the telescope, instead
of a single star twelve showed themselves (a not uncom-
mon circumstance). Three of them almost touched one
another, and, with four others, shone through a nebula,
so that the space around them seemed far brighter than
the rest of the heavens, which was entirely clear, and ap-
peared quite black, the effect being that of an opening in
the sky, through which a brighter region was visible."

The work thus inaugurated by Huygens for some reason
or other did not seem to attract the attention of astrono-
292

NEBULAE AND STAR CLUSTERS 293

mers for many years, altho Lacaille, while at the Cape of
Good Hope, 1750-1754, observed and described 42 nebulae,
nebular stars and star clusters, and altho Charles Messier
(1730-1817), who devoted himself to the detection of
comets, found he was liable to mistake nebulae for comets,
and recorded in 1781 the positions of 103 of the former. In
the meantime, in 1755 Immanuel Kant, the famous philoso-
pher, advanced the theory on purely theoretical and specu-
lative grounds that a single nebula or star cluster was an
assemblage of stars, comparable in magnitude and struc-
ture with the aggregation which we now term the Milky
Way and the other separate stars which can be seen. Ac-
cording to this theory, the Sun would be but one star of a
cluster and every nebula a system of the same order. This
was known as the "island universe" theory and was first
accepted by Sir William Herschel.

In the course of his indefatigable investigation of the
stars with his large telescopes Herschel inaugurated a sys-
tematic study of the nebulae and star clusters. Altho he
found it difficult to draw a line between nebulae and star
clusters, yet he was able to state positively and correctly
that they were not identical. Herschel noted the position
of each nebula and its general appearance and marked the
positions on a star map. He published catalogues, the first
of which, prepared in 1786, contained 1,000 nebulae and
clusters, the second in 1789, of about the same extent, and
a third in 1802, comprizing 500. Herschel's observations
of nebulae enabled him to note their differences in bright-
ness and apparent structure so that he could divide them
into eight classes. In 1786 he published the following
interesting account of the varieties in form which he had
observed :

"I have seen double and treble nebulae, variously ar-
ranged ; large ones with small, seeming attendants ; narrow
but much extended, lucid nebulae or bright dashes ; some of
the shape of a fan resembling an electric brush, issuing
from a lucid point; others of the cometic shape, with a

294 ASTRONOMY

seeming nucleus in the center; or like cloudy stars, sur-
rounded with a nebulous atmosphere ; a different sort again
contain a nebulosity of the milky kind, like that wonderful
inexplicable phenomenon about & Orionis; while others
shine with a fainter mottled kind of light, which denotes
their being resolvable into stars."

Herschel's great problem was to determine the relation
between nebulae and star clusters. Often the difference
between the two was made apparent only by the use of a
telescope of sufficient power to resolve a bright glow in
the heavens into clusters of stars. But at the same time
there were bright places that still remained nebulous.
Hence Herschel wrote : "Nebulae can be selected so that
an insensible gradation shall take place from a coarse
cluster like the Pleiades down to a milky nebulosity like
that in Orion, every intermediate step being represented."
To Herschel it seemed that the power of the telescope was
the important consideration, and the gradation mentioned,
he writes, "tends to confirm the hypothesis that all are
composed of stars more or less remote."

As Herschel progressed with his investigations the views
of other astronomers, as well as those first entertained by
him, did not seem tenable. By 1791 he reached the point
of view that in certain cases at least the nebulae were
essentially different from star clusters. Referring to a
certain nebulous star, he wrote : "Cast your eye on this
cloudy star and the result will be no less decisive. . . .
Your judgment, I may venture to say, will be that the
nebulosity about the star is not of a starry nature." Her-
schel reasoned that if the phenomenon were due to an
aggregation of far-distant stars that there must be one
central star of extraordinary dimensions or that something
radically different, such as "a shining fluid of a nature
totally unknown to us," must be called upon to explain the
appearance. His observations proved that an individual
nebula was usually surrounded by a region of the sky com-
paratively free from stars, and that where clusters were

NEBULA AND STAR CLUSTERS 295

common near the Milky Way nebulae incapable of resolu-
tion were scarce, but were crowded together in parts of the
sky most remote from this region. In short, Herschel be-
lieved that nebulae and clusters were external "universes,"
and he early believed that both were objects of the same
kind at different stages of development, the result of a
"clustering power" working to convert a diffused nebula
into a brighter and more condensed body, thus indicating
the process of evolution or age.

Berry, in his 'Short History of Astronomy,' to which
we are largely indebted for this record of Herschel's work
in the nebulae, thus summarizes Herschel's last views of
this important phenomenon:

"His change of opinion in 1791 as to the nature of
nebulae led to a corresponding modification of his views of
this process of condensation. Of the star already referred
to he remarked that its nebulous envelope 'was more fit to
produce a star by its condensation than to depend upon the
star for its existence.' In 181 1 and 1814 he published a
complete theory of a possible process whereby the shining
fluid constituting a diffused nebula might gradually con-
dense— the denser portions of it being centers of attraction
— first into a denser nebula or compressed star cluster, then
into one or more nebulous stars, lastly into a single star or
group of stars. Every supposed stage in this process was
abundantly illustrated from the records of actual nebulae
and clusters which he had observed.

"In the latter paper he also for the first time recognised
that the clusters in and near the Milky Way really be-
longed to it and were not independent systems that hap-
pened to lie in the same direction as seen by us."

Herschel's observations were utilized by Laplace, who
was engaged in evolving a theory to explain the evolution
of the universe. While his nebular hypothesis in its rela-
tion to other theories and systems will be discussed more
fully in the following chapter, yet in this connection it is
desirable to explain how Laplace was able to fit the results

296 ASTRONOMY

of Herschel's observations to his theory. Laplace had
inferred that the planets and their satellites must have
been derived from some common source, and he suggested
that either they might have been condensed from a body
and be regarded as a sun with a vast atmosphere filling
the space now occupied by the solar system or that they
represented the results of condensation of a fluid mass
which now possessed a more or less condensed central
nucleus which at one time was not in existence. The
nebulae of Herschel accordingly suggested to Laplace a
suitable fluid mass from which a solar system could have
been condensed, and, furthermore, the evolution of the
fixed stars could be explained on a similar basis. This in-
genious theory of Laplace's was rather a scientific specula-
tion than an accurate conclusion founded on data he had
himself observed. As a theory, whether accepted or not,
it has proved of the most vital importance to science.

John Frederick William Herschel (1792-1871) published
a catalogue (1833) of about 2,500 nebulae, of which some
500 were new and 2,000 were his father's, a few being due
to other observers, and later reobserved about 500 known
nebulae while at the Cape of Good Hope (1833-1838), in-
cluding the nebulae surrounding the variable star Eta
Argus and the wonderful collection of nebulae, clusters
and stars known as the Nebuculae or Magellanic Clouds. In
1864 Herschel was able to present to the Royal Society a
valuable catalogue of all known nebulae and clusters,
amounting to 5,079. Later this great catalogue, which
contained a condensed description of each body, was super-
seded by Dr. Dreyer's general catologue, which was
based upon it, and contained 7,840 nebulae and clusters
known up to the end of 1887. A supplementary list subse-
quently published by the same authority contained 1,529
entries of discoveries made between 1888 and 1894. Hence
the two Herschels are responsible for more than half of
the total number of nebulae and star clusters now known to
astronomers.

Spiral Nebula in Messier 33 Trianguli.

NEBULAE AND STAR CLUSTERS 297

Sir John Herschel was of the opinion that no nebula ex-
isted that could not be resolved into a group of stars if a
sufficiently powerful telescope were employed. With the
various large reflectors in use during the first half of the
nineteenth century, gradually a limit was reached. Some
nebulas remained unresolved, which led to the conclusion
either that still more powerful instruments were required
or that they were in their nature unresolvable. Herbert
Spencer, believing that the principle of evolution must
operate universally and that the stars must be formed from
nebulae, ventured to oppose the astronomers in their belief
that only telescopic power was needed to resolve nebulae
into groups of stars. In fact, from the point of view of
the astronomer the study of nebulae had almost ceased
when a new method was introduced which served not only
to throw a vast amount of light on the question but practi-
cally to turn astronomical research into new channels.

In 1864 the first positive clue to the nature of nebulae
was gained by Sir William Huggins, when he was able to
obtain with the spectroscope a characteristic spectrum of
the planetary nebula in Draco. This discovery was inter-
estingly described by Sir William in the following account,
recorded in the 'Publications of the Tulse Hill Observa-
tory,' Vol. I :

"On the evening of August 29, 1864, I directed the spec-
troscope for the first time to a planetary nebula in Draco.
I looked into the spectroscope. No spectrum such as I had
expected ! A single bright line only ! At first I suspected
some displacement of the prism and that I was looking at a
reflection of the illuminated slit from one of its faces.
This thought was scarcely more than momentary ; then the
true interpretation flashed upon me. The light of the
nebula was monochromatic, and so, unlike any other light
I had as yet subjected to prismatic examination, could not
be extended out to form a complte spectrum. After pass-
ing through the two prisms it remained concentrated into a
single bright line, having a width corresponding to the

298 ASTRONOMY

width of the slit and occupying in the instrument a position
at that part of the spectrum "to which its light belongs in
refrangibility. A little closer looking showed two other
bright lines on the side toward the blue, all three lines
being separated by intervals relatively dark. The riddle
of the nebulae was solved. The answer, which had come to
us in the light itself, read: Not an aggregation of stars,
but a luminous gas."

This discovery marked a new era in astronomical
progress. Its importance was at once appreciated, and
the ability of the spectroscope to distinguish between
a glowing gas and a star-like mass of partially
condensed vapors solved the problem that had so seriously
concerned the elder Herschel and immediately brought the
spectroscope forward as the chief instrument that must be
employed in the larger problem of the evolution of the
stars in the universe. It was at once apparent that these
amorphous nebulae might supply the material from which
the stars were formed and that the process was one of evo-
lution and possibly mere condensation.

What the spectroscope actually has accomplished for
research on the nature of the nebulae was excellently
summed up by Sir David Gill in his presidential address
before the British Association for the Advancement of
Science at Leicester (1907). "Huggins' spectroscope," he
said, "has shown that many nebulse are not stars at all;
that many well-condensed nebulse, as well as vast patches
of nebulous light in the sky, are but inchoate masses of
luminous gas. Evidence upon evidence has accumulated to
show that such nebulse consist of the matter out of which
stars — i.e., suns — have been and are being evolved. The
different types of star spectra form such a complete and
gradual sequence (from simple spectra resembling those
of nebulse onward through types of gradually increasing
complexity) as to suggest that we have before us, written
in the cryptograms of these spectra, the complete story of
the evolution of suns from the inchoate nebula onward to

NEBULiE AND STAR CLUSTERS 299

the most active sun (like our own) and then downward to
the almost heatless and invisible ball. The period during-
which human life has existed upon our globe is probably
too short — even if our first parents had begun the work —
to afford observational proof of such a cycle of change in
any particular star ; but the fact of such evolution, with the
evidence before us, can hardly be doubted."

After the spectroscope, photography has been found a
most valuable method of nebular research, and many
discoveries have been recorded through its assistance. It
will be appreciated readily that an impression left by visual
observation in a matter of this kind would necessarily be
very vague, and, furthermore, that the record on a sensitive
plate by long exposure would show far more detail than
even the eye of the observer can perceive. Accordingly all
the great observatories of the world, particularly those
equipped with good reflecting telescopes, have been at work
on nebular photographs since about 1880, tho it was not
until 1883 that the first really excellent pictures were ob-
tained.

It was early seen that the light of nebulae was strongly
photographic, that it was really more actinic than visual.
A photograph with a long exposure showed the Merope
nebula in the Pleiades just as the best observers had drawn
it, and at the same time filled the entire group of stars with
an entangling system of nebulous matter which seemed to
bind together the different stars of the group with misty
wreaths and streams of filmy light, nearly all of which
detail is entirely beyond the keenest vision and the most
powerful telescope.

After Sir William Huggins with his spectroscope had
endeavored in vain to determine the radial movement of
nebulae, Professor James E. Keeler repeated the attempt
at Lick Observatory in 1890-91 and achieved great success.
Ten planetary nebulae showed satisfactory evidence of line-
of-sight motion, one of which, the well-known greenish
globe in Draco, was found to be moving toward the Earth

300 ASTRONOMY

at a rate of 40 miles a second, while the Orion nebula was
receding at a rate of 11 miles. - Keeler's work demonstrated
that no longer should nebular fixity be looked upon as a
fact and that nebulae have a motion which can be studied.
Again the sensitive plate had scored a triumph.

The study of nebulae has led to their classification under
various heads into spiral, planetary, ring and irregular
nebulae. The first spiral nebulae was discovered by Lord
Rosse (1800-1867) witn his great 6-foot reflecting tele-
scope, which was mounted at Parsonstown, Ireland. A
typical spiral nebula consists of a central disk-shaped
portion or nucleus, with two long, curved arms projecting
from opposite sides, giving the effect of rapid rotary move-
ment and slightly suggesting the familiar "pin-wheel" of
firework displays. The chief characteristic of the spiral
nebulae is their white color as compared with the greenish
tinge of other types. Furthermore, they are more numer-
ous than all other types combined, it having been estimated
by the late Professor Keeler, of Lick Observatory, that at
least 120,000 of these spirals were within the grasp of the
Crossley reflector of that institution, while Professor Per-
rine, at the same observatory, increases the estimate to
half a million and believes that with a more sensitive pho-
tographic plate and a long exposure over a million could be
obtained. According to their spectra, they are largely in a
solid or liquid condition, but are of a tenuous character and
very transparent, so that it is inferred that they consist of
vast swarms of incandescent solid or liquid material, sur-
rounded by gaseous material. The nebula in Andromeda
is the greatest spiral known. Its diameter is more than
500,000 times the distance from the Earth to the Sun, or
8 light-years. It is:visible to the naked eye and is second
only to that of Orion in size and splendor. It has fre-
quently been mistaken for a comet. If it were one-twenty-
millionth as condensed as the Sun it would attract the
Earth as much as the Sun does. The extreme rarity or
tenuity of nebulae is evident from the fact that there is

NEBULAE AND STAR CLUSTERS 301

no evidence that we have the slightest disturbance of the
motions of even those stars nearest to them.

Planetary nebulae consist of small, round elliptical disks
of light, appearing much like the planets. Hence their
name. In the spectra of these nebulae are found bright
lines in the green portion, which are characteristic of
no terrestrial substance. Accordingly a hypothetical gas,
"nebulum," was assumed by Sir William Huggins (1868)
to correspond with these lines. At first Lockyer in
England believed that the green line of nebulum was due to
magnesium oxide, but this was entirely disproved by
Keeler. The nuclei of planetary nebulse are often bright,
as if to indicate that condensation is taking place at their
centers. The smaller and even brighter nebulae are pos-
sibly representative of a final stage of planetary nebulae,
or else they become stars and are then known as stellar
nebulae.

A few ring nebulae are found which have an annular
shape, the center of the ring being filled with nebulous
light. Hence it may be assumed that the ring nebulae are
brighter around the circumference than at the center.
The most striking and typical ring nebula is that of Lyra.

The Orion nebula, which, as we have said, is the largest
of nebulae, is typical of irregular nebulae and is also one of
the most beautiful bodies in the heavens. It is situated in
the center of the "sword" of Orion and from the time of
Sir William Huggins has been studied extensively with
the spectroscope. The bright lines of nebulum and hydro-
gen are noted in the spectrum, which shows conclusively
that it is a mass of glowing gas. This nebula represents a
vast amount of material, and if the theory that the con-
densation of such material produces stars needs proof, here
obviously are the sources of many suns in a very rough
stage of development.

The distribution of nebulae, as Herschel noted, is quite
the reverse of that of the stars. They are less numerous
in the Milky Way, increasing in number as we go from

302 ASTRONOMY

it in any direction. A speculative explanation advanced
is that the two regions of -maximum nebulae represent
places where they have not yet condensed into stars.

Not only the spectroscopic evidence of bright lines, but
the aspect of nebulae themselves shows that they are trans-
parent through and through. This is remarkable when
taken in connection with their inconceivable size. Leaving
out the large diffused nebulae which we have mentioned,
these objects are frequently several minutes in diameter.
Of their distance we know nothing, except that they are
probably situated in the distant stellar regions. Their
parallax can be but a small fraction of a second. We shall
probably err greatly in excess if we assume that it varies
between one-hundredth and one-tenth of a second. To as-
sign this parallax is the same thing as saying that at the
distance of nebulae the dimensions of the Earth's orbit
would show a diameter which might range between one-
fiftieth and one-fifth of a second, while that of Neptune
would be more or less than one second. Great numbers of
nebulae are, therefore, thousands of times the dimensions of
the Earth's orbit, and probably most of them are thousands
of times the dimensions of the whole solar system. That
they should be completely transparent through such enor-
mous dimensions shows their extreme tenuity. Were our
solar system placed in the midst of one of them, it is prob-
able that we should not be able to find any evidence of its
existence.

Considerably less conspicuous than the constellations,
hut composed of a far greater number of stars, are various
clusters and systems which often contain several thousand
individual members. It was the telescope of Galileo that
first made possible the study of these clusters, for he saw
in the Pleiades 36 stars where the ordinary eye could dis-
tinguish but six. In other parts of the heavens, as in por-
tions of the Milky WTay, he could observe clustered together
multitudes of fine stars, and in Praesepe, in the constel-
lation of the Crab, he was able to count 40 stars. With ad-

NEBULA AND STAR CLUSTERS 303

ditional telescopic power further clusters were noted, and
Halley (1656-1742), for example, discovered a great star
cluster in Hercules, which, now known as Messier 13, later
derived its name from the catalogue of the famous French
astronomer and comet hunter, Messier (1730-1817). But
it was Herschel, with his high-power telescope and his
keen powers of observation, who was able to give the sub-
ject further research, and he became, as we have seen,
greatly interested in the distinction btween nebulae and
star clusters. Herschel found that many of the nebulae
recorded by Messier could be resolved and he immediately
pushed observation in this field.

The most recent theory of the formation of star clusters
is due to Arrhenius, who states in "Worlds in the Making"
that most of the clusters are found in the neighborhood of
the Milky Way, where also the visible stars are unusually
crowded and where almost every year some new star is
discovered. It is in this region that collisions between
stellar bodies are most likely to occur and where gaseous
nebulae would be produced. "The nebulae," Arrhenius goes
on to state, "which are produced by collisions between two
suns, are soon crossed by migrating celestial bodies, such
as meteorites or comets, which there occur in large num-
bers ; by the condensing action of these intruders they are
then transformed into star clusters. In parts of the heav-
ens where stars are relatively sparse (at a great distance
from the Milky Way) most of the nebulae observed exhibit
stellar spectra. They are nothing but star clusters so far
removed from us that the separate stars can no longer be
distinguished. That single stars and gaseous nebulae are so
rarely perceived in these regions is no doubt due to their
great distance."

About 100 star clusters are known, and they comprize
either groups of the brighter stars or densely packed gath-
erings where a large number of faint stars, as a rule be-
tween the twelfth and sixteenth magnitudes, are collected.
Of the first class the Pleiades are typical and are perhaps

304 ASTRONOMY

the best known, tho the Hyades, Coma Berenices, Praesepe
in Cancer and Orion are also" representatives of this form
of cluster. The Pleiades contain seven visible bright stars
of about the fourth magnitude and cover nearly three
square degrees, in addition to which there are a large
number of fainter stars, some of which were first seen by
Galileo. Of these 53 have been examined and their mo-
tions determined. Nearly 2,000 fainter stars are in-
cluded in the cluster. They are less numerous here than
in most parts of the sky and even in the immediate neigh-
borhood. The Pleiades, it must be remembered, are distant
267 light-years, at which distance the Sun would appear
to the Earth as a star of the ninth magnitude; and while
these perhaps appear as a cluster, they are really something
like one-hundredth as far from each other as the group is
from us, or two or three light-years. In 1859 Wilhelm
Tempel discovered a faint nebulosity near Merope, one of
the brighter stars of the group, and on November 14, 1890,
Barnard at the Lick Observatory noticed a second nebulous
companion to this star and reached the opinion that the
whole cluster contained a number of faint nebulosities.
This opinion was confirmed by photographic observations,
which showed that the entire group was embedded in a
nebulous matrix which extended over a large section of the
heavens and indicated that the stars, many of which, as
brilliant as our own Sun, cannot be seen without high-
power telescopes, are intimately related in their origin and
in their evolution. In Coma Berenices, containing seven
fifth-magnitude stars visible to the naked eye and lying
east, south or west of the zenith on a spring or summer
evening, there is an unusual grouping of lucid stars within
a small area and in addition a large number of fainter
stars. The cluster Prsesepe, in the constellation of Cancer,
when seen by the naked eye, appears as a patch of nebulous
light, but is really a condensed group of stars of which the
brightest are of the seventh magnitude or just beyond the
range of visibility. An interesting cluster is contained in

NEBULA AND STAR CLUSTERS 305

Orion, where a circle 20 degrees in diameter, comprising
the brightest stars of the constellation, was found by the
late Professor Newcomb to contain 80 stars of magnitude
6.3. Of these six are of the first or second magnitude,
leaving 74 from the third to the sixth. But this remarkable
collection of bright stars has no unusual number of faint
stars associated with it, and the conclusion reached is that
the agglomeration of the brighter stars into clusters does
not extend to the fainter stars where it is noticeable to the
eye.

Globular clusters containing a vast number of stars
closely packed together are seen in the constellation of
Hercules. It contains over 5,000 stars and is so brilliant
that the naked eye can see it as a patch of light in the
heavens, tho the telescope of course can resolve it into a
number of individual stars. In fact, 5,482 have been counted
on a photograph taken at the Lick Observatory. Other clus-
ters of this class are that of Omega Centauri in the south-
ern heavens, which shows 5,000 more stars than the average
star density of the region, and in the northern sky Canes
Venatici and Pegasus. These globular clusters cover less
space than the apparent diameter of the Moon, or less than
one-half a degree, and the stars they contain vary from
the twelfth to the sixteenth magnitudes. Their composi-
tion is a disputed point — whether the feeble appearance of
the stars is due to great suns at immense distances or
whether there is a large number of small bodies among
which matter is fainty distributed. It is probable, however,
that they are distant about 400 light-years. The clusters
themselves have vast dimensions and the stars of which
they are composed are separated by great intervals. For
example, in a cluster containing 5,000 stars the average
distance of one from another might be 30,000 times the
distance of the Earth from the Sun, so that they can move
freely without danger of collision. The law of gravitation
does not require any greater velocity for them than is pos-
sessed by the average star. A number of the individual

306 ASTRONOMY

members of these clusters are variable, but the reason for
this variability has not yet been explained.

The late Professor Newcomb says that "Perhaps the
most important problem connected with clusters is the
mutual gravitation of their component stars. Where thou-
sands of stars are condensed into a space so small, what
prevents them from all falling together into one confused
mass? Are they really doing so and will they ultimately
form a single body? These are questions which can be
satsifactorily answered only by centuries of observation.
They must, therefore, be left to the astronomers of the
future."

CHAPTER XXV

COSMOGONY AND STELLAR EVOLUTION

The striking regularity and uniformity which prevails
in the planetary system was a mystery to Newton. He
was aware that the then known planets and satellites all
move in the same direction, all nearly in the same plane —
the ecliptic — and all in almost circular orbits. As Newton
did not believe in any vortex movement, such as that sug-
gested by Descartes, to carry the celestial bodies along
with it, he could not understand this peculiar regularity,
the less so because the comets, whose orbits seemed like-
wise to be dependent upon the attraction of the Sun, fre-
quently did not at all move in the same direction as the
planets. Without any justification, Newton drew the con-
clusion that the regularity of the planetary movements
could not have any primal cause.

The first who seems to have aspired to such an explana-
tion was BurTon, the ingenious author of the "Histoire
Naturelle" (1745). From the outset he emphasized the
extraordinary improbability that the inclination of the
ecliptic to the plane of the planetary orbits would, by itself
and merely owing to chance, never exceed J1/*0 or one-
twenty-fourth of the largest possible inclination of 1800.

That point had already been accentuated by Bernoulli.
The probability that this inclination is mere chance
amounts for every single planet only to one twenty-
fourth. For the five then known planets taken alto-
gether, the probability assumed the value of 24- B, or one-
307

308 ASTRONOMY

eight-millionth. They had to consider in addition that the
satellites, so far as then known — namely, the five moons
about Saturn, four about Jupiter, the one of the Earth and
the ring of Saturn — were all moving in planes which devi-
ated little from the ecliptic. A mechanical cause had to be
found in explanation of these facts.

In order to explain the movement of the planets, Buffon
supposed that they had resulted from a collision between
the Sun and comets. Assuming that a small amount of the
Sun's mass was knocked off, he fancied that as the result
of such collisions planets and their moons were formed.
The possibilityof such an impact between a comet and the
Sun seemed to him proved by the comet of 1680, whose
orbit, Newton had calculated, passed the Sun's luminous
surface at a distance of only one-third the solar radius,
and on its return 600 years later would fall into the Sun.
The fragments of the Sun formed from such a collision
would not drop back into that body, but would fly off with
a rotary motion in the same direction. The fragments
which possessed the smallest density would attain the
greatest velocity and would therefore be hurled farthest
away from the Sun before their orbits began to curve. The
enormous heat produced by the collision would probably
render the planets liquid, but they would rapidly cool owing
to their small size, tho remaining incandescent for longer
or shorter periods. Thus to the Earth Buffon assigned a
period of 75,000 years for cooling down to its actual tem-
perature, for the Moon 16,000 years, for Jupiter 200,000
and for Saturn 131,000, while the Sun required ten times
as long again as Jupiter. In passing through the atmos-
phere of the Sun after their separation the planets would
absorb from it air and water from which seas would later
on be condensed. This atmosphere would resemble that of
the Earth, for, except that he thought the Sun was a solid
incandescent body, Buffon regarded it as otherwise re-
sembling the Earth in its essential characteristics. The
Earth he considered had ceased to be incandescent because

COSMOGONY AND STELLAR EVOLUTION 309

no air could penetrate into it to feed the internal fire. Yet
at the same time he believed that only 2 per cent, of the
terrestrial heat was due to the radiation from the Sun, 'the
remainder being the Earth's own heat.

Buffon's theory was followed by that of Thomas Wright,
whose hypothesis of the evolution of the universe as well
as the solar system, published in 1750, supplied an ap-
proximately scientific explanation of the whole sidereal
universe. According to Wright, the solar system is but
one of a vast number of such gravitating systems which
form the Milky Way. These are arranged in a great
double ring, forming a stratum or disk of stars which
rotates about an axis perpendicular to its plane.

The ideas of Wright met with the approval of Im-
manuel Kant, who in 1755 explained his theory of the evo-
lution of the solar system. According to him, cosmic
space was empty, and the planets could not be carried
through it by any vortices in the sense of Descartes. But,
provided the planets had once been set in motion, no other
impelling force was needed for them in this empty space.
Why could it not be assumed that the vortex which had
started these planets on their trajectories had once existed
and disappeared afterward ? Kant then states "that in the
beginning all the matter which is now in the Sun, planets
and in the comets must have been spread through the
space in which these bodies now circulate." The attrac-
tion of the particles was directed toward the center of this
mass of dust, where the Sun stands now. The material
particles at once began to fall toward the center of the
mass. Thus resulted movements in closed paths or cir-
cular orbits about the center. As the different bodies
collided with one another they eventually grouped them-
selves and moved in circular paths in the same direction
about their common center. Some of the bodies which
were falling toward the center would likewise assume the
same movement and would cause the Sun to rotate about
its axis in the same direction. Kant maintained that the

310 ASTRONOMY

central body would be specifically lighter than those nearer
to it. In this respect he was in error, as we see in the
case of the Earth and the Moon.

Kant advanced various explanations for such points as
the deviation of the planetary orbits from circles and their
inclination to the ecliptic, for the formation of Saturn's
rings, for the interpretation of the deluge and other bibli-
cal facts. Finally he explained the extinction of the Sun,
which, according to the view of that age, was a celestial
body in the process of burning. The end of the Sun was
destined to ensue from want of air and from accumulations
of ashes. In this process of burning the Sun had lost its
most volatile and finest particles, constituting the cloud of
dust which he assumed to be the seat of the Zodiacal
Light. Kant makes the significant observation that "the
law concerning the extinction of the Sun includes a germ
for the reunion of dispersed particles, even if the latter
should have intermingled with the chaos." This is inter-
esting as indicating that matter passed through a cycle
of evolution, being now condensed into suns and now
scattered into chaos, the idea partaking somewhat of that
advanced centuries before by Democritus, who believed
that matter was in constant motion and that the atoms
were eternal and indestructible. In short, the cosmogony
of Kant is typical of those theories which assume the
planetary system to have originated from cosmic dust or
from a collection of small meteorites. In more modern
times ideas based on this hypothesis have been advanced
by Nordenskiold and Lockyer, while Sir George H. Dar-
win has devoted much thought to the mathematical con-
sideration of such a theory.

Few philosophers or astronomers were in a better posi-
tion to frame a basis for cosmical theory than Laplace,
whose work in this field must be mentioned after that of
Kant. At the end of his "Systeme du Monde" (1796) he
advances a mechanical explanation of the evolution of the
solar system, accompanying it, however, with the explana-

COSMOGONY AND STELLAR EVOLUTION 311

tion that it had not been subjected to rigorous mathemati-
cal analysis and proof and v/as advanced in a more or less
tentative way. Despite the hesitancy with which Laplace
offered this contribution to cosmical theory, it soon se-
cured wide consideration and was found acceptable as a
fundamental basis to explain the origin and evolution of
the universe. In fact, this nebular hypothesis was, without
doubt, during the nineteenth century the guiding idea in
cosmical philosophy as well as the philosophical basis
underlying astronomy. The work itself has been likened
very properly to that of Charles Darwin.

Laplace starts with the assumption of a glowing mass
of gas which from the very first was in vortex motion
from right to left (as viewed from the north) about an
axis passing through its center of gravity. "In its primi-
tive assumed condition the Sun resembled those nebulae
which are shown by the telescope to be composed of a
more or less brilliant nucleus surrounded by a nebulosity
which is condensing upon the nucleus, transforming it into
a star." The solar mass cannot have extended out indefi-
nitely. Its limits were the points at which the centrifugal
force due to its motion of rotation were balanced by the
gravitation of its action. On cooling the nebulae would
contract, and in this contraction of the glowing mass of
gas a gaseous disk would be split off when the centrifugal
force of the rotating mass would be equal to the inwardly
directed force of gravitation, so that the solar nebulae were
divided into rings of glowing gases which would rotate
as a whole and cool down in a solid or liquid ring.

Laplace says: "It may be conceived that the endless
variety in the temperature and density of the various parts
of this great mass produced eccentricity of their orbits and
deviation of their motion from the plane of its equator."
Laplace mentions the presence of comets, which he states
are strangers to the planetary system, as colliding with the
planets during their process of formation and causing
them to deviate, while other comets entering the solar

312 ASTRONOMY

system near the condensation of gaseous matter, so nearly
completed, became retarded in their motion and were in-
corporated into the solar system.

His conclusion was as follows : "Whatever be the
masses of the planets, it is only owing to the circumstance
that they all move in the same direction and in almost
circular orbits, at small inclinations to one another, that
the secular variations in their orbits are periodical and
within narrow limits, and that the system therefore oscil-
lates about a mean condition, from which it never deviates
by more than an insignificant amount."

While Laplace's theory was open to criticism and objec-
tion at numerous points, yet it played a useful and
remarkable part in the development of science. The New-
tonian doctrine of the wonderful stability of the solar sys-
tem was more firmly established and the permanence of
the planetary system was guaranteed. While it was con-
fined only to the solar system, yet the idea of the nebular
hypothesis was readily applicable to the universe itself
and formed what long was considered a satisfactory means
of accounting for its origin and evolution.

In its mechanical aspect Laplace's idea of the segrega-
tion of small dust particles during the cooling of nebulae
and their aggregation by the condensation of gases into a
ring or a single big mass was found physically impossible
by later astronomers, notable among whom were Stock-
well and Newcomb. There would result, on this hypothe-
sis, a collection of small meteorites such as circulated in
the Saturn rings, and the transformation of the Neptune
belt into a planet would have required no less than 120
million years. The retrograde movement of the moons
of Uranus and Neptune and also that of the most remote
moons of Saturn and Jupiter formed another serious ob-
jection to the thesis of Laplace.

The first important cosmical theory seriously to modify
the nebular hypothesis of Laplace was that first presented
on November 17, 1887, by Sir Norman Lockyer. This

COSMOGONY AND STELLAR EVOLUTION 315

was known as the meteoritic hypothesis and in its more
complete form was published in 1890. The fundamental
principle involved in this theory was that "all self-luminous
bodies in celestial space are composed either of swarms of
meteorites or of masses of meteoritic vapor produced by
heat." This theory was the result of spectroscopic studies.
Lockyer claimed that the original nebulae were composed
of meteoric material or cosmical dust rather than gases.
In other words, nebulae were vast swarms of meteors and
their light resulted from continual collisions between
constituent particles. Such a collision might take place
between two stars, shattering both and producing a vapor
or vapor combined with meteoric fragments from which
other stars might be derived. Thus a dark star might be
transformed into a bright and glowing star and pass
through successive changes until it was dissipated by some
long-delayed collision. Lockyer assumed that the meteor-
ites could be regarded as analogous to wandering mole-
cules of gas moving indiscriminately in all directions and
at widely different velocities. He believed that the heat
produced by the collision already referred to would vola-
tize certain constituents of the meteorites and render them
luminous. These luminous materials would supply a spec-
trum, and the discussion of the properties of the spectra
of nebula was an important consideration in Lockyer's
work. In some respects this theory has been found unten-
able. The meteoritic hypothesis was never considered seri-
ously as supplanting the fundamental ideas involved in
Laplace's great generalization.

Sir George H. Darwin, whose work on the tides has had
considerable bearing on cosmical theory, also discussed the
physical and mathematical properties of the meteoritic
swarm, and demonstrated that its behavior closely resem-
bled that of a gas and that the meteorites would move
about, colliding with one another in much the same fash-
ion as the molecules of gas according to the kinetic theory.
Darwin further demonstrated that when a mass was widely

314 ASTRONOMY

extended it would revolve as a solid. But such ideas as
those of Lockyer and Darwin involved modifications of
the nebular hypothesis rather than an overthrow.

At the very end of the nineteenth century F. R. Moulton
and T. C. Chamberlin, of the University of Chicago, de-
nied some of the fundamental ideas involved in the nebular
hypothesis. Not only was their work destructive in that
they advanced phenomena which the theory of Laplace
could not explain, but which were in fact controverted di-
rectly by observation, but they constructed a cosmical the-
ory in which these elements were taken into consideration
and in which, in large part, the conditions were met. The
chief objections to the theory of Laplace, as stated by
Professor Moulton, are as follows:

"i. The considerable mutual inclinations of the planes of
the planetary orbits and the inclination of the plane of the
Sun's equator to the general plane of the system are not
to be expected on the basis of the ring theory.

"2. The eccentricities of the planetary orbits are not to
be expected on the basis of the ring theory.

"3. The orbits of the planetoids contradict the ring
theory.

"4. The rapid revolution of Phobos and of the particles
of the inner ring of Saturn cannot be satisfactorily ex-
plained.

"5. The presence of light elements in the Earth is not to
be expected.

"6. A series of rings could not have been left off.

"7. A ring could not have been condensed into a planet.

"8. The moment of momentum of the present system is
less than 1/200 of that of the supposed initial nebula.

"9. The retrograde revolutions of the ninth satellite of
Saturn and (probably) of the seventh satellite of Jupiter
flatly contradict the theory."

Of these objections one of the most important is the
question of the leaving off of the rings at certain intervals
during the contraction of the nebulae. On this point Pro-

COSMOGONY AND STELLAR EVOLUTION 315

fessor Moulton writes in his ''Introduction to Astronomy" :
"It is easy to overlook the fact that the postulated nebula
must have been excessively rare. According to the hy-
potheses made, it must have been denser at its center than
near its periphery. But if we suppose that it was homo-
geneous and that it reached out to Neptune's orbit, we find
that its density was only 1/25mmoo that of air at the sea level.
Neptune's ring could not have been so dense as this, which
is many times rarer than the best vacuum yet produced in
our laboratories. Now a ring of such rarity would have
had no cohesion and would not have separated except
particle by particle. When the process was once started
it seems that it should have been continuous instead of in-
termittent as the theory supposes. The theory postulates
that when a ring was left behind, the nebula was made
stable for a long period of contraction. Roche has at-
tempted to show that rings of considerable dimensions
would be abandoned at certain intervals, but his work on
this point is far from conclusive. Every other writer on
the subject has keenly felt this difficulty. Thus Faye, in
his modification of the Laplacian theory, supposed that the
whole nebula broke up into rings simultaneously.

"We may assume for the sake of argument," he says,
"that rings are abandoned and inquire whether they will
unite into planets or not. The matter of the ring would be
very widely spread out and mutual gravitation of its parts
would be very feeble. The appropriate investigation shows
that the tidal forces coming from the interior mass would
more strongly tend to scatter the material than its gravita-
tion would to gather it together into a plane. Consequently
a ring could not even start to condense into a planet. It
would be something like a comet which becomes utterly
dissipated by tidal forces.

"To give every possible advantage to the ring theory,
we may assume that all the matter has been gathered into
a planet except a ring of very small particles, and then ask
ourselves whether this minute remainder will be brought

316 ASTRONOMY

to the planet. Investigation shows a strong probability
that only that part of the ring which is within 6o° of the
planet could be brought on to it in any time however long.
That is, if we assume that the process of formation of a
planet out of a ring is almost finished, we find that it can-
not complete itself. This shows the strong improbability
that the assumed stage could ever have been reached by
condensation from a more uniform ring."

Then it must be considered that the ninth satellite of
Saturn and the seventh satellite of Jupiter revolve in a
direction contrary to the other revolutions and rotations
of the solar system, which would be an impossibility if the
Laplacian doctrine were true. Finally, notwithstanding a
careful and thoro study of nebulae, developed by photogra-
phy to a point of great refinement, there has been found
no trace of a nebula which is in the course of breaking up
into concentric rings. The spiral nebulae, such as the
theory propounded by Professors Chamberlin and Moulton
required, are of the normal type of nebulae, and it is this
fact, brought out in the brilliant discoveries of Professor
Keeler, that has largely produced the new theory which
has been termed by its originators the "planetesimal the-
ory."

This planetesimal hypothesis to-day is attracting wide-
spread and favorable attention from many astronomers and
geologists. While it explains the formation of the solar
system and other systems, it does not eliminate the question
of primitive matter, which doubtless must have been nebu-
lous. While spiral nebulae may have been formed by the
collision or interaction of large suns, yet these in turn
demand for their origin a similar occurrence, so that one
is led rather strongly to the belief that the formation of
systems of suns and worlds may be a continuous process.

The planetesimal hypothesis has been summarized in the
following simple statement by Prof. James F. Kemp in the
course of a lecture in which its application to the origin
and development of the world is discussed. He writes:

COSMOGONY AND STELLAR EVOLUTION 317

"Instead of a highly heated and subsequently cooled and
solidified gaseous original, minute particles of matter,
which may have been molecules, are believed to have
moved in orbits around a common center in a manner
analogous to the solar system of to-day. In their evolu-
tion they became aggregated into larger bodies, such as the
planets and the Earth, continuing in groups the motions
and relations which they possessed when individuals. As
the mass gradually increased the pressure of the outer
layers consolidated the core and by the mechanical changes
involved produced those internal stores of heat with which
we are familiar in volcanoes and in deep borings and mines.
Vapors or liquids in the original cold particles are believed
to have been gradually squeezed out by this pressure. The
little particles are called planetesimals or diminutive
planets.

"Like all attempts to formulate primeval conditions, the
data of this new conception are partly matters of observa-
tion, partly assumptions. Speculation enters in a very
large degree, and, as in the case of various and widely dif-
fering estimates of the age of the Earth based on assumed
rates of cooling, once the data were provided, mathemati-
cal reasoning goes to a conclusion with unerring accuracy.
But the correctness of the solution turns on the reliability
of the original data, and where these are so largely as-
sumptive the conclusions are from time to time subject to
change. Yet we must have a starting point, and the strik-
ing contrasts of the older and later views cannot but
impress every one who reflects upon them. The former
postulates a highly heated original, the latter a cold one.
The one begins with gaseous matter, the other with solid.
The one draws upon an original but diminishing store of
heat, the other develops heat continuously by mechanical
processes. In many ways the two are diametrically op-
posed ; yet some have raised the question whether, in order
to obtain a swarm of separate cold particles, we must not
in our thought go still farther back to a gaseous or nebu-

318 ASTRONOMY

lous source, and it is not clear that we have yet escaped
the necessity of at least the essential features of the
nebular hypothesis."

Whatever theory is studied or adopted, it is clear that
the building of suns and the building of worlds is a process
of evolution in which the original matter must undergo
transformation. The process may be continuous and may
extend through infinite time. The collision of suns may
have produced nebulae and these nebulae in turn may grad-
ually develop themselves into suns again. It seems reason-
ably certain that nebulae are the stuff from which the stars
are made. Of the many types of nebulae the spiral forms
are considered the more primitive. There is indeed evi-
dence to show that even such apparently irregular nebulae
as that in Orion still preserve traces of original spiral
formation.

Professor Dewar has experimentally proved that, at the
low temperature prevailing in the outer regions of a
nebula, it is one of the properties of a dust particle to at-
tract and condense on its surface whatever gas may be
within its immediate vicinity. It may be safely assumed,
therefore, that each particle of dust hurled out by the ex-
plosion of two colliding suns bears its charge of gas. That
these gas-laden dust particles should collide with one an-
other would follow from their motion and their excessive
number, and that these particles should be cemented to-
gether by the film of liquid gases on their surfaces would
in turn follow from the fact of their collision. Thus, in
the course of centuries, large grains or aggregations of
matter constituting meteorites are formed, which, by gravi-
tational attraction, gather about them part of the remain-
ing dust and gas. Stars, comets and meteorites from
other systems will also wander into the nebula, attracting
the gas and dust of the nebula. Evidence of this draining
of a nebula by an immigrant star is to be found in many a
constellation, notably in the Swan, where a distinct lane
has been plowed through a nebula by an errant star. By

COSMOGONY AND STELLAR EVOLUTION 319

this upbuilding of meteorites from colliding gas-laden dust
particles and the concentration of nebular matter by bodies
which have strayed into the nebula, the entire mass of gas
and dust is converted in the lapse of ages into clusters of
stars, slowly revolving about the central Sun in periods of
thousands of years. Such star clusters are familiar objects
in the heavens. They may be found in the Pleiades, in
Pegasus and in other constellations.

Each star in a cluster may be regarded as either the
center of a new solar system or as a new-born world — not
a rock-bound, sea-swept world as yet, such as the Earth,
but a glowing chaotic mass, an embryo which will cool
and shrink as it cools and which will eventually chill into
a solid sphere. The central Sun, because of its greater
size, will blaze on for centuries after the clusters about it
have congealed into planets. Because the shrinking and
cooling are not of a year's nor even of a century's dura-
tion, but are extended over millions of years, it is possible
to study the various stages of the process in stars that
have been formed from nebulae at widely different epochs.
The method of investigation bears some relation to the
system devised by the evolutionists in tracing the develop-
ment of life on this Earth. With the aid of fossils dug out
of the Earth the paleontologist has ingeniously bound the
living present to the dead past with the chain of evolution,
and shown very convincingly how the creatures that now
roam the Earth are inevitably linked with the extinct ani-
mals of prehistoric time. An analogous system of stellar
classification, formulated by Prof. W. W. Campbell, throws
not a little light on the ancestry of the myriad of stars
whose rays pierce the heavens. Such a search indubitably
points to nebulae as the stuff of which worlds are fashioned.

In estimating the period which probably elapses before
a star in a cluster will shrink into an incrusted world
the color of the star's light plays no small part. Red-hot
iron is not nearly so hot as white-hot iron. The color of
the molten metal gives the iron-founder a clue to its tern-

320 ASTRONOMY

perature. Similarly a star's temperature may be gaged
by its tint. Since temperature, and therefore color, alter
with age, it is possible to state in a rough way that red
stars are the oldest of luminous bodies and that the orange,
yellow and white stars follow in respective antiquity.
Youngest and hottest of all are the bluish-white stars.

The spectroscope draws nicer distinctions than those
which can be drawn by color alone. Professor Campbell
has discovered from his study of stellar spectra that when
the spectrum of a star is rich in the bright lines of the
gases hydrogen and helium it is certain that the star in
question is young. Indeed, the presence of nebulous
masses about many stars in this stage of their evolution
proves how true is Professor Campbell's conception. In
the constellation of Orion, for example, is a nebula in
which stars are plunged. The spectra of the nebula and of
the stars exhibit the same lines. Can there be any doubt
that the stars were formed from the nebula?

In passing from the white stage of infancy the star
shrinks more and more. The result is a change in the
spectrum. The metals calcium and iron appear in their
characteristic lines. Vega and Sirius are stars of this
type. As the star ages the hydrogen lines thin out and the
lines of the metals become stronger and more numerous.
When a stage is reached corresponding with that of our
own Sun, a yellow stage, which may be considered the
very summit of stellar life, some twenty thousand metallic
lines appear. Then comes a gradual cooling. As it
changes in hue from yellow to red, more complex chemical
combinations are found in the star, and carbon makes its
appearance. Then follows a stage . represented by the
planet Jupiter, still gaseous, still hot, but no longer mark-
edly luminous without the aid of the Sun's borrowed light.
A further step brings with it external solidification, the
formation of a crust enclosing a hot interior, at which
point the Earth has arrived. The last and most pathetic
period is represented by the Moon — frozen, desolate, dead.

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