MODERN ASTRONOMY
. H. TURNER F.HS
/
MODERN ASTRONOMY
MODERN
ASTRONOMY
By
HERBERT HALL TURNER F.R.S
PRESS NOTICES OF THE FIRST EDITION
" This highly interesting work."
Athcnteum.
11 Prof. Turner's wholly admirable and
concise little book." Spectator.
"A valuable book of a popular kind."-
Da'ily Chronicle.
" His book is one which should interest
all who have even the most moderate
knowledge of astronomy. It is clearly and
brightly written, and is illustrated by a
number of good photographs and dia-
grams." Scotsman.
MODERN ASTRONOMY
BEING SOME ACCOUNT OF THE
REVOLUTION OF THE LAST
QUARTER OF A CENTURY
B 7
HERBERT HALL TURNER F.R.S
SAVILIAN PROFESSOR OF ASTRONOMY AND
FELLOW OF NEW COLLEGE IN THE
UNIVERSITY OF OXFORD
POPULAR EDITION
LONDON
CONSTABLE & COMPANY LTD
10 ORANGE STREET W.C
Published January 1901
Reprinted April 1902
Cheaper Reissue 1909
TO
ANDREW AINSLIE COMMON
AN EFFICIENT VOLUNTEER OF THE
ASTRONOMICAL ARMY
THIS BOOK IS AFFECTIONATELY INSCRIBED
BY
A COMRADE
IN THE EEGULARS
486703
PREFACE
IN the following pages an attempt is made to
show how powerfully Astronomy has been
affected by the scientific events of the last
quarter of a century, and .especially by the
invention of the photographic dry-plate. So
great are the changes in method which either
have actually been made, or are rendered
immediately possible, that the word Revolu-
tion is used in referring to them ; and though
so strong a word may cause surprise even to
astronomers, who might be expected to be
fully conscious of the magnitude of such
changes, it must be remembered that their
attention has been much occupied by the
additional work involved ; they may easily
have been swept along a considerable distance
in twenty-five years without realizing how
far they have come.
My object is accordingly rather to point
out the nature and magnitude of the changes
than to give a complete account of them. I
ix
PEEFACE
would represent myself as conducting a party
of visitors over an establishment where large
additions and improvements have recently
been made ; not stopping to examine every-
thing, and perhaps dwelling unduly over
things with which I am personally most
familiar.
The book owes its origin to three lectures
given at the Royal Institution in February,
1900 ; but what was then said has been con-
siderably expanded and added to.
I am indebted for permission to reproduce
illustrations to the Eoyal Astronomical Society,
the Astronomer Eoyal, Sir Eobert Ball, Prof.
E. E. Barnard, Sir David Gill, Mr. McClean,
Prof. E. C. Pickering, Prof. E. A. Sampson,
and Dr. Max Wolf.
H. H. TUBNEB.
UNIVERSITY OBSERVATORY,
OXFORD, December 20, 1900.
CONTENTS
PAGES
PREFACE . . . .-'-_. . . ix. x
Section I
MODERN INSTRUMENTS
An Astronomical Revolution Previous to 1875
Lunar Theory Double Stars Birth of
Astrophysics Routine Since 1875 Illus-
tration from the Royal Observatory, Green-
wich The Reconstruction by Airy, 1835-1859
Recent Reconstruction The Transit-Circle
Its Uses The Altazimuth Its Discom-
forts The New Instrument Importance of
Observations of the Moon's Place Longitude
at Sea Foundation of Royal Observatory
The Large Equatorials at Greenwich
Domes The Thompson Telescope The
Photographic Instruments The Spectre-
scopes The New Buildings These changes
typical of changes elsewhere The Almu-
cantar The Durham Almucantar Photo-
meters Equalization Extinction Largo
Refracting Telescopes Reflecting Telescopes
The Pulkowa Telescope The Lick Tele-
xi
CONTENTS
PAGES
scope The Yerkes Telescope The Bruce
Telescope The McClean Telescope Rising
Floors The Gelatine Dry-plate The
Camera of the Astronomer -Relative advan-
tages of Refractors, Reflectors and Doublets
How Photographs are taken Clock-work
for following the Stars The Coelostat
Measuring Plates The Telescope-Micro-
meter Photographic Micrometers The
Reseau Rectangular Co-ordinates Photo-
graphic Photometers Ahney's Sector The
Spectroscope The Objective-Prism Giving
width to the Spectrum The Slit Spectro-
scope Gratings The Spectroheliograph
Concluding Remarks 3-92
Section II
MODERN METHODS
Transits of Venus Sun's Distance from Obser-
vations of Mars Parallax Gill's Expedition
to Ascension The Diurnal Method Defect
of Mars Observations Parallax from Obser-
vations of Minor Planets Eros Heliometer
Observations of the Planets generally The
Heliometer Comparison with Transit-Circle
New Planetary Tables Photometric Ob-
servations of Jupiter's Satellites Measures
of Saturn's Satellites Changes of Structure
in large Telescopes The Equatorial Coude
The Sheepshanks' Telescope at Cambridge
The Telescope for Amateurs Photography
xii
CONTENTS
PAGES
Pictures of the Moon Pictures of the Sun
Comets and Nebulae Planets Discovery
of Minor Planets Discovery of Comets
Photographs of Meteors Eclipses Relief
of the Observer Automatic Methods
Star-Charting Former Distrust of Photo-
graphs The Astrographic Conference of 1887
Scale of Chart Measures of Star-places on
Photographs Measures of Lunar Photo-
graphs Co-operation Frequent Charting of
the Sky at Harvard Discovery of Eros
The Librarian Policy Kapid Examination
of Plates The Film to Film Device
Paucity of Peculiar Stars Photography in
Meridian Astronomy Longitudes by Photo-
graphy Photographic Transit Circles The
Photochronograph Variations of Personal
Equation Photographic Determination
Star Magnitudes Images out of focus
Motions in the Line of Sight Confirmation
from the Sun Algol Saturn Spectroscopic
Binaries Concluding Remarks . . . 95-194
Section III
MODERN RESULTS
Variation of Latitude Euler's ten-months' period
Kiistner's Observations Chandler's four-
teen-months' period Physical Explanation
Amount of Motion The Sun's Distance
Habitability of the Planets Canals in Mars
"La Lune a un Metre" Limitation of
Magnifying Power Changes in Moon What
xiii
CONTENTS
PAGES
can be seen on Mars Real Work with large
Telescopes Discovery of Satellites of Mars
Fifth Satellite of Jupiter Capella as a
Binary Extended Observations of Comets
Eesults due to Photography Stars which
cannot be seen New Asteroids New
Satellite of Saturn New Comets Comets'
Tails Forms of Nebulas The Nebular
Hypothesis confirmed Spiral Nebulae Dis-
tribution of Nebulae New Nebulas Diffused
Nebulas Variables in Star Clusters Tenny-
son's Astronomy The Spectroscope Classi-
fication of the Stars The Harvard Survey
McClean's Survey Temperature of the Stars
Discovery of Oxygen in the Stars Dis-
covery of Helium on Earth Distribution of
Wolf-Eayet Stars Distance of Nebulas 197-254
Section IV
MODERN MATHEMATICAL
ASTRONOMY
Planetary Theory Periodic Disturbances
Secular Disturbances Illustration G. W.
Hill's New Methods Variational Orbit-
Periodic Orbits The Gegenschein Forms
of Planets History of Earth and Moon
Effect of Tides Alteration of Day and
Month Separation of Moon from Earth 257-277
XIV
LIST OF ILLUSTRATIONS
The Royal Observatory, Greenwich, Flamsteed's
Observatory, 1676 35
The Eoyal Observatory, Greenwich, The New
Physical Observatory, 1899 .... 35
The Durham Almucantar in the Workshop . . 39
Apertures of the " Largest Telescope in the
World," at successive dates .... 45
Setting up the McClean Telescope at the Cape of
Good Hope 58
An Astronomer's Long Camera (40 feet) . . 61
The Coelostat 70
An Ordinary Photograph of the Sun, taken at
Greenwich 89*
The Sun, as shown by the Spectroheliograph
(Hale) .89
Orbit of Eros 107
A Photograph of Eros, taken at the University
Observatory, Oxford, on October 12th, 1900 . 109-
Principle of the Heliometer 113
The Sheepshanks Telescope at Cambridge (outside
view) 129
The Sheepshanks Telescope at Cambridge (inside
the Observing Tower) ..... 129-
Russell's Drawing of the Moon, 1795 . . . 133-
The Moon as Photographed at Paris, 1895 . . 133
Discovery of the Planet Svea .... 142
Two Drawings of the same Corona (1878) . . 148
Two Photographs of the same Corona (1893) . 149
Observed Motion of the North Pole, 1890-98 . 203-
xv
LIST OF ILLUSTRATIONS
PAGES
Four Drawings of Mars, by Barnard . . .214
The Andromeda Nebula, Trouvelot's Drawing . 228
The Andromeda Nebula, Eoberts's Photograph . 228
De la Rue's Drawing of Saturn . . . . 229
The Spiral Nebula in Canes Venatici . . .. 229
Diffused Nebula near the Pleiades ... . . 236
The Cluster eo Centauri . . . . . .238
Light Curves of Variables in M.5 . . ^ . 240
Distribution of the Wolf-Rayet Stars . . , 251
Some Possible "Periodic Orbits" . . 267
xvi
Section I
MODERN INSTRUMENTS
Section I
MODERN INSTRUMENTS
DURING the last quarter of a century there An As -
tronomical
has been a revolution in almost all depart- Revolution
ments of Astronomy, theoretical and practical.
Bsfore 1875 (the date must not be regarded
too precisely), there was a vague feeling that
the methods of astronomical work had reached
something like finality : since that time there
is scarcely one of them that has not been con-
siderably altered, or is not on the point of
alteration ; and entirely new departures have
been taken. Such a sudden development in
a science which had apparently reached a
" sticking- place " justifies a brief review of
the situation.
Two or three quotations will first be given Previous
to 1875
to show that the feeling above referred to
(that we had almost come to the end of our
3
MODERN ASTRONOMY
astronomical resources) really existed. We
have had such good cause to think differently
of late years that this state of mind has been
forgotten probably many would deny that
it ever existed. But it was so real that it
found its way into print, and so can be re-
called. Take first the words of a President
of the Royal Astronomical Society in 1887.
Lunar " The belief has been prevalent," he says,
" that the mathematical portion of the treat-
ment of lunar theory has been worked out,
and that there was no scope for the display
of mathematical skill, or the employment of
modern mathematical methods." 1 These are
the deliberate words of Dr. Glaisher in present-
ing the gold medal of the Society to Dr. Gr. "W.
Hill, who had swept away this misconception
by his brilliant investigations, so that the
President went on to characterize his work
as the "dawn of a new day in the history of
the lunar problem."
Double The same belief, that the field had been
Stars
worked out, was prevalent in practical
astronomy ; and an excellent illustration of
the fact is given by that fine observer of
1 Mon. Sot. R.A.S. vol. xlvii. p. 217.
4
MODERN INSTRUMENTS
double stars, Mr. S. W. Burnham. In his
recently published catalogue 1 he writes :
"For many years prior to 1870 it seems to
have been practically accepted that the field
for the discovery of new pairs had been sub-
stantially worked out by the Herschels and
the Struves. and that so little had been over-
looked by these eminent pioneers in this work
that there was little chance for later observers
to make many important additions. . . .
The late Rev. T. W. Webb, author of Celestial
Objects for Common Telescopes, one of the most
eminent English amateur astronomers, in a
letter written to me in 1873, after the publica-
tion of my first three catalogues, said : ' It
will hardly be possible for you to go on for
any great length of time as you have begun,
because the number of such objects is not
interminable, and every fresh discovery is
one less to be made ; still, what you have
already done is so much more than any man
now living has accomplished that your high
position as an observer is fully secured/
Since that time more than 1,000 new double
1 Vol. I. of the Yerkes Observatory publications,
p. xiii. of the " Introduction."
5
MODEEN ASTEONOMY
stars have been added to my own catalogues, and
the prospect of future discoveries is as promising
and encouraging as when the first star was found
with the six-inch telescope"
The italics for the last sentence are mine ;
it seems well to emphasize this opinion of so
capable an observer and discoverer. It may
further be remarked that Mr. Burnham has
demonstrated the vast possibilities for future
discoveries in spite of a considerable limitation
which he has himself imposed on the field of
work : nine-tenths of what were called double
stars he rejects as irrelevant, because the com-
ponents are too far apart to be physically con-
nected. If he had adopted the standard of
the Herschels and Struves he could have
' ' increased the number of pairs to hundreds
of thousands by sweeping with a very
moderate aperture." This limitation greatly
enhances the value of his discoveries and
his testimony : the field of work was by no
means thoroughly explored a beginning had
in reality scarcely been made; but the
astronomers of the middle of the century
were content to assume that their predecessors
had done nearly all that could be done.
6
MODEEN INSTRUMENTS
Take again the words of Sir William Hug-
gins in a recent article 1 in which he gives
an account of the birth of " The New of B ^ t h ro .
Astronomy." He tells us first how, about physics
t'he middle of the century, he decided to devote
himself to observational astronomy, " after
a little hesitation, for I was strongly under
the spell of the rapid discoveries, then taking
place, in microscopical research in connection
with physiology." He accordingly built him-
self an observatory, obtained what was at the
time a very fine telescope, and did some ex-
cellent work. But then he tells us plainly
(the italics are mine) :
" I soon became a little dissatisfied with the
routine character of ordinary astronomical work,
and in a vague way sought about in my mind
for the possibility of research upon the heavens
in a new direction or by- new methods. It
was just at this time, when a vague longing
after newer methods of observation for attack-
ing many of the problems of the heavenly
bodies filled my mind, that the news reached
me of Kirchhoff s great discovery of the true
nature and the chemical constitution of the
1 Nineteenth Century, June, 1897.
7
MODERN ASTEONOMY
sun from his interpretation of the Frauenhofer
lines. The news was to me like the coming
upon a spring of water in a dry and thirsty
land. Here at last presented itself the very
order of work for which, in an indefinite way,
I was looking namely, to extend his novel
methods of research upon the Sun to the other
heavenly bodies."
How Sir William Hugghis took advantage of
the opportunity is well known. We must date
the birth of a new science, which is now called
Astrophysics, from this time about 1860.
This is a good deal earlier than our date 1875 ;
but we need not, therefore, abandon the latter
as a reference epoch ; for it was not until about
that time that the invention of the dry-plate
gave the spectroscope entirely new powers. 1 In-
deed, without disparaging the importance of
what was done with the spectroscope before that
time, we may remark that some eminent as-
tronomers did not accept it as a serious and
1 Sir William Huggins in the article quoted, writes :
" The great and notable advances in astronomical
method and discoveries by means of photography since
1875, are due almost entirely to the great advantages
which the gelatine dry-plate possesses for use in the
observatory, over the process of Daguerre, and even
over that of wet collodion."
8
MODERN INSTRUMENTS
permanent addition to astronomical equipment.
" When the novel and entertaining observa-
tions with the spectroscope have received their
natural abatement," said one of them, a little
jealous for the welfare of older work, " and
have been assigned their proper place, it is to
be hoped that some of the powerful telescopes
recently constructed may be devoted to the
observations of satellites. 1 This was in 1878,
and the words are not quoted as an instance
of individual error of judgment, but as a
reflection of the spirit of the times in the
frank utterance of an eminent man. The
important lines of work in astronomy had
come to be considered as settled, and any in-
terference with them was regarded with a
little impatience.
The routine work which wearied Sir William Routine
Huggins had become very strong ; it had,
perhaps, gained in strength, rather than lost, in
the years between 1860 and 1875. The transit-
circle had come to be regarded as the instru-
ment of the professional astronomer, who
could not do better than observe with it the
positions and motions of the planets and fixed
1 Mon. Not. K.A.S. vol. xxxix. p. 308.
9
MODERN ASTRONOMY
stars, on established lines. He would supple-
ment such work with the aid of the equatorial
for double stars and faint objects ; but the equa-
torial was regarded rather as the instrument
for amateurs, who should make drawings of
planets, comets and nebulae. The size of a re-
fracting telescope was supposed to have nearly
reached its limit at about 18 or 20 inches.
Something had already been done in pho-
tography, and some observations of great
value made with the spectroscope ; but there
was a feeling that these new departures were
not likely to lead very far. We get a notion
of the state of affairs by a glance at the list of
awards of the gold medal of the Royal Astro-
nomical Society for more than a quarter of a
century. From 1848 to 1879, from the re-
cognition of the discovery of Neptune to that
of the discovery of the satellites of Mars, the
awards are nearly all for achievements on
well recognised lines rather than for original
discovery ; work of conspicuous merit, but
lacking a little in that novelty to which
we have recently become well accustomed.
Ill-natured critics, if such there be, might
almost have accused astronomers of a gentle
drowsiness.
10
MODERN INSTRUMENTS
But we have recently been supplied with Since 1875
numerous excellent reasons for the most wake-
ful activity. Not only Hill but Gylden and
Poincare have proposed new methods of at-
tacking the great problems of lunar and
planetary theory. G. H. Darwin began in
1877 his patient work of attempting to
unravel the history of our solar system, and
thus organized a new department of theo-
retical astronomy which had been repre-
sented previously only by Laplace's sketchy
speculation called the Nebular Hypothesis.
In practical astronomy new instruments of
precision have been suggested to take their
place alongside the transit circle ; the size
cf telescopes has increased by leaps and
bounds, and the important step has been
taken of placing large telescopes in climates
which enormously multiply their efficiency ;
the spectroscope has created a new astronomy
of its own, and finally the invention of the
dry-plate has provided astronomy with a
weapon whose use seems to be inexhaustible.
In all these directions there have been vast
additions to astronomical work and responsi-
bilities, and were the shade of an astronomer
who died in the middle of the century to re-
11
MODERN ASTRONOMY
visit his familiar haunts, he would scarcely
recognise many of the pursuits of his suc-
cessors.
illustration ^ particular example of this general course
from the
Royal of events is afforded by the history of our own
Observatory,
Greenwich national Observatory at Greenwich. It has
twice undergone reconstruction in the present
century : the first time in the years 1835-59
by Sir George Airy, who fitted it admirably
for the " routine work " characteristic of the
middle of the century ; the second time by
Mr. Christie, the present Astronomer Royal,
who has equipped it for playing its part in
" modern astronomy."
The recon- When Airy succeeded Pond as Astronomer
Airy, Royal in 1835, he set himself to provide the
Observatory with first-class instruments, and
to organize its work on a thoroughly sound
basis of routine, so that it should be in the
front rank of observatories. He set up a first-
rate, transit-circle for meridian work, which
has never been surpassed, if equalled ; an
altazimuth for observations of the Moon at
times when it could not be caught on the
meridian ; a reflex-zenith tube, and finally
the south-east equatorial of 13 inches aper-
12
MODERN INSTRUMENTS
ture. Reorganization was extended also to
the staff and buildings ; and after more than
twenty years' work he announced that the
reconstruction was complete so complete that
not a single person was still employed, or
instrument used, which had been there when
he came. 1
He had, indeed, brought the equipment well
up to date, and had every reason to be well
satisfied with the result. It would have been
interesting to see what would have happened,
1 Extract from the Astronomer Royal's Eeport for
1859 :
" There is not now a single person employed or
instrument used in the Observatory which was there
in Mr. Pond's time, nor a single room in the Observa-
tory which is used as it was used then. In every
step of change, however, except this last, the ancient
and traditional responsibilities of the Observatory
have been most carefully considered ; and in the last
(viz., the erection of the S.E. equatorial), the substitu-
tion of a new instrument was so absolutely necessary,
and the importance of tolerating no instrument
except of a high class was so obvious, that no other
course was open to us. I can only trust that while
the use of the equatorial within legitimate limits
may enlarge the utility and the reputation of the
Observatory, it may never be permitted to interfere
with that which has always been the staple and
standard work here."
13
MODERN ASTBONOMY
if, for instance, the dry-plate had been in-
vented just about this time or a little earlier,
so that its influence on Astronomy was begin-
ning to be felt. It would have been a sore
trial for Airy to begin afresh just when he
had completed his reconstruction ; but we can
scarcely doubt that he would cheerfully have
done so. As it turned out, the next twenty
years were so generally quiescent that his
new instruments and staff were able to settle
down into steady, useful work, and it would
be difficult to overestimate the value of the
work done with the transit-circle during this
period, and of the natural corollary to it in the
similar work carried out by Mr. Stone at the
Cape of Good Hope. Of Airy's other instru-
ments it must be admitted that the altazi-
muth and reflex-zenith tube did not turn out
altogether satisfactory. As regards the equa-
torial (for which he almost apologised, hoping
that its use would never be allowed to inter-
fere with the staple work of observation with
the transit-circle and altazimuth), it need not
cause surprise that but little was done with it.
It was not till 1873 that a spectroscope was
obtained to be attached to it ; the same year
in which photography was recognised in the
14
MODERN INSTRUMENTS
Observatory by the mounting within its walls
of the Kew photoheliograph for taking daily
photographs of the Sun. In these new depar-
tures we may trace the influence of the
present Astronomer Royal, then Chief Assist-
ant.
When Christie succeeded Airy in 1881, he Recent Re-
began a reorganization of the Observatory,
much as Airy had done half a century before
him. The transformation has taken him also
some twenty years, and it is now very nearly
complete.
I have classified the recent changes under
four heads, typical of similar changes in the
astronomical world generally. The classifi-
cation is not wholly satisfactory, but it may
serve, as that useful phrase goes, to fix the
ideas.
In the first place, the position of the work
for which Airy showed such concern, so far
from being weakened, has been strengthened.
Airy's altazimuth, for observing places of the
Moon away from the meridian, to which he
attached very great importance, was never
very satisfactory, and the present Astro-
15
MODEEN ASTEONOMY
nomer Royal has substituted an instrument
of a higher grade of precision, which is, in
fact, a new transit-circle with additional
features, and as a preliminary to describing
it, we may recall the characteristics of the
transit-circle in its ordinary form, an in-
strument which has been already mentioned
several times, and which is of the very first
importance in the work of a great national
observatory. The casual visitor is usually
surprised to learn of its importance ; he is
prepared to be shewn the largest telescope in
the Observatory as the chief item of interest,
and receives with obvious bewilderment the
explanation that work with large telescopes
does' not take a prominent place in the pro-
gramme. But for the work of a national
observatory this is certainly the case ; the
accurate shape of the pivots of the transit-
circle is of far more importance at Greenwich
than the size of the largest equatorial, and
the reason will now be briefly explained.
The Tran- The transit-circle it* a telescope mounted
something like a gun with a firmly fixed car-
riage. It can be elevated properly by turning
it round a horizontal axis, but the amount of
IB
MODERN INSTRUMENTS
elevation required in the case of a gun is
never very large, and this is a limitation which
we must suppose removed in the case of the
transit-circle, which can be pointed to stars at
all elevations, and even downwards, so as to see
star images formed in a mercury-trough an
important class of observation. But the
fixed gun-carriage is a permanent limitation,
at once the strength and the weakness of the
instrument. It is its strength because in this
way great steadiness is secured. The gun-
carriage is represented by massive stone
pillars ort very firm foundations, and though
these are by no means immoveable (it was
found by Airy to his great astonishment that
in spite of all his care to get a fixed support in
this way, the massive piers themselves swung
slowly backwards and forwards throughout
the year, from some obscure cause, probably
meteorological), yet their changes in position
are small and can be allowed for. The instru-
ment is virtually quite steady, and is used for
determining the most accurate positions of
the heavenly bodies.
The limitation is, however, a weakness, in
that these "positions can only be observed at
17 c
MODERN ASTRONOMY
special times. The gun is, so to speak, perma-
nently laid north and south, and only commands
an enemy who places himself directly north or
south : it could not be used on an army which
refused to move on to this line at whatever
range, but would be effective on one which
was incautious enough to march past. Now,
the heavenly bodies are continually marching
past, owing to the daily rotation of the earth ;
and so each of them in turn comes within
view of the transit-circle at some elevation,
and at this opportunity the accurate observa-
tion of its position is made. To notice the
moment when the opportunity occurs is indeed
an observation in itself, and if we note at the
same time the elevation given to the telescope
in order to see the object, we have all the
material necessary for specifying the place of
the heavenly body on the celestial sphere.
its Uses Almost all our knowledge of the movements
of the planets and their satellites, as well as of
the stars, is obtained in this way, by observa-
tions with the transit-circle. The orbits of
the planets and their distances from us are
traced from these observations, and when
Neptune was discovered by the calculations of
18
MODEBN INSTBUMENTS
Adams and Leverrier, from its disturbing
effect on the planet Uranus, it was the transit-
circle observations of the latter planet which
formed the starting-point for their work. We
learn whether the length of the day or year is
changing : whether the earth's axis remains
parallel to itself or is slowly changing its
direction : and so we get glimpses into our
history in ages past, and some idea of what we
may expect in ages to come. By patient
observation we test the accuracy and univer-
sality of the wonderful law of gravitation, and
may hope in time to advance towards the
explanation of its origin. These are a few of
the ways in which the transit-circle helps us ;
and in all this work it is the accuracy of the
measurement rather than penetrating power
of the telescope, which is important : and so
the steadiness of the instrument and the exact
circularity of its pivots are more important
than the size of its lens ; and the stranger is
puzzled when he is shown the chief instru-
ment in the Observatory to find it a compara-
tively small one.
We have said that, for steadiness, the in- ThG
Altazimuth
strument must be mounted on firm and
19
MODERN ASTRONOMY
massive piers the gun-carriage is to be fixed
as rigidly as possible. Attempts to remove
this limitation have generally failed ; and a
conspicuous instance of this is to be found in
the fact that Airy, who designed a perfectly
satisfactory transit-circle, was unsuccessful in
his altazimuth. The need for this latter
instrument arose in this way: there is a
special importance attaching to observations
of the Moon's place, as will presently be ex-
plained. Now the transit-circle, with its
limited opportunities, did not get enough
observations : it was for instance an aggrava-
ting, but in our erratic climate too frequent
experience for the Moon to shine brightly
until the time came for making the observa-
tion with the transit-circle, and then for clouds
to come up and obscure her, only to roll away
again when the opportunity was gone for that
day. More than this, in the first and last
quarters the Moon is so near the Sun that
the time for the meridian (or transit-circle)
observation is too near noon, and the Moon
cannot be seen owing to the brightness of the
surrounding sky. When the Sun has set and
the Moon is shining against a dark sky, she is
away in the west quite out of range of the
20
MODEBN INSTEUMENTS
transit-circle. Hence Airy made the attempt
to set up an altazimuth, which may be com-
pared to a gun capable of being elevated at
any angle as before, with a carriage which
could be swung round to any point of the
compass, the exact compass-point or " azi-
muth " being read off on a carefully divided
horizontal circle. With this instrument the
Moon could be observed at any time, if she
would only show for a brief interval ; but, oh !
the cost to the observer on a fitful night !
With the transit-circle there is disappoint- Its
Discomforts
ment enough for him when he comes to the
instrument at the proper time (which may be
at any hour of the night, let us say 3 a.m. on a
cold morning) and the Moon hides her face
behind the clouds at the critical moment ; but
he can at least go back to bed knowing that
no other chance of observation will occur for
24 hours. But with the altazimuth there is a
chance whenever the Moon will show herself
for a sufficient interval, and he must wait and
watch her " dodging in and out of the clouds
on the horizon like a water-rat " as a patient
observer once expressed it to me, hoping to
secure his set of four observations at the times
when she comes up for a rather longer breath
21
MODERN ASTRONOMY
than usual ; and perhaps after getting three of
them finding them rendered useless by the
impossibility of getting the fourth! Small
wonder that the weary watcher was sometimes
discovered next morning asleep on his uncom-
fortable and rather perilous perch !
Such discomforts might have been borne
more cheerfully if the observations had been
of sufficient value ; but it must be accepted as
a fact that while Airy's transit-circle was first-
rate, his altazimuth was a failure. It gave
considerable, but not sufficient, accuracy ; and
the observations have not helped the construc-
tion of lunar tables as it was intended they
should. The accuracy of the transit-circle
was lost when the carriage was pub on a
swivel so that the telescope had, in mechanical
language, two degrees of freedom instead of
one.
The new But though this particular attempt to
observe the place of the Moon off the meridian
must be acknowledged a failure, the need of
such observations is still pressing, and en-
deavours to obtain them must not be relaxed.
There are several promising ways in which
attempts may be made (I may perhaps express
22
MODEEN INSTEUMENTS
the personal opinion that some photographic
method will ultimately be found most suc-
cessful), and of these the present Astronomer
Eoyal has very appropriately chosen for trial
a compromise between the transit-circle and
altazimuth, There is no doubt of the ex-
cellence of Airy's transit-circle : cannot more
of this accuracy be carried into observations in
other azimuths ? Christie's suggestion is very
simple : instead of having permanent freedom
for the carriage to swing in any azimuth, he
proposed to lay the instrument permanently
east and west, or north-east and south-west,
or in some other selected direction ; fixing the
carriage just as firmly as in the case of the
transit-circle. The instrument is, in fact, what
is called in text-books a " transit-circle out of
the meridian," and is not new in conception,
only in the way in which the mechanical
arrangements are carried out. It has as yet
to be thoroughly tested by trial ; but the ex-
periment is an important and promising one,
and the results will be awaited with interest,
though some years must elapse before they
can be arrived at.
Before leaving this instrument, which may
23
MODERN ASTRONOMY
importance be taken as typical of modern changes in what
tionsofthe has been called "meridian astronomy," I
Place should like to explain why observations of the
Moon's place among the stars have always
been considered so important at Greenwich.
'Of the planets or " wanderers " which change
their places among the stars, the Moon does so
the most rapidly and consistently by far,
because she is much the nearest to us and
revolves round the Earth instead of round the
Sun. The planet Mars, one of our next nearest
neighbours, moves on the average more than
twenty times as slowly among the stars : and
so irregularly that at times he may be seen in
nearly the same position for a fortnight or
more, while in a fortnight the Moon regularly
makes half the circuit of the heavens. The
planets are not always so " stationary," but
they do not move with anything approaching
the speed or regularity of the Moon.
Longitude Now any object in regular and determinate
motion can be utilized as a clock. If we know
where it will be at a given time, we can tell
the time from its observed position ; and to a
sailor this is most important for determining
his longitude, which may be regarded as the
24
MODEEN INSTRUMENTS
time shown by a Greenwich clock when it is
noon at the place where the sailor is " noon
at the ship," as it is called. The sailor can
tell when it is noon at the ship by watching
when the Sun reaches his highest point (or by
an equivalent process) ; but it used to be a
very difficult matter to find the Greenwich
time at that moment, and thus infer how far
he was east (or west) of Greenwich. Now-a-
days he carries this information with him in
the shape of a very accurate chronometer,
which he sets to Greenwich time before he
starts on his voyage, and which keeps to
Greenwich time all the way, allowing for a
certain " rate " which can easily be deter-
mined. But before the present century this
was not possible, for no one knew how to
make a clock or watch which was not alto-
gether upset by changes of temperature ; and
the sailor found the determination of his
longitude a considerable difficulty. He could
readily determine how far north or south he
was of his intended port, but not how far east
or west. Sometimes he was fain to dispense
with this knowledge entirely, and sail due
south (or north) until he reached the proper
latitude, and then sail due east (or west)
25
MODERN ASTRONOMY
until his destination came in sight. Even
then he was sometimes in such ignorance of
his longitude that he believed himself to be
due east when he really was due west, and
sailed for days in the exactly wrong direction
before discovering his mistake. The problem
of " finding the longitude at sea " was a very
serious one indeed ; and the best chance
of solving it seemed, in the seventeenth cen-
tury, to be afforded by using the Moon as the
clock to indicate Greenwich time.
Foundation There were two difficulties to be reckoned
of Royal
Observatory with. The first, that though the Moon moves
much faster than any other planet among the
stars, its motion is still very slow, and to tell
the time by it is like trying to read the exact
time to a second from a clock with an hour-
hand only. Still, even a rough knowledge of
the Greenwich time would be useful to a
sailor, and this difficulty was not vital. The
second was far greater, viz., that at this time
the motions of the Moon could not be pre-
dicted. Not only had no sufficiently good
calculations been made to form tables of the
Moon's future place, but no observations had
been made as a starting point for the calcula-
26
MODERN INSTRUMENTS
tions. The first thing to do, obviously, was to
start regular observation of the Moon, and for
this expresss purpose the Royal Observatory
at Greenwich was founded in 1676. The two
centuries which have since elapsed have given
us much information, but not by any means
all that is wanted ; it is still necessary to go
on with these observations, and to improve
them if possible. The object for which the
Royal Observatory was founded is still rightly
regarded as one of the main objects of its
work. Airy considered it still the chief ob-
ject, and he had a drastic way of bringing
his opinion home to others. When, for in-
stance, a luckless observer had some accident
with an observation of the Moon, overslept
himself, perhaps, on a cold morning, or other-
wise lost an opportunity, Airy's rebuke would
be conveyed in some such terms as these, writ-
ten out on a piece of paper (as was his custom
in all business transactions), and laid on the
desk of the delinquent : " The Royal Observa-
tory was founded for observation of the Moon.
We get about 300 observations of the Moon
during the year in all ; and the Observatory
costs the nation 6,000 a year. Hence each
observation of the Moon is worth 20 ; and by
27
MODERN ASTRONOMY
losing one last night you have cost the nation
20 ! " The effect of such a rebuke has been
described as " prodigious."
The large It has been remarked that when Airy
Equatorials
at mounted a 13-inch telescope at Greenwich in
the fifties, he did so almost apologetically,
and in the earnest hope that its use should
not interfere with the proper work of the
Observatory, which was to be done with
instruments distinguished for their steadi-
ness rather than their size. Recently,
however, the equipment of the Greenwich
Observatory has been developed not only in a
manner which Airy would have thoroughly
approved, but also in the way of large
equatorials. In the first place, Airy's 13- inch
refracting equatorial has been replaced by
one of more than double the aperture a tele-
scope with a lens 28 inches in diameter. The
largest in the world l at the present moment
is 40 inches in diameter, so that in this respect
our national Observatory is some way behind
the record ; but there are only three larger
refractors in existence than the Greenwich
1 At the moment of writing, the lens for the Paris
telescope is not yet made.
28
MODERN INSTRUMENTS
28-inch one at St. Petersburg of 30 inches,
and two in America of 36 and 40 inches, of
which mention will presently bo made. The
28-inch is economically attached to the old
mounting built by Airy for the 13-inch al-
ready spoken of ; it was originally intended to
house it actually under the same old dome, or Domes
rather dram, though the telescope, being
necessarily longer as well as larger in cross-
section, would have in that case projected
through the shutter opening. This arrange-
ment was not one which any one would have
chosen if he could help it ; but it meant ask-
ing our Government for a small sum instead
of a larger one, and the Government is not
fond of giving large sums to science. Accord-
ingly, the Astronomer Royal decided to put up
with inconvenience so as to get his large
telescope in any case, and to house it in the
old drum. He might have successfully done
so, but, fortunately, he was saved from having
to make the attempt ; for the old drum, which
had stood thirty years' wear and weather in
the interests of its old friend the 13-inch,
struck work at the new proposal, and refused
to rotate any longer. A new dome was neces-
sary in any case ; and though the tower
29
MODERN ASTRONOMY
foundation was also rather small, by intro-
ducing a novelty in dome-shapes, Mr. Christie
got a house large enough to contain his big
telescope in all positions. The new dome is
shaped rather like the head of a small mush-
room, projecting beyond the tower, which
forms its stalk, and it has the additional novel
feature of opening by the sliding apart of two
halves, instead of having a series of shutters.
The work has been admirably carried out by
Messrs. Cooke & Sons, of York, who were the
first to suggest making domes of papier-
mache, which has allowed of a great reduction
in their weight.
It may be asked why a new tower and
mounting were not provided as well. The
answer is that, not only was there a saving
of expense, but also the old mounting for the
13-inch was known to be a good one. All
Airy's mountings were firm and steady, and
no better instance of this can be given than
the fact that this particular mounting, de-
signed to carry a telescope half the size, now
carries the new 28-inch with the perfection of
steadiness.
But this is not the only addition to the
30
MODERN INSTRUMENTS
Greenwich equipment in the shape of a large The
equatorial. Sir Henry Thompson has pre- Telescope
sented to the Observatory a large photo-
graphic refractor of 26 inches aperture, and
attached to the same mounting a beautiful
reflecting telescope of 30 inches aperture,
made by Dr. Common, F.R.S. Not only is
this addition noteworthy in itself, but it is
a representative instance of the way in
which many of the largest telescopes have
come into the possession of astronomers, as
gifts from rich men, and, I am glad to add,
rich women. In such cases there is not the
same need to study small economies, for these
generous benefactors give with a free and
open hand. The addition of the reflector to
the refractor (which alone was originally
contemplated) is a case in point. When it
was seen that this addition would materially
enhance the value of his gift, Sir Henry
Thompson hastened to increase his original
estimate, and the result is a magnificent in-
strument, forming a fitting crown to the new
buildings at Greenwich.
The mention of the Thompson twin-telescope The photo-
. h<
31
brings us to the third head of the four under instruments
MODERN ASTRONOMY
which I have classed the recent developments
at Greenwich the use of photography, A
beginning was made with the Kew photo-
heliograph in 1873 for taking daily records
of the Sun's surface ; and it is to be remarked
that the photographs were from the first,
not taken and then stored away, but studied
and measured at once, so that the " Green-
wich Observations " are in this, as in other
fundamental work, the standard place of
reference for records of Sun-spots. A larger
instrument for solar work has since been
presented to the Observatory, also by Sir
Henry Thompson.
In 1887 an International Conference, of
which more will be said later, met at Paris,
to consider the project of mapping the whole
sky by photography ; and one of its most
important resolutions decided the pattern of
instrument to be employed by the eighteen
observatories which undertook to share the
work. Of these our national Observatory
was one, and an instrument of the standard
pattern was soon afterwards erected at Green-
wich. The total photographic equipment
is thus considerable. It is not necessary to
32
MODEEN INSTRUMENTS
describe it here in detail, as much of what
will be said later of the general course of
astronomical photography will apply to
Greenwich.
Nor is it necessary to say more of the The Spectro-
. scopes
spectroscopes which have been acquired, than
that the Observatory is fully equipped for
taking its share in that new department of
astronomy which has been called astro-
physics.
It remains only to notice that all these The New
Buildings
new instruments have necessitated large
additions to the buildings. Not only new
domes for protecting the instruments have
sprung up like mushrooms (one of them at
least rather like a mushroom), but new
libraries, computing-rooms, storehouses, and
workshops have been added, so that the
Octagon Tower, which was the whole Ob-
servatory at its foundation in 1676, and
remained its chief architectural feature for
two centuries, is now rivalled by a stately
building of cruciform shape at the south end
of the grounds ; and the " Seven Domes of
Greenwich." when seen from a distance, have
33 D
These
Changes
typical of
Changes
elsewhere
The Almu
cantar
MODERN ASTKONOMY
the appearance of a small astronomical city
set on a hill.
And now I shall endeavour to show that
each of these lines of development of our
national Observatory is thoroughly repre-
sentative of what has been done elsewhere.
In the astronomical world generally new in-
struments of precision have been erected or
invented : telescopes have increased in size
and number, new observatories have been es-
tablished private munificence having especi-
ally helped in this direction ; photography
has come to the aid of astronomy in numerous
ways, and the spectroscope has founded a new
science of its own.
And, first, as regards instruments of pre-
cision. We saw that at Greenwich the
transit-circle, the standard instrument for
observing the places of the planets and fixed
stars, has been supplemented by another in-
strument very nearly resembling it, but not
restricted to use on the meridian. Elsewhere
the lonely supremacy of the transit-circle has
been more seriously attacked. I do not here
dwell on Dr. Gill's use of the heliometer for
determining planetary positions ; for the helio-
34
Flamsteed's Observatory, 1676.
The new Physical Observatory, 1899.
THE KOYAL OBSERVATORY, GREENWICH.
35
MODERN INSTRUMENTS
meter is not a new instrument, and the
novelty will be dealt with in the next
section as a change in method. But an
entirely new instrument has been suggested
and used with success by" Mr. S. C. Chandler,
of Boston, U.S., to which he has given the
name of Almucantar. It is essentially a
telescope firmly fixed to a float, which can
thus rotate round a vertical axis, but always
remains accurately at the same angle with
the vertical. The accuracy of the transit-
circle is secured by restricting observations
to a vertical circle of the sky, usually the
meridian ; the almucantar is similarly re-
stricted to a horizontal circle. But there is
this essential difference between the methods
of securing the restriction : in the case of
the transit-circle it is secured by human skill
and labour in turning two pivots to an
accurate shape. Each of the pivots of Airy's
transit-circle took six weeks of unremitting
and tedious toil * before its shape was perfect
1 In a paragraph, describing the celebration of Sir
George Airy's ninetieth birthday (July 27th, 1891)
occurs the following passage :
"There were many other guests whose presence
was significant, as for instance, Mr. Biddell, who was
forty years ago charged by Messrs. Ransomes &
37
MODEEN ASTEONOMY
enough to satisfy the exacting astronomer.
For the almucantar, on the other hand, no
art or labour is required. The faithfulness
with which it keeps to its prescribed path de-
pends on the innate properties of floating bodies,
and is thus liable to no human imperfections.
I have no hesitation in predicting a great
future for some form of almucantar : for
the instrument may be modified in various
ways so long as the essential principle of
flotation is retained. In one sense it is
not a new instrument, for it is closely
allied to the old floating collimator. Mr.
Chandler cannot claim entire originality
for his instrument any more than he can
May with the construction of the present transit-
circle. He described to a small knot of most interested
guests the dismay of the workmen and their em-
ployers at the demands of Sir G. B. Airy, especially
those relating to the pivots. These were to be of
chilled iron, six inches in diameter, and perfect
cylinders to within 3^00 i ncn No error of this
magnitude was to be discernible with a delicate
spirit-level ; and after trying all the most delicate
methods of turning then known, the requisite accuracy
was obtained by sheer labour rubbing down bit by
bit all the places which this same spirit-level indi-
cated as too high. Each of the pivots cost six weeks
of such labour ! " The Observatory, vol. xiv. p. 291.
38
MODERN INSTRUMENTS
claim to have discovered the principles of
floating ; hut he has at any rate shown us a
practical working form of instrument, and
made a valuable series of observations with it.
Though he has himself ceased to observe with The
Almucantar
such an instrument, and though no one else
has yet taken advantage of the opportunity
THE DURHAM ALMUCANTAR IN THE WORKSHOP.
to follow him, Professor Sampson, of Durham,
is on the point of beginning work with an
almucantar, which has been made by Messrs.
Cooke & Sons, of York. In this instrument
an ingenious alteration of Chandler's original
design has been introduced at the suggestion
of Dr. Common, F.R.S., the telescope being of
39
MODERN ASTRONOMY
the " broken " kind, with a mirror at the
elbow joint, so that the observer always looks
in a horizontal direction. But the principle
of any form of almucantar is the same fix a
telescope to a float so that it always remains
at the same angle with the vertical ; and the
details can be varied in many ways. The
telescope may be a photographic one for in-
stance, and I shall show later how the photo-
graphic method can be used with either transit-
circle or almucantar. The advantage of the
latter, which may ultimately recommend it in
preference to the transit-circle, is its indepen-
dence of workmanship.
Photometers Among new instruments of precision due to
the last quarter of a century, we may fairly
include those for measuring the brightness of
a heavenly body, instead of, or in addition to,
its positions. It is curious how little had been
done in this direction previously, for the study
of changes in brightness is a most fascinating
one. It was, for instance, the sudden appear-
ance of a new bright star which attracted the
attention of Tycho Brahe to astronomy ; and
the observation of variable stars has always
been a department of the science in great
40
MODEBN INSTEUMENTS
favour with, astronomers. But they seem to
have been reluctant to provide themselves
with an instrument for accurately measuring
the brightness ; the usual plan has been to
estimate differences of brightness between the
variable and neighbouring stars, and it is only
quite recently that photometers have been
extensively used. Of such there are two
classes, one depending on the equalization of
two lights, the other on the extinction of
light. In the first class a standard light, Equaliza-
tion
either of a particular star or of an artificial
star, is varied by some device, in a measurable
manner, until it is equal to the light of the
star selected for examination. The device
usually adopted depends on the properties of
polarized light ; but as a simple example we
may suppose a wedge of neutral-tinted glass
passed in front of the standard light, which is
thus obscured more and more as there is a
greater thickness of glass to pass through,
until it is found equal to the star under exam-
ination. The wedge should be graduated in
the direction of increasing thickness, so that
the reading of its position, when the two lights
are judged equal, may be registered as a
measure of the brightness of the selected star.
41
MODEEN ASTRONOMY
If this is originally brighter than the standard
we must reverse the process and pass the wedge
gradually in front of the former until it is
diminished to agreement with the standard.
Such a wedge, however, has hitherto been ex-
clusively used as the second kind of photo-
meter, which depends on the principle of
Extinction extinction. If the range of thickness be great
enough, then as the wedge is gradually passed
in front of a star the light becomes dimmer
and dimmer, until at last the eye fails to per-
ceive it at all. The reading at which this
" extinction " occurs varies with the brightness
of the star, and is therefore a measure of the
brightness ; and we get rid of the necessity
for a comparison star to a large extent not
entirely, for in our variable climate we must
make allowance for the state of the sky, and
it is advisable therefore to make constant
measures of some star of known brightness to
determine this allowance ; but in each separate
observation we are concerned with only one
star instead of two, and this is an advantage
which recommends the " wedge photometer "
as an extinction photometer. It was intro-
duced by the late Professor Pritchard, in 1882.
In the following few years his assistants at
42
MODEEN INSTEUMENTS
Oxford accomplished a remarkable piece of
work in cataloguing the brightness of nearly
3,000 stars, and for this work he obtained in
1885 the gold medal of the Eoyal Astronomical
Society jointly with Professor E. C. Pickering,
of Harvard University Observatory, who had
commenced a similar though larger work
in 1879 with an equalization photometer.
Pickering has largely added to this since
1885, and a fine research has also been carried
out by Miiller & Kempf at Potsdam with a
Zollner photometer, which is of the equali-
zation class. The instruments of Zollner,
Pickering, and Pritchard are the three chief
photometers at present in existence, but there
are also photographic methods of measuring
the brightness of the stars of which mention
will presently be made.
Passing now to the development of large Large
Refracting
telescopes, we find that the '28-inch lens of Telescopes
Greenwich, large as it is, has been considerably
surpassed in America by the 36-inch lens at
the Lick Observatory, in California, and the
giant 40-inch lens at Yerkes Observatory, near
Chicago. Indeed, nowhere is the difference
between the third and fourth quarters of our
43
MODERN ASTBONOMY
century more clearly indicated than in the
history of large refracting telescopes. In the
diagram is shown the size of the largest tele-
scope (of this kind) in existence at different
dates ; and the periods of relative inactivity in
the middle of the century, and of sudden de-
velopment recently are both plainly marked.
The date of the discovery of Neptune is in-
dicated on the diagram ; for it might be
supposed that a great event like this would
have a stimulating effect on almost all branches
of astronomy ; but it will be seen that no such
influence is discernible. The size of telescope
is indicated in the usual way by the aperture
of the lens in inches, and not by the length of
the tube. It should be remarked that a tele-
scope of over 50 inches aperture is being con-
structed by that thoroughly able optician,
M. G-autier, of Paris ; but although the tube is
erected in the Paris Exposition, at the time of
writing (August, 1900), the lens is not made,
and hence the line is dotted in the diagram.
Reflecting Before saying anything in detail about these
Telescopes
fine instruments, it may be remarked that
there is another kind of telescope altogether
(not included in this diagram), in which a
44
si
1 1 1
1//
I/ MOM ~IHd
x
*
3 ^
19.
. I ' '
-ii
45
MODERN ASTRONOMY
concave mirror takes the place of the lens.
This form has several distinct advantages over
the other ; in the first place, all the colours
forming white light can be brought accurately
to the same focus, whereas with a lens the
focussing can only be approximate. Even the
approximation is only secured by combining
two or three lenses together, and a different
combination is required according to the use
to which the telescopa is to be put whether
for gazing with the eye, or for taking photo-
graphs. A refracting telescope suitable for
visual work cannot be used for photography,
and vice versd, whereas a reflecting telescope
can be used indifferently for both purposes.
In the second place, a reflecting telescope has
only one surface to be made optically perfect,
whereas a compound lens has at least four.
Thirdly, the surface, and not the substance, is
of chief importance. The light does not pene-
trate the substance of the mirror, and thus, so
long as the surface is perfect the interior may
be faulty. The lens, on the other hand, must
not only have its four surfaces without flaw,
but the substance of the glass as well. For all
these reasons it is much easier to make a large
reflector than a refractor of the same size, and
46
MODERN INSTRUMENTS
the largest telescopes in the world are reflectors,
and do not appear in the above table of re-
fracting telescopes. The largest is still Lord
Rosse's giant reflector, with a 6-foot reflector
(a mirror 6 feet in diameter), at Parsonstown,
in Ireland ; but the mirror being of speculum
metal is not so highly reflective as the modern
silver on glass, and this instrument is therefore
not so effective as Dr. Common's 5-foot re-
flector, made by himself and mounted in his
garden at Ealing, which may be regarded as
effectively the most powerful instrument in
the world.
But the size of a reflecting telescope has
not been regarded with the same interest as
that of a refractor why, it is rather difficult
to say. The latter is in many respects a
handier and more trustworthy instrument,
the former being subject to curious moods
which render it at times difficult to work
satisfactorily. (Dr. Common has been heard
to compare the reflector to the female sex
for uncertainty.) But this disadvantage is
more than counterbalanced by the advantages
above mentioned. Perhaps the difficulty of
making a lens has invested the refractor
47
MODEEN ASTEONOMY
with a special interest. It has several times
been suggested that the limit of what is prac-
ticable in the size of lenses has been reached,
and thus there is a special kind of record-
breaking in making a larger one. Whatever
the reason, there is no doubt that when a
large telescope is mentioned it generally means
a refractor ; and American millionaires and
other generous benefactors to astronomy have
usually spent their money on refractors such
as those in the above list.
The The three largest are worthy of special
Pulkowa
Telescope notice. First comes the 30-inch erected at
Pulkowa by the Czar of Eussia. About 1835
the Czar Nicolas determined to have the
finest observatory in the world, and he in-
structed the famous Wilhelm Struve to see
that it was built, giving him practically carte
blanche as regards expense. It was built at
Pulkowa, twelve miles from St. Petersburg,
and a straight road leads from the observa-
tory to the city so straight that a telescope
placed in the central tower of the observatory
can look into the market-place. The build-
ings and equipment are magnificent, and the
observatory contained a large refracting
48
MODERN INSTRUMENTS
telescope for that epoch aperture 15 inches.
When all was finished (in 1839) the Czar
came to inspect it, and, after being shown
over the observatory, turned to the director
and asked simply whether he was satisfied ;
to which the diplomatic astronomer replied
that he was for the moment. 1 And excellent
use he made of the noble observatory and in-
struments. But in course of time telescopes
of greater size were built elsewhere, and the
15-inch was no longer in the front rank. So
that after forty years the Czar Alexander III.
instructed Wilhelm Struve's son and suc-
cessor, Otto Struve, to provide him the largest
refractor in the world ; and in consequence
the 30-inch Pulkowa telescope (the joint work
of the Alvan Clarks, of Washington, who
made the lens, and the Repsolds, of Ham -
burg, who made the mounting) was erected
in 1884.
It did not, however, long enjoy its proud The Lick
supremacy. The Czar of all the Russias was
outbidden twice successively by American
millionaires. First by James Lick, whose
1 This story was told me by Otto Struve at Pulkowa
in 1887. The actual words used by him were: "Struve,
sind Sie zuf rieden ? " " Augenblicklich."
49 E
MODERN ASTRONOMY
bones lie beneath the great 36-inch on Mount
Hamilton, in California. The establishment
of the Lick Observatory, with its wonderful
equipment and climate, may be classed among
fortunate accidents. The story goes that Mr.
Lick, having acquired considerable wealth,
was desirous of erecting a permanent memo-
rial of himself and his wife ; and his first idea
took the form of two immense statues on the
Pacific coast, which should be a landmark
visible for a considerable distance. About
this time, however, an enthusiastic astronomer
suggested the much better plan of building a
giant telescope as a mausoleum. He adroitly
pointed out that in case of war landmarks of
the sort contemplated would be liable to bom-
bardment by the enemy ; whereas a telescope
high up on Mount Hamilton would be quite
safe, and the instrument would be kept in
good repair, and the memory of its founder
ever fresh, by the devoted astronomers. This
excellent advice was taken, and though the
munificent benefactor of astronomy did not
live to see his monument completed, he died
in the happj^ assurance that his bones would
ultimately be interred under the largest tele-
scope in the world, placed in the finesb situa-
BO
MODERN INSTRUMENTS
tion as yet selected for an observatory. There
is reason to believe that the Pyramids were
both astronomical observatories and the tombs
of Pharaohs. In modern times James Lick
was fired with the same ambition as the
Pharaohs, and realized it with something of
the same magnificence. By choosing the site
for the observatory with judgment, the tele-
scope, large as it is, has been rendered more
effective still to an extent which it is difficult
so overestimate ; and it will be recognised
that the expense was at the same time con-
siderably increased, for a good road had to be
made to the top of a mountain 4,250 feet
high, and all the materials taken up. It was
well worth doing. A large telescope in a poor
climate is useless, for the rays of light which
fall on different parts of the large lens are
differently disturbed by atmospheric tremors,
and produce a confused image when combined
at the focus. When the air is not steady a
small telescope gives better images than a
large one, and is much easier to handle. But
there is no question about the excellence of
the climate on Mount Hamilton, " God's own
climate," as one of the observers has rever-
ently described it, and the legacy of James
51
MODEEN ASTRONOMY
Lick has a double claim on the gratitude of
astronomers. It should, however, be remem-
bered that to those in actual charge of the
telescope the situation is not without its dis-
advantages. They are at some distance from
the nearest town, and without many of the
comforts of civilization. The winter on the
mountain is severe, and brings with it at
times considerable privations. In one winter
there was actually no water to drink except
what had passed through the engines.
In spite of such discomforts, and of some
faults in administration which robbed the
observatory of some of its ablest observers,
the little colony on Mount Hamilton has
worked cheerfully and enthusiastically from
the first, and thoroughly justified expecta-
tions.
^Telescope 8 H> i g t be hoped that the discomforts of the
Lick observers will diminish as civilization
creeps round the base of Mount Hamilton.
The value of land in the neighbourhood began
to go up when the telescope was erected, so
that very soon the telescope and observatory,
costly as they were, could have been built out
of the profits on land sales. So I have heard
52
MODEEN INSTRUMENTS
on good authority; and further (though this
has been contradicted), that to this circum-
stance is due ultimately the existence of the
Yerkes telescope, which is four inches larger
than the Lick. For some enterprising gentle-
men in another neighbourhood, desiring to
test the generality of the law that if a large
telescope were built the value of land in the
neighbourhood would go up, announced a still
larger telescope, and ordered two 40-inch discs
of glass for the lens. The experiment suc-
ceeded admirably, and they were so well satis-
fied with the rise in price which followed on
the mere announcement (so the story goes)
that they considered it unnecessary to proceed
further with the experiment. Be this as it
may, the fact is undoubted that two beautiful
40-inch discs were produced to order, not ulti-
mately claimed, and being left on the maker's
hands were to be had comparatively cheap.
No one was, however, able to take advan-
tage of the unique opportunity, until another
millionaire, Mr. Yerkes, of Chicago, came to
the rescue. Two eminent astronomers of the
city called upon him one day and explained
the situation, and suggested that it would be
53
MODERN ASTRONOMY
a fine chance for Chicago. He responded in
the most generous manner, and the construc-
tion of the telescope was undertaken at once
by Alvan G. Clark, the only remaining mem-
ber of the firm which had made the Pulkowa
and Lick object-glasses three times called
upon to beat their own previous record. The
complete instrument was exhibited at the
Chicago World's Fair in 1893, and is now
mounted in a splendid observatory at Williams
Bay, about eighty miles from Chicago. Here
it is in the hands of three men of world- wide
fame : G. E. Hale, director of the observatory,
who has perfected a new line of research in
the study of the Sun's surface ; S. W. Burn-
ham, the first authority on Double Stars, and
E. E. Barnard, who discovered the fifth satel-
lite of Jupiter. Burnham and Barnard were
both formerly at the Lick Observatory, and
are thus able to compare the performances of
these two giant telescopes. As yet they have
made no invidious declaration in favour of
either; and such a decision would, no doubt,
be difficult, for while advantage of size is in
favour of the Yerkes, that of climate is with
the Lick. The climatic advantage even ex-
tends to material comforts, for severe as are
54
MODERN INSTRUMENTS
the winters at Mount Hamilton, it seems that
those at Williams Bay are severer still. A
temperature of 40 below zero is not unknown,
and though the recent winter (1899-1900) was
described as mild, the lake was frozen over in
March to a depth of fifteen inches!
To these great endowments by Americans The Bruce
there are many smaller ones to be added ; and
notably those by an American lady, the late
Miss Catherine W. Bruce, of New York. Her
benefactions have been numerous, and (al-
though something of a digression) one especi-
ally is worthy of notice, because of its felicitous
form. Instead of giving some immense gift to
one particular observatory, she had the happy
idea of offering a number of small sums of
about 100 to astronomers in any part of the
world, to supply any wants which they might
be otherwise unable to satisfy. The scheme
was a great success ; it brought to light a
number of pressing needs which were delaying
good work for the want of comparatively
small sums, and both the generous donor and
Professor E. C. Pickering, who organized the
distribution of the gifts (and who probably
originated the idea), must have been immensely
gratified by the result.
55
MODERN ASTRONOMY
But this is, as above remarked, a digression.
The special gift to astronomy which intro-
duces the name of Miss Bruce here is her pre-
sentation to the Harvard University Observa-
tory of the largest photographic doublet in
existence with a lens made up of four single
lenses, like a portrait lens. The telescope is
thus in a different category from the Lick and
Yerkes telescopes, but being the largest in
existence of its own class, may be mentioned
alongside them. It also resembles the Lick
telescope in having the advantage of a first-
rate climate ; for Professor Pickering, instead
of keeping it at Harvard (where the climate is
by no means bad, but not specially good), has
sent it to the branch observatory at Arequipa,
in Peru. The very existence of this branch
establishment is due to another benefaction ;
for Mr. Boyden left a sum of money for ex-
peditions and experiments to determine the
most suitable climate for astronomical observa-
tions, and the station at Arequipa is the
outcome of the investigation. Until recently,
far too little attention has been paid to this
vital matter of climate. Observatories have
been built in or near large towns from force of
circumstances quite independent of astronomi-
56
MODEEN INSTRUMENTS
cal considerations, and it is only at this latter
end of the nineteenth century that attempts
have been made, as in the case of the Mount
Hamilton and Arequipa Observatories, to get
the inestimable advantage of a good climate
for observation.
Passing from America to our own country, The
McClean
it is pleasant to note that, although a little Telescope
behind the United States in the matter of
millionaires, we in England have also our *
munificent benefactors. I have already men-
tioned Sir Henry Thompson's generosity to our
Royal Observatory at Greenwich, and I may
now remind you that one of the Visitors of the
Royal Institution l has, besides founding the
Isaac Newton Studentships at the University
of Cambridge, which have already done much
for mathematical astronomy, recently pre-
sented a pair of large telescopes to the Royal x
Observatory at the Cape of Good Hope, with
apparatus providing for visual, photographic,
and spectroscopic work. From these fine in-
struments, in the able hands of Sir David
Gill, we may look for important results in the
near future.
1 This book is an expansion of three lectures
delivered at the Royal Institution.
57
Rising
Floors
MODERN ASTRONOMY
Before leaving the subject of large telescopes,
I would call attention to one new depart urer
which their recent great development in size
has necessitated, viz., the rising-floor. When
SETTING UP THE McCLEAN TELKSCOPE
AT TttE CAPE OF GOOD UOPE.
using a small telescope the observer can
accommodate himself to its different positions
by using a small step ladder. But as the
instrument increases in size, the dimensions of
this ladder become inconvenient, and finally
58
MODERN INSTRUMENTS
impossible. To meet the new requirements, Sir
Howard Grubb, the well-known astronomical
instrument maker in Dublin, hit upon the idea
of a moveable floor, which should rise and fall, in
the manner of a lift or elevator, by machinery
under the electric control of the observer. The
device has been applied to the Lick and Yerkes-
telescopes ; to the Washington 26-inch ; to the
telescope . presented by Mr. McClean to the
Cape ; and to several others, and with conspicu-
ous success : but it is not without considerable
attendant dangers. When the rising-floor of
the Yerkes telescope had just been erected,
owing to faulty engineering l it slipped from
its fastening wire-ropes, and fell the whole dis*
tance from the top of its range to the bottom.
Most fortunately there was no one on the floor
at the time, but several people had only just
left it, and photographs of the wreckage make
one shudder to think of what might have
happened. There is no doubt that the accident
was due to negligence, and that it is easy to
make the device thoroughly safe ; and we may
1 I should make it clear that though the idea of the
rising-floor originated with Sir H. Grubb, he was not
responsible for the workmanship of this particular
floor in any way whatever.
59
MODERN ASTRONOMY
-hope that this serious warning has lessened
the possibility of any such accident in the
. future.
The Gelatine We now come to the most important new
Dry-plate
weapon with which astronomy has been pro-
vided since the invention of the telescope the
gelatine dry- plate. It may seem strange to
particularize in favour of the dry-plate over
photography generally ; many people are
doubtless accustomed to regard the dry-plate
as a mere gain in convenience of manipulation.
But in astronomy the dry-plate is all-impor-
tant ; it removed a limitation under which
photography previously suffered. With a wet-
plate, a photograph could not be exposed for
more than a short time ; the film dried, and
ceased to be sensitive. "With the advent of
the dry-plate came not only a gain in sensitive-
ness, but the possibility of prolonging exposures
indefinitely. For they are not limited by the
duration of a single night ; when dawn or
cloud comes, it is only necessary to put the cap
on the telescope and wait for the next fine
night, when the exposure may be resumed.
Plates have been left in the telescope for
weeks or even months. Hence the very
60
MODERN INSTRUMENTS
faintest objects can be photographed and it
is a common experience now to photograph
objects too faint to be seen in the largest
telescope.
The camera of the astronomer does not The Camera
of the
differ in essentials from that which takes Astronomer
.
AN ASTRONOMER S LONG CAMERA
(40 feet).
Drawn by a Japanese Artist.
your portrait ; indeed, much astronomical
work is now being done with portrait lenses.
But to get a picture on a large scale, he must
have a long camera, and he uses one as long as
a telescope. [In the illustration is shown a
camera 40 feet long used by Professor Schae-
61
MODERN ASTRONOMY
berle for the eclipse of 1896 in Japan. He
found a rock placed very fortunately to act as
a support. The drawing was made for me on
the spot by a Japanese artist.]
His instrument is indeed usually called a
photographic telescope, though this is some-
what of a misnomer; for a " telescope," an
instrument for seeing things at a distance, is
a combination of object-glass (or mirror) and
eye-piece, connected by the tube; and for
photography we remove the eye-piece and
substitute a photographic plate. There is no
need, however, to be very critical of nomencla-
ture ; and the name photographic telescope has
the advantage of reminding us that the instru-
ment may be either a refractor or a reflector,
as in the case of the visual telescope. In the
latter case a concave mirror is used to form
the image in the former a lens or series of
lenses. The special form of refractor in which
two compound lenses are arranged as in a por-
trait lens is generally distinguished as a
photographic doublet, and it is a large instru-
ment of this kind that Miss Bruce gave to the
Harvard Observatory. Such instruments as
are being used for the International Chart of
62
MODEBN INSTRUMENTS
the Heavens nave only one compound lens,
and the name photographic refractor is usually
applied to these only.
All three forms of instrument have their
special advantages and disadvantages. The
reflector, has the great advantage that since
light is not analysed into colours by reflection,
the images formed have absolutely no colour
fringes at all, whereas with any form of lens
there is some colour, though the skill of the
optician may reduce its effects nearly to the
vanishing point. But the reflector has only a
very limited field ; if we attempt to photo-
graph with it the stars scattered over a con-
siderable area of the sky, we shall find that
only within a small circle of less than a degree
in radius are the images even approximately
round ; outside this they become elongated,
and the images are useless for purposes of
refined measurement. The field of the re-
fractor is rather larger, though with an
ordinary object-glass it is not possible to extend
it very far. Mr. Dennis Taylor, of Messrs.
Cooke & Sons, York, has lately invented a
form of triple object-glass, which constitutes a
great advance in this direction. When, how-
63
Relative
Advantages
of
Refractors,
Reflectors,
and
Doublets
MODERN ASTRONOMY
ever, we come to the portrait-lens or doublet,
the field is greatly extended. Instead of one
or two degrees' radius we can get ten or even
twenty. So great is the difference that the
portrait-lens has hitherto been regarded with
some mistrust ; it is feared that the image of
such a large field cannot be accurate because
other instruments, known to be accurate, are
so strictly limited in field. But much of this
mistrust is ill-founded. We have recently
been measuring at Oxford some plates kindly
lent by Professor Pickering, of Harvard, who
has championed the portrait-lens from the
first, and we find them wonderfully accurate.
"We find that although a certain allowance
must be made for what is called " optical dis-
tortion," yet the law of this distortion is so
simple and well-defined (it varies as the cube
of the distance from the centre of the plate)
that there is no difficulty in measuring the
position of any trail on the plate to a degree oi
accuracy determined only by the size of the
grains of the photographic film. I make this
statement with some little reserve, for the in-
vestigation is so new that it is still proceeding ;
but I also make it with considerable confi-
dence, and I venture to predict for the portrait-
64
MODERN INSTRUMENTS
lens, as for the almucantar, an important place
in the astronomy of the future.
To those readers who are unfamiliar with HOW
. n -, ., .11- Photographs
astronomical work, it may not be obvious how are taken
photographs are taken. One fact deeply im-
pressed on the minds of those accustomed to
have their portraits taken is that there must
be a good light ; if daylight is not strong
enough (as happens in the winter), magnesium
or electric light is used. By what light, then,
are the stars photographed at night? The
answer, " By their own light " will no doubt
come as a surprise to many, but a few moments'
reflection will show the difference between
the two cases. A human being gives out no
light of his own he must be illuminated to be
photographed ; and if he is to be photographed
quickly the illumination must be bright.
Given patience on the part of operator and
sitter, and the latter could be photographed
by candlelight or moonlight perhaps, even
by starlight ; but it would take a long time,
and the operation is one which all concerned
are usually glad to get over quickly. Now,
with the heavenly bodies, such illumination as
is possible is already provided ; and we can use
65 F
MODERN ASTRONOMY
no other. Astronomers would be only too glad
to illuminate some of the objects a little more
strongly if they could, so as to get the operation
of photographing them over more quickly, but
no searchlight that we could flash on a
heavenly body would add appreciably to its
illumination. We cannot get beyond the
light sent us from the sky, and hence we must
make up our minds for a long sitting. Other-
wise the operation is much the same as is
already familiar to those who take portraits
or have them taken.
There is, however, one important respect in
Clock-work
for following which the camera of the observatory differs
from that of the studio. When taking a por-
trait, the photographer can bid his subject
remain still, with more or less success. There
is record of a man who commanded the Sun to
stand still, but he is no longer available ; the
Sun and stars refuse to modify their courses
for the camera. The alternative is for the
camera to follow them in their movements,
and good clock-work machinery is accordingly
devised to secure this following of the stars
with wonderful exactness. Recently this
machinery has been perfected by what is
66
MODEEN INSTEUMENTS
called electrical control, of which there are
two forms. In one case the photographer
holds in his hand two buttons connected with
electric circuits, by pressing one of which he
can accelerate the machinery, by the other
retard it. "While the plate is being exposed
in one telescope, he looks through another
telescope furnished with cross wires rigidly
attached to the former, and keeps some selected
star on the cross wires. Should it show a
tendency to move in either direction, he
presses the appropriate button and checks this
tendency. Such watching is very tiring when
continued for long ; and so another form of
" control " has been devised, in which a good
clock plays the part of the watcher. As the
clock pendulum swings to and fro it expects
to find a certain state of matters in an electric
circuit at each beat, which will mean that the
machinery is going at the proper pace. Any
failure to find this state of things sets up an
electric current which amends the pace in the
right direction. In this way there is but little
difficulty in making the largest camera follow
the stars with great nicety, so that the photo-
graph is taken with as much ease as though
they could actually be bidden to stand still.
67
MODEEN ASTRONOMY
The But an instrument has been recently brought
Coelostat
into use which would have interested Joshua
not a little ; for although it cannot actually
do what he did, it has the appearance of doing
much more. The name Coelostat, which has
been given to it, may be new to you (it only
dates from 1895), though you may have heard
of heliostat and siderostat. A siderostat is a
mirror rotated by machinery so that the re-
flection of a particular star remains fixed in
direcion. Any one looking into this mirror
at the image of this star would imagine it
stationary, though stars near it would be seen
to revolve slowly round it. The heliostat is
essentially the same instrument put to the
slightly different use of counteracting the
Sun's motion. Looking into its mirror the
Sun would appear to stand still, though im-
perfectly ; for he would be all the time twisting
round his centre as though impatient under
the restraint. Now the coelostat is an instru-
ment which makes the whole sky appear to
stand still. Looking into its mirror we should
imagine, not merely the Sun's centre, but all
his disc to be stationary ; not merely one star
but all the stars, to be reduced to rest. For
photographic purposes this is clearly a great
68
'MODERN INSTRUMENTS
gain. The idea of the instrument is simple,
and was expounded long ago by a Frenchman
called August ; but its advantages have been
little recognised until recently. It has been
used with great success on the British eclipse
expeditions. Beautiful 16-inch plane mirrors
for these instruments were made by Dr. Com-
mon, whose experience of grinding mirrors,
acquired in making his own 5-foot telescope,
already mentioned, has been so often placed at
the service of others. These mirrors are
mounted with their plane surfaces parallel to
the Earth's axis, and are made to rotate round
an axis parallel to the Earth's axis once in
forty-eight hours no other machinery or ad-
justment is necessary. Looking into a mirror
which fulfils these conditions, a star seen in any
direction will remain in that apparent direction
until the mirror has turned so far round that
the star is lost at the edge of the mirror. Thus
the photographer can point his camera at the
image of the sky in the mirror, just as though
it were a fixed object fixed, that is, with
reference to the Earth. Hence the camera can
be fixed, all the necessary movement being
supplied by the mirror ; and this is a great
convenience when the camera is 100 feet long,
69
MODEBN ASTEONOMY
such as was used in America to observe the
last eclipse of the Sun (May 28th, 1900). The
illustration shows the first coelostat made in
England for eclipse work, being set up for
trial in the garden of the University Observa-
tory at Oxford. The mirror was made by Dr.
THE COELOSTAT.
Common, and the instrument designed by him.
The camera pointed to it is a double one, two
tubes side by side, each with its own lens and
its own image. It is pointed at the mirror so
as to receive the reflected images of the Sun,
which may be seen upon the ground glass.
70
MODEKN INSTBUMENTS
The observer is prepared to test the going
of the instrument by seeing whether these
images remain stationary whether the Sun
really " stands still."
When a celestial photograph is taken, the Measuring
Plates
astronomer's work is not over; it is rather
only beginning. He has only brought down
into his study a little bit of the sky for
examination ; but the examination is still to
be performed, though it is done more con-
veniently in the study than at the telescope.
Suppose, for instance, that he is concerned with
a " double star," a pair of stars revolving
round each other in a time and manner which
it is required to determine. Before the days Ww
Telescope
of photography he measured the relative Micrometer
positions of these stars with a micrometer at
the eye-end of the telescope. Looking into the
eyepiece he sees not only the two star images,
but two parallel spider-lines, which can be
rotated like the hands of a watch to any
position-angle, the angle being read off on a
circle graduated like a watch dial but more
elaborately. One* of the observations to be
made consists in setting the spider-lines
parallel to the line joining the stars (which is
71
MODERN ASTRONOMY
best done by making one of them actually
pass through the star-images), and then reading
the graduated circle. In this way it is ob-
served that the line of junction points success-
ively, as years roll on, to all the figures on the
watch-dial, and so the revolution of one star
round the other is proved, and the time of
revolution found. For a complete description
of the motion, however, we must observe also
the distance between the stars, which is simi-
larly subject to variation. This is the use of
the second spider-line. The distance between
the spider-lines can be varied in a measurable
manner by a screw very carefully made ; the
head of the screw is expanded into a large
circle and graduated round its rim, so that
fractions of a whole turn can be read off with
exactness ; and hence, if the spider-lines be set
at such a distance apart that one falls on the
image of one star, and the other on the other,
we have the means of measuring the distance
of the stars from each other, not, of course,
the actual distance in millions or billions of
miles, of which we may know nothing, but the
angle subtended at the Earth, which varies
proportionally to it.
72
MODEEN INSTEUMENTS
These observations of "position-angle and
distance " with a micrometer at the eye-end of
a large telescope play an important part in
astronomy. Not only are the movements of
double stars followed in this way, but those of
satellites, especially faint satellites ; and the
places of small planets and comets are found
by measuring their position-angles and dis-
tances from neighbouring stars.
Now, when photography comes to the aid of- Photo-
graphic
the astronomer, it does not make these mea- Micrometers
surements for him. He can take two photo-
graphs of a pair of stars at different dates,
and see by inspection of the photographs that
they have changed their relative positions,
just as we may see that a person has grown
older by comparison of two portraits taken at
different dates. But in astronomy we want
far more than this general knowledge, and we
cannot evade the necessity for accurate mea-
surement. The photographs must still be
submitted to the micrometer, though this is
no longer at the eye end of the telescope, but
placed conveniently in the study. More im-
portant still is the advantage that the measures
can be made at any time when once the
73
MODERN ASTRONOMY
photograph, is secured ; they can be repeated if
any mistake is suspected, and repeated many
times, and by several persons, for the elimina-
tion of accidental and systematic errors. We
gain immensely by taking the photograph,
but we do not avoid the necessity for a micro-
meter.
[To prevent misunderstanding, it should
be remarked here that photography has not
as yet rendered much service in double-
star work. The most interesting pairs are so
close together that their images on the photo-
graph are generally inseparable, and the
measurement must be done by eye at the
telescope ; but the example given sufficiently
illustrates a general principle.]
There are many forms of micrometer for
measuring plates. The most obvious form
closely resembles that already described for use
on the telescope ; for measures can be made
precisely in the same way on the photographed
images of stars as on the images themselves.
But to take this instrument as it stands is
to lose some of the opportunities opened up
to us by photography, which we are only
gradually realizing.
74
MODEEN INSTBUMENTS
One most important addition to the micro-
meter since it has been used on photographs is
the reseau, a network of lines forming small
squares impressed on the plate by a second
exposure, independent of that to the sky, and
developed along with the star images. In-
stead of measuring the position of one star
with respect to another, we measure the posi-
tions of all stars with reference to this reseau ;
and this is found to be an immense gain in
convenience and accuracy. For instance, if
two stars are very far apart we should have to
turn the micrometer screw a large number of
times in order to measure the distance between
them ; and not only does this take time, but
errors are apt to creep in when a long screw is
used. "With a reseau on the plate, each star
is referred to the cross lines of the network in
its immediate neighbourhood, which only re-
quires a small portion of screw ; and the
relation of the different parts of the reseau is
determined once for all.
As a consequence of the introduction of this Rectangular
Co-ordinates
reseau, the method ot measuring position-
angles and distances has been almost given up ;
and there has been substituted the measure-
75
MODEEN ASTRONOMY
ment of " rectangular co-ordinates," that is,
of two distances at right angles, instead of an
angle and a distance. The method may be
compared with that by which a person finds
his way in American cities, where the streets
cross at right angles and at nearly uniform
distances apart. He knows how many " blocks "
to pass going eastward, and how many " blocks"
to pass going north ; and when he comes to
the particular block containing the person he
wants, there is a further subdivision into
houses and rooms which guides him. On a
star photograph the streets are the cross lines
of the reseau, and the " blocks " are the squares
formed by them. The place of a star in a par-
ticular block is all that need be measured by
the micrometer, and the simple knowledge of
the place of the block completes the desired
information. The subdivision of the "block"
may be conducted by a micrometer screw as
before ; or better by two screws at right
angles. But there is one form of micrometer
in which screws are dispensed with, and there
is instead in the eyepiece a small scale of
equal parts, on which the observer reads the
distances. He thus gains considerably in ra-
pidity ; for to screw backwards and forwards
76
MODERN INSTRUMENTS
takes an appreciable time, which is saved by
the use of a scale ; and although with a fine
screw greater accuracy can undoubtedly be
secured than with a scale, it has been found
that even steel screws are liable to serious
wear, which necessitates constant examination
and application of troublesome corrections to
the readings, whereas a scale does not, of
course, alter in the same way.
Suppose again that the brightnesses of the Photo-
stars are under examination. A photograph Photometers
shows at once which are the brightest by the
size of their images. This size has nothing
whatever to do with the actual size of the
bodies which send us the light they are so far
off that all the light comes as if from a single
point it only represents the imperfections of
our instruments. A star image 1 ought to be a
mere point of light in the largest telescope,
but for a variety of reasons (firstly, because the
lens, however large, is finite in area, and th^s
introduces what are called " diffraction " phe-
nomena ; secondly, because no combination of
1 The case of the planets must be carefully dis-
tinguished from that of the fixed stars. The planets
show sensible discs, which increase in size as a higher
magnifying power is used.
77
MODERN ASTEONOMY
lenses can focus all the colours making up
white light accurately to the same point ; and
thirdly, because human workmanship is not
perfect, and even the focussing of one colour
is only approximate) the image only approxi-
mates to this form, and though there is a
central point of light it is surrounded by a
circular patch, which rapidly decreases in
brightness and is not bounded by any well-
defined edge, but merely becomes too faint to
be further perceived ; and in a star photograph
bright stars give larger images than faint ones,
because more of this patch leaves a record on
the plate. "We can get a very fair measure of
the brightness of the star by measuring the
size of the image ; but the indefinite character
of the edge introduces difficulties ; for instance,
different persons would make quite different
measures. And again, images of the faintest
stars shown on the photograph do not differ in
size, but only in blackness (or grey ness). Thus
even with star-images it is better to measure
the density of the image in some way rather
than its size ; and when we pass to pictures of
the planets, or of the Sun's corona, the density
of deposit is the only guide to the brightness
of the source of light. The true photographic
78
MODEEN INSTRUMENTS
photometer should thus measure the density of
deposit in some way ; and one of the simplest
instruments for doing so is Sir "W. Abney's Abney's
J Sector
revolving sector, an instrument of beautiful
simplicity. The principle is as follows : If
from an opaque circular disc we cut out a piece
of sector form, and then rotate the disc round
its centre in a beam of light, the beam is some-
times stopped by the remainder of the disc,
sometimes allowed to pass through the sector
cut away ; so that a screen placed in the beam
is sometimes illuminated and sometimes not.
When the rotation of the disc is slow, these
alternations of light and darkness can be re-
cognised separately ; but when the rapidity of
rotation is increased, there comes a time when
the eye entirely fails to notice the alternations,
and the effect is that of a steady illumination
of the screen by a beam of less intensity than
before. The apparent intensity is simply pro-
portional to the angular opening of the sector.
If then we have side by side two beams of
light of equal intensity forming patches of
light on a screen of equal brilliance, and if we
obstruct one of them by the photograph whose
density we wish to measure, and the other by
the revolving sector, we can obtain a measure
79
MODERN ASTRONOMY
of the density by varying the aperture of the
sector until its obstruction is precisely equal to
that of the photograph. By an ingenious
device this variation of aperture can be effected
while the sector is in rapid rotation ; and even
the angular opening read off without stopping
the rotation. This instrument is a most
valuable addition to the astronomer's equip-
ment, though its uses are by no means
confined to ustronomy.
The Spectro- j^ j^g k een remarked that the spectroscope
scope
has already created a new department of
astronomy, so vast that it is practically a
separate science, and has been called Astro-
physics. The spectroscope can, however,
scarcely be regarded at this date as a mod-
ern astronomical instrument. It is as much
the instrument of the chemist and physicist
as of the astronomer. The general principle
of analysing the light received from a body
into . its constituent colours, and thereby
recognising the nature of the source of light,
is by this time probably quite familiar to
those whose scientific knowledge is of the
slightest ; and instrumental details are com-
paratively unimportant. Still, to give a
MODEEN 1NSTEUMENTS
proper idea of astronomical methods of work,
and of discoveries with the spectroscope which
will follow later, it will be advisable to de-
scribe briefly the arrangements of the instru-
ments used in astronomy.
The simplest form of spectroscope, but one The Objec-
tive Prism
with limited applications, is a simple glass
prism, such as used to be hung from a chan-
delier. Most children of a generation ago,
and some of the present, have seen with de-
light the patches of " rainbow" thrown on the
walls by the Sun shining through such prisms.
These patches are, however, not of much use
for scientific observation, for they represent
the confusion of overlapping images. If there
were seven distinct and exact colours, red,
orange, yellow, etc., as in the ordinary lan-
guage, then there would be seven distinct and
exact images of the Sun in these colours ; but
even then there would be confusion, owing to
the overlap of the images. As a matter of
fact, the colours are by no means separate and
exact, but have every gradation : there are
millions of colours, and so millions of over-
lapping images, and great confusion.
If, however, we look at a star through the
81 G
MODERN ASTRONOMY
prism, there is no such, confusion, for the
images are points of light which do not
overlap ; and the total result is a thin line
of light of different colours. We may use
a telescope to magnify the effect, and we
then have one of the forms of astronomical
spectroscope, a prism with a telescope behind
it. The prism is of course much larger and
more accurately made than one from a glass
chandelier ; it must be as large in area as
the lens of the telescope behind it, and is a
good deal thicker than the lens at its thickest
part. A prism of this kind is therefore a
costly thing, but its work is well worth
the cost. It is essentially the spectroscope of
the astronomer, for chemists and physicists
generally deal with flames and other sources
of light, which are not points, but require
limitation by a slit ; it is only the astronomer
who finds in the stars, and on occasions of an
eclipse, sources of light which are already
points and lines, and thus require no artificial
limitation. Moreover, this form of lt objective
prism, "as it is called, has been hitherto chiefly
used by the astronomer who is also a photo-
grapher, and wishes to photograph the spectra
of the stars (or of the disappearing crescent
82
MODEEN INSTEUMENTS
of the Sun at an eclipse), especially if lie
wishes to make a comprehensive survey. If a
telescope be pointed to the heavens with a
photographic plate placed at the focus, then the
images of a number of stars will fall on the
plate and be photographed as points at wide
distances from each other. But if the prism
be placed in front of the object glass, each
little star image becomes a line instead of a
point, so that we get by one exposure the
photographic spectra of a number of stars,
whereas with the slit-spectroscope we can only
get one at a time that of the particular star
on the slit.
Before passing to the slit-spectroscope, we
may remark how a defect already noticed in
star spectra (viz., that the spectrum is only a
line of light, difficult to read because of its
narrowness) may be removed when dealing
with it photographically. Instead of using a
cylindrical lens (which answers the purpose
of giving width to the spectrum, but is apt
to introduce other errors), we may allow the
star to vary its place on the plate slightly
during the exposure. Such variation must
not take place in the direction of the length
83
Giving
Width to
the
Spectrum
MODEEN ASTEONOMY
of the spectrum, or we shall introduce just
that "confusion" noticed at first, but in the
perpendicular direction, the effect of which is
thus to repeat the spectrum above or below its
previous position, and so give it a sensible
width. This method can also be used with
the slit-spectroscope, which is arranged as
follows :
The sut The light from the Sun or other heavenly
scope body is allowed now to pass first through the
telescope and form an image at the focus. Of
this image all but a line of light is there
stopped by a screen, the line of light passes
through the slit in the screen, which must
be very carefully made, with edges as straight
and parallel as possible. Its width can be
varied according to requirements.
Now this light cannot be passed through a
prism as it is, for it is diverging from the
focus to which the telescope made it converge.
A collimating lens makes the rays parallel,
as they were before entering the telescope,
and we are then ready to use an apparatus
exactly similar to the former (viz., a prism and
. a telescope), though of smaller dimensions, so
that the prism can be relatively thicker at its
84
MODERN INSTRUMENTS
or we may have two or three prisms,
without running to enormous expense. In
this way we can get much greater dispersion
in the spectrum, i.e., separate the different
colours more effectively, so that the slit-
spectroscope, besides being the only possible
form of instrument for objects of sensible
area like the Sun, is also the instrument for
work requiring the greatest accuracy.
It should be remarked that in both the slit- Gratings
spectroscope and the objective prism form, we
may substitute for the prism a " grating " a
series of lines ruled close together with the
greatest nicety, many thousands of them to
the inch. Such an apparatus analyses white
light in the same way as a prism, but with
special advantages over the prism, and also
some disadvantages. Generally speaking, the
spectrum is more regular but fainter with a
grating. 1
1 It may be added that a good grating is also much .
more difficult to get than a prism. The best gratings
are made under the direction of Professor Rowland, of
the Johns Hopkins University, and each one takes some
months to make. The ruling is done by a diamond-
point on silver, the spacing between the lines being
effected by a screw moved by machinery. Needless to
MODERN ASTRONOMY
The Spectro- There is one form of the slit - spectro-
Heliograph . . .
scope which is as essentially astronomical as
the objective-prism, and is called the spectro-
heliograph. It is used to photograph parts of
the Sun's surface, which in the ordinary way are
lost in the surrounding glare, but owing to
their peculiar light can be made to reveal
themselves under appropriate conditions. The
ordinary light of the Sun when displayed as a
spectrum is found to be " continuous," i.e., there
is light of all colours, except for the dark
Frauenhofer lines. There are parts of the Sun,
>
say, the screw must be of an accuracy which tests
the maker's art to the utmost ; indeed, an accuracy
is required which the maker cannot attain, and his
workmanship is supplemented by an ingenious device.
When the screw has been made it is carefully tested
by refined measurements capable of detecting its
errors, and these are all noted. It is then possible to
say, for a given position of the screw, how much the
diamond-point is in advance of or behind its proper
position ; and apparatus is arranged to correct the
error. Those who have the opportunity of seeing this
. beautiful mechanism at work at the Johns Hopkins
University (in Baltimore) should not neglect it, but
it may be well to tell the following little story by
way of caution. It should be premised that one of the
main difficulties in making a grating is to find a
suitable diamond-point, which can only be done by
trial. The experienced workman in charge of the
86
MODEEN INSTEUMENTS
however, which do not give such a spectrum,
but one of only one or two colours. Such
are the "red flames" seen at the edge of the
Sun's disc on the occasion of a total eclipse : the
spectrum of these consists of a few bright
lines, one of which is a brilliant red line. Now,
though the light of these flames is faint com-
pared with the total light of the ordinary sur-
face,which is made up of so many colours, if we
arrange to stop out all colours but this parti-
cular red, we diminish enormously the bright-
ness of the continuous spectrum, without
affecting that of the red flames appreciably ;
machine turns a diamond ovei and over until he finds
. a likely-looking point, but on trial it turns out un-
satisfactory. It is a matter of " luck "; he must go
on trying until he finds one, and sometimes it is months
before the trials prove satisfactory. Now on one
occasion he had had a particularly long series of dis-
appointments, and had at last succeeded, and an
important grating was about half-ruled. A party of
visitors were admitted to see the beautiful machine
and duly admired it, but unfortunately the party in-
cluded one who had been there before, and who was so
anxious to show his familiarity with these matters
that he pointed out the " thing that was doing the
ruling " with his finger, touching it gently as he
spoke. Alas ! the gentlest touch was sufficient to set
the poor workman again looking for a good diamond-
point.
87
MODEEN ASTEONOMY
and in this way we can actually make the
red flames appear brighter than the rest of the
surface. We could stop out the other colours
in a rough sort of way by looking through red
coloured glass, but this is not sufficiently
accurate for our purpose. "We must use a
spectroscope to separate all the colours into a
band : then put a slit so as to admit the exact
red we want only, and stop all the rest : and
on this principle the spectro-heliograph is con-
structed. Other parts of the Sun, besides the
red flames, or chromosphere, which can be
photographed in this way are the so called
"faculse," bright patches seen generally in the
neighbourhoood of spots near the edge of the
Sun's disc. When a photograph of the Sun
is taken in the ordinary way (see illustration),
faculse are only seen near the edge : for in the
middle of the disc they are lost in the glare
of the ordinary light. With the spectro-
heliograph, however, they can be photographed
all over the Sun's disc, as will be seen in the
illustration given. In this case it is not red
light that is used but two lines near the violet
end of the spectrum. This beautiful instru-
ment has been used with great success by its
inventor, Professor G. E. Hale, now "Director of
AN ORDINARY PHOTOGRAPH OP THE SUN,
TAKEN AT GREENWICH.
THE SUN AS SHEWN BY THE SPECTRO-HELIOGRAPH (HALE).
89
MODEEN INSTBUMENTS
the Yerkes Observatory. I believe it was dur-
ing a trip to Europe, and after a chat with Sir
"William Huggins, that he thought of the in-
strument, and promptly returned to the United
States to put his idea into practice. It re-
warded him by complete success , and it is to
be hoped that when the new Yerkes Observa-
tory is completed, which is now nearly the
case, we shall have from it as regular a
record of the prominences and faculse as is
obtained of the spots at Greenwich and else-
where.
Such, then, are some of the new instruments Concluding
Remarks.
which have come into our hands quite recently
for the exploration of the heavens. They are
so numerous that the description of each has
necessarily been scanty, and I fear the general
effect of the enumeration may be confusing.
The director of a large observatory of many
departments, where it is necessary to use most
or all of them, may well sigh for the old days
of simplicity, when he would have been fully
equipped with a transit-circle and an equator-
ial. But for those who can limit their atten-
tion to one or two, the variety for choice is
splendid. There are instruments to suit all
91
MODERN ASTRONOMY
tastes, and, it may be added, all pockets. Those
who feel, as Sir William Huggins tells us he
felt in 1859, dissatisfied with the routine cha-
racter of ordinary astronomical work, may
follow him into the new science of Astro-
physics. An ordinary man cannot buy a tele-
scope like the Yerkes 40-inch ; but this large
telescope must clearly be devoted to work
which his 4-inch or 6-inch cannot reach, and
therefore leaves a clear field for him in depart-
ments which he can work at. Or, if he has no
telescope at all, he can for a few pounds get a
micrometer or photometer, which will enable
him to do first-rate work in the measurement
of photographs ; and there are vast stores
of photographs already taken awaiting mea-
surement. It is only necessary to make an
earnest beginning, and work and means will
find themselves.
92
Section II
MODERN METHODS
93
Section II
MODERN METHODS
IN the preceding section we have taken stock
of the various new instruments recently put
into the hands of astronomers. Most of these
have necessitated quite new methods of work,
which we now proceed to consider. But before
doing so, we may notice one or two "modern
methods " which are the natural outcome of
progress and experience, and would have been
adopted if these, new instruments had not
come into being. A conspicuous example is
the change in attitude towards the old problem
of finding the Sun's distance.
Transits
Thirty years ago, although the Sun's dis- of Venus
tance was known to be between ninety and
ninety-five millions of miles, the actual figure
was in doubt by some millions of miles. But
two transits of Venus were in prospect (the
transits of 1874 and 1882), and it was hoped
95
MODEEN ASTEONOMY
that by observations of these the margin
might be reduced to one-tenth of its width.
The hope was terribly earnest : it was scarcely
realized that the method itself might break
down, and leave us no better informed as to
the Sun's distance than we were before ; but
there was of course always the chance of bad
weather at so many of the stations that the
opportunity would practically be lost, not to
come again for over a century (for transits of
Venus occur in pairs at long intervals, and the
next pair will not come till the twenty-first
century). It is recorded of Mrs. Somerville,
who was able to carry on scientific work after
she had passed her ninetieth year, that one
of her chief regrets in dying was that she
should not u live to see the distance of the
Earth from the Sun determined by the transit
of Venus, and the source of the most renowned
of rivers, the discovery of which will immor-
talize the name of Dr. Livingstone." Those
who have already attained middle life will
remember the excitement caused by the
transits of Venus in 1874 and 1882. Ob-
servers who had been most carefully drilled
beforehand were sent out to all quarters of the
globe to make the requisite observations, and
96
MODERN METHODS
the result was awaited with, almost breathless
expectation. It was a great disappointment.
Unforeseen difficulties robbed the observations
of their expected accuracy, and though the
margin of our uncertainty as to the Sun's
distance was reduced, the reduction was trivial
compared with what had been hoped for. The
next pair of transits of Venus will occur in
2004 and 2012, but it is doubtful whether they
will attract much attention, for we no longer
look to these opportunities as the best for
determining the Sun's distance. The excite-
ment of 1874 has left the name "Transit of
Venus " ringing in the ears of the present
generation, but henceforward the name will
probably only be familiar to astronomers.
For not only has this method of determining
the Sun's distance failed, but another one has trom Ob -
servations
already attained a large measure of success, oi Mars
and is likely to be more successful still. "We
know very precisely the relative distances in
the solar system from one of Kepler's laws,
which originates with the law of gravitation
itself. This law connects the distances of
the planets from the Sun with the times
in which they revolve round the Sun ; and
97 H
MODEBN ASTRONOMY
these are known with extreme accuracy from
comparison of observations made long ago
with those of modern times. We can there-
fore make a most accurate map of the solar
system at any moment, leaving unknown just
one thing, the scale of the map ; and if we
know any one length in miles, this supplies
the scale and therefore all the other lengths.
Hence it is not necessary to measure the actual
distance of the Sun ; that of a planet will do
equally well, and the best distance to choose
for actual measurement is the shortest ; just
as we can judge with our two eyes of the dis-
tance of near objects, though we find it more
difficult for distant ones : indeed, for objects so
far away as the heavenly bodies we entirely
lose our perception of relative distance, and
the Moon seems as far away as the stars.
Parallax The method of gauging distances in astron-
omy closely resembles that by which we
estimate them with two eyes. To look at a
near object with both eyes we must draw the
pupils together, closer and closer as the object
approaches us. If it is brought very close the
convergence of the pupils becomes ugly and
painful, and is called a "squint"; but the
98
MODERN METHODS
squint is only an extreme form of what
happens in all cases our muscles turn the
pupils, or the axes of the two eyes, so that
both point to the object, and the muscular
effort varies with the distance of the object,
and so gives us a notion of that distance.
When the muscular effort is entirely relaxed,
the eyes have the well-known appearance of
gazing u into vacancy," or at an object at an
infinite distance.
This apparatus which we carry in our heads
for gauging distances is copied on a larger
scale by the astronomer. For two eyes at a
couple of inches apart, he substitutes two
telescopes as far apart as he can get them,
which on our earth is something under 8,000
miles ; since we increase the power of our
apparatus directly in proportion to the length
of this " base," as it is called, the distance
apart of the two stations from which observa-
tions are made. (If our eyes were two feet
apart instead of two inches, we should be able
to gauge distances twelve times as well.) For
the muscular effort in turning the eyes to
convergence on the object, the astronomer
substitutes the reading of graduated circles, or
99
MODERN ASTRONOMY
some other method of telling how much the
telescopes are inclined to each other. This is
the quantity he wishes to measure, the " par-
allax," as it is called, being simply the angle
between the directions in which the telescopes
are pointed. The angle is zero when the
object is at an infinite distance, the telescopes
then being parallel, and it increases with the
proximity of the object. Since it is easier to
measure a large angle within a given percent-
age of error, than a small one, the astronomer
chooses for observation the nearest object he
can get, because its parallax is largest. Until
recently the nearest suitable object for deter-
mining in this way the distance of the Sun
was the planet Mars.
Gill's In 1877, therefore, Dr. (now Sir David) Gill
Expedition
to Ascension took a heliometer to the lone island of Ascen-
sion, and spent six months there determining
the distance of Mars. In the first chapter I
ventured to specify the year 1875 as something
of an epoch in astronomical history, from
which new developments in various directions
may be dated. There is nothing beyond curi-
ous coincidence to establish ; but as a curious
coincidence we may remark that the value of
100
MODERN METHODS
transits of Venus practically ended with 1874,
and within a few years the modern method of
determining the Sun's distance was initiated.
Unlike the projects for observation of the
transit of Venus, which began so hopefully
and ended in disappointment, Gill's expedition
began with disaster and ended successfully.
The instrument he was to take with him to
Ascension was constructed for European lati-
tudes, and as he -thought it possible that when
set up for a tropical latitude it might not
work well, he set it up for trial in the rooms
of the Royal Astronomical Society some little
time before starting. His doubts proved only
too well founded. Scarcely had the instrument
been erected as it would be in Ascension, when
it overbalanced and came crashing down on
its delicate eye end, practically a wreck. The
curious may see to this day a bruise on the
table in the meeting-room of the Society, which
is a reminiscence of this disaster. But Sir
David Gill is a man not lightly discouraged ;
he got all the instrument-makers in England
to lend their aid, and the instrument was
repaired (impossible as it seemed immediately
after the accident) in time for him to start as
101
MODERN ASTBONOMY
he had intended. He arrived at Ascension
without further mishap, and then other
troubles began : there were thick clouds to
prevent his seeing Mars, and his health was
not good. But he moved his point of observa-
tion and got beyond the cloud-bank, and he-
and his devoted wife eventually triumphed
over other difficulties. The whole story is
vividly told in Lady Gill's 'Six Months in
Ascension. 1 Here we can only add that the
expedition was thoroughly successful in its
main object ; and not only was the margin of
uncertainty in the Sun's distance considerably
reduced, but the observations called attention
to other matters of importance, as will presently
appear.
The Diurnal One point has not been made clear. In
Method
describing the method of gauging the distance
of Mars, reference was made to a pair of tele-
scopes placed at the ends of the base, and Sir
David Gill only took with him one instrument,
his heliometer. The fact is, one telescope can
be made to serve as a pair if observations be
made both morning and evening, for the rota-
1 Six Months in Ascension. An unscientific account
of a Scientific Expedition. 8vo, London, 1878.
102
MODERN METHODS
tion of the Earth carries the instrument round
continually to different places, and we can thus
get a succession of observations from different
points, which, though not made simultaneously,
can readily be treated as though they were
simultaneous. We have said that our percep-
tion of the relative distances of objects in
ordinary life is largely due to our possessing a
pair of eyes, and that a man with only one
eye loses this perception in a marked degree.
But this is because his head remains "steady.
If his head rotated as the Earth does, and
he directed his one eye persistently towards
the same object so long as it remained in
view, he would be able to gauge its distance
by the apparent change of direction as the
eye passed across, a change which would be
large for near objects, and small for distant
ones. The idea of a rotating head is perhaps
too uncomfortable for sensitive nerves ; but
the principle can be sufficiently illustrated
by merely twisting the head from side to
side as far as it will go, keeping one eye
closed. It will be seen at once how near ob-
jects change their places relatively to distant
ones ; and precisely in the same way a single
telescope can observe the change of direction,
103
MODEEN ASTEONOMY
and hence the parallax, of Mars or another
planet with reference to the stars, and thus
serve the purpose of a pair.
Defect of The observations of Mars in 1877, though
Mars Ob-
servations thoroughly successful, were made under one
disadvantage. In measuring the angular dis-
tance of Mars from neighbouring stars, Sir
David Grill was comparing a disc of light with
points of light ; and, moreover, the disc was of
a different colour from the stars, being of a
reddish tinge. Any one familiar with obser-
vational or experimental work will recognise
that such differences as these are liable to
introduce small systematic errors. When, for
instance, the ordinary balance is used to weigh
any substance, the weights in one scale usually
differ in size and shape from the substance to
be weighed, and we must apply a small correc-
tion for the different buoyancy of the air.
We can estimate this with considerable ex-
actness, but we may not be able to gauge the
effect of air currents, or something else depend-
ing on the difference of shape and size which
may escape attention. We must be prepared,
in fact, for minute errors which would not
exist if the two scales were filled exactly alike,
104
MODERN METHODS
though we may not be able to assign the
cause of these errors very definitely. And so
in other cases : it is generally better to have
things which are to be compared in any one
particular as nearly as possible the same in
other , particulars, for we can often see how
differences may affect our result ; and even if
we cannot see this at the time, there is no
harm in guarding against the possibility that
we may have overlooked some way in which
an apparently irrelevant difference may act.
In the observations of the place of Mars among
the stars, the dissimilarity of the planet and
the stars was an undoubted disadvantage ; but
this disadvantage disappears if a minor planet
be taken instead of Mars, for a minor planet is
so small as to present no sensible disc it is
strictly comparable with a star in fact. On
the other hand, the minor planets are all con-
siderably farther away than Mars, with an
important exception to be presently men-
tioned ; and so we lose something by taking
one of these instead of Mars. Nevertheless,
Sir David Gill found it advantageous to do so,
and, with the help of several other astrono-
mers, he determined the Sun's distance three
times more from observations of the minor
105
Parallax
from Ob-
servations
of Minor
Planets
MODERN ASTRONOMY
planets Victoria, Iris, and Sappho, getting
results on all three occasions beautifully in
accord, and much more accurate than that
derived from Mars. These three results un-
doubtedly constitute a great advance on any-
thing that has ever been done. Combining
the results in the best way, Sir David Gill
finds for the Sun's distance
92,874,000 miles.
To show the close accordance of the observa-
tions, it may be stated that the observations
of Victoria alone give a result greater than
this by only 7,000 miles. Sappho alone indi-
cates 40,000 miles less, and Iris alone 100,000
miles greater. [The Mars observations in 1877
indicated a distance 200,000 miles greater ;
but, in Sir David Gill's own words, " the ac-
curacy of the Mars observations is very in-
ferior to that realized in the observations of
the minor planet Victoria in 1889." 1 ] Thus,
while the long-expected transits of Venus left
us uncertainty to the extent of perhaps a
million miles, Sir David Gill's observations
have reduced this margin to within 100,000
miles.
1 Mon. Not., R.A.S., vol. liv. p. 844.
106
MODEEN METHODS
Striking as is this advance, we are probably Eros
on the eve of a still further step, owing to the
recent discovery (in 1898) of the minor planet
Eros. This is the exceptional minor planet
above referred to which comes nearer the Earth
than Mars. By its occasional near approaches
to the Earth, it presents new opportunities for
789Q
S&o
1300
OCU
OKBiT OF EROS.
determining the Sun's distance, one of which
has come upon us while this book is in press.
During the present winter (1900-1) astrono-
mers are as busy as they were at the transits
of Venus, with the same object and nearly
a hundred times better chances of success.
There may not be so much stir 'to reach the
107
MODERN ASTRONOMY
ears of the public ; there will be no expedi-
tions for instance ; photographic telescopes
are sufficiently scattered over the Earth's
surface, without moving them from the fixed
observatories where they regularly dwell. Nor
is any very startling result to be expected as
the outcome of the work : the adopted value
of the Sun's distance cannot be much in error,
though if the small correction required can be
obtained with certainty, it is a matter of the
first importance to astronomy. But though
the occasion may yield nothing sensational to
the general public, it is of exceptional interest
to astronomers, who are hard at work photo-
graphing the planet on every fine night.
A photograph taken at the University Ob-
servatory, Oxford, on October 12th, 1900, is
here reproduced. Each star is shown nine
times over, the plate being exposed nine
separate times in slightly different positions
five of them soon after sunset, and four of
them just before sunrise. Under the letter B
are seen the nine separate images of a pair
of stars, which maintain their relative posi-
tions throughout. Under the letter A are
the five evening images of Eros and a star,
108
MODERN METHODS
and it will be seen how Eros moves away
from the star.
The uppermost pair of images are close
together, but the lowest pair is considerably
A PHOTOGRAPH OF EROS
(Taken at the University Observatory, Oxford, on
October 12th, 1900).
separated, Eros being above the star. The
morning four images of the star are shewn
to the right of these five, but the planet had
by that time moved quite out of the picture.
109
MODERN ASTRONOMY
The lines crossing the picture are the reseau
lines, which have several times been men-
tioned.
There will not be so favourable an oppor-
tunity again for thirty years, unless another
unexpected discovery of a small planet should
be made. Thus the history of the last quarter
of a century, in reference to this great pro-
blem of the Sun's distance, may be summed
up as follows : Our period opens with the
failure of the method of transits of Venus,
which had been looked forward to with such
eagerness and confidence for half a century at
least. But almost immediately the modern
method was employed with a fair measure of
success on the planet Mars. Much better
results were soon after obtained from three
minor planets, Victoria, Iris, and Sappho (all
this work chiefly by Sir David Gill, though
others heartily co-operated with him) ; and
within a few months a still more favourable
opportunity will probably give us the best
knowledge of the Sun's distance we are likely
to get for the next quarter of a century.
Heiiometer In explaining this change of attitude with
tions of the regard to the method for attacking this
110
MODEBN METHODS
problem, something has necessarily been said Planets
of the results obtained, anticipating perhaps
what would naturally come in Section III.
But we now turn to a " modern method " of
which the results cannot yet be quoted because
they belong to the future. The method was
suggested by Sir David Gill, as an outcome of
the work which has just been described. It
was thereby made manifest with what won-
derful accuracy the positions of the planets
among the stars could be determined an ac-
curacy far greater than can be attained with
the transit-circle. Commenting upon the ob-
servations of Mars, Professor Simon Newcomb
(the greatest living authority on such a matter)
remarks that " a minute inequality, which
would never have been noticed in even the
best meridian observations (i.e., observations
with the transit-circle) was brought to light,
and mapped in a diagram so as to be unmis-
takeable." And the observations of the minor
planets were more accurate still. Hence Sir
David Gill puts the pertinent question : why
should a method capable of this accuracy not
be used more generally instead of only on
special occasions? Has not the time come
when the transit-circle observations of the
111
MODEEN ASTBONOMY
planets should be superseded by observations
with the heliometer?
The The heliometer is not a new instrument. It
Heliometer
might quite reasonably have been added by
Airy to the Greenwich equipment; but Airy
had little sympathy with equatorial work, and
the heliometer is a form of equatorial, if we in-
clude the micrometer at the eye end. The use
of a micrometer has already been described : it
will measure the angular distance between a
pair of stars and the direction in which they
are separated. The heliometer does exactly this
work in a more complete and efficient manner
than the micrometer. Speaking generally, we
may say that the heliometer can measure
points further apart than the micrometer. Its
..**'*'''
principle of construction is as follows :
The image of a star at the focus of a tele-
scope is formed by light coming from all por-
tions of the lens. If we cover up a part of the
lens tl\e image remains in the same place, but
is less bright, because we have taken away
part of the light which went to form it. It is
also slightly different in shape, the "diffraction
phenomena" being affected by the shape of the
lens, which is now no longer circular ; but these
112
MODERN METHODS
effects we will neglect for the present. If, then,
an object-glass be cut in two along a diameter,
each half may be considered as forming its own
image, the two images being superposed to
form one only so long as the two halves of
the glass are in their usual position. But now,
PRINCIPLE OF THE HEL1OMETER.
suppose we slide them asunder in the direction
of the division, each half will carry with it
its own image, and we shall get two images
of a star, separated by just the interval sepa-
rating the halves of the object-glass. Suppose
now that there are two stars, E and G, in the
field of view, and that we have rotated the
object-glass so that its divided diameter is
113 i
MODERN ASTRONOMY
parallel to the line EG. and let EeGg be the
pairs of images formed by the divided object-
glass. As we separate the halves further and
further the distances Ee and Gg increase in
exactly the same way ; and there comes a
position when one image e of one star coincides
with one image G of the other. The separa-
tion is then exactly equal to the distance be-
tween the star images. We can measure this
separation with great nicety, and so get the
information we want. The position-angle of
the line EG is got by reading on a graduated
circle the position of the divided diameter
which allows us to make this coincidence of
images ; for it can only be made when the
divided diameter is exactly parallel to the line
joining the stars.
Thus the heliometer does what the micro-
meter does, only in a better way. To explain
the superiority completely would involve
some rather technical matters ; but two ad-
vantages can be stated with tolerable sim-
plicity. Firstly, the observations with the
heliometer are less dependent on the good
going of the driving clock ; for if two star
images are made to coincide they will remain
114
MODERN METHODS
coincident even if the clock drives badly.
"With the micrometer, on the other hand, the
observer has to put one of the wires on one
star and then to attend to the other star with
*
the other wire : meanwhile, bad clock-driving
may have upset his careful adjustment on the
first star. Secondly, as already remarked, the
heliometer can measure larger distances ; be-
cause the separation of the lenses is watched
by a scale instead of a screw ; and the scale
has several advantages for one thing it is not
liable to wear like a screw which is much used.
But it is with the advantages of the helio- Comparison
meter over the transit-circle with which we Transit-
are chiefly concerned for the moment, rather
than its advantages over the ordinary micro-
meter, and the chief difference is this : The
transit-circle can only make an observation
at a particular time, viz., when the object
is on the meridian. We can, on the other
hand, with the heliometer go on all night
(or as long as the planet is visible) measuring
its position with respect to neighbouring
stars. It is true that we must, in order to
get all the information we require, proceed
then to find the places of these particular
115
MODEEN ASTEONOMY
stars on the celestial sphere : that is, we must
make transit-circle observations of them, and
at first sight it might seem as though we
had gained nothing, but rather lost : for in-
stead of one planet we have now several stars
to observe with the transit-circle (one com-
parison star not being regarded as sufficient,
for reasons which may be passed over for
the present). But the difference is this : the
stars are practically fixed in the sky, their
movements during years, or even centuries,
being very small ; and hence we can accumu-
late observations of them at leisure. But the
place of a moving planet is constantly chang-
ing, and an observation missed at one time
cannot be compensated by another subse-
quently.
Hence Sir David Gill has proposed that we
should enter upon a new method of planetary
observation. Instead of merely observing the
planets when they come to the meridian with
the transit-circle, he proposes to make a
number of measures throughout the night
with a heliometer, of the place of the planet
among the neighbouring stars, and then
determine the places of these stars with the
116
MODERN METHODS
transit-circle. The change may be illustrated
from everyday life. Suppose we had a watch
whose going we wished to test. One way of
doing so would be to wait till the church clock
struck at each hour and see what the watch
read. , Sometimes we might miss an hour by
accident, but we should get a fairly good idea
of the going of the watch even if it went
somewhat irregularly. It might gain in the
morning for instance, and lose in the afternoon ;
or go much faster in summer than in winter :
all this we could find out. But suppose it
went irregularly in between the hours : we
should perhaps be unable to notice this from
the hourly comparisons with the church clock.
If, however, we had an intermediary in the
shape of a clock known to be really good,
which could be compared with the watch at
any time, we could make our knowledge of
the watch's going as complete and accurate
as we pleased, checking the general rate of
the good clock by the striking hours as
before.
In this illustration the striking of the church
clock at fixed times represents transit-circle
observations limited to occasions when the
117
MODERN ASTRONOMY
planet is on the meridian. Our erratic watch
is a planet, whose general going we may
observe by transit-circle observations, but
about which we may obtain much more and
better information by comparing it with the
neighbouring stars our intermediary good
clock. We know that the stars go regularly,
and their movements are sufficiently checked
by the intermittent observations with the
transit-circle.
New This proposal is of considerable importance.
Tables It is made at a most appropriate time : for not
only are we on the eve of a new century, but
our knowledge of the planetary movements
has just been put on a new footing by the
formation of new tables of the planets. The
basis of any set of tables is the accumulated
knowledge at the date of construction. Start-
ing from this, tables are made predicting the
places of the planets for the future. The com-
parison of these predictions with observations
gives materials for correcting the tables. Such
corrections are not made until a considerable
mass of new observations has been accumulated
since the last edition : then the great work is
undertaken of discussing and co-ordinating all
118
MODERN METHODS
these observations, made in different countries,
with essentially different instruments, and by
different observers, and of deducing from them
the best possible revision of the old tables.
This task is not only very laborious, but re-
quires mathematical skill of the highest order
and the astronomical world owes a great debt
of gratitude to Simon Newcomb, of Wash-
ington, for having just completed such a re-
vision of the tables, which will enable them
to start the new century with a new and
closer approximation to the truth. Sir David
Gill has already put his proposal for doing
justice to the occasion into practice at the
Cape Observatory, and there can be but one
opinion as to the value of his work. The only
question seems to be whether the photogra-
phic method might not give sufficiently good
results with less labour. This is as yet to be
tried ; but in any case the principle remains
the same : observations of the planets will
almost certainly be made more often and more
accurately than before.
A principle of the same kind has been adop- Photometric
ted by Professor E. C. Pickering, of Harvard tions of
University Observatory, in the observation of satellites
119
MODERN ASTRONOMY
the eclipses of Jupiter's satellites. To watch
these eclipses is one of the simplest and most
interesting observations which a beginner,
equipped with a small telescope, can make.
Jupiter, with his line of four moons, like little
beads on an invisible string, can be seen in a
very small telescope. At times which are
given in the Nautical Almanac, one of them
is seen to fade and ultimately disappear ; it
has passed into the shadow of the planet and
lost the Sun's illumination. The moment
when it is thus eclipsed is an accurate indica-
tion of its place in its orbit round Jupiter,
and the observation is independent of any
measuring apparatus. Such observations have
therefore been regarded as giving us the best
information we can get for calculating the
orbits of the satellites ; and from their sim-
plicity and acknowledged value they have
been made in considerable quantities. But
they are found to be disappointingly in-
accurate. The moment of disappearance de-
pends a good deal on the observer, and cer-
tainly on the instrument employed, and there
is a large " probable error " of observation.
Professor Pickering has carried into practice
a method of observing these eclipses which
120
MODERN METHODS
gives very much more accurate results than
the old method. He measures with a photo-
meter the brightness of the eclipsed satellite
compared with one of the others. Before it
passes into the shadow the ratio of bright-
nesses is constant, but when it enters the
shadow the ratio begins to diminish, and if
observations are made (say) every ten seconds,
the resulting ratios show. a steady progression
towards zero, which may be exhibited in the
form of a curve or diagram. It is clear that
by observing a large number of points on this
curve instead of one only (the vanishing
point), we enormously increase the accuracy
of observation. Suppose, for instance, that a
light cloud gradually passed in front of Jupi-
ter near the critical moment. The observer
using the old-fashioned method might lose
sight of the almost eclipsed satellite without
noticing (from the slightly diminished light
of Jupiter) that the disappearance was due to
cloud. But in the new way previous observa-
tions would be a guide to an accidental cir-
cumstance of this kind : cloud would obscure
both satellites to the same extent, and leave
the ratio little affected, or if it blotted out
the fainter altogether would indicate an
121
MODERN ASTRONOMY
abrupt termination to the curve which the
rest of it would show to be accidental. A
large number of these observations, extending
over many years, are now in the hands of
Professor Sampson, of Durham, who hopes to
get from them material for new tables of
Jupiter's satellites, though it will take a con-
siderable time to do this.
Measures of There is another new plan of satellite
Saturn's
Satellites observation which is certainly modern in the
sense that it was first used recently, but there
is no particular reason why it should not have
been adopted long ago. It was initiated
by Dr. Hermann Struve in observing the
places of the satellites of Saturn with
the large telescopes of the Pulkowa Obser-
vatory.
The change consisted in this : previous ob-
servers had measured the distance of each
satellite from the body of the planet Saturn,
and observed the position-angle of this dis-
tance. Now Saturn is a very bright object with
a large disc, and a satellite is a faint point of
light ; and it is very difficult to make measures
of two such dissimilar objects without con-
siderable bias in one direction or the other
122
MODEKN METHODS
a bias which is different for different observers
and not easy to determine. Dr. Struve, there-
fore, gave up this method, and measured the
position of one satellite with reference to
another. Since both were moving round the
planet, the changes of position became more
complex, depending on the motions of both,
instead of on that of one alone as in the old
method. But Dr. Struve showed himself fully
equal to dealing with the more complex cal-
culations, and found several new and re-
markable relations between the movements of
the satellites, which former observations had
been quite unable to detect. I well remember
showing his early results to the late Professor
Adams, who found them so astonishingly accu-
rate that he could scarcely believe them. The
work which followed showed, however, that
there had been no mistake, and the whole
investigation, which has been quite recently
published, has put our knowledge of the
Saturniaii system on a new basis. This in-
creased activity in a department of astro-
nomy which belongs essentially to the old
times is, perhaps, an even better indication of
the new life which has been poured into the
science than some of the essentially new
123
MODERN ASTBONOMY
enterprises. There is no particular reason
why the " triangulation " of Saturn's satel-
lites, as this new method of measurement is
called, or of Jupiter's satellites, or the photo-
metric observation of the eclipses of the latter,
should not have been commenced much earlier
in the century : except that astronomers had
settled down into a groove, and regarded
novelties with some disfavour. They have
been a long time in recognising properly the
uses of photography, but having been once
roused, they are readier to accept changes such
as those just mentioned, which might have
been made long ago.
Changes of If we keep to the classification adopted in
Structure in
large the first section, our attention is next claimed
Telescopes . .
by changes in method which the increase in
size of telescopes has suggested or necessi-
tated. The most noteworthy recent improve-
ments are mainly concerned with the comfort
of the observer, which, though in some aspects
a mere detail, is of the greatest importance
in delicate observation, when the nerves and
muscles should be as free from strain as
possible. Sir Howard Grubb's invention of
the rising-floor has already been mentioned.
124
MODEBN METHODS
This does a good deal towards keeping the
position of the observer the same relatively
to his instrument.
M. Loewy, however, now director of the The
Paris Observatory, has done this much more coude
completely with his equatorial coude, though
this involves a radical change in the form of
the instrument. The observer sits in an arm-
chair in a comfortably warmed room much as
a man sits with a microscope. He looks in a
fixed direction down the polar axis of the in-
strument, and the light from the star he
wants to see is reflected to him by an arrange-
ment of mirrors or prisms, passing through
the proper lenses, of course, on its way. The
parts of the telescope which follow the stars
are out in the open air. He can direct them as
he pleases by mechanism, close to his hand, of
a very simple character. The actual shape of
the instrument and its movements may be
illustrated with the human arm. Let any one
hold steady the portion from shoulder to
elbow, which is to represent the fixed tube
down which the observer looks : he is sup-
posed to be perched at the shoulder. Now
crook the elbow (which gives the name coude
125
MODEEN ASTRONOMY
to the telescope) to a right angle, and imagine
a mirror placed at the elbow, so that light
coming from the fingers down the forearm is
reflected by the mirror at the elbow to the
observer at the shoulder. The lens of the
telescope is placed at the fingers, where there
is also a second mirror, which gives command
over different stars A telescope, in order to
reach any part of the sky at a given moment,
must have at least two motions. In the
equatorial coude one of these is given by this
mirror near the lens (at " the fingers"), the
other is obtained by rotating the whole in-
strument round the elbow-to-shoulder portion
as axis. The rotation is effected by clock-
work in the same way as for any other equa-
torial, so that the complete instrument now
stands as follows : A mirror at the fingers,
set at 45 with the forearm (and capable of
rotation round it as axis), reflecting stars
of different declinations through the object-
glass down the forearm ; another mirror
at the elbow, which reflects the light up to
focus at the shoulder, and the whole move-
able by clock-work in right ascension, i.e.,
round the axis of the elbow -to -shoulder
portion.
126
MODEEN METHODS
There are other devices of a similar kind in ' rhQ sheep-
shanks Tele-
which the same principle is adopted viz., that scope at
,, , , i -, . . -, Cambridge
the observer should remain in a constant and
comfortable position, while the movements of
the instrument are still under his control. A
fine telescope of this class has recently been
set up at the Cambridge Observatory ; but it-
differs from the coude of M. Loewy in having
only one mirror at the elbow, the motion in
declination being supplied by varying the angle
at which the forearm is crooked, and at the
same time varying the position of the elbow-
mirror at half the rate ; a condition which
involves a little mechanical ingenuity, but has
been found perfectly feasible. Two illustra-
tions of this telescope are given. In one the
portion of the telescope out in the open air is
shown, the elbow being considerably crooked.
To cover up this part of the instrument when
not in use, there is a shed running on rails,
which has been pushed aside in order to take
the photograph. The upper part of the instru-
ment disappears into the small tower for the
observer, the interior of which is shown in the
second illustration. Instead of lying on his
back, or twisting his neck according to the
vagaries of the object to be observed, he can
127
MODERN ASTRONOMY
sit permanently in the position occupied at a
writing-table ; and there is another point of
material comfort the room can be comfortably
warmed without affecting the definition of the
star images too much ; whereas in the ordinary
dome the temperature must be kept as
nearly as possible the same as that of the
outside air, in order to avoid air currents,
which spoil the definition of the images ; and
in the winter this makes observing cold work.
It is not easy to represent this difference of
temperature in an illustration ; but the general
gain in material comfort is symbolized by the
introduction into the picture of a table on
which apparatus is arranged for afternoon tea.
The Tele- To divide a telescope into two portions, one
Amateurs of which can be fixed, is not a new idea, having
been adopted in the " broken-transit " and
other instruments. Professor Pickering, who
uses the principle in his " meridian photo-
meter," remarks with surprise how little use is
made of it by amateurs ; for it is specially im-
portant to those who regard astronomy as an
amusement rather than a profession that they
should not be gradually weaned from it by
discomforts, as is so often the case. Probably
128
Outside View.
Inside the Observing Tower.
THE SHEEPSHANKS TELESCOPE AT CAMBRIDGE,
129
MODEBN METHODS
they buy their instruments when their astro-
nomical knowledge and acquaintance is slight ;
they buy a telescope of the ordinary pattern,
and set it up some distance from the house in
a small and uncomfortable building. Their
first enthusiasm carries them out to look at
the wonders of the sky, though the night may
be cold, and the position for observing cramped ;
but there comes a time when the key of the
observatory is not often required. With some
device for bringing the eye end of the telescope
into the comfortable study, the fascination of
the work would be much more lasting. Pro-
fessor Pickering, with his meridian photometer,
does four hours' observing with the greatest
comfort on every fine night. M. Loewy had
given up observing at all before he invented
the equatorial coude, owing to its discomforts
and inclemencies ; he had left it to younger
men, and occupied himself with indoor astro-
nomy. But when his instrument was built,
he went back to it with delight. The matter
is worth the attention of amateurs. There is
no reason why instruments of this kind should
be more costly than 'others : the only necessary
extra is a plane mirror or perhaps two ; and it
should be easy to devise a form of covering for
131
MODERN ASTRONOMY
the part of the instrument which is in the
open, which will be cheaper than the ordinary
" observatory."
g rea test changes in all methods of work
have followed on the introduction of photo-
graphy. The patient delineation of objects by
hand has been largely (though not entirely)
superseded ; discoveries are made automatically
instead of by toilsome searching ; objects too
faint to be seen are rendered visible and
measurable ; sudden phenomena are seized and
retained for detailed examination : in numerous
ways the work of the astronomer has under-
gone the most fundamental modifications.
The new instruments introduced into the ob-
servatory, of which some account was given in
the first section, are of themselves sufficient to
change its aspect ; but the changes in the
actual processes of work are even greater.
Pictures of The most obvious use of photography is to
the Moon . . , . ,
give us at >nce a picture which formerly had
to be elaborately and patiently drawn by
hand. The observer need no longer sit for
hours, perhaps months or years, at the tele-
scope, peering through it for a moment,
transferring his impressions to paper ; again
132
RUSSELL'S DRAWING OF THE MOON, 1795.
THE MOON AS PHOTOGRAPHED AT PARIS, 1895.
133
MODERN METHODS
looking for a little more information, and so
alternating between telescope and sketch-book.
He has only to put in the plate, watch that
the telescope is properly guided, and at the end
of the allotted time he can develop a picture
far more accurate than he could ever draw.
There is at the Radcliffe Observatory, Oxford,
a beautiful crayon drawing of the Moon bear-
ing the date 1795. It is five feet in diameter,
and was made by John Russell, R.A., who de-
voted to the work all suitable nights which he
could possibly spare for nearly eighteen years.
His telescope was a 6-inch reflector lent him
by Herschel, and he undertook the work at the
suggestion of the President, of the Royal
Society. There is no doubt that it represents
the utmost that skill and labour could produce
a century ago. In some ways the value of
this drawing will never be superseded by
photographs ; but a photograph can now be
obtained in a second or two which is for many
purposes much better than the result of these
eighteen years of an artist's work. In the
illustration a copy of Mr. Russell's drawing is
shown alongside one of the beautiful pictures
of the Moon taken at the Paris Observatory
with the equatorial coude] a good deal of
135
MODERN ASTRONOMY
detail is, of course, lost in reproduction, but a
fair comparison of the two can be made. The
photograph is not always superior to the draw-
ing ; there are small details which the eye can
catch that are lost in the photograph. The
very faithfulness of the photograph is in some
respects a disadvantage : it catches the whole
surface of the Moon just as it is at a particular
moment, but only at that moment. Now the
artist cannot do this : the sunlight and shadow
on the Moon's surface are continually changing,
and while he is drawing one part another will
have altered. Hence he cannot rival the ac-
curacy of the photograph. On the other hand,
he can, by watching the changes under different
illuminations, get an idea of the real shape of
objects, and perhaps convey it in his drawing,
which thus becomes to some extent the equiva-
lent of several photographs.
Pictures of This advantage of the artist only exists,
the Sun J
however, when the changes are regular and
due to known causes, such as the rising and
setting of the Sun on the lunar mountains.
But there may be irregular changes which he
cannot allow for in this way, even on the Moon.
And there are other bodies subject to un-
136
MODEKN METHODS
doubted changes in form or structure. The
surface of the Sun, for instance, undergoes real
variations with sensible, and some of them
with startling, rapidity. Most people have
heard of Sun-spots, though they may not know
of their caprices. "We find from observation
that the average number of spots on the Sun
is subject to periodic fluctuations in about
eleven years, but we are almost as far off as ever
from knowing what the spots are, or what
causes them. A spot appears on the disc,
grows, lives a time, fades away and dies, like
something organic ; or perhaps we might say
as a storm comes and passes. During its life-
time the spot is carried across the Sun's disc by
the rotation of the Sun ; it may be carried
across several times, disappearing for the
interval when it is on the face turned away
from us. Thus the Sun's surface is constantly
changing, and a photograph, which can be
taken in the hundredth part of a second, or
even much less, has obvious advantages over
a drawing requiring a considerable time to
make.
In the case of comets and nebulae also the Comets and
Nebulae
draughtsman has been almost entirely super-
137
MODERN ASTRONOMY
seeled by the photographer ; for in these objects
there are delicate details which it is beyond
the skill of the draughtsman to portray.
Especially is this the case with regard to the
relative brightnesses of different parts : photo-
graphs show these to have been strangely mis-
represented by the draughtsman. There is a
very good reason for this fault : the draughts-
man only sees a very little of the object at one
time. The eyepiece of a large telescope only
admits light from a very small portion of the
"field"; and when another portion is to be
examined, the eyepiece must be moved or the
telescope bodily moved. Professor Barnard
devised a few years ago a striking lecture ex-
periment.. He threw on the screen what he told
us was the Great Nebula in Orion ; but all that
we could see was a small patch, the rest of
the screen being dark. He explained that the
small patch was all that could be seen of this
nebula at one time with the great 36-inch of the
Lick Observatory ; but he showed us that the
nebula was really there by moving about the
small illuminated patch (which was formed by
an aperture in an opaque cover) to different
portions of the picture, much as the eyepiece
could similarly be moved about. We were in
138
MODERN METHODS
this way able to follow the well-known form
of the nebula. But what a difference when he
removed the opaque cover and showed us the
photograph in all its grandeur ! It was a
veritable revelation ! The photographic plate,
which receives the impression of the whole
picture simultaneously, is at an immense
advantage compared with the eye, which must
wander from point to point, forgetting, or
modifying by recollection, what it has already
seen.
But having shown how severely the Planets
draughtsman is handicapped in his competi-
tion with the photographer, we must now
notice one case in which he still holds his
own ; which is in pictures of the major planets.
The reason is, that owing to the restlessness of
our atmosphere the very fine detail on these
planets is only visible by glimpses. Now the
photographic plate can take no advantage of
glimpses ; it goes on steadily recording during
the whole time of the exposure ; good views
and bad views are superposed, and the result
is a confused or blurred image, in which the
fine detail is lost. It has, indeed, been pro-
posed that an observer should attempt to take
139
MODERN ASTRONOMY
advantage of glimpses even photographically ;
that he should watch the surface through
another telescope, and only open the shutter of
the camera at the good moments. But this
would require marvellous quickness of hand
and eye, and even then might not attain its
object ; for the air tremors which ' blur an
image are very local, and there might, for
instance, be disturbance of the image in the
photographic telescope just when the rays
coming through the other were steadiest. I
do not know of the method -having been tried
with any success.
Discovery of Besides giving us faithful pictures of known
Planets objects, photography affords a simple method
of discovering previously unknown ones ; and
this is well illustrated in the case of the minor
planets or asteroids. So many of these are
now known that it is amazing to think that
search was made for such objects for years
before even one was discovered. The first dis-
covery was made on the first day of this cen-
tury 1801, Jan. 1, and three others were found
within a few years ; but for three decades after
this no new planets were found, in spite of
considerable labour spent in looking for them.
140
MODERN METHODS
Knowing as we do now, that there are hun-
dreds, perhaps thousands of these bodies, this
ill-success seems strange. But it is by no
means an easy matter to recognise these objects
without the aid of photography. They are
mere points of light, indistinguishable from the
surrounding stars except by their slow motion
among the stars ; and to detect this, methodical
measurements and very careful observation
indeed are required. There are few people
who have the faculty of remembering a con-
figuration of stars so accurately for a length of
time that they can recognise the movement of
one individual ; and yet this was the only way
of discovering minor planets. With photo-
graphy to help the case is now very different.
If a prolonged exposure is made on a region of
sky, and the telescope is properly guided, so that
the fixed stars show round images, an object
which is moving among them will leave a
" trail," as shown in the illustration. (A ring
has been drawn round the trail to draw atten-
tion to it : this ring must not be mistaken for
part of the photograph.) In this way minor
planets betray themselves automatically. Dr.
Max "Wolf, of Heidelberg, has already discovered
over three score in this way ; and the illustra-
141
MODERN ASTEONOMY
tion is a copy of his photograph which revealed
the planet Svea. There is a delightful sim-
plicity about this method of exposing a plate
and looking to see what trails are caught : Sir
Robert Ball has called the arrangement a
DISCOVERY OF THE PLANET SVEA (MAX WOLF).
u star- trap." There is one attendant danger:
that an accidental mark or stain on the film
may be mistaken for a planetary trail ; but
this is easily guarded against by exposing
another plate and comparing the two,
142
MODEEN METHODS
It may be added that comets have been Discovery of
Comets
caught in the " star-trap " by exposing a
plate at a venture and examining the objects
photographed but not often. Perhaps more
will be done in this way in the future : there
is no d.oubt of the feasibility of the method.
Sometimes, too, meteors have been caught by Photographs
the watchful plate as they flash by ; and here
the superiority of the plate over the eye is
obvious. The record on the plate is perman-
ent : that on the retina begins to fade at once,
and unless immediately converted into a note
or a diagram on paper is soon irretrievably
lost. Even then the transference to paper
carries with it serious imperfections. The
photograph is superior in every point save one,
viz., if the meteor is not a bright one, or flies
across too quickly, its. light may not leave any
impression on the plate. Films are still increas-
ing in sensitiveness, and it is not known with
accuracy what we can do with our present
ones. Much was hoped from photographs of
the Leonids last November ; but as you are
aware the result was a failure, not so much
from want of sensitiveness in the films as from
want of meteors to experiment upon ; and this
143
MODERN ASTRONOMY
department of photographic astronomy is still
quite in its infancy.
Eclipses But there are other important phenomena
occupying short intervals of time, in recording ;
which photography has already proved itself
of immense value for instance, eclipses.
There . are four kinds of eclipses : total and'
partial eclipses of the Sun, and total and
partial eclipses of the Moon. But when an
astronomer speaks of eclipse work or an
eclipse expedition, he generally refers to a f
total eclipse of the Sun only, which far'
transcends all the others in importance, being. I
an opportunity of a special kind. "When the
ordinary Sun is completely hidden by the
Moon, there flashes into view the glorious
appendage to the Sun called the Corona. It
does not appear until the last trace of ordinary
sunlight is gone, and it vanishes with the first
returning ray. The interval between is only
a minute or two, and the occasion only occurs
about once in two years in strictly limited
localities, which may involve journeys of
thousands of miles to reach them. Yet these
brief and rare intervals are the only oppor-
tunity we have for studying what is really
144
MODEEN METHODS
far the most interesting part of the Sun. In
the year or two between eclipse and eclipse
the corona is for us entirely hidden from
view by the glare of the sunlight on our
atmosphere ; and we can only conjecture its
listory from the brief glimpses afforded us on
he occasions of a total solar eclipse. Small
Bonder then that we desire to make the
r ery most of our opportunities, and to this
nd photography has been invaluable. Eye-
bservations of the corona and its spectrum
fere hurried and affected by intense excite-
nent ; the photographic plate records its im-
oressions with lightning rapidity, but without
.he least flurry. Drawings of the corona
were even more unsatisfactory than those of
nebulse ; for they could not even be made
patiently. Photographs of the corona are
better than those of nebulae, for the object
is brighter, and shorter exposures can be
given.
Look, for instance, at the illustration show-
ing two of a series of drawings of the same
corona (that of 1878), which are bona fide
attempts to represent the same phenomenon.
They are not exceptional : in the Washington
145 L
MODERN ASTRONOMY
Observations for 1876 a whole series of similar
drawings was published, resembling each other
no more than this selected pair. Now look
at the two photographs of the 1893 eclipse
taken at places some 2,000 miles apart, and
you will see the closeness of the similarity.
Indeed, it is so close as to be in a sense
disappointing. We had hoped that in the
interval of an hour between totality at these
two widely separated stations, some minute
change of structure might have declared
itself. But on the most careful examination
no trace of such change has been found.
Drawings of the corona have recently shewn
a considerable improvement, but this is in
part due to the education afforded by photo-
graphs.
Eclipse work does not consist merely in
taking pictures of the corona ; the spectrum
of the corona is also scrutinized, and its
light is examined for polarization. But in
this and other eclipse work photography has
become almost imperative. The eclipse ob-
server no longer trusts his eyes and excited
brain during the few precious minutes of
totality : his skill is exercised in preparing
146
148
149
MODERN METHODS
beforehand, with the utmost care, a pro-
gramme of work which shall give him the
test photographs. This programme, which
may require the help of several persons to
carry out, is rehearsed by them over and
over again, until the operations can be per-
formed like clockwork ; and then, when the
critical time comes, the observer and his
assistants become as like machines as possible,
and are relieved from the strain of making
observations.
There is a story of Sir Greorge Airy which Relief of
brings home to us the difference between the Observer
old order of things and the new. A dis-
tinguished American astronomer, who had
come to Europe to observe a total eclipse,
paid a visit to Greenwich on his way. In
taking leave of him Airy said, " Well ! good-
bye ; and I think the best wish I can wish
you is cloudy weather ! I have been on
several eclipse expeditions, and on the whole
the most satisfactory ones have been those
when we saw nothing because of clouds."
In this rather cyuical remark there was
considerable wisdom. Eye-observations made
so quickly, and under the strain 'of anxiety
151
MODERN ASTRONOMY
and excitement, were liable to errors and un-
certainties which sometimes rather hindered
than helped the progress of our knowledge.
The observer felt bound to do his best, but
if clouds prevented any observations at all,
he had at least the 'satisfaction of know-
ing that he had made no mistakes. With
photography to help him, however, and
proper time for preparation, he need have
less fear. He can make the operations almost
automatic : indeed, one enterprising eclipse
Automatic observer, Professor D. P. Todd, has devoted
Methods
himself to proving that they can be made
absolutely automatic. He arranges an
immense amount of apparatus so that it
can be played like an organ, by pneumatic
arrangements controlled by clockwork ; and
after putting all this in order he can, as
he says, go quite away at the time of the
eclipse, knowing that his machinery will
take all the photographs he wants in the
proper way. Thus equipped he did any-
thing but hope for cloudy weather; but, by
the irony of fate, cloudy weather seemed
to dog his footsteps. Three times he set
up his apparatus at three different eclipses
and was disappointed by the weather ; he
152
MODERN METHODS
merely had the satisfaction of knowing that
all had gone perfectly, and if there had not
been clouds he would have got excellent
photographs. At last, however, after having
carried his apparatus some 50,000 miles
altogether, the weather ceased to persecute
him, and in May last he had the satisfaction
of seeing his astronomical organ play a real
tune for the first time, in Tripoli. No one
else has yet adopted his plan so far as I know;
but we all of us make ourselves as nearly like
machines as we can during the few minutes
of totality. I am tempted to mention" a trivial
circumstance in illustration. For the total
eclipse of August, 1896, we had pitched our
astronomical camp- in the hotel yard of a
Japanese fishing village. The officers and
men of H.M.S. Pique were told off to assist
us, which they did in the ablest manner.
Some of them were to hand the plate-holders
to the operators and others to receive them,
so that no precious fraction of a second should
be lost : others were to screen the lenses until
the word was given to make the exposure :
and two were to count out in a loud voice
the seconds of total eclipse, " One, two, three,"
etc.
153
MODERN ASTRONOMY
All these operations were practised several
times on the days before the eclipse, so that
everything should go without a hitch : and
a crowd of merry little Japanese round our
enclosure watched and listened with the
greatest interest. Daring one of our sub-
sequent walks we were startled to hear the
voices of the time-keepers imitated with
wonderful distinctness " One, two, three,"
etc. ; and on looking round saw a party of
Japanese children saluting us in these familiar
terms. We had rehearsed so often that they
had learnt the performance !
star- To the above-mentioned uses of photo-
charting
graphy (for quickly getting pictures or por-
traits of complex structures, for discovery of
new planets and comets, and for recording
transient phenomena), we have now to add
its uses in the domain of exact astronomy,
where until recently only measurements made
by the eye of a skilled observer were trusted ;
and especially in recording the relative places
of the stars. It may seem as though this
were not a new use ; that to take a picture
of a group of stars is not essentially different,
from taking a picture of a nebula or the
154
MODEEN METHODS
Moon ; and this is quite true. The difference
between the cases lies in the degree of
accuracy required. It was long thought
(and the notion was only dislodged with
difficulty, indeed it is largely prevalent
still), that while photography was capable
of making a picture in which the features
were exhibited roughly in their true relations,
the resemblance to the original was not
accurate enough for astronomical purposes.
There are curved mirrors in which the
curious may see their own faces grossly
distorted, and yet recognise the likeness : the
face is too long for its width perhaps, but
eyes, nose, and mouth still occur in their
proper sequence. So it was thought that
there might be distortion in the pictures
obtained photographically not gross distor-
tion, bat still enough to render measures
made on a photograph rather than on the
sky only approximate. It seems difficult,
now that we know how accurate a photograph
really is, to understand this old mistrust of
it ; but of course it was only prudent to make
sure of the new weapon, before preferring
it to the old.
J55
MODEBN ASTEONOMY
Although confidence in the accuracy of
photographs is now firmly established, there
is one material symbol of the old lack of
faith which will probably remain with us,
because it is eminently useful, though in a
different way from that which was expected.
The reseau (a network of lines at right
angles which is impressed on star photo-
graphs, of which mention was made on pp.
75, 110), was originally proposed as a safe-
guard against the distortion of the film one
of the faults of which photographs were
suspected. Experience shows that it is un-
necessary for this purpose, for such distortion
is insensible ; but it has been found so useful
in measuring the plate that it will probably
become, if it has not already become, a per-
manent feature in photographs intended for
accurate measurement. It dates from the
Astrographic Conference of 1887, of which
something will now be said.
The Astro- The Astrographic Conference of 1887 met
graphic
Conference in Paris to consider the project of making
a complete map of the whole sky by inter-
national co-operation. The credit for initia-
tive falls chiefly to two men Sir David Gill,
166
MODEEN METHODS
of the Cape Observatory, and the late Admiral
Mouchez, Director of the Paris Observatory.
At both their observatories work had been done
which showed the great promise of the new
method, and they felt that the time had come
for decisive action. The Conference was a
great success. A standard pattern of instru-
ment was chosen, and eighteen observatories
undertook to get instruments of this pattern
and take part in the work.
There were about 45,000 plates to be taken,
and accordingly the share of each observatory
is to take about 2,500 plates, half of which
are to have a long exposure of nearly an hour,
showing on the average 1,000 stars per plate ;
the other half a short exposure, which, there-
fore, only gives the brighter stars, to the num-
ber of about 300 per plate on the average.
Even these "short exposure" plates will ex-
hibit an enormously greater number of stars
than have ever been recorded before in any
way.
A complete map of the heavens is not, of Scale of
course, an entirely new thing ; this Inter-
national Chart will only be new in the scale
on which it is made and the detail shown.
157
MODERN ASTRONOMY
At the beginning of an atlas of geography we
find a map of the whole world on a small
scale, and of the towns in England perhaps
only London is marked on this map. After
this we find a map of Europe, in which the
chief English towns are noted ; but later
comes a map of England itself, with many
more towns and even villages ; and if we care
to get the large scale Ordnance map we get
vastly more information still. The Inter-
national Chart is to bear the same relation to
previous charts of the heavens as the large
scale Ordnance map does to a small one from
a school-atlas ; it is to be immensely bigger
and very much more accurate. The most ex-
tensive map of the stars at present in exist-
ence, due to Argelander and Schonfeld (and
this is only made for about half the sky
the northern hemisphere and a little of the
southern), would contain about eighty sheets
of paper (size 29 in. x 21 in.), and weigh about
a stone if completed for the whole sky. The
International Chart, if completed, will contain
ten thousand sheets, forming a pile of paper
twenty feet high and weighing nearly a ton ;
and it is not too much to say that the star
positions will be indicated with an accuracy
158
MODEKN METHODS
ten times as great as that of Argelander, and
there will be at least ten times as many.
Such figures will give some idea of the mag-
nitude of the undertaking. It is far from
completion as yet; but its course has been
prosperous enough to give every hope of ulti-
mate success, and all those who helped its
inception are to be congratulated on the real-
ization of a noble work, which is bearing quite
unexpected fruits in various directions.
The names of Admiral Mouchez and Sir
David Gill have already been mentioned, but
France generally deserves a great deal of
credit. It was the brothers Henry, working
at the Paris Observatory, who devised the
form of instrument adopted as a general pat-
tern, and made it with their own hands ; and
it may be added that it was in the course
of completing a previous French enterprise
(Chacornac's ecliptic charts) that they thought
of using photography at all. The French,
with their well-known hospitality, have enter-
tained in Paris not only the original Confer-
ence of 1887, but several subsequent meetings
of the Executive Committee, which is almost
the same thing ; and they contribute a larger
159
MODERN ASTEONOMY
number of participating observatories to the
eighteen than any other country, unless we
include Colonial observatories as English.
Measures of A most valuable outcome of the enterprise
Star Places
on Photo- has been the demonstration of the rapidity
and ease with which stellar positions can be
determined by measures made on photographic
plates. As an instance in point, the Cam-
bridge catalogue of stars, published a few
years ago, gives the positions of 14,000 stars
in a certain narrow belt of the heavens. This
represents twenty years' work of two people
with the transit-circle. It falls to our lot at
Oxford to explore the same belt of the heavens
by photography. We shall, perhaps, have six
people at work at Oxford, but to give a
simpler comparison I will divide their work
by three. With a staff equal to Cambridge
we shall, in five or six years, obtain photo-
graphically the places of two or three times
as many stars ; in other words, the work is
done five or six times as quickly, and the
results are even more accurate.
Much of this gain in rapidity is due to the
fact that the study of star photographs has
taught us the inconvenience, in some connec-
160
MODEEN METHODS
tions, of a method of work which has been
hitherto universal in astronomy. Maps of the
stars have hitherto been made by the use
of instruments, especially the transit-circle,
which utilize in some form or other the
rotation of the Earth ; and, in consequence,
they are covered with sets of lines like those
on maps of the Earth, one set converging to
the poles and another set curved in some way
or another. The use of such reference lines for
star positions complicates the calculations, and
it is much simpler to use straight lines at
right angles, such as those given by the
reseau (see p. 75) on a flat plate. We cannot
use such lines in geography because the Earth
really is spherical and has two very real poles;
and accordingly our reference lines must be
curved over its surface, and the poles are im-
portant points. But the poles do not belong
to the stars, they only seem to belong to them
because we use terrestrial instruments, and
we can make an equally good, or better, map
of the stars with poles in quite different
quarters of the heavens from those to which
our terrestrial poles point. And, further,
though it has been customary to consider the
stars displayed upon a sphere, this is only a
161 M
MODEBN ASTEONOMY
convention which can be discarded in favour
of any other more suitable. In dealing with
photographs it is far more suitable to regard
the stars as displayed or projected on a flat
surface indeed the very operation of photo-
graphing them produces such a " projection "
(in mathematical language the stars are pro-
jected through or from a point, the centre of
the lens, on to a plane, the photographic
plate) ; and the introduction of the rseau,
originally for quite a different purpose, has
shown clearly how much is gained by shaking
off the trammels of the old system, and work-
ing with plane rectangular co-ordinates, in-
stead of with the right ascension and decli-
nation to which astronomers had become so
thoroughly accustomed.
Measures of With some modifications this change of
Lunar
Photo- method is applicable also to measures on the
graphs
Sun and Moon, and may make a considerable
difference in our knowledge of the Moon's
surface. Accurate " selenography," as it is
called, is in a most backward state, owing to
the laborious nature of the necessary calcula-
tions, when any measures have been made.
Even with photography to help not much
162
MODERN METHODS
has been done until quite recently. The late
Professor Pritchard, of Oxford, made a large
number of measures of lunar photographs, but
never published the results ; the calculations
were so laborious that I do not think they
were ever completed.
With the methods of calculation above re-
ferred to, which have since been developed,
Mr. S. A. Saunder has, within the last year,
obtained very satisfactory results from mea-
sures on lunar photographs, and he is now
entering on a campaign which may result
in putting our knowledge of selenography on
a more accurate footing.
He has another great advantage over earlier Co-opera
measurers of lunar photographs. Professor
Pritchard measured the comparatively small
pictures, of 1J inches in diameter, taken
with the De la Rue reflector at the Oxford
University Observatory in the years 1876-
1879. These are rather small judged by mod-
ern standards ; and pictures taken with a
reflector have apparently some defect of dis-
tortion which does not appear with a refractor.
(Such, at any rate, is the experience at Green-
wich, at Oxford, and elsewhere.) Now Mr.
163
MODERN ASTRONOMY
Saunder has been able to measure some of the
wonderful 6-inch pictures of the Moon taken
with the large equatorial coude at the Paris
Observatory, which were kindly lent by the
director, M. Loewy, for the purpose. It is
not the least of the advantages of photo-
graphy that it affords new opportunities for
collaboration between observatories. The
staff of an observatory may have fine in-
struments and good opportunities generally
for taking the plates, but not time to measure
them in detail. This work can, however, be
undertaken by some one else less fortunately
situated as regards cameras, but with conse-
quently the more leisure for measurement and
examination of plates. A. conspicuous ex-
ample of the success of this new bond between
astronomers at a distance is afforded by the
Cape Photographic Durchmusterung, which is
a huge catalogue of stars in the southern
hemisphere, constructed by Sir David Gill at
the Cape of Good Hope, who took the photo-
graphs, and Dr. Kapteyn, of Groningen, in
Holland, who measured them. At the Uni-
versity Observatory, Oxford, we have recently
carried out a considerable investigation by
measuring plates taken at Harvard University
164
MODEBN METHODS
Observatory, U.S., and so on. Such instances
will doubtless multiply rapidly in the future
as the new conditions are better realized.
The mention of the Harvard University Frequent
Charting of
Observatory reminds us that the charting of the Sky
at Harvard
the whole heavens is being conducted at that
observatory, in a manner differing widely
from that initiated by the International Con-
ference of 1887. "We have already compared
this latter scheme to an Ordnance Survey
extended to the whole earth. Minute details
are to be shown (i.e., not only the brighter
stars, but the very faintest), and the scale is
to be considerable. The total labour and cost
will also be considerable. There is no doubt
of the value of such a survey, but it is liable
to one defect : a large scheme like this, when
carried through, leaves a feeling that we may
now rest on our oars for a while ; astronomers
may feel that when the chart is made, their
work is done, instead of only beginning ; their
real business is to note changes in the heavens,
for which the chart is only the starting
point.
Now the energetic director of the Harvard
Observatory has begun to accumulate material
165
MODEEN ASTRONOMY
for noting changes. He charts the whole sky
once a month ! Not, of course, on the large
scale chosen for the International Chart, but on
a scale quite sufficiently large to give valu-
able information. More than this, with a
smaller instrument, and on a smaller scale
still, he charts the brighter stars every fine
night ! So that if a star brighter than the
sixth magnitude appeared in any quarter of
the heavens, he would have a record of it on
the first fine night. Already he has had the
satisfaction of such an experience, for when
a new star was noticed in the constellation
Auriga on February 2, 1892, Professor Picker-
ing found the star on thirteen of his plates
between December 10 and January 20. We
may remark that he did not accelerate the
actual discovery of the star, which was made
independently ; but when made, he was able
to say how long the star had been shining
before Dr. T. D. Anderson, of Edinburgh,
actually noticed it. Had it been possible to
completely examine each plate soon after it
was taken, the actual discovery might of
course have been made from the plates them-
selves. But this is an immense labour, exactly
equivalent, of course, to examining the whole
166
MODEBN METHODS
sky each fine night, and beyond the limited
resources of the staff of any observatory.
The point is an important one, and worth Discovery
full consideration. Let us first take another
illustration of a striking kind. Reference is
made several times in these pages to the dis-
covery of the minor planet Eros, which has
come very close to the Earth this winter,
1900-1, and thereby gives us an exceptional
opportunity for determining the Sun's dis-
tance. The planet was discovered in the
autumn of 1898, and soon afterwards it became
clear that it had made a very near approach
to the Earth in 1894. Professor Pickering
turned over his vast store of photographs,
to see whether he had caught the planet on
any of the plates unknowingly ; and after a
little trouble, which need not be noticed for
our present purpose, he found it on several
plates taken in 1894, and on others taken in
1896. Copies of some of these plates are
shown in the Paris Exposition this year, and
on one of them especially the planet is shown
by a conspicuous trail. If by a happy chance
this trail had attracted sufficient attention,
Professor Pickering would have discovered
167
MODERN ASTRONOMY
Eros in 1894, in time to make some use of the
great opportunity ; and one cannot help dwell-
ing a little regretfully on this "if." It may
be asked how a trail of this kind came to be
overlooked. The answer is that the number
of plates taken at Harvard is so great that
systematic examination of them is impossible.
An enormous amount of examination is car-
ried out. Professor Pickering and his assistants
have done more than all the rest of the
astronomical world put together to indicate,
and to carry out, schemes for the comprehen-
sive and rapid survey of numbers of photo-
graphs ; but even then more plates are ob-
tained than can be examined. If he had
made it a rule not to take plates without
thoroughly examining them, the only result
would have been, not that Eros would possi-
bly have been found in 1894, but that the
plates on which it was found subsequent to
the discovery would never have been taken
not a possible gain, but a positive loss. Professor
Pickering has the courage to take thousands
of plates, on the chance that they may turn
out useful, though he can only, for the
The present, put them on the shelves. The find-
Poiicy ing of Eros in 1894 is a conspicuous instance
168
MODEKN METHODS
of the success of this policy, which is, after all,
very similar to that of the librarian. Some
men never buy a book unless they can read it
at once ; others form a library of thousands of
volumes, most of which they know they can
never open ; and both policies have their
merits. Professor Pickering is, par excellence,
the astronomical librarian, and that the first
book telling of the existence of Eros passed
on to the shelves without being read, was
all in the way of business. He and his
assistants glance through all the books as they
come in, but they naturally cannot read more
than a few; and very thorough reading would
have been necessary to find Eros in this way.
For it must be remembered that the existence
of a trail on a plate, though it means a planet,
does not always mean a new planet. There
are more than 400 already known, and it is
necessary first to make sure that it is none of
these in itself a laborious piece of work. Then,
again, no one suspected that any of these
bodies was going to ttirn out particularly
important interest in them was beginning to
flag. Would it be pushing an analogy too
far to compare minor planets with minor
poets? We may liken Eros to the one poet
169
MODERN ASTRONOMY
of a century who suddenly emerges from the
crowd. Professor Pickering is the librarian,
who then finds he has on his shelves several
early productions of this new " star," but this
does not mean that he ought to have thereby
u discovered " him.
This point is dwelt on at some length,
because, as above remarked, it is a most
important one, as those familiar with the
history of astronomy will recognise. One of
the commonest faults in the past has been
the accumulation of observations which were
never reduced and published. The temptation
was to use the fine nights at the telescope
and trust to luck for the reduction of the
results : too often it turned out that if only a
fraction of the observations had been made,
and the time and energy saved from the re-
mainder devoted to the discussion and publica-
tion of these few, the world would have been
the wiser. One of the great reforms intro-
duced by Airy was the prompt publication of
the observations made ; and though his exam-
ple has had a most salutary effect, lapses are
by no means unknown in modern times. Is
the taking of photographs and placing them
170
MODEBN METHODS
on the shelves a repetition of the old error ?
There may be some who would reply in the
affirmative, but I fancy the general answer
even now would be a distinct negative, for the
reasons above given ; and I feel sure that in a
very few years there will be no doubt on the
subject at all. If so, we could not have a
more striking instance of contrast between the
old methods and the new.
At the same time, it is clear that methods Rapid Ex-
amination
of examining photographs rapidly must be of Plates
devised, if possible. "We have at our disposal
material of which we can only take a fraction
of the full advantage ; and it behoves us to
look about for more efficient methods of deal-
ing with it. The pioneers in this respect are
to be found also at Harvard, as has been
already remarked ; and one of their ingenious
devices is worthy of notice. If two photo-
graphs be taken at different times and
superposed, so that the two images of the
same star are close together, each pair of
images may be made to form a " double star,"
with the images similarly separated ; and so
long as the two plates are merely examined
by eye, which has an adaptable focus, both
171
MODEEN ASTEONOMY
images of the pair can be fairly focussed.
But we really see one set of images through,
the glass of the upper plate, and if we tried to
make accurate measures with a microscope,
this would be found a serious difficulty : the
focus would be sensibly different for the mem-
The Film b ers o f each pair. If we put the plates film
to Film
Device to film, we should not, of course, be able to
make more than two images coincide, and to
make a positive from one plate and put it
film to film with the other is not satisfactory.
Professor Pickering's device is to put one of
the plates into the telescope with film away
from the object-glass^ so that the rays of light
pass through the glass plate to the film. The
difference of focussing is very slight, and may
be neglected for the purpose of finding con-
spicuous changes ; and the resulting picture
has the same sort of inversion with regard to
an ordinary plate as a positive has to a
negative, viz., right is changed for left, while
up and down remain the same. Thus it can
be placed film to film with a plate taken in
the ordinary way on another night, of the
same region, and all the pairs of double stars
formed by the two sets of images will be
very approximately at the same distance from
172
MODERN METHODS
the examining microscope, and hence in focus
together. Any case of large proper motion
or parallax in a star would give an unusual
distance or position-angle to the corresponding
" double star " ; and any case of variability
in brightness would make the two components
unequal. That a rapid review of a region
can thus be successfully conducted has been
proved by the results : for instance, the well
known star 61 Cygni, which has a large
proper motion, was recognised at once, al-
though the reviewer had no idea even of the
region of sky under examination. Again,
variable stars have been actually discovered
during a review of this kind : one of them
being of special interest as a short period
variable.
That there are not many examples to be Paucity of
f Peculiar
quoted is in some ways disappointing, but stars
has a definite significance : it must mean that
the number of exceptional objects in the sky
is not great. This particular kind of dredge-
net has been dragged over portions of the sky
without catching much. The net is presum-
ably a good one, since it has caught auto-
matically what we knew to be there already,
173
MODERN ASTRONOMY
and even made new captures. If the haul
has not been a large one, we may conclude
fchat there is not much to catch ; and, after all,
there is considerable comfort in this thought.
"We already have our hands very full, and if
it were rendered probable that discoveries
might easily be made in large numbers, other
work would undoubtedly suffer, unless the
astronomical army were suddenly reinforced
by a large body of new workers, which does
not seem immediately probable. As it is,
some method of this kind offers to those who
may be willing to undertake a straight-
forward, though laborious search, a practical
certainty of sooner or later making some
interesting discovery without any apparatus
at all beyond a simple lens or microscope.
There are plenty of photographs already in
existence to occupy several lifetimes in such
examination. The Harvard Observatory alone
has a practically inexhaustible store, and Pro-
fessor Pickering has already and several times
expressed his willingness to furnish material
for any earnest worker. As has been already
remarked, a man of any length of purse, or
with no purse at all, may be an astronomer
and a discoverer now-a-days.
174
MODERN METHODS
So far we have considered photography
chiefly as an aid to discovery and record : dis- Meridian
Astronomy
covery of new minor planets or other objects ;
discovery of changes proper motions or vari-
ations in brightness. Star-charting is the
first half of a discovery in a sense, but may
be better called a record ; pictures of nebulae
are records, though often also considerable
discoveries.
But photography has also lent aid already,
and is cartain to lend much more in the
future, in the astronomy of exact measure-
ment. Sooner or later the photographic plate
is bound to replace the observer at the eye
end of the transit-circle, though as yet there
is little progress in this direction to report.
Photographic transit- circles are indeed already
in existence, and other forms have been pro-
posed, but most of the meridian work in the
world is still done by eye. It must not be
too hastily assumed that this is to our dis-
credit. The merits of the visual transit-circle
have been established by a century of work :
it is not to be lightly abandoned for an
instrument as yet scarcely designed. True,
it has defects (or rather the man who uses it
175
MODERN ASTRONOMY
has defects), which can be obviated by a
photographic method ; but we have yet to
learn the defects which must also inevitably
come with the new instrument, and it will be
wise to know, and if possible to cure them
before abandoning the existing and proved
instrument.
Longitudes Before describing the general principle on
graphy which I believe a photographic transit-circle
will ultimately be constructed, I will refer
to a case in which photography has been
actually used with success for one of the
problems of the old astronomy. It has been
remarked that a traveller or sailor finds it
tolerably easy to determine his latitude, but
a difficult matter to find his longitude. For
the latter he wants to know the Greenwich
time ; and if he has not carried it with him
from civilization, in the shape of a good-going
watch or chronometer, his best way of finding
it (a poor one at the best), is by observing
the place of the Moon among the stars. He
may do this in a variety of ways so long
as the main object is attained. The sailor is
provided in the Nautical Almanac with tables
of "lunar distances," i.e., distances of the Moon
176
MODEEN METHODS
from certain well-known bright stars. Owing
to the Moon's motion these distances are
constantly changing, and the Greenwich times,
when they have certain specified values, are
known. Hence, if the traveller measures one of
these distances at a certain moment, he knows
the Greenwich time. Let us suppose it is
exactly midnight with him, and he finds
that the distance between the Moon and
Aldebaran is that given by the tables for
10 p.m. at Greenwich : then he knows that
he is two hours East of Greenwich. The
defect of the method is its inaccuracy, owing
to the slow (and, it may be added, to the im-
perfectly known) motion of the Moon, and to
the difficulty of making exact observations
with the sextant, which is only provided with
a very small telescope. If we could use a
larger telescope we should increase the accu-
racy ; and here, as elsewhere, we gain by
having an automatic record by photography
instead of human observations. Capt. Hills,
R.E., has accordingly devised a method of
making this observation photographically.
Remembering that the problem is to deter-
mine the place of the Moon among the stars
at a given instant photographically, there are
177 N
MODERN ASTRONOMY
certain difficulties to be overcome. The Moon
is so bright an object that it is almost out
of the question to expose a plate to the Moon
and surrounding stars and expect to find them
both shown. If the exposure were long
enough to show the stars the moonlight would
in general fog the plate all over. The pho-
tograph is therefore taken in two operations:
first, a snap-shot is taken of the Moon with
a camera which is left firmly fixed ; after an
interval (say one hour), the Moon's image has
passed off the plate, and if the shutter be now
again opened, stars will shine through the
lens not those immediately round the Moon,
but others at a known distance from them,
and these can be photographed without dis-
turbance from moonlight. By choosing the
interval properly we can also get bright stars.
And by measuring the place of the Moon
among these stars which we have artificially
brought alongside it, we can infer its real
place, and so find the Greenwich time, when
there is no telegraph line.
In order to leave a trail on the plate a star
must exceed a certain brightness. We can
photograph very faint stars when the camera
178
MODERN METHODS
is made to follow them by means of clock-
work ; but it is the essential part of the above
method that the camera should remain fixed.
This limitation can be removed by an Photogra-
phic Transit-
artifice. Let the camera be firmly fixed, with circle
the exception of the plate, which' can be
moved in a slide by clockwork so as to
follow the motion of the stars. We can
then keep the image of a star shining on
the same point of the plate for several
minutes, and so faint stars will no longer
be lost. "We have, however, introduced an
uncertainty, for the plate will not be in the
same position when the Moon is photographed
as when the stars are photographed, and we
must know the relation between these posi-
tions. For this purpose let a spot of light
from a fixed source fall on the plate ; and let
there be a screen which can cut off this beam
every second (a good clock can be arranged
to do this automatically). Then, as the plate
moves along, the spot will form a line of
dots, which register the position of the plate
at each instant, so that we know its relative
positions at any two moments In this way
the advantages of a moving plate and a fixed
179
MODERN ASTRONOMY
plate are combined. This principle is of wide
application, though it has not yet been used
in practice.
It may be applied to either the transit-circle
or the almucantar. Thus : a camera is fixed
in a vertical position, lens downwards. The
plate can move in a slide, which can be
turned into any azimuth, and the rate of
motion of the plate is controlled by a lever.
Under the lens, reflecting the light from
stars into the camera, is a plane mirror. If
this mirror is mounted on pivots, and capable
of rotation round a horizontal axis placed
east and west, we have a photographic
transit- circle. If it is floated at an angle
with the vertical we have a photographic
almucantar.
The Photo- Photography has, however, been applied to
Chronograph
the transit-circle in other ways, usually by
allowing stars to trail across the plate and in-
terrupting the trail at regular intervals. Ex-
cellent results have been obtained in this way
at the Georgetown Observatory by Father
Hagen. The method is limited to the brighter
stars, but positions of the fainter stars can be
obtained by other methods. If the difficulties
180
MODERN METHODS
of photographing stars in the day-time could
be satisfactorily got over, I should not hesitate
to prophesy that some form of photographic
instrument would entirely supersede the visual
transit-circle in the near future.
For visual observations, accurate as they are, variations
of Personal
have one great defect, that no two observers Equation
make them in the same way. Every one has
an idiosyncrasy called a " personal equation,"
which causes him to observe a transit always
too early, or always too late, as the case may
be. If this were the whole of the story it
would not matter much, for we can easily
compare the personal equations of different
observers and make the proper allowance
accordingly. This has been done year by year
at Greenwich for nearly a century, assuming
throughout that the personal equation of each
of the observers remained sensibly constant
during each year, and was the same for all
stars. But we have lately found that this
quantity is not the same for all stars : it
depends, even for the same observer, on the
brightness of the particular star he is observ-
ing. Generally speaking, if it is a bright star
that is crossing the transit wires he signals its
181
MODERN ASTRONOMY
crossing too early, as if the brightness of the
star accelerated the message to his brain. As
yet no satisfactory reason why this should be
so has been formulated, but there is no doubt
whatever about the fact, to which serious
attention was first called by Sir David Gill
in 1878. He had compared the relative places of
certain stars of different brightness with his
heliometer, an instrument which does not intro-
duce personal equation of the kind we are
considering ; and he found that his observa-
tions could not be reconciled with transit-circle
observations except by assuming a variation
in personal equation according to brightness
of the star. Since then several laborious in-
vestigations have been undertaken which have
demonstrated this variation, and measured its
Photo- amount with more or less success. But during
Determina- the l ag t year or two it has been shown how
easy it is to measure this quantity by means
of photography. For instance, the belt of the
heavens which was observed for twenty years
with the transit-circle at Cambridge, is now
under survey by the photographic method at
Oxford. The distance between a bright star
and a faint star as found at Cambridge is
slightly erroneous, because the bright star was
182
MODEBN METHODS
signalled relatively too soon ; but this error
does not exist in the photographic measures ;
and so a comparison of the two surveys detects
it. It is so small that we must compare as
many stars as we can to get a good result on
the average : but we have plenty of material
many thousands of stars and when they
were examined it was found how clearly the
variation can be brought out in this way.
Down to magnitude 8O the signals given by
the eye -method are given earlier and earlier
by about l-50th second for each magnitude
brighter. For stars fainter than 8'0 the
change is much larger than this. The observer
at Cambridge was the same throughout (Mr.
A. Graham), and these results show his in-
dividual change of personal equation. But a
comparison was made with the general average
of the observers at Greenwich, and it was found
that Mr. Graham's habits agreed minutely with
the average of all the Greenwich observers.
Hence we infer that observations made at
Greenwich are all affected in this way. There are
reasons, which I need not here enter into, why
this is not so serious a matter as it looks at first
sight ; but it is sufficiently serious to demand
immediate attention. A photographic transit-
183
MODEEN ASTEONOMY
circle would remove the cause of error root and
branch.
star The point just referred to reminds us that
Magnitudes
the stars differ in brightness as well as in
position ; and it is important to measure these
differences in lustre. Most of the stars remain
constant in magnitude, and occasional measures
to confirm the constancy are all that are neces-
sary. Others, called variable stars, are con-
tinually changing ; and from the history of
their changes we may hope to learn something
of the nature of these wonderfully interest-
ing objects. The few references I can
make in this rapid review of the last quarter
of a century are out of all proportion to the
immense amount of work recently done in
photometry. (Harvard Observatory is in a sense
devoted entirely to photometry.) And at pre-
sent we are considering what is new in equip-
ment and method rather than gauging the
amount of work done. We accordingly select for
notice a method conspicuous for its novelty,
although the results obtained by it arc as yet
few. In the history of telescopes up to the
last ten years, probably very few observers
have ever deliberately put their telescopes out
184
MODERN METHODS
of focus for a useful purpose. The minutest
deviation from perfect focus has been uniformly
regarded as an instrumental defect, to be avoided
if possible. Recently it has occurred to two
people independently (Professor Pickering, of
Harvard, and Dr. Schwarzschild, of Vienna)
to take photographs of stars considerably out
of focus, for the purpose of measuring their
brightnesses : and the method seems promising
for the following reason. When a camera is
accurately focussed the image of a star changes
in several respects as the brightness increases.
The brightest stars give large black discs with
indefinite borders : those less bright give
smaller discs ; and the size of the disc is a good
indication of the star's brightness. But for the
faintest stars the disc does not diminish in size,
it only becomes less black : and thus we must
observe not only the size of disc but the black-
ness of it to get complete knowledge of the
star's brightness. Now, if we can reduce these
two things to one only, it is a distinct gain :
and this object is attained by taking the stars
out of focus. The size of the disc does not
then vary appreciably, only the density ; and
we have thus only one element to observe in-
stead of two.
185
MODEEN ASTEONOMY
Motions in Accurate photometry on some plan is very
the Line of J
Sight important for the study of variable stars ; but
perhaps the greatest advance in our knowledge
of these objects has come, not from any photo-
meter at all, but from the spectroscope ; and
the use of this instrument for studying, not
the chemical constitution of bodies, but their
movements, has been one of the most wonderful
developments of modern astronomy. It occurred
to Sir William Huggins early in his work
with the spectroscope, that if a star were mov-
ing towards or from us, the lines in its spectrum,
which are like particular notes in a musical
scale, should slightly change their positions,
much as the pitch of a railway whistle alters
when it passes us and so recedes instead of
approaching. The general principle was not
new, especially in its application to sound,
where it is called Doppler's principle : but the
application of it to light was quite new. In-
deed, in 1868 it was not an experimental fact,
but only a theoretical probability that the lines
in a spectrum would shift their positions
according to the motion of the star, and Sir
William Huggins entered upon the difficult
task of testing it practically. The observa-
tions were extremely delicate : the shifting of
186
MODERN METHODS
the lines is undoubted but very small. The
analogy of sound is rather misleading as to the
amount of change, because the velocity of
sound is so small compared with that of light
little more than a millionth part of it. To
get an alteration of colour equivalent to the
alteration of pitch of a railway whistle, the
train should move a million times as fast, and
even the heavenly bodies do not go at this pace.
They may move 100 miles a second, but this is
not quite 10,000 times the pace of a train.
Hence the effects of even these high velocities
on the spectra of the heavenly bodies are very
slight : one hundred times slighter than the
effects on musical pitch of the motion of an
express train.
Still they are measurable, when extreme
care is used ; and the announcement of their
detection caused the greatest interest, and in
some cases, where it was not quite understood,
even alarm. I remember some fifteen years
ago, when I was at Greenwich, having to
answer for the Astronomer Royal an anxious
inquiry whether any danger was to be appre-
hended from the rapid approach of these stars
to us.
187
MODERN ASTRONOMY
"When it was explained that they were not
necessarily moving straight towards us, and
that, even if they were, some millions of years
at least would be occupied in reaching us, the
inquirer was so obviously relieved that she
called down a fervent blessing upon the
Astronomer Royal. She would have doubtless
been even more gratified by the recent dis-
covery that some of these high velocities are
not continuously in the same direction ; for by
this same method it has been found that some
of these stars are revolving. We presume that
they are revolving round some star which we
cannot see; but that they are moving alter-
nately backwards and forwards there is no
doubt. The analogy of sound again helps us
here. If any source of sound be in rapid
rotation, so that it alternately approaches the
hearer and recedes from him, a distinct alter-
nation of pitch can be noticed. There is a
schoolboy's implement called a " bull-roarer,"
which is easily made from a piece of a cigar-
box and a bit of string. If in a flat piece of
wood, say three inches long and about an inch
wide, but taparing slightly, a hole be bored
near the broad end and' a string pushed
through and knotted on the far side, then by
188
MODEBN METHODS
whirling the wood from the other end of the
string a buzzing noise or roar is produced by
the rapid twisting of the wood. But th
sound heard by another person is not a steady
one, except in one particular case : it is usually
an alternating sound. The alternations are
most marked if the observer stands so that the
whirling circle is edgeways to him, for then he
gets the full effect of the alternate approach
and recession at the top and bottom of the
circuit. If, however, he stands with his ear in
the axis of the whirling circle, i.e., in the
straight line through its centre perpendicular
to its plane, so that the source of sound is
always at the same distance from his ear, he
will hear a steady note : the alternation will
disappear. The alternation may be made
plain to a large number of people at once, but
only a limited number can be placed so as to
hear the steady note.
If further experimental evidence is needed
to show that similar phenomena occur with the Sun
light, it is afforded by bodies which are known
to be rotating or revolving. For instance, the
Sun is seen to be rotating on his axis by the
motions of the spots across his disc. The
189
MODERN ASTKONOMY
spectrum of sunlight on one side of the disc is
therefore that of an advancing body, and on
the other side of a retreating body ; if the two
are compared, the lines should show relative
displacements accordingly. This experiment
was made very early in the history of this new
method, and found to boar out theoretical
reasoning completely.
AJgo1 A more interesting case is that of the star
Algol, where the revolutionary motion was
not certainly known but only suspected.
Algol is a variable star of a peculiar kind. Its
light remains constant for two-and-a-half days,
then it begins to fade quickly, and after three-
and-a-half hours reaches a minimum value, at
which it remains constant for twenty minutes :
then it brightens again in ' three-and-a-half
hours to its original value. It was shrewdly
suspected that the star was really composed
of two bodies, a bright one and a dark one,
revolving round each other ; the variations in
brightness being caused by the dark body
passing in front of the other so as to obscure
it partially though not entirely. It is easily
seen how the observed variation of brightness
could be explained on this hypothesis, but we
190
MODEBN METHODS
had no direct evidence of the dual existence or
revolution. The first attempt to detect a
revolutionary motion in the star with the
spectroscope on the principle above sketched
was*made by Mr. Maunder at the Royal
Observatory, Greenwich, and he got varying
velocities which were distinctly favourable to
the hypothesis, though the accidental errors
were large. Later Dr. Vogel, of Potsdam, got
better observations by photography, and
proved the revolution beyond a doubt.
Another instance is afforded by the planet Saturn
Saturn and its ring. It was shown years
ago by Clerk - Maxwell that the ring of
Saturn could not be a solid body, or it would
fall on to the planet. It must be a collection
of small satellites, revolving round the planet
according to the law of satellites, viz., the
outermost going most slowly, instead of most
quickly if the ring were solid. The honour of
confirming this mathematical demonstration
by the direct evidence of the spectroscope
belongs to Professor Keeler, the present Direc-
tor of the Lick Observatory. A spectrum of
Saturn and his ring obtained by him shows
clearly the rotation of the planet according to
191
MODEEN ASTEONOMY
the law of solid bodies (as in the case of the
Sun quoted above), and that of the ring as a
collection of small particles ; and this is a most
beautiful and complete confirmation of this
principle for observing " motion in the lifie of
sight."
spectro- The principle having thus been confirmed
Binaries experimentally by observation of bodies known
to be rotating, and having confirmed the
hypothesis of rotation in suspected cases, it
was an obvious further step to detect rotation
or revolution where it was not known previ-
ously to exist.
Double stars have been discovered by this
method which cannot be seen directly as double
stars even with the previous knowledge of
their binary character. When two bodies re-
volving round each other are both bright (and
not one bright and one dark, as in the case of
Algol), the lines of the spectrum of one move
to the right, while those of the other move to
the left, so that what should be a single line
appears double. A new method of finding
double stars has thus come into existence, just
as simple as the old method of looking at a star
to see if it is double,' viz., we look at the lines
192
MODERN METHODS
in its spectrum and see whether they are
double at any time.
The foregoing account, however imperfect. Concluding
Remarks
will at least have made it clear that there
are many new methods of work claiming
attention at the present moment. A word
or two may be added concerning the difficul-
ties and dangers of changing old methods for
new, even when the latter are obviously better.
Astronomy occupies a peculiar position among
the sciences owing to the vast number of
observations of the same kind which it is
necessary to make. All observers multiply
their experiments to some extent a 'careful
physicist or chemist may repeat the same ex-
periment twenty or even a hundred times, but
no one save the astronomer deals in millions
of measures of the same kind. It follows that
he must not too readily change his instruments
and methods when along series of observations
is in hand ; he will do better to make up the
tale of measures on one uniform system, even
though this involves working for years on an
old-fashioned plan, while his colleagues are
reaping the benefits of modern appliances. *
Again, it is prudent to be sure before taking
193 o
MODEBN ASTBONOMY
up a new tool or new method, that it has
reached a sufficiently stable form ; it is aggra-
vating to begin work and find within a year
or two that a better plan still might have
been adopted. Hence astronomers are slow
to change their methods for good reasons,
even in the presence of such obvious advan-
tages as have been described.
194
Section III
MODERN RESULTS
195
Section III
MODERN RESULTS
IN the previous sections we have considered
the new instruments recently put into the
hands of astronomers, and the new methods
which they have suggested. The interest of
the general public is more generally mani-
fested in the results of astronomical work
than in the processes in the actual dis-
coveries rather than in the way they are
made ; and it will no doubt appear to some
readers that the present section, dealing with
modern discoveries and results, might with
advantage have been expanded at the ex-
pense of the other two. It is, however, my
object in the present work to lay more stress
on the new methods than on the results al-
ready obtained r for two reasons: firstly, be-
cause discoveries are announced in other ways,
while the important and interesting changes
197
MODERN ASTRONOMY
in method are unnoticed ; and, secondly, be-
cause the crop of results yielded up to the
present, although of goodly size, is but a
fraction of what we may expect in the near
future, as the new tools grow more familiar
and the new methods are better understood
and perfected. The examples which follow,
though fairly representative, are not in-
tended as a complete collection even of
specimens.
Variation of Let us take, first, the movements of the
poles on the surface of the Earth, or the
"variation of latitude " as it is technically
called. Though this discovery is in great
part an outcome of old observations, it is
also largely due to the almucantar ; for it
was his observations with this instrument
which directed Mr. Chandler's attention to
the subject, and ultimately led to the eluci-
dation of a difficult problem. The history of
the problem is curious. The question, Does
the latitude of a given place vary? or, in
other words, Does the North Pole, which our
explorers go to seek, remain accurately in the
same place on the Earth's surface? has been
before the minds of astronomers for a long
198
MODEBN RESULTS
time. It was soon recognised that if the
North Pole does not remain quite stationary,
its excursions are very small. It never wan-
ders down into Europe, for instance, or we
should have a different climate ; its excur-
sions cannot carry it very far on the way
towards Europe, or the length of day and
night would be sensibly affected. But are
there any very minute excursions which
might not be noticed in such ways as these,
and which yet might be detected by astro-
nomical measurements of great precision?
The mathematician Euler investigated many Euier's
J Ten Months'
years ago what would be the general cha- Period
racter of such movements if they existed.
He found that a rigid body like the Earth,
which was spinning about an axis once a
day with nearly complete steadiness, might
have a slight " wabble," which would mean
that the North Pole was in motion, but that,
if so, the motion would complete a circuit
every ten months. There seemed to be no
doubt about this result, and astronomers ex-
amined their observations of latitude to see
whether there was any change of ten months'
period. None was found, although long series
of observations were cut up into chapters of
199
MODERN ASTRONOMY
ten months and added together to magnify
any possible small disturbance ; and after
several attempts of this kind, the question
was regarded as settled in the negative. The
North Pole did not move at all. So confident
did astronomers feel on this point that when
Mr. Chandler, who ultimately demonstrated
the real facts so clearly, found an apparent
movement of the Pole by observations with
the almucantar in 1885, he himself thought
he must have made some mistake, and did
not follow up the matter. A year or two
later ' nowever > Dr. Kustner, of Berlin, pub-
tions lished some observations made about the same
time as Mr. Chandler's, which seemed also to
show that the North Pole had moved in the
same way. This independent confirmation
aroused attention. In Germany a long series
of special observations was initiated, by which
it was hoped that the motion of the Pole
would be detected and measured for a num-
ber of years. But this deferred the complete
solution of the problem to a future time,
when sufficient of these special observations
should have been accumulated. Mr. Chandler,
in America, adopted a different method. He
felt that if the motion of the Pole was real, it
200
MODERN RESULTS
ought to be just as plainly revealed by ob-
servations already made and published, as by
those to be made in the future, for which we
should have to wait. It was true that old
observations had been already examined to
no purpose ; but possibly the examination
had not been sufficiently thorough, es-
pecially as it had always proceeded on the
assumption (following Euler's mathematics)
that the disturbance, if any, must have a
period of ten months. Mr. Chandler said,
" Let us give up this limitation, and see
whether the observations perhaps indicate
some other period " ; and the answer came
almost at once, that they indicated a
period of fourteen months instead of ten.
Plainly, however, as the answer was written
in the published observations, and plainly as
Mr. Chandler indicated it, it was received
with universal disbelief. The mathematicians
replied that such a period was impossible :
Euler's work had shown what period the
motion must have, and any appearance of
another period must be due to some error in
the observations. Chandler replied to the
effect that he did not care for Euler's
mathematics : the observations plainly showed
201
Chandler's
Fourteen
Months'
Period
MODEBN ASTRONOMY
fourteen months, and if Euler said ten he
must have made the mistake. I do not exag-
gerate the situation in the least : it was a
deadlock : Chandler and observation against
the whole weight of astronomical opinion and
theory. No better instance could be given of
the spirit which lately animated astronomy,
which was referred to in the first section,
amounting to a tacit assumption that certain
problems had been worked out and settled,
and had almost lost interest. But Chandler
Explanation n bly stuck to his guns, and he had at last
the satisfaction of capturing a mighty oppo-
nent in Simon Newcomb. It ultimately oc-
curred to the latter that Euler had assumed
the Earth to be a rigid ~body, and possibly it
was this assumption which made the apparent
discrepancy between theory and observation.
A preliminary investigation convinced him
that he had found the key to the rusty old
lock ; and singe that time the reconciliation
between theory and observation has been
rapidly effected, though not without other
pitched battles in which Chandler exhibited
the same resolute bravery as before.
The Earth is not rigid ; and when allowance
202
MODERN RESULTS
is made for its yielding to stress, we see that
ten months is merely the minimum period in
which it can wabble. The period is greater
and greater according to the extent of yield-
ing ; so that Chandler, by his discovery of the
true period of fourteen months, not only set-
tled the question of variation of latitude, but
0-
OBSERVED MOTION OF THE NORTH POLE, 1890-1898.
gave us a measure of the plasticity of the
Earth. The amount of the wabble is very
small. The North Pole is never a dozen yards Amount of
. . Motion
away from its mean position ; and its move-
ments might almost be executed on the floor
of this lecture hall. 1 There will thus not be
any serious difficulty in identifying the spot
if any of our brave explorers penetrate to
1 At the Eoyal Institution.
203
MODERN ASTRONOMY
the North Pole. To the dangers from cold and
hunger there will not be added the mortifi-
eation of finding, when the supposed North
Pole is reached, that it has removed for the
season to another locality; but at the same
time its movements affect astronomical ob-
servations quite sensibly, and must be regu-
larly taken account of in the future. The
accompanying illustration shows the observed
wanderings of the North Pole during several
years. It will be seen that the motion is by
no means simple, and the fourteen-months'
period is not recognisable by a mere glance,
even with this observed motion before us.
The fact is that there are annual changes as
well, mixed up with the others. The cause
of these is not yet wholly understood ; they
are probably due to annual meteorological
changes (such as the regular melting of the
north polar ice every year, for instance), but
the most important agent has not yet been
identified. The discovery, like most others,
by no means concludes our work in that par-
ticular direction ; it rather opens up new
fields of work. It is, however, a considerable
triumph to have made such a step towards
the solution of this old problem of the varia-
204
MODEEN EESULTS
tion of latitude. For the complete solution
we must wait till the end of time ; but we
may regard the provisional answer obtained
by Mr. Chandler with considerable satisfac-
tion.
There is another problem of long standing
of which a satisfactory provisional solution has
recently been obtained ; the question of the
Sun's distance, or, as it is more technically
called, of the Sun's " parallax." Enough has
been said in the second section (see p. 106) to
show the present state of our knowledge on
this head. Thirty years ago the distance
was uncertain by some millions of miles, a
margin of error which it was hoped to ma-
terially reduce by the transits of Venus of
1874 and 1882. But this method failed.
Fortunately, however, a better one was found
almost immediately, and we may now give
the Sun's distance as 92,874,000 miles, from
Sir David Gill's beautiful observations, feeling
some confidence that this is within 100,000
miles of the truth. This margin looks large
in these actual figures, but it is equivalent
to less than the thickness of a stump in a
cricket pitch of twenty-two yards ! The best
205
MODEEN ASTRONOMY
measures agree as well as measures of a
man's height would if he alternately put on
and off a pair of very thick socks !
It might seem that astronomers could
therefore rest satisfied with their knowledge
of the Sun's distance ; but we hope to im-
prove it still further during the present
winter (1900-1), when the planet Eros is pay-
ing us a very neighbourly visit. He is such a
tiny planet that even at his nearest approach
he cannot be seen without a powerful tele-
scope, but he will be the " observed of all
observers" for several months.
Hawtabiiity The mention of our neighbours naturally
of the
Planets suggests the old question of the existence of
life on other planets, which is of such para-
mount interest to all of us. Have the large
telescopes recently erected brought us any
nearer to the solution of this problem ? I fear
that what there is to report will be a disap-
pointment. The recent increase in size of
telescopes, and even their installation in vastly
better climates for observation, is quite inade-
quate to take us any nearer such knowledge.
We have heard a good deal in late years of the
Canals in
Mars canals in Mars ; and there is no doubt at all
206
MODERN EESULTS
that certain straight markings on the planet's
surface have been detected. Many of us have
sufficient faith in that wonderful observer
Schiaparelli, to believe that these are occasion-
ally seen double. But as regards the inter-
pretation of such markings, the notion that
because they are called canals it is implied
that there are inhabitants in Mars who have
dug them for irrigation purposes, we must
exercise much more caution.
To realize the value of our information, con-
sider first how much farther away Mars is
than the Moon about 200 times at least, and
generally much more. Now 200 is about the
magnifying power of a good telescope, that is
to say, the magnifying power which can be
used with advantage. It follows, then, that
whatever a fair telescope enables us to see on
Mars could be seen on the Moon with the
naked eye ; and it may be added that what-
ever the largest telescope in existence would
enable us to see on Mars, could be seen on the
Moon with a pocket opera-glass : for our gain
from the recent increase in size of telescopes
is well within that represented by a small
opera-glass as compared with the eye. Hence,
207
MODERN ASTRONOMY
let any one look at the Moon, with the naked
eye, or even with a small opera-glass, 1 for
traces of canals or other signs of life of any
kind, and he will begin to understand the
caution which must be exercised in drawing
conclusions, however attractive, as to the habi-
tability of the planets. We want, in fact, an
increase of our optical resources by a thousand
times at least to get any satisfactory intelli-
gence of this kind : whereas the advances of
the last century would be represented by a
factor not greater than 10, and there seems 110
chance at present of our getting to 100 : we
might manage 20, perhaps, by slow and costly
advances, but 100 seems impossible.
A brief statement, and especially a numerical
statement, of this kind should not be criticised
too closely in detail ; but it may be accepted
unhesitatingly as giving a general idea of the
situation. The usual numerical form of state-
ment is to give the apparent distance at which
1 Professor E. E. Barnard, who has had probably more
experience of the largest telescopes in favourable con-
ditions than any one, is of opinion that the naked eye
view of the Moon better represents the view of Mars
through the best telescope. He kindly gave me this
opinion, after a little consideration, in reply to a
definite question.
208
MODEBN EESULTS
a body is viewed by a telescope. In the adver-
tisements of the great telescope built for the
Paris Exposition this year, the phrase " La lune La Lune
, ,, -i mi i a un Metre
a un metre was used. This obvious extrava-
gance is thus corrected in the guide to the Palais
d'Optique as follows :
" La lune n'est pas rapprochee a un metre
mais a environ 70 kilometres, ce qui est deja
fort joli."
To bring the Moon within 70 kilometres,
instead of its actual 380,000, simply means
that a magnifying power of gJ2.2. or 5,400
can be used on the telescope. Now, there is
no physical impossibility in putting a mag-
nifying power of this amount on this telescope Limitation
,, ., . , , ,., . of Magnify-
or any other it is done by putting in a ing Power
suitable eyepiece, and if none suitable is at
hand it is only a matter of some shillings to
make one. But the real question is whether
this power can be used with advantage. A.
similar case arises in the enlargement of a
miniature portrait : by a moderate enlarge-
ment we see the features more clearly, but after
a certain point we lose rather than gain by
further enlargement. We cannot, for instance,
by enlarging 10,000 times get a picture on
209 p
MODEEN ASTBONOMY
which, the hairs of the head are all shown
separately ; what would be shown separately
would be the grains of the film or paper form-
ing the miniature portrait, and the huge image
might be quite unrecognisable. If the ori-
ginal miniature was very clearly denned, we
could carry the process of enlargement further
than if it were not; but in all cases there
would come a somewhat indefinite limit beyond
which we could not enlarge with advantage.
With a telescope, the big lens or mirror
forms a miniature image, and the eyepiece
enlarges or magnifies it, to an extent con-
trolled by the focal length chosen for the eye-
piece. The larger the lens or mirror, the more
clearly defined is the miniature image, and
therefore the more we can magnify it with ad-
vantage : but there is in all cases a limit be-
yond which it is futile to go. This limit is not
very definite and is differently assigned by
different observers. It depends, too, on the cli-
mate in which the telescope is used. "Whether
a power of 5,400 can be used on the big Paris
telescope is doubtful ; at the present moment
the big lens for visual observations is not made,
and so no one can speak with certainty. As-
210
MODERN RESULTS
suming for a moment that it is possible, and that
our atmosphere did not exist to trouble the image
with its air currents, we should be able to study
the surface of the Moon as we should a map, on
a scale of one inch to four miles, held at a foot
from the eye. A glance at a bicycle map will
give some notion of the apparent size of objects
under these conditions. We should not be able
to see human beings, but we could recognise
towns, villages, and roads, and might infer the
existence of life from changes in these products
of activity.
In spite of difficulties such as these it is Changes in
Moon
probable that we can detect, and have detected,
changes going on in the Moon. The text-books
usually call the Moon " dead," and say that
there is no sign of change ; but the following
pregnant remark recently made * by Professor
W. H. Pickering is well worthy of notice.
" In reviewing the history of Selenography,
one must be impressed by the singular fact
that, while most of the astronomers who have
made a special study of the Moon, such as
Schroeter, Maedler, Schmidt, Webb, Neison
and Elger, have all believed that its surface
1 Harvard Annals, vol. xxxii. p. 175.
211
MODERN ASTRONOMY
was still subject to changes readily visible
from the Earth, the great majority of astrono-
mers, who have paid little attention to the sub-
ject, have quite as strenuously denied the
existence of such changes."
The consensus of expert testimony in favour
of change is indeed remarkable. "What then is
the reason of the ordinary statement ? It is
doubtless due to the fact that there is known
to be no appreciable l atmosphere on the Moon
such as could support any life that we know
of ; and the changes noticed are therefore not
considered significant of life. It is indeed
quite likely that they are due to volcanic
activity on the Moon : Professor W. H. Picker-
ing considers that the lunar volcanoes are more
active than those on the Earth. But again it
is quite difficult to be certain even of these
changes. The same object on the Moon's sur-
face looks so different at different times, from
the varying illumination of the Sun, that it
needs very careful records indeed to make
certain that a change is real.
1 In the volume just referred to, a good case is made
out for an extremely tenuous atmosphere, especially
on the side of the Moon illuminated by the Sun.
212
MODEEN EESULTS
With photography now to help us (and
M. Loewy is devoting himself specially to the
photography of the Moon with the large
equatorial coude at Paris ; but he gets very few
plates during the year which satisfy his ex-
acting requirements) we shall presently attain
greater certainty on this point. But mean-
while it is well worthy of note that those who
have studied the Moon all agree that changes
are going on, while those who have studied Mars
do not by any means agree that the canals
are double. It is far better established that
changes are going on in the Moon than that
the canals in Mars are double. In saying this
I do not wish to disparage the keen sight of
Schiaparelli : but others do not always confirm
his observations. Look, for instance, at the
drawings of Mars in the illustration. They
represent all that Professor Barnard, one of
the keenest sighted observers, could see with what can
be seen on
one of the largest telescopes under the very Mars
best conditions. The diameter of the disc of
Mars is more than 4,000 miles, which gives the
scale of the map. With these pictures and
this information any one of intelligence is in
practically the same position as regards draw-
ing conclusions concerning life on the planet.
213
MODERN ASTRONOMY
Does it seem likely that any ingenuity of in-
terpretation will avail anything ? No : if
any one could construct, and use with advantage,
a telescope one hundred times larger than the
largest in use say three miles long, with a
lens a quarter of a mile in diameter we might
FOUR DRAWINGS OF MARS BY BARNARD.
learn something about the habitability of the
planets ; but while we are still in difficulties
about telescopes measured in yards, miles
or furlongs seem out of the question.
Heal Work But it must not be thought that the per-
Teiescopes formances of large telescopes have been dis-
appointing because they have told us nothing
of the habitability of the planets. We might
as well be disappointed with the electric light
214
MODERN EESULTS
for not having given us perpetual daylight.
All that astronomers could fairly expect from
large telescopes was that they should render
certain difficult observations rather easier, and
render possible some that had been previously
just beyond the limit of possibility ; and such
expectations they have amply fulfilled. The
second test is the more definite ; and it is not
a little remarkable how immediately some of
the large telescopes have justified their con-
struction by making a discovery early in
their history.
Thus it was very soon after the erection of Discovery of
Satellites
the big Washington refractor that Asaph Hall of Mars
discovered with it the two minute satellites of
Mars. Was this a genuine instance of an
increase in size of the telescope rendering a
previously impossible observation just possible?
The answer is a little uncertain. There is no
impossibility (though considerable difficulty)
in seeing these satellites with smaller instru-
ments, now that they have been discovered ; but
the words in italics are important. There is
a vast difference between seeing a thing when
it is known where to look for it, and actually
seeing it for the first time ; and so it is not
215
MODEEN ASTRONOMY
unfair to say that an instrument which can
now see the satellites could not have dis-
covered them. In this sense the Washington
refractor rendered possible what was pre-
viously just outside the limits ; and if an
accident had destroyed it immediately after-
wards, the cost and labour of erecting it would
have still been amply repaid.
Fifth So too with the great Lick telescope : we
Satellite of
Jupiter may take the discovery of the minute fifth
satellite to Jupiter by Barnard, in 1892 (an
object so minute that only the largest tele-
scopes on favourable nights can reveal it even
when its place is known), as an achievement
fully justifying the existence of the telescope
if it had done no more. These new satellites
are not mere tiresome additions to the numer-
ous family of the solar system, as the im-
patient might call many of the 400 or 500
minor planets ; in both cases they present
new features which seem likely to guide us in
unravelling the past history of the solar
system.
capeiia as a Another instance of rather a different kind
Binary
may be given. It has been explained (see p. 192)
that a star may be found to be u double " by
216
MODEBN RESULTS
means of the spectroscope, although no exist-
ing telescope can separate the components. We
see the lines in the spectrum of the star
double, which means that there must be two
bodies sending light to us, one of which is
approaching and the other receding ; and we
infer that there are two stars revolving round
each other, though in the most powerful tele-
scope we can only see one point of light.
Until the present year this inference had
never been confirmed by actual observation
at the telescope. This did not mean that
there was any flaw in the argument, but
simply that the revolving stars were always
so close together that it was beyond the
power of our best telescopes to distinguish
them apart : and so, although confidence in
the inference was not shaken, the interest
in finding a particular case where it could
be independently confirmed grew naturally
keener. The large refractor at Greenwich
has been the first to detect such an instance,
in the well-known star Capella. Credit is not
due altogether to the telescope ; much is due
to Mr. Newall, of Cambridge, who shares with
Professor Campbell, of the Lick Observatory,
the honour of making spectroscopic observa-
217
MODERN ASTRONOMY
tions which showed Capella to be double, and
who suggested, from a study of these observa-
tions and of the parallax found by Dr. Elkin, of
Yale, that here might at last be a case where
the duplicity of the star could be actually
detected with a powerful telescope. The
suggestion was not lost on the Greenwich
observers : they applied themselves diligently
to watch Capella, and found that although
they could not actually see two stars sepa-
rated, they could detect an elongation of the
image in a definite direction. By repeated
observations they found that this direction
changed, gradually performing a complete
circuit in the time in which it was indicated
by the spectroscopic observations that the
stars should be revolving round one another ;
and the chain of evidence was thus completed.
This most interesting discovery, while not
entirely due to a large telescope, could not
have been made without one, and it is not un-
fair to include it in our category of justifica-
tions for the construction of large instruments.
Extended One more instance may be given. Our
tionsTof" observations of comets have been usually
Comets confined to the brief period when they are
218
MODERN RESULTS
near the Sun and near us, which represents
only a very small portion of their whole
orbits. Such observations are sufficient to
enable us to predict the return of certain
comets, even though we may lose sight of
them for many years ; but what happens to
them in the meantime we have not been able
to say, for ordinary telescopes completely lose
sight of them. "With the Lick telescope and
the Californian climate Barnard has been able
to follow a comet so far on its outward journey,
so long after other telescopes had lost it, as to
raise hopes that we may perhaps soon be able
to follow some comet all round its orbit. If so,
we may learn something more of the nature of
these mysterious visitors. This is as yet only
a possibility ; still, it is by following up such
possibilities that advances in knowledge are
made, and without big telescopes we should
not have even the possibility.
One of the facts which has most arrested Results due
attention of late years is that objects can be tograpny
photographed which cannot be seen. Since
the discovery of the Rontgen rays, by which
photography triumphs over the eye in quite a
novel manner, it seems less extraordinary that
219
MODEEN ASTEONOMY
we should be able to detect by the sensitive
plate light that is merely too faint for the eye ;
but not many years ago this was a new and
remarkable advance. It really dates from the
discovery of the dry plate, which has allowed
us to prolong exposures indefinitely ; so that
by giving longer and longer exposures of a
photographic plate at the end of a telescope
pointed to any region of sky, we find that we
get on the plate fainter and fainter stars ;
and we can continue the process until we
have certainly got stars on the plate (and
nebulae too), which the eye cannot see with
the best telescope ; and even then we can go
on further. The only limit to the time of
exposure which has yet been reached is that
assigned by the patience (or impatience) of
the observer. If the night clouds over, or the
dawn comes, he can shut up the camera and
wait till the next fine night, when he can
open it again and continue the exposure ; and
so on for months or years. Up to the present
I do not know of any photographs having
been taken with a cumulative exposure of
more than fifty hours ; but there is no par-
ticular reason for stopping there, and doubt-
less we shall have longer ones in the future.
220
MODERN RESULTS
These discoveries, however, of stars which stars which
cannot be
have not previously been seen because they
are too faint for the eye even with the largest
telescope, though they are in a very real sense
discoveries, are not of any very great impor-
tance,' for the reason that the vast majority of
the stars are of no special interest. They are
to be found in the same position, looking
exactly the same for centuries to come as in
centuries past. They are not quite so uninter-
esting as Euclid's " points " ; for though they
have no " parts " (so far as we can see at such
a great distance) they have " magnitude," in
the astronomical sense of brightness. We have
good reason for assuming them to be bodies as
large as our Sun some of them much larger ;
but they are so far off that it is quite hopeless
to detect any dimensions, as hopeless as to see
objects about the size of human beings in the
Moon. In the largest telescope they are mere
points of light ; and their position and bright-
ness remain nearly constant in the great
majority of cases. The number of " variable
stars " known is not twenty in a million of
those which can be seen, and these must be
excepted ; there are also stars which are of
interest because they are double, are compara-
221
seen
MODERN ASTRONOMY
tively near us or are in comparatively rapid
motion, or have a peculiar spectrum. Except-
ing all these, which form only a minute
fraction of the total number, we may say that
the average star is of no particular interest,
and hence the occupation of discovering new
ones is profitable only so far as that among
the number there may by chance be one of
these exceptional cases a new variable or a
new binary ; but then the vast number of
stars easily visible in any telescope have only
as yet been searched very inadequately for
such objects. Hence, though the discovery of
new stars is undoubtedly one of the achieve-
ments of the photographic dry plate it cannot
be regarded as one of its greatest triumphs.
New In thus disparaging the importance of any-
thing new, however, we are always likely to
be proved at fault ; and in a closely allied field
of work we have recently had a conspicuous
instance of the danger of such judgments. It
has been already remarked that photography
has been applied with great success to the
discovery of minor planets. The old laborious
process of watching each star, to see whether
it was moving, has been superseded by the
222
MODERN EESULTS
simple expedient of exposing a photographic
plate, so that any planet in the field may
betray itself by its little trail. It was men-
tioned that the number of these bodies had
accordingly rapidly increased, and interest in
them had begun to flag ; for most of them are
just as uninteresting as stars. It is true they
are in motion while the stars remain fixed,
but this adds to the difficulty of keeping
track of them rather than to our interest in
them ; for beyond the fact that they are
moving round the Sun under the well-known
law of attraction, there is no special feature
in their movements. And we cannot see any
detail 011 their surfaces, not because they are
far away like the stars, but because they are
actually so small the surface of most of
them would not be so large as that of the
British Isles. Hence, the occupation of find-
ing new minor planets was beginning to be
regarded with disfavour, as adding a new
burden without any compensating advan-
tage. But in the autumn of 1898 one of these
little bodies was discovered worth hundreds of
others put together ; for it turns out to be
our nearest neighbour in the solar system,
after the Moon and Venus. The importance
223
MODERN ASTRONOMY
of this new planet Eros has been already
explained (see p. 107) ; it makes periodical near
approaches to the Earth, and on these occa-
sions we hope to obtain an accurate measure
of the Sun's distance. The discovery is men-
tioned here as a reminder that there is a
danger in regarding any field of work as
unprofitable. We may find a rare gem in
what is supposed to be a pan of ordinary
gravel.
New satci- A few months ago photography discovered
Saturn its first satellite a ninth satellite of Saturn.
All previous discoveries were made by eye, in-
cluding those of the other eight satellites to
Saturn ; but on developing a series of photo-
graphs of the planet and its neighbourhood,
Professor W. H. Pickering (brother of E. C.
Pickering) found traces of an object, which oc-
cupied different positions on different dates, in
the manner of a satellite. There seems to be
sufficient evidence of the existence of this new
satellite ; but as yet it has not been detected
visually even with the largest telescope;
and it has not been photographed again, for
specially good conditions are required, and
they did not recur in time. For further con-
firmation we await a favourable opportunity.
224
MODERN RESULTS
It was mentioned in the last section that New Comets
new comets had been discovered by photo-
graphy ; and beyond this general statement
no particular discovery calls for remark. If
among the number had occurred one of those
exceptional comets which becomes a striking
object in the heavens easily visible to the
naked eye, like those of 1858 and 1882, we
might have dwelt a little on the discovery.
But we have had none of these for some time
(and in answer to the inquiry sometimes put,
" When are we going to have another bright
comet ? " I may reply, u We cannot tell in the
very least: there is no regular recurrence of
such which enables predictions to be made ") ;
and both photographic discoveries and those
made in the old-fashioned way (which are still
the great majority), have been limited to those
small comets which are only visible in a
telescope and do not interest the public. Of
these there are generally half a dozen found
every year.
Photography has, however, told us a good Comets'
Tails
deal of which we were previously quite
ignorant the striking changes in comets'
tails. These are subject to sudden convul-
225 Q
MODERN ASTRONOMY
sions which shatter them into extraordinary
forms, the origin of which is not yet under-
stood. Professor Barnard is fond of telling
the story of a lady to whom he showed
one of his photographs of a comet, with the
tail in a peculiarly ragged and disreputable
condition : whereon she remarked, " Why !
that comet looks as if it had been out all
night ! "
Forms of The reason why such changes of form were
Nebulae
little known before the days of the dry-plate
is that it is very difficult to get an idea of
the form of such objects with the visual tele-
scope : firstly, because they are faint, and
secondly, because so little of them can be seen
at one time, the field of an eyepiece being
small. This point has been already referred
to (see p. 138) ; and we now pass to an illus-
tration of its importance, not in the case of a
comet, but in that of a nebula, which is in
some respects an object of the same kind. We
shall presently refer to the discovery of new
nebulae by photography ; but the present ex-
ample is intended to show how much photo-
graphy has taught us about an old and well-
known nebula that in Andromeda. It is a
226
Roberts' Photograph.
THE ANDROMEDA NEBULA.
228
DK LA RUE S DRAWING OF SATURN.
THE SPIRAL NEBULA IN CANES VENATICI (LICK OBSERVATORY).
229
MODEKN RESULTS
large bright object, visible even to the naked
eye, and is a tolerably easy object to draw
at the telescope. But the drawings, even of
the best observers, left us in ignorance of an
essential feature of the object, which was
revealed directly it was photographed. Look
first at the drawing shown alongside the
photograph, and notice specially the two
dark rifts. The draughtsman has made them
straight, whereas it is seen in the photograph
that they are slightly but sensibly curved. The
draughtsman is not very far wrong, but just so
far as to miss the whole point of the formation
which we see so admirably in the photograph.
The rifts are really the separation between
the central nebula and a ring thrown off from
it, seen in perspective ; and we see here actu-
ally in the sky the state of things which
Laplace suggested in his famous Nebular The Nebular
Hypothesis a central nebula, which in its confirmed
rotation throws off a series of rings, some of
which break up to form satellites. There are
two satellites already formed, and others are in
course of formation. The system closely
resembles (except in crispness of outline) that
of the planet Saturn, as we see by the draw-
ing annexed, which is an actual drawing
MODERN ASTRONOMY
made by Dr. De la Rue in 1852, not altered
in any way beyond being turned to an
orientation resembling that of the nebula.
By a curious coincidence two satellites of
Saturn are shown by Dr. De la Rue in
somewhat the same positions relative to the
central body as the satellites of the nebula
but this is merely a coincidence of detail.
The general evolution of such a system,
with rings and satellites thrown off by the
rotation of a central nebula, all becoming
more definite by condensation, was long ago
suggested by Laplace as the possible history
of our solar system ; but we had no direct
evidence of the occurrence of such pheno-
mena, beyond that indicated by the system of
Saturn, until this nebula in Andromeda was
photographed. The first photograph to show
the true nature of the nebular system, which
was taken by Dr. Isaac Roberts, thus stands
in the same relation to Laplace's nebular
hypothesis as do some of Galileo's first glances
through his telescope to the Copernican theory.
Copernicus made the Moon a satellite to the
Earth ; but no confirmation was forthcoming
in the snaps of similar satellites to other
planets, until Galileo looked through his
232
MODERN RESULTS
"optic tube " at Jupiter, and discovered satel-
lites to that planet, and then the idea of the
Moon being a satellite was no longer strange.
On the Copernican theory Mercury and Venus
ought to exhibit phases like the Moon ; but
there was no evidence of this until Galileo
looked through his telescope and saw them.
The invention of the telescope supplied just the
evidence wanted for the Copernican system ;
and the invention of the dry-plate has put in
evidence Laplace's nebular hypothesis as an
actual fact. Many beautiful pictures have
been taken of other nebulae, and some of them
are very similar to the Andromeda nebula: Spiral
Nebulas
others show a spiral structure (see illustration),
which suggests a rather different historical
development. The full significance of all the
information thus acquired is scarcely yet
realized, but it all tends to throw light on the
history of the universe. The information is
accumulating on our hands so rapidly that
there has been no time to arrange and study it
properly ; but it seems quite probable that
when the forms are classified, we shall learn
something new about the history of stellar
systems. And besides the study of individual
forms there is that of the distribution of
233
MODERN ASTRONOMY
nebulae over the sky. They do not occur in
all quarters of the sky equally, but seem
specially to avoid the Milky Way : while, on
the other hand, star-clusters congregate in the
Milky Way. What is the meaning of this ?
At present we cannot say : I do not think that
even a good guess has been made ; but it is
clear that the more we learn of nebulas and
clusters the more likely we are to interpret
this most important fact.
New It has been said that the information is
Nebulae . .
accumulating rapidly ; and precision can be
given to this statement by a few figures. Pro-
fessor J. E. Keeler, 1 the Director of the Lick
Observatory, is finding on his photographic
plates about three new nebulae per square
degree, which would lead him to expect about
120,000 in the whole sky ! It will be obvious
that to arrange and digest such a mass of in-
formation is not a light matter.
The instrument used by Professor Keeler
is a 3-foot reflector, the same instrument with
which the pioneer in this work, Dr. A. A.
1 While these sheets were in the press, the sad
intelligence reached England of the sudden death of
Professor Keeler.
234
MODERN RESULTS
Common, of Ealing, took his photographs of
the Orion nebula in 1882-3, for which the
Royal Astronomical Society awarded him its
gold medal. His instrument was afterwards
purchased by Mr. E. Crossley, of Halifax, who
subsequently presented it to the Lick Obser-
vatory. Here, in the clear air of Mount Hamil-
ton, and in the able hands of Professor Keeler,
it is doing the magnificent work above men-
tioned. The reflector seems to be specially
fitted for the work of photographing nebulae.
Dr. Isaac Roberts, to whom fell the honour
of taking the first photograph showing the
character of the Andromeda nebula, and who
has published two volumes of beautiful photo-
graphs of nebulae, has used a reflector through-
out.
But there is another kind of nebula for which Diffused
the reflector is comparatively useless, and the
portrait-lens is the proper instrument. The
above objects, which are being photographed
at the Lick Observatory by the thousand, are
all small and well defined, or at least well
separated from each other. It has, however,
gradually become apparent that there are
whole regions of the sky over which extremely
235
MODERN ASTRONOMY
faint nebulous matter extends, with marvellous
ramifications and interlacings. These objects
are too faint to be seen by the eye even with
the best telescopes, but if a long exposure is
given with a powerful portrait-lens they may
be photographed. Look, for instance, at the
DIFFUSED NEBULA NEAR TIIE PLEIADES (MAX WOLF).
picture of the Pleiades shown in the illustra-
tion. With the largest telescope little more
can be seen in this region than stars, which
the picture shows as round dots. A few wisps
of nebula were detected in the very middle of
the picture some years ago, but so little that
236
MODEEN EESULTS
we may neglect it entirely, and say that the
eye can only see the stars. The picture shows
us what a portrait-lens can reveal to us a
revelation indeed ! All over the region is a
nebulous structure to which there seems no
limit. "We begin to wonder whether there is
not an invisible veil of nebula over the whole
sky, which would betray itself with a long
enough exposure. Here, again, we are getting
information which we have only had time as
yet to marvel at not to interpret.
The actual picture is drawn from his photo-
graphs by Dr. Max "Wolf, of Heidelberg ; but
it was Professor Barnard who first drew atten-
tion to these diffused nebulse by his beautiful
pictures taken at the Lick Observatory with
a portrait-lens (the Willard lens), and even
with a cheap magic-lantern lens. These first
revealed the large diffused nebulse of which
that round the Pleiades is a striking illustra-
tion. Valuable work at an observatory is not
always done with its largest telescope. The
achievements of the Crossley reflector and of the
Willard lens even of a small lantern lens, in
Barnard's skilful hands range worthily along-
side those of the giant refractor, which many
237
MODERN ASTRONOMY
people regard as essentially constituting the
Lick Observatory.
Variables It was mentioned above, that while the nebulae
in Star-
Clusters seem to avoid the Milky Way, star-clusters
THE CLUSTER U3 CENTAURI.
seem t6 prefer it. A notable discovery about
star-clusters has been made by Mr. S. I. Bailey,
of Harvard, viz., that a very large proportion
of the stars in them are variable. In one
238
MODERN RESULTS
cluster 85 stars are variable out of 900, which
is a very large proportion compared with the
ordinary sky. Moreover, the variations of light
show a striking similarity : each goes through
a regular cycle of changes in something like
twelve hours, some have a period as short as
ten hours, and some as long as fourteen, the
average being about twelve and a half. Each
begins by a gradual diminution of light to
about half its maximum, remains at half-
brightness for a sensible time, and then
springs back again to full brightness. The
suddenness of the return to full brightness is
quite remarkable, as may be seen from the
curves for some of these stars shown in the
illustration. Now, here again is a striking
fact which as yet we cannot explain. "What
is the cause of this peculiar variation of light ?
In the case of the variable star Algol, it is toler-
ably certain that the light-variation is due to
the existence of a dark body circulating round a
bright one, and regularly eclipsing it at inter-
vals. Not only does this hypothesis fit the
facts, but there is independent spectroscopic
evidence of its truth. But neither this nor
any allied supposition will fit the light curves
of these cluster-variables. We have here
239
A/2
70 20 30 40
ISO
LIGHT CURVES FOR 8 OUT OP 85 VARIABLES IN THE CLUSTER
MESSIER 5, AS DETERMINED BY S. I. BAILEY, OF HARVARD, 1899.
240
MODEEN EESULTS
another discovery of which we are not as yet
able to take full advantage.
Although this is all so new, it is at the same Tennyson's
Astronomy
time curiously old. In 1835, Tennyson wrote
in his " Palace of Art " the following stanzas.
They were omitted later because he thought
that the poem was " too full," so his son
tells us in the Life of Tennyson, from which
I have taken the stanzas. In the centre of
the four quadrangles of the palace is a tower,
and
"Hither, when all the deep unsounded skies
Shudder'd with silent stars, she clomb,
And as with optic glasses her keen eyes
Pierced thro' the mystic dome.
Kegions of lucid matter taking forms,
Brushes of fire, hazy gleams,
Clusters and beds of worlds, and bee-like swarms
Of suns and starry streams.
She saw the snowy poles and moons of Mars,
That mystic field of drifted light
In mid Orion, and the married stars."
Enormous additions to our knowledge have The Spec-
been made by the spectroscope, but many of
them do not admit of a brief and crisp state-
241 B
MODEEN ASTEONOMY
merit as definite discoveries or results. When
the spectroscope was turned, in 1864, for the
first time to one of the nebulae, a definite step
in advance was made. It was seen at a glance
that the spectrum consisted of a few distinct
bright lines, and hence that the nebula was
a mass of glowing gas. Before this it had
been doubtful whether it might not be
merely a star-cluster, the stars so small and
close together as to be inseparable in the
best instruments. Sir William Huggins knew
before he put his eye to the instrument, on
August 29, 1864, that he was about to get
a definite answer to this question, and felt
an intense thrill of excitement accordingly.
But such opportunities are comparatively rare,
even in the early days of a new instrument.
It is a commoner experience to attain a definite
result only after accumulating masses of obser-
vations, and studying them carefully. Much
of the work done by the spectroscope has con-
sisted in obtaining pictures of the spectra of
different stars for comparison and classifica-
tion ; and as time goes on, the crispness and
definiteness of the early results is lessened
rather than increased.
242
MODERN RESULTS
For instance, the stars were divided at first Classifica-
tion of
into four or five well-defined classes or types, the stars
according to their spectra ; but with more in-
formation the divisions between the classes
have been broken down by the addition of
intermediate types ; and now, although we
can form a long string of spectra in regular
order, so that the main features change gradu-
ally and regularly as we pass along the series,
we can only cut up the series into sections
quite arbitrarily, as the milestones divide up
a road, to use the favourite illustration of an
eminent spectroscopist. Again, it does not
seem certain that the present arrangement
can be regarded as final. Various classifica-
tions have been proposed by different workers,
but for the moment we will consider two speci-
ally : those of Professor Pickering, of Harvard, The
and of Mr. McClean, of Tunbridge Wells. Both ^rvly
these earnest workers have surveyed the whole
sky, north and south hemispheres and both
used an object-glass prism to do the work ; but
in other respects they have differed. Professor
Pickering, the head of a great observatory,
which has the exceptional feature of a branch
establishment in Peru, so as to command both
hemispheres, naturally worked through his
243
MODERN ASTRONOMY
assistants. The prism was of small angle, so
that the spectra were short, and showed main
features rather than great detail ; but a large
number of stars were photographed on each
plate, so that altogether the spectra of an im-
mense number of stars were obtained. These
were examined and classified by a compara-
tively rapid process.
Mcciean's M r McClean did the whole of the work him-
Survey
self with his own eyes and hands : the nor-
thern half in his own private observatory at
Tunbridge "Wells, the southern during a six
months' visit to Sir David Gill at the Cape of
G-ood Hope Observatory. He only photographed
the spectrum of one star at a time, limiting
himself to the brighter stars. But he was
thus enabled to use a prism of larger angle
and get much greater dispersion. His spectra
are given in considerable detail.
On comparing the two sets of results it is
sometimes found that a star which has been
assigned by Professor Pickering, guided by
the main features of the spectrum, to one
place in the series, is found, when the details
shown on Mr. McClean's photographs are
studied, to belong really to quite a different
'244
MODERN EESULTS
place : the greater dispersion shows cause for
some rearrangement of the classification. It
seems possible, then, that a future advance in
the same direction may call for further modi-
fications.
Yet again, there is some doubt about the Tempera-
ture of
general interpretation of the classification or stars
series. It may be that the stars so classified
are arranged according to their temperature.
Sir Norman Lockyer is a strong advocate of
this view, and points out very confidently
which are the hottest stars. On the other
hand, Sir William Huggins has shown recently
that certain features in a spectrum need not
be due, as had been supposed, to the high
temperature of the star, but to the quantity of
certain substances present ; and we shall see in
the next few paragraphs how seriously dif-
ferent may be the quantitative distribution of
an element in different bodies.
For such reasons as these much of the most
interesting part of spectroscopic work cannot
be stated in the form of definite results. But
there are some notable exceptions, and to
mention one or two of these will give a
general idea of the importance of the work,
245
MODERN ASTRONOMY
leaving completer knowledge to be sought
elsewhere.
Discovery of A good concrete example is afforded by the
Oxygen in J
the stars recent discovery of oxygen in the stars. It
was a strange thing that oxygen, so vitally
important to us, and so lavishly distributed
over our globe, could never be detected (by the
spectroscope) in any other celestial body. Its
lines have been looked for persistently, but
without success, in the spectrum of the Sun ;
twenty years ago Dr. Draper thought he had
found them there, but it was proved to be a
mistake. The veteran, M. Janssen (who came
out of Paris in a balloon during the siege of
1870, because he wanted to observe the total
solar eclipse in that year, and who is respon-
sible for the sensational project of building
an observatory on the top of Mont Blanc),
has spent a considerable portion of his life in
trying to detect oxygen in the Sun, or prove
its non-existence. The particular difficulty
with which he has done battle is this : since
sunlight comes to us through our atmosphere
which contains oxygen, its spectrum natur-
ally contains traces of the oxygen lines ; the
question is, are these traces exactly such as
246
MODEBN BESULTS
are due to our atmosphere, or is there an
excess which must be due to the Sun ? M.
Janssen has arranged striking experiments to
test this point. He has for observatory what
was once an imperial palace, reduced to ruins
in the war of 1870. The imperial stables,
however, are not damaged, and the stalls for
the imperial horses form efficient supports for
some very long tubes into which M. Janssen
compresses oxygen equivalent to what is
between us and the Sun. Looking then
through such a tube, at a source of light
known not to contain oxygen, he sees the
traces introduced by passage through the
oxygen in the tube, and can accordingly esti-
mate the allowance to be made in observing
the Sun.
Another and more sensational experiment
was to observe the lights on the Eiffel Tower
in a similar manner ; for between Meudon and
the Eiffel Tower there is just about as much
oxygen (compressed as it is close to the
Earth's surface) as between Meudon and the
Sun.
And finally, by getting to the top of Mont
Blanc he diminishes the amount of oxygen
247
MODERN ASTRONOMY
between him and the Sun in a known propor-
tion, and can draw conclusions from the effect
on the solar spectrum.
The conclusions from all these experiments
are, however, so far negative oxygen cannot
be detected in the Sun. Since the other
planets only shine to us reflected sunlight, we
cannot say whether they have oxygen or not ;
we presume so, but we have no direct evidence ;
and the uncertainty has an important bearing
on the possibilities of life on other planets.
Nor was oxygen discernible in any of the
stars. To all appearance we might be the
only body in the universe possessing oxygen
at all ; it seemed a possible but extraordinary
isolation that our very life-giving element
should not be shared by any other celestial
body.
This absolute isolation has been removed
within the last two years by Mr. McClean.
In the course of his survey above referred to,
he found certain lines in the spectrum of the
star ft Crucis, which did not correspond with
those of any other star, or element already
known in the stars. On search being made
they were found to be lines due to oxygen.
248
MODERN EESULTS
The discovery has been confirmed by others,
and the lines found also in other stars, so that
we are no longer peculiar, though possibly
specially favoured.
On the other hand, an element of which we Discovery of
Helium
have only a minute quantity, and until a few on Earth
years ago were believed not to possess at all,
seems to be of the first importance in the
sidereal universe the element helium. It was
known to us until recently only by a single
line in the spectrum of the Sun ; an impor-
tant line which did not occur in the spectrum
of any known substance. A few years ago,
soon after the discovery by Lord Rayleigh and
Professor Ramsay of the new gas argon in our
air, and during experiments undertaken with
the view of obtaining argon, Professor Ram-
say discovered the element helium in the rare
mineral cleveite. The whole of its spectrum
was thus identified not merely the one line;
and it was then seen that this element oc-
curred in a very large number of stars, and
the progressive variation of the intensity of
bhe lines representing it was one of the chief
features guiding the arrangement of spectra
in series. Helium is in some intimate way
249
MODERN ASTRONOMY
connected with the evolutional history of the
stars; and that we have so very little of it
is a very significant fact, of which we do not
yet know the full meaning.
Distribution Let us now turn to a different kind of dis-
of the
Woif-Rayet covery. In arranging or classifying the stars
by their spectra, there is a peculiar class
which does not readily take its place in the
series. These are usually . called the Wolf-
E/ayet stars, from the names of the two astro-
nomers who first noticed them in 1867 : they
have bright lines in their spectra, and are
probably not so much stars as nebulae. More
than 100 of these stars are now known, and
they nearly all congregate close to the Milky
Way, the only exception being one group in
what may be called a detached portion of the
Milky Way. The discovery of many of these
stars, and the establishment of this fact, are
some of the many achievements of the Har-
vard University Observatory under its present
able Director, to whom I am indebted for the
information shown in the diagram. This is
another remarkable fact which seems to show
that the Milky Way is in some sense the
backbone of our sidereal system. But in what
250
251
MODEEN ASTRONOMY
sense? The ordinary nebulae avoid it, the
star-clusters and these Wolf-Rayet stars are
found there and not elsewhere. What general
idea of the stellar universe will co-ordinate
these facts? It has been suggested that the
whole system of the stars is spinning round an
axis, of which the Milky Way represents the
Equator, and that one class of bodies is driven
away from the axis, while another class is
sucked towards it, much as when a cup of tea
is stirred heavy particles are flung to the sides,
and light ones drawn into the middle. There
may, of course, be such a rotation, even com-
paratively rapid, without our being conscious
of it. We can detect the rotation of the
Earth, because the stars do not partake in it
and we see them sweep past ; but if all the
universe is rotating, where are the landmarks
by which to observe it ? The only signs may
be just these signs we have noticed. But the
suggestion is a vague and difficult one ; it
does not take us appreciably further than the
hope that the real explanation may not be
very far away it may be that some happy
thought will reveal it almost any day. We
know too little as yet we know, for instance,
nothing of the distances of the nebulae, or
252
MODERN EESULTS
their changes in form. They are certainly
so far away that their parallax is very
small, and even vast changes might appear
minute to us ; and until recently such
changes were hopelessly masked by the un-
certainties of drawing. Now, with photo-
graphy to help, we may hope that the com-
parison of photographs taken at long intervals
may tell us something of the changes, though
as yet this page in our observation book is
blank.
As regards the distance, a very important Distance of
Nebulas
step has been taken by Sir William and Lady
Huggins in the last ten years. In the Orion
nebula, as in many other nebulae, there
appear to be involved various stars ; but it was
impossible to say whether these stars were in
front of, in, or behind the nebula. By a
skilful observation with the spectroscope, Sir
William and Lady Huggins have made it
most probable that the stars are really part
of the nebula ; for the spectrum of the star is
found to closely resemble that of the nebula.
This being the case, it may be possible to
learn something of the distance of the nebula ;
for though the delicate measurements neces-
253
MODERN ASTRONOMY
sary to determine parallax cannot be made
on the vague and indefinite image of a
nebula, they can be made on a star ; and if we
can find the distance of the star we shall
know that of the nebula. It is significant
that this investigation has not (so far as I
know) been yet undertaken ; the attention of
astronomers has recently been claimed in so
many new directions that they cannot pos-
sibly do justice to all, and some of the most
attractive problems have accordingly failed to
attract solvers. The astronomical standing-
army is a very small one, and much of it is
wanted for home-defence for keeping a watch
on the objects already discovered, and doing
routine work that must be done. It is nobly
reinforced by volunteers ; and there is a perfect
accord between'the regulars and the reserve
forces. But we are in the presence of a vast
extension of the astronomical empire, and we
begin to find how small our numbers really
are. Is it a vain hope that our ranks may be
materially increased shortly ?
254
Section IV
MODERN MATHEMATICAL
ASTRONOMY
255
Section IV
MODERN MATHEMATICAL
ASTRONOMY
IT was remarked at the beginning of the first
section that new developments had recently
taken place not only in practical astronomy
but also in theoretical. To give any adequate
idea of the mathematical progress of the last
quarter of a century would be quite beyond
the scope of a work like the present. But
an attempt will be made to give a brief ac-
count of the kind of changes which have
revolutionised theoretical astronomy ; for they
are quite as striking as any already men-
tioned.
There are two great historic problems in
theoretical astronomy the Planetary Theory
and the Lunar Theory, as they are called. The Planetary
Theory
first problem may be thus stated : if no other
257 s
MODEEN ASTEONOMY
bodies existed but the Sun and a single planet,
it has long been known that the planet would
describe an ellipse round the Sun under the
influence of his attraction, the Sun being in
one focus of the ellipse. The fact that the
planets described ellipses in this way round
the Sun very approximately, was discovered in-
the seventeenth century by Kepler from a
study of Tycho Brahe's observations ; and the
explanation of the fact was given by Newton
in his famous law of gravitation.
But since other planets exist this is only
an approximate and not an exact statement.
The attractions of the other planets disturb
this simple state of things, and the problem
of the planetary theory is, how to find the
actual motion of a planet when others exist
to perturb it by their attractions. It is an
extremely difficult problem, and cannot be
completely solved with our present mathe-
matical machinery. We are enabled, how-
ever, to get aii approach to the solution for
our solar system, because all the planets are
small compared with the Sun, and their per-
turbations are minute compared with the
main attraction of the Sun.
258
MODEEN MATHEMATICAL ASTEONOMY
Hence, although any one planet does not
describe persistently an ellipse round the Sun,
as it would if all the others were removed,
it follows this path very nearly for some time.
small influence of the perturbations is
manifested in two distinct ways :
Firstly, instead of keeping accurately on
the track prescribed by the Sun, the planet
deviates now to one side and then to the
other. Part of the attraction of the disturb-
ing planets acts in contrary directions at
different times, and so keeps the body oscil-
lating, as a pendulum oscillates about a mean
position. If this were the only effect of per-
turbations, the ellipse round the Sun, though
not the actual orbit of a planet, would still
be its average orbit, from which it would
never be very far removed. The deviations
from side to side are called " periodic inequali-
ties."
Secondly, the average path does not remain
the. same; it slowly changes, under the in-
fluence of a part of the disturbing attractions
which always acts in the same direction, and
thus steadily accumulates. The path deviates
259
Periodic
ances
ances
MODERN ASTRONOMY
more and more from the original ellipse,
which ultimately ceases to represent the orbit
even approximately. Such changes are called
" secular."
illustration Perhaps an elementary illustration may
help in elucidating these ideas. Suppose we
have a steamer travelling on a perfectly flat,
calm ocean, with the helm firmly fixed truly
amidships. It will describe a perfectly
straight line, and this we may call the un-
disturbed orbit, corresponding to the fixed
ellipse round the Sun as focus described by
a planet if no other planets exist.
Now suppose the helm is slightly interfered
with. We may imagine two kinds of inter-
ference. In the first kind the helm is not
fixed amidships, but is occasionally slightly to
port, and as often slightly to starboard, in
the sort of way that would occur in actual
steering, owing to the irregularities of wind
and wave. The steamer would take a slightly
serpentine course, sometimes to one side of her
" undisturbed orbit," sometimes to the other ;
but, on the whole, she would follow the same
course as before, or at least would never de-
viate far from it.
260
MODEEN MATHEMATICAL ASTEONOMY
If, however, the helm were by a slight
mistake fixed not truly amidships, we should
have a very different kind of interference.
The steamer would no longer pursue a
straight course, but would turn continuously,
and so describe a circle. The smaller the
deviation of the helm the larger would be
the circle described ; but in all cases (provided
the ocean were large enough) the complete
circle would ultimately be described, and the
steamer would find itself sooner or later going
over its old track. It would start very nearly
along its old straight track, but there would
come a time when the vessel was steaming at
right angles to its original track ; another
time, when it was steaming parallel to it but
in the precisely reverse direction, and so on.
The whole character of the motion is gradually
changed by this sort of interference with the
helm, which corresponds to the perturbations
of a planet called " secular."
Now we may proceed to use this illustration
for the purpose of representing the way in
which mathematicians were accustomed to
treat the planetary and lunar theories, and
the important change in method recently in-
261
MODERN ASTRONOMY
troduced by Mr. G. W. Hill. The old method
of considering the motion of the steamer may
be stated as follows :
If the helm were truly amidships the
steamer would describe a straight line.
The error of the helm is very small.
Hence, the actual path will be very nearly a
straight line, though this straight line will
continually change in direction.
Let us, therefore, take the path as a straight
line, but allow this line slowly to revolve ;
and let us apply ourselves to determine the
rate of revolution, and then we can predict
the place of the steamer at any time.
This corresponds to the old methods of
planetary and lunar theory, which were by
no means unsuccessful. By laborious calcu-
lations fairly satisfactory tables of the planets
and the Moon were formed, and it was
thought that the interest had almost evapo-
rated from the problems.
G. w. Hill's Suddenly, however, new life was put into
Methods them by a complete change of method, which
allows of much -less laborious, and much more
262
MODERN MATHEMATICAL ASTRONOMY
complete solutions. Mr. Gr. W. Hill's argu-
ment may be stated for the case of the
steamer thus :
It is true that if there were actually no
error, of the helm the steamer's path would
be a straight line; but it is equally true that
the slightest error changes this path into one
of quite different character, viz., a circle.
It will, therefore, be better to accept the
situation at once, and call the path a circle
from the start. To cling to the notion of a
straight line only hampers the work, and we
can get on much better without this notion,
By actual experience this was found to be
the case. The idea was a simple one, but
fundamentally important ; it has effected a
revolution in the methods of the planetary
and lunar theories; so that those who studied
them thirty years ago in what was thought
to be practically their final form, would barely
recognise the modern treatment.
We will now repeat for a planet the state-
ments corresponding to those given for the
illustration.
263
MODEEN ASTRONOMY
The old methods started with the ellipse
round the Sun, which is the undisturbed
orbit.
The perturbations due to other planets
being small, it was assumed that the actual
orbit would be very nearly an ellipse, though
the shape and position of the ellipse would
continually change.
The orbit was, therefore, taken as an ellipse,
constantly, though slowly, changing in shape and
position, and the mathematicians set them-
selves to determine these changes, and thence
to find the place of the planet at any time.
The changes are, of course, far more complex
than the simple turning of the steamer's path,
but some of them resemble it closely. The
axis of the ellipse, for instance, instead of
remaining fixed in direction, as it would if no
other planets existed, in actual fact revolves
slowly in its own plane. So also the plane
in which the orbit lies revolves instead of
remaining fixed. The very smallest disturb-
ing planet that could be imagined would
make all the difference : without it the axis and
the plane would remain permanently fixed,
with it they would revolve, and, however slow
264
MODERN MATHEMATICAL ASTRONOMY
the revolution might be, it would ultimately
carry them right away from their original
position.
Now, although this idea of starting with
the ellipse, the " undisturbed orbit," and fol-
lowing its modifications from perturbation,
led to the solution of the problems of planet-
ary and lunar theory with a considerable
measure of success (so much so that thirty
years ago it was felt that probably all the
success had been attained which could be ex-
pected in such a difficult matter), still, Gr. "W.
Hill and other mathematicians have recently
shown that it is much better to discard the
ellipse at the outset. To cling to it is a loss
rather than a gain, for a fixed ellipse is really
different in essentials from a revolving one.
It was in lunar theory, and not in planetary,
Variational
that Gr. "W. Hill first introduced this reform, Orbit
but it applies equally in both cases. In lunar
theory we consider the orbit of the Moon round
the Earth. This would be an ellipse with the
Earth in one focus if no other bodies existed ;
but other bodies, and especially the Sun, per-
turb this orbit. The perturbations are small,
even for the Sun ; for though he is very much
265
MODEEN ASTBONOMY
larger than the Earth, he is so far away as to
more than counterbalance his size. Hence,
the problem is in many respects similar to the
planetary problem : to find the small disturb-
ances produced by a third body. It is found
possible to separate the disturbances into
several types, and one important type is called
the " variation." Hill included this in his first
approximation, and hence the name " varia-
tional orbit."
It would be difHcult to attach too much
importance to this simple but fundamental
reform : others who have worked in the same
field have testified emphatically to the fruitful-
ness of the idea. M. Poincare, who received
the gold medal of the Eoyal Astronomical
Society in 1900 for his splendid work in
mathematical astronomy, wrote of Hill's
papers. " Dans cette ceuvre il est permis
d'apercevoir le germe de la plupart des progres
que la science a faits depuis." And these
words were quoted 1 with approval by the
President of the Society, who had himself
received the medal for work in neighbouring-
fields, in his address on the occasion.
1 Hon. Not. R.A.S. vol. Ix. p. 413.
266
MODEBN MATHEMATICAL ASTRONOMY
c
One of the most
striking advances
which, have re-
sulted is repre-
sented by the
study of " periodic
orbits." The ac-
tual orbits of the
Moon and planets
are all very nearly
circular ; and the
methods of the old
lunar and planet-
ary theories, de-
signed as they
were with refer-
ence to these ac-
tual cases, were
not capable of ex-
tension to cases
differing very
much from them.
But it is easy to
imagine cases es-
sentially different,
though mathema-
ticians had until
Periodic
Orbits
267
MODERN ASTRONOMY
lately been able to do little or nothing in the
way of studying them. Hill first showed
how to get some general information on the
subject, and from his work, and that of
Gr. H. Darwin, Poincare, and others, we are
gradually getting a general analysis of the
possible orbits which may be described by one
body in the presence of two others : orbits en-
tirely unlike ellipses or circles, as will be seen
by reference to the diagram taken from one of
Professor Darwin's papers. It lay quite outside
the old planetary theory to consider even for
a moment a " figure of eight " orbit, such as
that marked A, or a bell-shaped orbit, like that
marked C, or an orbit such as that marked 6,
where the satellite or Moon does not go round
the Earth or Sun at all, but oscillates outside
them ! The mathematician will, on seeing
such orbits drawn, naturally ask about the
stability ; and, as a matter of fact, none of
those drawn in this diagram are stable ; but
they are possibilities, and we cannot get a
complete idea of planetary motions without
paying some attention to them. They may
even be actually occurring in Nature more
often than we think. It has been suggested
by Gylden, and recently quite independently
268
MODEEN MATHEMATICAL ASTEONOMY
by F. E" Moulton, that the Earth has a num-
ber of satellites of the oscillating class & very
small satellites, such as when they fall on the
Earth we call meteors, but making up in
number to some extent for what they lack in
size. The suggestion was made to explain
the phenomenon known as the " Gegenschein " The
Gegenschein
or counterglow. There may be seen on a fine,
dark night an extremely faint patch of light
in that part of the heavens just opposite the
Sun that is, in the direction from the Earth
to the cross within the orbit 6 in the diagram.
What is the explanation of this appearance?
G-ylden suggested that it is the sunlight
reflected back to us from a number of meteors
at that spot, which are not absolutely fixed,
but are describing in the neighbourhood of the
cross oscillating orbits like that marked b ;
and since these orbits are unstable, any par-
ticular meteor does not remain permanently in
the neighbourhood, being after a time swept
away in some different orbit ; but this is com-
pensated probably by the arrival of another
meteor, which joins the general dance for a few
turns. Near the cross there is always there-
fore a little crowd of meteors, each of which is
arrested for a time in the midst of its more
269
MODERN ASTRONOMY
extended travels, to jig up and down, back-
wards and forwards, for such time as will keep
the "pot a-boiling," or, rather, the G-egenschein
a-shining. The idea is, so far as can be seen
at present, a satisfactory explanation of the
phenomenon, and is the first application of the
new investigations on orbits hitherto un-
studied. There may be many others to come ;
for although the orbits of our own solar system
are all nearly circles, there are among the stars
cases of three or more bodies forming a stellar
system ; and when we know more of their
motions we may find all sorts of curiosities in
the way of orbital motions. At present such
systems have not been watched long enough
to tell us much of their complete orbits ; and it
may be added that we have scarcely had time
to apply the new ideas to observations already
made. Here, as elsewhere in astronomy, we
are only on the threshold of the new depar-
ture.
Forms of The lunar and planetary theories, although
Planets their methods have lately been revolutionized,
were already worked out with more or
success. But there is another department of
mathematical astronomy in which very little
270
MODERN MATHEMATICAL ASTRONOMY
had been done before the last quarter of a
century, and in which a good deal has been
done since, viz., hat which treats of the
changes of form of the heavenly bodies in long
periods. A few elementary propositions on the
u Figure of the Earth " had been arrived at,
and the mathematics of the tides had been
studied and found extremely difficult ; but any
one who takes up Professor Gr. H. Darwin's book
on u The Tides " 1 will see from the references
there given how much of the work in this new
department of astronomy is quite recent.
This book is an admirable popular exposition
of such work, and its existence renders any
extended account of it here more than un-
necessary. I will, however, venture to give,
in illustration of the kind of work referred
to (much as a reviewer might quote from the
book he is reviewing while recommending it
for full perusal), the striking results arrived
at by Professor Darwin regarding the past
history of the Earth and the Moon.
According to Laplace's nebular hypothesis, History of
the whole solar system has been generated " Moon
1 The Tides and Kindred Phenomena in the Solar
System. By George Howard Darwin. London:
John Murray, 1898.
271
MODEEN ASTBONOMY
from a single nebula, the greater part of which
now forms the Sun. As this nebula contracted
from its original diffused form, rotating faster
and faster (according to the well-known law of
conservation of moment of momentum), it
threw off rings which broke up and formed
the planets. In the same way these generated
their satellites, circulating round them as the
planets circulate round the Sun, and so the
Moon was formed out of the Earth.
Effect of Now. we know that the Moon causes tides
Tides
on the Earth, and though we usually think of
tides as affecting the ocean only, it has been
recognised recently that there must be tides in
the solid Earth as well. The same forces of
attraction which are able actually to move the
liquid water, must produce strains in the solid
earth, and though the effect may be small now,
it would be larger when the Earth was more
plastic, as it must have been if it was formed
by condensation from nebulous matter. Fur-
ther, not only does the Moon cause tides in the
Earth, but the Earth could cause tides in the
Moon. It does not cause ocean tides, because,
so far as we know, there is no water on the
Moon ; nor does it now cause bodily tides in the
272
MODEEN MATHEMATICAL ASTKONOMY
solid Moon ; but this is merely because in time
past it caused such enormous tides of this kind
that it has reduced the Moon to a state where
they are impossible. The tides, in fact,
whether ocean or bodily, caused by a satellite
in its planet, or a planet in its satellite,
gradually cause changes in the relative
motions of the pair, and in their distance
apart. There are " strained relations " between
them, in consequence of which they gradually
separate, and the rotation of each is modified
especially that of the smaller. The effect on
the rotation is this : each of the pair tries to
make the other rotate on its axis in the same
time which is occupied by the revolution of
the pair round each other. In the case of the
Earth and Moon we call this time a month,
and the Earth, being the bigger and stronger,
has already succeeded in making the Moon
revolve on her axis in exactly one month ; so
that we always see the same face of the Moon.
The Moon is trying to do the same thing to
us, and will ultimately succeed no doubt. At
present the Earth rotates in a day very much
faster than the Moon likes : she is reducing Alteration
of Day and
this speed as much as she can, but her power is Month
very small, so small that we can scarcely
273 T
MODERN ASTRONOMY
detect any appreciable diminution of speed in
historic times. But the diminution must be
there, and will ultimately make our day a
month long. The Earth will then always
turn the same face to the Moon as the Moon
does to us ; and the Americans may have a
monopoly of the Moon they may see it
always, while it will never be seen in Europe,
or vice versd. How far ahead this is we
cannot say ; for we can scarcely measure the
rate of diminution of the Earth's velocity as
yet : we must wait for a little more informa-
tion.
But it is easier to go back in time than to
go forward, because the effect was greater in
the past than it will be in the future. And
following up such clues as* are available, Prof.
Gr. H. Darwin has made a very fair guess at
the past history of the Earth and Moon. Some
years ago he gave in one of his papers the
following interesting little table of dates. It
must not be taken too accurately, but it gives
at least a general idea of past events :
274
MODEBN MATHEMATICAL ASTEONOMY
1
Time in
Millions of
Moon's
Moon's
Years
Sidereal
Sidereal
No. of
distance in
(dating back-
day in
period in
days in
Earth's
wards).
M.S. hours.
M.S. days.
Monr-h.
mean radii.
h
d
d
o-oo
23-93
27-32
27-40
60-4
46-30
15-50
18-62
28-83
46-8
56-60
9-92
8-17
19-77
27-0
56-80
7-83
3-59
11-01
15-6
56-81
6-75
1-58
5-62
9-0
5-60
0-23
1-00
1-5
The lengths of the day and month have been
changing because of this influence of each
body on the time of rotation of the other. It
will be seen from the second column of the
table that the Moon's recent influence on the
Earth's time of rotation is, however, slight :
in the last forty-six million years it has only
changed the day from a fifteen-and-a-half hour
day to a twenty-four hour day. Going back
beyond this time the change was more rapid.
But the most interesting columns are the
fourth and fifth. Looking at the bottom of
the fourth column, which refers to a time
something like fifty-seven million years ago,
there was one day in the month only, i.e., the
Moon was running round the Earth as quickly
as the latter rotated on its axis. This must
have been the time when the Moon was just
275
MODERN ASTRONOMY
flung off the Earth, and is an interesting
separation birthday. "We see from the fifth column that
of Moon
from Earth her distance from the Earth was then only
one-and-a-half radii, so that the Earth was
only very little greater than its present size.
It has since contracted in size, and the mutual
tidal action of the two bodies has carried off
the Moon to its present distance ; but it is
interesting to see how these figures come out.
The table is the result of most laborious cal-
culations, and several hypotheses have to be
made in the course of them. If Professor
Darwin had got a result at the end which
was impossible or absurd, it would have been
disappointing, but perhaps only such a disap-
pointment as he should have been prepared
for. If, for instance, the last figure in the fifth
column instead of coming out 1'5 had come out
O9, meaning that the Moon separated from
the Earth when the latter was less than its
present size, the whole work would have been
discredited, for we know of no causes which
would produce gradual expansion of the Ear-th
as time went on. But the figure 1-5 is
eminently reasonable, and we may accept with
some confidence the result that the Moon
separated from the Earth when the latter was
276
MODERN MATHEMATICAL ASTRONOMY
not much bigger than now, and about fifty-seven
millions of years ago. The date is subject to
alteration when we have measured the pre-
sent rate of tidal retardation more accurately,
though it is unlikely to be reduced below
twenty millions, or extended to more than one
hundred millions. But those interested in such
glimpses into the past should certainly refer
to the book itself, where they will find it stated
clearly, and with due caution, what is the best
information we have yet acquired.
277
Index
Abney, Sir W., 79
Adams, J. C., 123
Airy, Sir G. B., 12, 14, 15,
17, 20, 21, 30, 37, 38,
112, 151, 170
Aldebaran, 177
Algol, 190, 239
Almucantar, 34-40, 180
Altazimuth, 12, 15, 20, 23
Amateurs, telescope for,
128
Anderson, T. D., 166
Andromeda nebula, 226-
233, 235
Arequipa, 56, 57
Argelander, 158
Argon, 249
Ascension, expedition to,
100-106
Astrographic chart, 32, 62,
156-162
Astrophysics, 7, 8, 80, 92
Aurigae, nova, 166
Automatic methods, 152
Azimuth, 21
Bailey, S. I., 238-240
Ball, Sir K., 142
Barnard, E. K, 54, 138,
208,213,214,216,219,
226, 237
Biddell, G-., 37
Boyden fund, 56
Broken-transit, 128
Bruce, Miss C. W., 55, 56,
62
telescope, the, 55, 56
Bull-roarer, 188
Buriiham, S. W., 5, 6, 54
Cambridge Observatory,
127-128, 160, 182, 217
Camera of the astron-
omer, 61, 69, 70
Campbell, W. W., 217
Canals in Mars, 206, 208
Capella, 216-218
279
INDEX
Cape of Good Hope Boyal
Observatory, 14, 57,
58, 59, 119, 164, 244
Chacornac, 159
Chandler, S. C., 37, 198-
205
Chart, astrographic, 32,
62, 154-162
Charting of the sky, fre-
quent, 165
Chicago, 53, 54
Christie, W. H. M., 12, 15,
23, 29, 30
Chromosphere, 88, 91
Clark, Alvan, 49
Classification of stars,
243
Clerk-Maxwell, J., 191
Climate, 51, 52, 56
Clockwork, 66, 114, 115
Clusters, 234, 238
Coelostat, 68-70
Collimating lens, 84
Comets, 137, 143, 218, 22
Common, A. A., 31,39, 47,
69, 70, 234
Conference, astrographic,
32, 156
Control, electric, 67
Cooke & Sons, Messrs., 30,
39,63
Copernican theory, 232,
233
Corona, 144-153
Coude, Equatorial, 125,
126, 131, 135, 164
Crossley, E., 235, 237
Crucis, /3, 248
Cygni, 61, 173
Cylindrical lens, 83
Czar Alexander III., 49
Nicolas, 48
D
Darwin, 'G. H., 11, 267,
268, 271-277
Day, alteration of, 271-
277
Dela.Bue, W., 229, 232
Diffraction, 112
Distortion of photographs,
64, 155
Diurnal method for par-
allax, 102
Domes, 29, 33
Doppler's principle, 186
Double stars, 4, 54, 71, 73,
74, 192
Doublet, photographic, 56,
62-64, 235-237
Draper, Dr. H., 246
Driving clock, 66, 114, 115
Dry plate gelatine, 8, 14,
60, 233
Durham Observatory, 39,
122
280
INDEX
E
Earth and moon, 271-277
Eclipses, solar, 62, 70,
144-153
of Jupiter's satellites,
120-122
Eiffel tower, 247
Elger, 21X
Elkin, W. L., 218
Equalization photometer,
41
Equatorials, 12, 13, 28
Equatorial coude, 125-
126, 131, 135, 164
Eros, 107, 110, 167-169,
206, 224
Euler, 199-202
Exposition, Paris, 44, 167,
209
Extinction photometer,
42
Eyes used to gauge dis-
tance, 98, 103
Faculse, 88, 91
Film to film device, 172
Focus, images out of, 184
G
Galileo, 232, 233
Gegenschein, 269
Georgetown Observatory,
180
Gill, Sir D., 34, 57, 100-
106,110,111,116,119,
156-159, 164, 182, 205,
244
- Lady, 102
Glaisher, J. W. L., 4
Graham, A., 183
Gratings, 85
Greenwich Royal Obser-
vatory, 12, 26-34, 57,
89, 91, 151, 181, 183,
187, 191, 217-218
time, 24, 25, 176-178
Grubb, Sir H., 59, 124
Gylden, H., 11, 268
Habitability of planets,
206
Hagen, Father, 180
Hale, G. E., 54, 88-91
Hall, Asaph, 215
Hamilton, Mount, 50, 51,
52,57
Harvard University Ob-
servatory, 43, 56, 62,
64, 119-122, 164-174,
184, 185, 243, 250-252
Heidelberg, 141, 236, 237
Heliometer, 34, 100-102,
110-119, 182
281
INDEX
Heliostat, 68
Helium, 249
Henry Bros., 159
Herschel, Sir J., 5, 6
Sir W., 5, 6, 135
Hill, G. W., 4, 11, 262-265
Hills, Capt., E.E., 177
Huggins, Sir W., 7, 8, 91,
92, 186, 242, 245, 253
Lady, 253
Image of stars, 77, 78, 184
Iris, 106, 110
Janssen, J., 246-248
Japan, 62, 153, 154
Johns Hopkins Univer-
sity, 85, 86
Jupiter's satellites, 54,
119-122, 124, 216
K
Keeler, J. K, 191, 234, 235
Kempf, 43
Kepler's laws, 97, 258
Laplace, 11, 231, 271
Large telescopes, 28-31,
43-59, 124, 206-219
Latitude, variation of, 198
Leonids, 143
Lick, James, 49-52
Observatory or tele-
scope, 43, 45, 49-52, 59,
138, 191,216-218,234-
237
Line of sight, motions in,
186
Livingstone, Dr., 96
Lockyer, Sir J. N., 245
Loewy, M., 125-128, 131,
135, 164
Longitude at sea, 24
Longitudes, photographic,
176
Lunar theory, 4, 257, 265,
267
distances, 176
M
Maedler, 211
Magnifying power, limita-
tion of, 209
Mars, 100-106, 111, 206-
208, 213-215
Kew photoheliograph, 15, Maunder, E. W., 191
32
Kiistner, F., 200
McClean, F., 57, 58, 59 7
243, 244, 248
282
INDEX
Measurement of plates,
71-80, 160-163
Meridian photography, 175
photometer, 128, 131
Meteors, 143, 269
Meudon Observatory, 247
Micrometer, 71-74, 112,
114, 115
Milky Way, 234, 238, 250-
252
Minor planets, 105-110,
140,167-169,205,222-
224
Mont Blanc Observatory,
246, 247
Month, alteration of, 271-
273
Moon, 15, 20, 24-27, 135,
136, 162, 163, 176, 207,
211-213, 271, 277
Mouchez, Admiral, 157,
159
Moulton, F. E., 269
Mliller, 43
N
Nautical almanac, 120, 176
Nebulee, 137, 226-238, 242,
253
Nebular hypothesis, 11,
231, 271
Neison, E., 211
Neptune, 18, 44
Newall, H. F., 217
Newcomb, Simon, 111, 119,
202
Nile, sources of, 96
Nova Aurigae, 166
Objective prism, 81, 243
Orbit, periodic, 267
variational, 265
Orion nebula, 138, 235,
253
Oxford University Obser-
vatory, 43, 64, 70, 108-
110, 160, 182
Oxygen in the sun and
stars, 246-248
Parallax, 95-110, 167, 205,
254
Paris Exposition, 74, 167,
209
large telescope for, 44,
209-210
Observatory, 125-128,
131, 135, 136, 159, 164
siege of, 246
Peculiar stars, paucity of,
173
Periodic disturbances, 259
orbits, 267
Personal equation, varia-
tions of, 181-183
Pharaohs, 51
283
INDEX
Photocronograph, 180
Photographs, how taken,
65
distortion of, 64, 155
Photography, 32, 60-80
former distrust of, 155
Photometers, 40^3, 77-80,
128, 131
Photometer, meridian, 128,
131
Photometric observations
of Jupiter's satellites,
119-122, 124
Pickering, E.G., 43, 55,64,
119-122, 128, 131, 164-
174, 185, 243, 250-252
W. EL, 211, 212, 224
Pique, H.M.S., 153
Pivots of transit circle, 19,
37,38
Planets, habitability of,
206
photographs of, 139
Planetary theory, 257, 267
Pleiades, nebula in, 236
Poincare, H., 11, 266, 268
Pond, J., 12, 13
Portrait lens, 56, 62-64,
235-237
Potsdam Observatory, 43,
191
Pritchard, C., 42, 163
Publication of observa-
tions, 170
Pulkowa Observatory, 45,
48,49
Pyramids, 51
Radcliffe Observatory, Ox-
ford, 135
Eamsay, W., 249
Eansomes & May, 37
Rapid examination of
plates, 171
Eayleigh, Lord, 249
Rectangular co-ordinates,
75-77, 160-162
Red flames round sun, 87,
91
Reflector, 31, 44-48 62-
64, 234, 235
Reflex zenith tube, 12
Refractor, 31, 43-59, 62-64
Repsold, 49
Reseau, 75, 156, 161, 162
Rising floors, 58, 59, 124
Roberts, Isaac, 232, 233,
235
Rosse, Lord, 47
Rowland, H. A., 85
Royal Astronomical So-
ciety, 4, 10, 43, 101,
235, 266
Royal Institution, 57, 203
Royal Society, 135
Russell, J., 135
284
INDEX
Sampson, E. A., 39, 122
Sappho, 106, 110
Satellites, observation of, 9
- of Jupiter, 119-122, 124
of Mars, 215
- of Saturn, 122-124, 224
Satellite V. of Jupiter, 54,
216
Saturn's ring, 191, 231
- satellites, 122-124, 224
Saunder, S. A., 163
Scale for measurement, 76
Schaeberle, J., 61
Schmidt, 211
Schonfeld, 158
Schroeter, 211
Schwarzschild, Dr., 185
Screws, 72, 75, 76, 85, 86
Sectors, Abney's, 79
Secular disturbances, 259
Sheepshank's telescope at
Cambridge, 127-128
Siderostat, 68
Somerville, Mrs., 96
Spectroheliograph, 86-91
Spectroscopes, 14, 33, 80-
91, 241-253
Spectroscopic binaries, 188,
192, 216-218
Star charts, 154-166
- images, 77, 78, 184-185
"Star-trap," the, 142, 143
Stars which cannot be
seen, 221
Stone, E. J., 14
Struve, H., 122-124
- O., 5, 6, 49
- W., 5, 6, 48, 49
Studentships, Isaac New-
ton, 57
Sun, distance of, 95-110,
167,205
eclipses of, 144-153
photographs *of , 88, 137
rotation, 189
spots, 32, 89, 137
- surface of, 86-91, 136-
137
Tables of planets, new, 118
Telescopes, large refract-
ing, 28-31, 43-59, 124,
206-219
reflecting, 31, 44-48,
62-64
Temperature of stars, 245
Tenn y son's astronomy, 241
Thompson, Sir H., 31, 32,
57
Tides, 271, 273
Todd, D. P., 152
Trails, planetary, 141, 167
Transit circle, 12, 16, 23,
34, 37-40, 111, 115,
161, 175, 182
285
INDEX
Transit circle, photo-
graphic, 179
Tychq Brahe, 40, 258
Variable stars, 40, 221,
238-240
Variational orbit, 265
Variation of latitude, 198
Venus, transits of, 95-97,
106, 107
Victoria, 106, 110
Vogel, Dr., 191
W
Washington, 59
observations, 145
Observatory, 215
Webb, Eev. G. W., 5, 211
Wedge photometer, 41
Width to spectrum, 83
Williams Bay, 54, 55
Wolf, Max, 141, 236, 237
Wolf-Eayet stars, 250
Y
Yale University Observa-
tory, 218
Yerkes Observatory, 5, 91,
92
telescope, 43, 45, 52-55,
59
Zollner photometer, 43
Butler & Tanner, The Sehvood Printing Works, Frome, and London.
286
r '
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