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LIBRARY

ASTRONOMICAL DISCOVERY

C T H F

ASTRONOMERS ROYAL.

ASTRONOMICAL
DISCOVERY

HERBERT HALL TURNER, D.Sc., F.R.S.

SAVILIAN PROFESSOR OF ASTRONOMY IN THE
UNIVERSITY OF OXFORD

OF THF

UNWEPT

.' iro

WITH PLATES

LONDON
EDWARD ARNOLD

41 & 43 MADDOX STREET, W.
1904

ASTWfr
LIBR>

EDWARD EMERSON BARNARD
i

ASTRONOMICAL DISCOVERER

THESE PAGES ARE INSCRIBED IN MEMORY OF
NEVER-TO-BE-FORGOTTEN DAYS SPENT WITH HIM AT THE

YERKES OBSERVATORY

OP
THE UNIVERSITY OF CHICAGO

PBEFACE

THE aim of the following pages is to illustrate, by
the study of a few examples chosen almost at
random, the variety in character of astronomical
discoveries. An attempt has indeed been made
to arrange the half-dozen examples, once selected,
into a rough sequence according to the amount of
''chance" associated with the discovery, though
from this point of view Chapter IV. should come
first ; but I do not lay much stress upon it.
There is undoubtedly an element of "luck" in
most discoveries. " The biggest strokes are all
luck," writes a brother astronomer who had done
me the honour to glance at a few pages, " but a
man must not drop his catches. Have you ever
read Montaigne's essay ' Of Glory ' ? It is worth
reading. Change war and glory to discovery
and it is exactly the same theme. If you are
looking for a motto you will find a score in
it." Indeed even in cases such as those in
Chapters V. and VI., where a discovery is made
by turning over a heap of rubbish — declared such
by experts and abandoned accordingly — we in-
stinctively feel that the finding of something
valuable was especially " fortunate." We should
scarcely recommend such waste material as the
best hunting ground for gems.

viii PREFACE

The chapters correspond approximately to a
series of six lectures delivered at the University
of Chicago in August 1904, at the hospitable
invitation of President Harper. They afforded me
the opportunity of seeing something of this
wonderful University, only a dozen years old and
yet so amazingly vigorous ; and especially of its
observatory (the Yerkes observatory, situated
eighty miles away on Lake Geneva), which is
only eight years old and yet has taken its place
in the foremost rank. For these opportunities I
venture here to put on record my grateful thanks.

In a portion of the first chapter it will be
obvious that I am indebted to Miss Clerke's
" History of Astronomy in the Nineteenth
Century " ; in the second to Professor R. A.
Sampson's Memoir on the Adams MSS. ; in
the third to Rigaud's " Life of Bradley." There
are other debts which I hope are duly acknow-
ledged in the text, My grateful thanks are due
to Mr. F. A. Bellamy for the care with which he
has read the proofs ; and I am indebted for per-
mission to publish illustrations to the Royal
Astronomical Society, the Astronomer Royal, the
editors of The Observatory, the Cambridge
University Press, the Harvard College Obser-
vatory, the Yerkes Observatory, and the living
representatives of two portraits.

H. H. TURNER.

UNIVERSITY OBSERVATORY, OXFORD,
November 9, 1904.

CONTENTS

CHAPTER I

PAGE

URANUS AND EROS i

CHAPTER II
THE DISCOVERY OF NEPTUNE 38

CHAPTER III

BRADLEY'S DISCOVERIES OF THE ABERRATION OF LIGHT

AND OF THE NUTATION OF THE EARTH'S AXIS . 86

CHAPTER IV

ACCIDENTAL DISCOVERIES . . . . . .121

CHAPTER Y

SCHWABE AND THE SUN-SPOT PERIOD . . . -155

CHAPTER VI

THE VARIATION OF LATITUDE 177

INDEX 221

ASTRONOMICAL DISCOVERY

CHAPTER I
URANUS AND EROS

DISCOVERY is expected from an astronomer. The Popular
lay mind scarcely thinks of a naturalist nowadays discovery,
discovering new animals, or of a chemist as find-
ing new elements save on rare occasions ; but it
does think of the astronomer as making dis-
coveries. The popular imagination pictures him
spending the whole night in watching the skies
from a high tower through a long telescope, occa-
sionally rewarded by the finding of something new,
without much mental effort. I propose to compare
with this romantic picture some of the actual facts,
some of the ways in which discoveries are really
made ; and if we find that the image and the reality
differ, I hope that the romance will nevertheless
not be thereby destroyed, but may adapt itself to
conditions more closely resembling the facts.

The popular conception finds expression in the Keats'
lines of Keats : —

Then felt I like some watcher of the skies
When a new planet swims into his ken.

Keats was born in 1795, published his first
volume of poems in 1817, and died in 1821. At

2 ASTRONOMICAL DISCOVERY

the time when he wrote the discovery of planets
was comparatively novel in human experience.
Uranus had been found by William Herschel in
1781, and in the years 1800 to 1807 followed the
first four minor planets, a number destined to
remain without additions for nearly forty years.
It would be absurd to read any exact allusion into
the words quoted, when we remember the whole
circumstances under which they were written ;
but perhaps I may be forgiven if I compare them
especially with the actual discovery of the planet
Uranus, for the reason that this was by far the
largest of the five — far larger than any other planet
known except Jupiter and Saturn, while the
others were far smaller — and that Keats is using
throughout the poem metaphors drawn from the
first glimpses of " vast expanses " of land or water.
Perhaps I may reproduce the whole sonnet.
His friend C. C. Clarke had put before him
Chapman's " paraphrase " of Homer, and they
sat up till daylight to read it, "Keats shouting
with delight as some passage of especial energy
struck his imagination. At ten o'clock the next
morning Mr. Clarke found the sonnet on his
breakfast-table."

SONNET XI

On first looking into Chapman's " Homer "

Much have I travell'd in the realms of gold,
And many goodly states and kingdoms seen ;
Round many western islands have I been
Which bards in fealty to Apollo hold.

URANUS AND EROS 3

Oft of one wide expanse had I been told

That deep-brow'd Homer ruled as his demesne ;

Yet did I never breathe its pure serene

Till I heard Chapman speak out loud and bold :

Then felt I like some watcher of the skies

When a new planet swims into his ken ;

Or like stout Cortez when with eagle eyes

He star'd at the Pacific — and all his men

Look'd at each other with a wild surmise —

Silent, upon a peak in Darien.

Let us then, as our first example of the way in
which astronomical discoveries are made, turn to discovery
the discovery of the planet Uranus, and see how °
it corresponds with the popular conception as
voiced by Keats. In one respect his words are
true to the life or the letter. If ever there was a
" watcher of the skies," William Herschel was
entitled to the name. It was his custom to watch
them the whole night through, from the earliest
possible moment to daybreak ; and the fruits of
his labours were many and various almost beyond
belief. But did the planet " swim into his ken" ?
Let us turn to the original announcement of his
discovery as given in the Philosophical Transac-
tions for 1781.

ASTKONOMICAL DISCOVERY
PHILOSOPHICAL TRANSACTIONS, 1781

XXXII.— ACCOUNT OF A COMET

BY MB. HERSCHEL, F.R.S.

(Communicated by Dr. Watson, jun., of Bath, F.R.S.)
Read April 26, 1781

"On Tuesday the I3th of March, between ten

Announce- -A. • i,-i T

ment. and eleven in the evening, while I was exam-
ining the small stars in the neighbourhood of
H Geminorum, I perceived one that appeared
visibly larger than the rest; being struck with
its uncommon magnitude, I compared it to H
Geminorum and the small star in the quartile
between Auriga and Gemini, and finding it to be
so much larger than either of them, suspected it
to be a comet.

" I was then engaged in a series of observations
on the parallax of the fixed stars, which I hope
soon to have the honour of laying before the
Koyal Society; and those observations requiring
very high powers, I had ready at hand the several
magnifiers of 227, 460, 932, 1536, 2010, &c., all
which I have successfully used upon that occasion.
The power I had on when I first saw the comet
was 227. From experience I knew that the
diameters of the fixed stars are not proportionally
magnified with higher powers as the planets are ;
therefore I now put on the powers of 460 and 932,
and found the diameter of the comet increased in
proportion to the power, as it ought to be, on a

URANUS AND EROS 5

supposition of its not being a fixed star, while the
diameters of the stars to which I compared it
were not increased in the same ratio. Moreover,
the comet being magnified much beyond what its
light would admit of, appeared hazy and ill-defined
with these great powers, while the stars preserved
that lustre and distinctness which from many
thousand observations I knew they would retain.
The sequel has shown that my surmises were well
founded, this proving to be the Comet we have
lately observed.

" I have reduced all my observations upon this
comet to the following tables. The first contains
the measures of the gradual increase of the comet's
diameter. The micrometers I used, when every
circumstance is favourable, will measure extremely
small angles, such as do not exceed a few seconds,
true to 6, 8, or 10 thirds at most ; and in the
worst situations true to 20 or 30 thirds ; I have
therefore given the measures of the comet's
diameter in seconds and thirds. And the parts
of my micrometer being thus reduced, I have also
given all the rest of the measures in the same
manner ; though in large distances, such as one,
two, or three minutes, so great an exactness, for
several reasons, is not pretended to."

At first sight this seems to be the wrong refer- called
ence, for it speaks of a new comet, not a new comet
planet. But it is indeed of Uranus that Herschel
is speaking ; and so little did he realise the full

6 ASTRONOMICAL DISCOVERY

magnitude of his discovery at once, that he
announced it as that of a comet ; and a comet
the object was called for some months. Attempts
were made to calculate its orbit as a comet, and
broke down ; and it was only after much work
of this kind had been done that the real nature
of the object began to be suspected. But far
more striking than this misconception is the
display of skill necessary to detect any peculiarity
in the object at all. Among a number of stars
one seemed somewhat exceptional in size, but
the difference was only just sufficient to awaken

other suspicion in a keen-eyed Herschel. Would any
other observer have noticed the difference at all ?
Certainly several good observers had looked at

at ail. the object before, and looked at it with the care
necessary to record its position, without noting
any peculiarity. Their observations were re-
covered subsequently and used to fix the orbit of
the new planet more accurately. I shall remind
you in the next chapter that Uranus had been
observed in this way no less than seventeen times
by first-rate observers without exciting their
attention to anything remarkable. The first
occasion was in 1690, nearly a century before
Herschel'.s grand discovery, and these chance
observations, which lay so long unnoticed as
in some way erroneous, subsequently proved to
be of the utmost value in fixing the orbit of the
new planet. But there is even more striking
testimony than this to the exceptional nature of

URANUS AND EROS 7

Herschel's achievement. It is a common experience
in astronomy that an observer may fail to notice in
a general scrutiny some phenomenon which he can
see perfectly well when his attention is directed
to it : when a man has made a discovery and
others are told what to look for, they often see
it so easily that they are filled with amazement
and chagrin that they never saw it before. Not
so in the case of Uranus. At least two great
astronomers, Lalande and Messier, have left on
record their astonishment that Herschel could
differentiate it from an ordinary star at all ; for
even when instructed where to look and what
to look for, they had the greatest difficulty in
finding it. I give a translation of Messier' s words,
which Herschel records in the paper already quoted
announcing the discovery :—

" Nothing was more difficult than to recognise
it ; and I cannot conceive how you have been
able to return several times to this star or comet ;
for absolutely it has been necessary to observe it
for several consecutive days to perceive that it
was in motion."

We cannot, therefore, fit the facts to Keats' NO
version of them. The planet did not majesti-
cally reveal itself to a merely passive observer: ken
rather did it, assuming the disguise of an ordinary
star, evade detection to the utmost of its power ;
so that the keenest eye, the most alert attention,
the most determined following up of a mere

8 ASTRONOMICAL DISCOVERY

hint, were all needed to unmask it. But is the
romance necessarily gone ? If another Keats
could arise and know the facts, could he not
coin a newer and a truer phrase for us which
would still sound as sweetly in our ears ?

Though I must guard against a possible misconception.

happenat I do not mean to convey that astronomical dis-
coveries are not occasionally made somewhat in
the manner so beautifully pictured by Keats.
Three years ago a persistent " watcher of the
skies/' Dr. Anderson of Edinburgh, suddenly
caught sight of a brilliant new star in Perseus ;
though here "flashed into his ken" would per-
haps be a more suitable phrase than " swam."
And comets have been detected by a mere glance
at the heavens without sensible effort or care on
the part of the discoverer. But these may be
fairly called exceptions ; in the vast majority of
cases hard work and a keen eye are necessary to
make the discovery. The relative importance of
these two factors of course varies in different cases ;
for the detection of Uranus perhaps the keen eye
may be put in the first place, though we must not
forget the diligent watching which gave it oppor-
tunity. Other cases of planetary discovery may
be attributed more completely to diligence alone,
as we shall presently see. But before leaving

Name Uranus for them I should like to recall the
circumstances attending the naming of the planet.
Herschel proposed to call it Georgium Sidus
in honour of his patron, King George III., and

URANUS AND EROS 9

as the best way of making his wishes known,
wrote the following letter to the President of the
Royal Society, which is printed at the beginning
of the Philosophical Transactions for 1783.

A Letter from WILLIAM HERSCHEL, Esq., F.R.S.,
to Sir JOSEPH BANKS, Bart., P.R.S.

" SIR, — By the observations of the most eminent
astronomers in Europe it appears that the new
star, which I had the honour of pointing out to
them in March 1781, is a Primary Planet of our
Solar System. A body so nearly related to us by
its similar condition and situation in the un-
bounded expanse of the starry heavens, must often
be the subject of conversation, not only of astro-
nomers, but of every lover of science in general.
This consideration then makes it necessary to
give it a name whereby it may be distinguished
from the rest of the planets and fixed stars.

" In the fabulous ages of ancient times, the
appellations of Mercury, Venus, Mars, Jupiter,
and Saturn were given to the planets as being the
names of their principal heroes and divinities. In
the present more philosophical era, it would hardly
be allowable to have recourse to the same method,
and call on Juno, Pallas, Apollo, or Minerva for
a name to our new heavenly body. The first
consideration in any particular event, or remark-
able incident, seems to be its chronology : if in
any future age it should be asked, when this last

to ASTRONOMICAL DISCOVERY

found planet was discovered? It would be a
very satisfactory answer to say, ' In the reign of
King George the Third.' As a philosopher then,
the name GEORGICTM SIDUS presents itself to me,
as an appellation which will conveniently convey
the information of the time and country where
and when it was brought to view. But as a
subject of the best of kings, who is the liberal
protector of every art and science ; as a native
of the country from whence this illustrious
family was called to the British throne ; as a
member of that Society which flourishes by the
distinguished liberality of its royal patron ; and,
last of all, as a person now more immediately
under the protection of this excellent monarch,
and owing everything to his unlimited bounty ; — I
cannot but wish to take this opportunity of ex-
pressing my sense of gratitude by giving the name
Georgium Sidus,

Sidus.

Georgium Sidus

- jam nunc assuesce vocari,

Virg. Georg.

to a star which (with respect to us) first began to
shine under his auspicious reign.

" By addressing this letter to you, Sir, as Presi-
dent of the Royal Society, I take the most
effectual method of communicating that name to
the literati of Europe, which I hope they will
receive with pleasure. — I have the honour to be,
with the greatest respect, Sir, your most humble
and most obedient servant, W. HERSCHEL."

URANUS AND EROS 11

This letter reminds us how long it was since a
new name had been required for a new planet, —
to find a similar occasion Herschel had to go to
the almost prehistoric past, when the names of
heroes and divinities were given to the planets.
It is, perhaps, not unnatural that he should have
considered an entirely new departure appropriate
for a discovery separated by so great a length of
time from the others; but his views were not
generally accepted, especially on the Continent.
Lalande courteously proposed the name of Her- Herschel.
schel for the new planet, in honour of the dis-
coverer, and this name was used in France ; but
Bode, on the other hand, was in favour of retain-
ing the old practice simply, and calling the new
planet Uranus. All three names seem to have
been used for many years. Only the other day
I was interested to see an old pack of cards, used
for playing a parlour game of Astronomy, in which
the name Herschel is used. The owner told me
that they had belonged to his grandfather; and
the date of publication was 1829, and the place
London, so that this name was in common use
in England nearly half a century after the actual
discovery; though in the " English Nautical Al-
manac " the name "the Georgian" (apparently pre-
ferred to Herschel's Georgium Sidus) was being
used officially after 1791, and did not disappear
from that work until 1851 (published in 1847.)

It would appear to have been the discovery of Uranus
Neptune, with which we shall deal in the next adopted.

12 ASTRONOMICAL DISCOVERY

chapter, which led to this official change ; for in
the volume for 1851 is included Adams' account
of his discovery with the title —

"Ox THE PERTURBATIONS OF UKANUS,"

and there was thus a definite reason for avoiding
two names for the same planet in the same work.
But Le Verrier's paper on the same topic at the
same date still uses the name "Herschel" for the
planet.

The discovery of Neptune, as we shall see, was
totally different in character from that of Uranus.
The latter may be described as the finding of
something by an observer who was looking for
anything ; Neptune was the finding of something
definitely sought for, and definitely pointed out
by a most successful and brilliant piece of metho-
dical work. But before that time several planets
had been found, as the practical result of a
definite search, although the guiding principle
was such as cannot command our admiration to
quite the same extent as in the case of Neptune.
To explain it I must say something of the relative
sizes of the orbits in which planets move round
the sun. These orbits are, as we know, ellipses ;
but they are very nearly circles, and, excluding
refinements, we may consider them as circles, with
the sun at the centre of each, so that we may
talk of the distance of any planet from the sun
Bode's as a constant quantity without serious error. Now
if we arrange the planetary distances in order, we

URANUS AND EROS

shall notice a remarkable connection between the
terms of the series. Here is a table showing this
connection.

TABLE OP THE DISTANCES OF THE PLANETS FROM
THE SUN, SHOWING " BODE's LAW."

Distance from

"Bode'sLaw"

Name of Planet.

Sun, taking
that of Earth
as 10.

(originally formulated
by Titius, but brought
into notice by Bode).

Mercury .

4+ o= 4

Venus

4+ 3= 7

The Earth

4+ 6 = 10

Mars

4+ 12= 16

( )

4+ 24 = 28

Jupiter

4+ 48= 52

Saturn

4+ 96=100

Uranus . »

192

4+192 = 196

If we write down a series of 4*3, and then add
the numbers 3, 6, 12, and so on, each formed by
doubling the last, we get numbers representing
very nearly the planetary distances, which are
shown approximately in the second column. But
three points call for notice. Firstly, the number
before 3 should be ij, and not zero, to agree with
the rest. Secondly, there is a gap, or rather was Gap in
a gap, after the discovery of Uranus, between suggest-
Mars and Jupiter ; and thirdly, we see that when J^wn
Uranus was discovered, and its distance' from the Planet-
sun determined, this distance was found to fall
in satisfactorily with this law, which was first
stated by Titius of Wittenberg. This third fact
naturally attracted attention. No explanation of

i4 ASTRONOMICAL DISCOVERY

the so-called "law" was known at the time; nor
is any known even yet, though we may be said
to have some glimmerings of a possible cause ;
and in the absence of such explanation it must
be regarded as merely a curious coincidence. But
the chances that we are in the presence of a mere
coincidence diminish very quickly with each new
term added to the series, and when it was found
that Herschel's new planet fitted in so well at
the end of the arrangement, the question arose
whether the gap above noticed was real, or
whether there was perhaps another planet which
had hitherto escaped notice, revolving in an orbit
represented by this blank term. This question
had indeed been asked even before the discovery
of Uranus, by Bode, a young astronomer of Berlin ;
and for fifteen years he kept steadily in view this
idea of finding a planet to fill the vacant interval.
The search would be a very arduous one, involv-
ing a careful scrutiny, not perhaps of the whole
heavens, but of a considerable portion of it along
the Zodiac : too great for one would-be discoverer
single-handed; but in September 1800 Bode suc-
Search ceeded in organising a band of six German astro-
nomers (including himself) for the purpose of
conducting this search. They divided the Zodiac
into twenty-four zones, and were assigning the
zones to the different observers, when they were
startled by the news that the missing planet had
been accidentally found by Piazzi in the constella-
tion Taurus. The discovery was made somewhat

URANUS AND EROS 15

dramatically on the first evening of the nineteenth
century (January i, 1801). Piazzi was not look- Acciden-
ing for a planet at all, but examining an error
made by another astronomer ; and in the course
of this work he recorded the position of a star of
the eighth magnitude. Eeturning to it on the
next night, it seemed to him that it had slightly
moved westwards, and on the following night this
suspicion was confirmed. Remark that in this
case no peculiar appearance in the star suggested
that it might be a comet or planet, as in the case
of the discovery of Uranus. We are not unfair
in ascribing the discovery to pure accident, al-
though we must not forget that a careless observer
might easily have missed it. Piazzi was anything
but careless, and watched the new object assidu-
ously till February nth, when he became danger-
ously ill; but he had written, on January 23rd, to
Oriani of Milan, and to Bode at Berlin on the
following day. These letters, however, did not
reach the recipients (in those days of leisurely
postal service) until April 5th and March 2Oth
respectively ; and we can imagine the mixed feel-
ings with which Bode heard that the discovery
which he had contemplated for fifteen years, and
for which he was just about to organise a diligent
search, was thus curiously snatched from him.

More curious still must have seemed the mtelli- Hegel's
gence to a young philosopher of Jena named fc
Hegel, who has since become famous, but who
had just imperilled his future reputation by pub-

1 6 ASTRONOMICAL DISCOVERY

lishing a dissertation proving conclusively that the
. number of the planets could not be greater than
seven, and pouring scorn on the projected search
of the half-dozen enthusiasts who were proposing
to find a new planet merely to fill up a gap in a
numerical series.

The The sensation caused by the news of the dis-

lost again, covery was intensified by anxiety lest the new
planet should already have been lost; for it
had meanwhile travelled too close to the sun
for further observation, and the only material
available for calculating its orbit, and so predict-
ing its place in the heavens at future dates, was
afforded by the few observations made by Piazzi.
Was it possible to calculate the orbit from such
slender material ? It would take too long to ex-
plain fully the enormous difficulty of this problem,
but some notion of it may be obtained, by those
unacquainted with mathematics, from a rough
analogy. If we are given a portion of a circle,
we can, with the help of a pair of compasses,
complete the circle : we can find the centre from
which the arc is struck, either by geometrical
methods, or by a few experimental trials, and
then fill in the rest of the circumference. If the
arc given is large we can do this with certainty
and accuracy ; but if the arc is small it is difficult
to make quite sure of the centre, and our drawing
may not be quite accurate. Now the arc which
had been described by the tiny planet during
Piazzi's observations was only three degrees ; and

URANUS AND EROS 17

if any one will kindly take out his watch and
look at the minute marks round the dial, three
degrees is just half a single minute space. If
the rest of the dial were obliterated, and only
this small arc left, would he feel much confidence
in restoring the obliterated portion ? This problem
gives some idea of the difficulties to be encountered,
but only even then a very imperfect one.

Briefly, the solution demanded a new mathe- Gauss
matical method in astronomy. But difficulties hovTto
are sometimes the opportunities of great men, findlt-
and this particular difficulty attracted to astro-
nomy the great mathematician Gauss, who set
himself to make the best of the observation avail-
able, and produced his classical work, the Theoria
Motus, which is the standard work for such cal-
culations to the present day. May we look for a
few moments at what he himself says in the pre-
face to his great work ? I venture to reproduce
the following rough translation (the book being
written in Latin, according to the scientific usage
of the time) : —

EXTRACT FKOM THE PREFACE TO THE
Theoria Motus.

" Some ideas had occurred to me on this sub- The
ject in September 1801, at a time when I was
occupied on something quite different; ideas
which seemed to contribute to the solution of the
great problem of which I have spoken. In such
cases it often happens that, lest we be too much

1 8 ASTRONOMICAL DISCOVERY

distracted from the attractive investigation on
which we are engaged, we allow associations of
ideas which, if more closely examined, might
prove extraordinarily fruitful, to perish from
neglect. Perchance these same idea-lets of mine
would have met with this fate, if they had not
most fortunately lighted upon a time than which
none could have been chosen more favourable for
their preservation and development. For about
the same time a rumour began to be spread abroad
concerning a new planet which had been detected
on January ist of that year at the Observatory of
Palermo ; and shortly afterwards the actual obser-
vations which had been made between January ist
and February nth by the renowned philosopher
Piazzi were published. Nowhere in all the
annals of astronomy do we find such an impor-
tant occasion ; and scarcely is it possible to
imagine a more important opportunity for point-
ing out, as emphatically as possible, the impor-
tance of that problem, as at the moment when
every hope of re-discovering, among the innumer-
able little stars of heaven, that mite of a planet
which had been lost to sight for nearly a year,
depended entirely on an approximate knowledge
of its orbit, which must be deduced from those
scanty Observations. Could I ever have had a
better opportunity for trying whether those idea-
lets of mine were of any practical value than if I
then were to use them for the determination of
the orbit of Ceres, a planet which, in the course

URANUS AND EROS 19

of those forty-one days, had described around the
earth an arc of no more than three degrees? and,
after a year had passed, required to be tracked
out in a region of the sky far removed from its
original position? The first application of this
method was made in the month of October 1801,
and the first clear night, when the planet was
looked for by the help of the ephemeris I had
made, revealed the truant to the observer. Three
new planets found since then have supplied fresh
opportunities for examining and proving the effi-
cacy and universality of this method.

"Now a good many astronomers, immediately
after the rediscovery of Ceres, desired me to publish
the methodswhichhad been used in my calculations.
There were, however, not a few objections which
prevented me from gratifying at that moment
these friendly solicitations, viz. other business,
the desire of treating the matter more fully, and
more especially the expectation that, by continu-
ing to devote myself to this research, I should
bring the different portions of the solution of the
problem to a more perfect pitch of universality,
simplicity, and elegance. As my hopes have been
justified, I do not think there is any reason for
repenting of my delay. For the methods which I
had repeatedly applied from the beginning ad-
mitted of so many and such important variations,
that scarcely a vestige of resemblance remains
between the method by which formerly I had
arrived at the orbit of Ceres and the practice

20 ASTRONOMICAL DISCOVERY

which I deal with in this work. Although in-
deed it would be alien to my intention to write
a complete history about all these researches
which I have gradually brought to even greater
perfection, yet on many occasions, especially
whenever I was confronted by some particularly
serious problem, I thought that the first methods
which I employed ought not to be entirely sup-
pressed. Nay, rather, in addition to the solutions
of the principal problems, I have in this work
followed out many questions which presented
themselves to me, in the course of a long study
of the motions of the heavenly bodies in conic
sections, as being particularly worthy of attention,
whether on account of the neatness of the analysis,
or more especially by reason of their practical
utility. Yet I have always given the greater care
to subjects which I have made my own, merely
noticing by the way well-known facts where con-
nection of thought seemed to demand it."

These words do not explain in any way the
methods introduced by Gauss, but they give us
some notion of the flavour of the work. Aided
by these brilliant researches, the little planet was
° found on the last day of the year by Von Zach at
Gotha, 'and on the next night, independently, by
Olbers at Bremen. But, before this success, there
had been an arduous search, which led to a curious
consequence. Olbers had made himself so fami-
liar with all the small stars along the track which
was being searched for the missing body, that he

URANUS AND EROS 21

was at once struck by the appearance of a stranger Another
near the spot where he had just identified Ceres, found!
At first he thought this must be some star which
had blazed up to brightness ; but he soon found
that it also was moving, and, to the great bewilder-
ment of the astronomical world, it proved to be
another planet revolving round the sun at a dis-
tance nearly the same as the former. This was
an extraordinary and totally unforeseen occurrence.
The world had been prepared for one planet ; but
here were two !

The thought occurred to Olbers that they
were perhaps fragments of a single body which Hypo-

, , , * , ,F , , . . thesis of

had been blown to pieces by some explosion, and many
that there might be more of the pieces ; and he
therefore suggested as a guide for finding others
that, since by the known laws of gravitation,
bodies which circle round the sun return perio-
dically to their starting-point, therefore all these
fragments would in due course return to the point
in the heavens where the original planet had
exploded. Hence the search might be most pro-
fitably conducted in the neighbourhood of the
spot where the two first fragments (which had
been named Ceres and Pallas) had already been
found. We now have good reason to believe
that this view is a mistaken one, but nevertheless
it was apparently confirmed by the discovery of
two more bodies of the same kind, which were
called Juno and Vesta ; the second of these being
found by Olbers himself after three years' patient

ASTRONOMICAL DISCOVERY

work in 1807. Hence, although the idea of
searching for a more or less definitely imagined
planet was not new, although Bode had conceived
it as early as 1785, and organised a search on this
plan, three planets were actually found before the
first success attending a definite search. Ceres,
as already remarked, was found by a pure acci-
dent ; and the same may be said of Pallas and
Juno, though it may fairly be added that Pallas
was actually contrary to expectation.

MINOR PLANETS, 1801 TO 1850.

Number.

Name.

-
Discoverer.

Date.

Ceres

Pia/zi

1801

Pallas

Gibers

1802

Juno

Harding

1804

Vesta

Olbers

1807

Astraea

Hencke

1845

Hebe

Hencke

1847

Iris

Hind

1847

Flora

Hind

1847

Metis

Graham

1848

Hygeia

De Gasparis

1849

Parthenope

De Gasp;iris

1850

Victoria

Hind

1850

Egeria

De Gasparis

1850

Here now is a table showing how other bodies
were gradually added to this first list of four, but
you will see that no addition was made for a long
time. Not that the search was immediately aban-
doned ; but being rewarded by no success for some
years, it was gradually dropped, and the belief
gained ground that the number of the planets

By permission of Messrs. Macmillan &• Co.

I.— I. C. ADAMS.

II.— A. GRAHAM.

DISCOVERER OF THE NINTH MINOR PLANET (.METIS).

URANUS AND EROS 23

was at last complete. The discoverers of Uranus
and of these first four minor planets all died before
any further addition was made ; and it was not
until the end of 1845 that Astraea was found by
an ex-postmaster of the Prussian town of Driessen, Hencke'
by name Hencke, who, in spite of the general search,
disbelief in the existence of any more planets, set
himself diligently to search for them, and toiled
for fifteen long years before at length reaping his
reward. Others then resumed the search ; Hind,
the observer of an English amateur astronomer
near London, found Iris a few weeks after Hencke
had been rewarded by a second discovery in 1847,
and in the following year Mr. Graham at Markree
in Ireland (who is still living, and has only just
retired from active work at the Cambridge Obser-
vatory) found Metis ; and from that time new
discoveries have been added year by year, until
the number of planets now known exceeds 500,
and is steadily increasing.

You will see the great variety characterising
these discoveries ; some of them are the result
of deliberate search, others have come accident-
ally, and some even contrary to expectation. Of
the great majority of the earlier ones it may be
said that enormous diligence was required for
each discovery ; to identify a planet it is necessary
to have either a good map of the stars or to know
them thoroughly, so that the map practically exists
in the brain. We need only remember Hencke's
fifteen years of search before success to recognise

24 ASTRONOMICAL DISCOVERY

what vast stores of patience and diligence were

required in carrying out the search. But of late

The years photography has effected a great revolution

graphic in this respect. It is no longer necessary to do
method. more than get what gir Robert Bau has called a

" star-trap," or rather planet-trap. If a photograph
be taken of a region of the heavens, by the methods
familiar to astronomers, so that each star makes a
round dot on the photographic plate, any suffi-
ciently bright object moving relatively to the stars
will make a small line or trail, and thus betray its
planetary character. In this way most of the
recent discoveries have been made, and although
diligence is still required in taking the photo-
graphs, and again in identifying the objects thus
found (which are now very often the images of
already known members of the system), the tedious
scrutiny with the eye has become a thing of the past.

TABLE SHOWING THE NUMBER OF MINOR PLANETS DISCOVERED
IN EACH DECADE SINCE 1850.

1 80 1 to 1850 — altogether 13 discoveries.
1851 to 1860 — , 49 „

1861 to 1870 —
1871 to 1880—
1881 to 1890 —
1891 to 1900 —
In 1901

49 >»
1 08

83
1 80 announcements.

36 „

1902 „ 50

1903 » 4i

Total 609

[N.J2. — Many of the more recent announce-
ments turned out to refer to old discoveries.]

URANUS AND EROS 25

The known number of these bodies has accord-
ingly increased so rapidly as to become almost
an embarrassment ; and in one respect the embar-
rassment is definite, for it has become quite
difficult to find names for the new discoveries. Scarcity
We remember with amusement at the present °
time that for the early discoveries there was
sometimes a controversy (of the same kind
as in the case of Uranus) about the exact
name which a planet should have. Thus when
it was proposed to call No. 12 (discovered in
1850, in London, by Mr. Hind) "Victoria,"
there was an outcry by foreign astronomers
that by a subterfuge the name of a reigning
monarch was again being proposed for a planet,
and considerable opposition was manifested,
especially in America. But it became clear,
as other discoveries were added, that the list
of goddesses, or even humbler mythological
people, would not be large enough to go round
if we were so severely critical, and must
sooner or later be supplemented from sources
hitherto considered unsuitable ; so, ultimately,
the opposition to the name Victoria was with-
drawn. Later still the restriction to feminine
names has been broken through ; one planet has
been named Endyniion, and another, of which we
shall presently speak more particularly, has been
called Eros. But before passing to him you

26 ASTRONOMICAL DISCOVERY

may care to look at some of the names selected
for others : —

No. Name.

248 . . Lameia

250 . . Bettina

261 . Prymno

264 . . Libussa

296 . . Phaetusa

340 . . Eduarda

341 . . California
350 . . Ornamenta
357 . . Ninina
385 . . Ilmatar

No. Name.

389 . . ludustria

391 . . Ingeborg

433 • • Eros

443 . . Photograpliica

457 . . Alleghenia

462 . . Eri phyla

475 . . Ocllo

484 . . Pitt>burghia

503 . . Evelyn

Bettina. In connection with No. 250 there is an interest-
ing little history. In the Observatory for 1885,
page 63, appeared the following advertisement : —
" Herr Palisa being desirous to raise funds for his
intended expedition to observe the Total Solar
Eclipse of August 1886, will sell the right of
naming the minor planet No. 244 for ,£50." The
bright idea seems to have struck Herr Palisa, who
had already discovered many planets and begun to
find difficulties in assigning suitable names, that
he might turn his difficulty into a source of profit
in a good cause. The offer was not responded to
immediately, nor until Herr Palisa had discovered
two more planets, Nos. 248 and 250. He found
names for two, leaving, however, the last dis-
covered always open for a patron, and on page
142 of the same magazine for 1886 the following
note informs us how his patience was ultimately
rewarded: — "Minor planet No. 250 has been

URANUS AND EROS 27

named ' Bettina ' by Baron Albert de Rothschild."
I have not heard, however, that this precedent has
been followed in other cases, and the ingenuity of
discoverers was so much overtaxed towards the
end of last century that the naming of their
planets fell into arrears. Recently a Commission,
which has been established to look after these
small bodies generally, issued a notice that unless
the naming was accomplished before a certain
date it would be ruthlessly taken out of the hands
of the negligent discoverers. Perhaps we may
notice, before passing on, the provisional system Thepro-
which was adopted to fill up the interval required letter^
for finding a suitable name, and required also for
making sure that the planet was in fact a new
one, and not merely an old one rediscovered.
There was a system of numbering in existence as
well as of naming, but it was unadvisable to
attach even a number to a planet until it was
quite certain that the discovery was new, for
otherwise there might be gaps created in what
should be a continuous series by spurious dis-
coveries being struck out. Accordingly it was
decided to attach at first to the object merely a
letter of the alphabet, with the year of discovery,
as a provisional name. The alphabet was, how-
ever, run through so quickly, and confusion was
so likely to ensue if it was merely repeated, that
on recommencing it the letter A was prefixed,
and the symbols adopted were therefore A A,
AB, AC, &c. ; after completing the alphabet

28 ASTRONOMICAL DISCOVERY

again, the letter B was prefixed, and so on ; and
astronomers began to fear that they had before
them a monotonous prospect of continually add-
ing new planets, varied by no incident more excit-
ing than starting the alphabet over again after
every score.

Fortunately, however, on running through it
for the fifth time, an object of particular interest

Eros. was discovered. Most of these bodies revolve
at a distance from the sun intermediate between
that of Mars and that of Jupiter, but the little
planet which took the symbol DQ, and afterwards
the name of Eros, was found to have a mean
distance actually less than that of Mars, and
this gave it an extraordinary importance with
respect to the great problem of determining the
sun's distance. To explain this importance we
must make a small digression.

Transit of About the middle of the last century our
knowledge of the sun's distance was very rough,
as may be seen from the table on p. 32 ; but there
were in prospect two transits of Venus, in 1874
and 1882, and it was hoped that these would give
opportunities of a special kind for the measure-
ment of this important quantity, which lies at the
root of all our knowledge of the exact masses and
dimensions of not only the sun, but of the planets
as well.

The method may be briefly summarised thus :
An observer in one part of the earth would see
Venus cross the disc of the sun along a different

URANUS AND EROS 29

path from that seen by another observer, as will
be clear from the diagram. If the size of the

EARTH

SUN

FIG. i.

earth, the distance of the sun, and the relative
distance of Venus be known, it can be calcu-
lated what this difference in path will be.
Now the relative distance of Venus is known
with great accuracy, from observing the time of
her revolution round the sun; the size of the
earth we can measure by a survey ; there remains,
therefore^ only one unknown quantity, the sun's
distance. And since from a knowledge of this we
could calculate the difference in path, it is easy
to invert the problem, and calculate the sun's
distance from the knowledge of the observed
difference in path. Accordingly, observers were
to be scattered, not merely to two, but to many
stations over the face of the earth, to observe the
exact path taken by Venus in transit over the sun's
disc as seen from their station ; and especially to
observe the exact times of beginning and ending
of the transit ; and, by comparison of their results,

30 ASTRONOMICAL DISCOVERY

it was hoped to determine this very important
quantity, the sun's distance. It was known from
previous experience that there were certain diffi-
culties in observing very exactly the beginning
and end of the transit. There was an appear-
The ance called the " Black Drop," which had caused

"Black , , . .

Drop." trouble on previous occasions ; an appearance as
though the round black spot which can be seen
when Venus has advanced some distance over the
sun's disc was reluctant to make the entry and
clung to the edge or " limb " of the sun as it is
called, somewhat as a drop of ink clings to a
pen which is slowly withdrawn from an inkpot.
Similarly, at the end of the transit or egress,
instead of approaching the limb steadily the
planet seems at the last moment to burst out
towards it, rendering the estimation of the exact
moment when the transit is over extremely
doubtful.

These difficulties, as already stated, were known
to exist; but there is a long interval between
transits of Venus, or rather between every pair
of such transits. After those of 1874 and 1882
there will be no more until 2004 and 2012, so
that we shall never see another ; similarly, before
that pair of the last century, there had not been
any such occasion since 1761 and 1769, and no
one was alive who remembered at first hand
the trouble which was known to exist. It was
proposed to obviate the anticipated difficulties
by careful practice beforehand ; models were

URANUS AND EROS 31

prepared to resemble as nearly as possible the
expected appearances, and the times recorded by
different observers were compared with the true
time, which could, in this case of a model, be
determined. In this way it was hoped that the
habit of each observer, his "personal equation" as
it is called, could be determined beforehand, and
allowed for as a correction when he came to
observe the actual transit. The result, however,
was a great disappointment. The actual appear-
ances were found to be totally different in
character from those shown by the model ; Failure.
chiefly, perhaps, because it had been impos-
sible to imitate with a model the effect of the
atmosphere which surrounds the planet Yenus.
Observers trained beforehand, using similar in-
struments, and standing within a few feet of each
other, were expected, after making due allowance
for personal equation, to give the same instant for
contact; but their observations when made were
found to differ by nearly a minute of time, and
after an exhaustive review of the whole material
it was felt that all hope of determining accurately
the sun's distance by this method must be given
up. The following table will show how much
was learned from the transits of Venus, and
how much remained to be settled. They left
the result in doubt over a range of about two
million miles.

32 ASTRONOMICAL DISCOVERY

SUN'S DISTANCE, IN MILLIONS OF MILES, AS
FOUND BY DIFFERENT OBSERVERS

Before the Transits of Venus estimates varied
between 96 million miles (Gilliss and Gould,
1856) and 91 million (Winneche, 1863), a range
of 5 million miles.

The Transits of 1874 and 1882 gave results lying
between 93^ million (Airy, from British observa-
tions of 1874), 92^ million (Stone, from British
observations of 1882), and 91|- million (Puiseux,
from French observations), a range of if millions.

Gill's Heliometer results all lie very near 93
millions. The observations of Mars in 1877 give
about 100,000 miles over this figure : but the
observations of Victoria, Iris, and Sappho, which
are more trustworthy, all agree in giving about
100,000 miles less than the 93 millions.

It became necessary, therefore, to look to other
methods; and before the second transit of 1882
was observed, an energetic astronomer, Dr. David
Gill, had already put into operation the method
which may be now regarded as the standard
one.

Modem We have said that the relative distance of

fbr sun's Venus from the sun is accurately known from

distance, observations of the exact time of revolution. It

is easy to see that these times of revolution can

URANUS AND EROS 33

be measured accurately by mere accumulation.
We may make an error of a few seconds in noting
the time of return ; but if the whole interval
comprises 10 revolutions, this error is divided by
10, if 100 revolutions by 100, and so on; and by
this time a great number of revolutions of all the
planets (except those just discovered) have been
recorded. Hence we know their relative dis-
tances with great precision ; and if we can find
the distance in miles of any one of them, we
can find that of the sun itself, or of any other
planet, by a simple rule-of-three sum. By making
use of this principle many of the difficulties
attending the direct determination of the sun's
distance can be avoided ; for instance, since the
sun's light overpowers that of the stars, it is not
easy to directly observe the place of the sun
among the stars ; but this is not so for the planets.
We can photograph a planet and the stars sur- Photo-
rounding it on the same plate, and then by care- ^
ful measurement determine its exact position
among the stars ; and since this position differs
slightly according to the situation of the observer
on the earth's surface, by comparing two photo-
graphs taken at stations a known distance apart
we can find the distance of the planet from the
earth ; and hence, as above remarked, the dis-
tance of the sun and all the other members of the
solar system. Or, instead of taking photographs
from two different stations, we can take from the
same station two photographs at times separated

Ascen-
sion.

34 ASTRONOMICAL DISCOVERY

by a known interval. For in that interval the
station will have been carried by the earth's rota-
tion some thousands of miles away from its former
position, and becomes virtually a second station
separated from the first by a distance which is
known accurately when we know the elapsed
time. Again, instead of taking photographs, and
from them measuring the position of the planet
among the stars, we may make the measurements
on the planet and stars in the sky itself; and
Dr. Gin's since in 1878, when Dr. Gill set out on his enter-
tkmto" prise of determining the sun's distance, photo-
graphy was in its infancy as applied to astronomy,
he naturally made his observations on the sky
with an instrument known as a heliometer. He
made them in the little island of Ascension, which
is suitably situated for the purpose ; because,
being near the earth's equator, it is carried by
the earth's rotation a longer distance in a given
time than places nearer the poles, and in these
observations for "parallax," as they are called,
it is important to have the displacement of the
station as large as possible. For a similar reason
the object selected among the planets must be as
near the earth as possible ; and hence the planet
Mars, which at favourable times comes nearer to
us than any other superior planet1 then known,
was selected for observation with the heliometer.
And now it will be seen why the discovery of

1 The inferior planet Venus conies closer, but is not visible
throughout the night.

URANUS AND EROS 35

the little planet Eros was important, for Mars
was no longer the known planet capable of
coming nearest to us ; it had been replaced by
this new arrival.

Further, a small planet which is in appearance
just like an ordinary star has, irrespective of this
great proximity, some distinct advantages over a
planet like Mars, which appears as a round disc,
and is, moreover, of a somewhat reddish colour.
When the distance of an object of this kind from
a point of line such as a star is measured with the
heliometer it is found that a certain bias, some-
what difficult to allow for with certainty, is intro-
duced into the measures ; and our confidence in
the final results suffers accordingly. After his
observations of Mars in 1878, Dr. David Gill was
sufficiently impressed with this source of error
to make three new determinations of the sun's
distance, using three of the minor planets instead Victoria,
of Mars, in spite of the fact that they were sen- Sappho.
sibly farther away ; and his choice was justified
by finding that the results from these three
different sets of observations agreed well among
themselves, and differed slightly from that given
by the observations of Mars. Hence it seems
conclusively proved that one of these bodies is
a better selection than Mars in any case, and the
discovery of Eros, which offered the advantage Eros,
of greater proximity in addition, was hailed as
a new opportunity of a most welcome kind. It
was seen by a little calculation that in the winter

36 ASTRONOMICAL DISCOVERY

of 1900-1901 the planet would come very near the
earth ; not the nearest possible (for it was also
realised that a still better opportunity had occurred
in 1894, though it was lost because the planet had
not yet been discovered), but still the nearest
approach which would occur for some thirty years ;
and extensive, though somewhat hasty, prepara-
tions were made to use it to the fullest advantage.
Photography had now become established as an
accurate method of making measurements of the
kind required ; and all the photographic tele-
scopes which could be spared were pressed into
the service, and diligently photographed the planet
and surrounding stars every fine night during the
favourable period. The work of measuring and
reducing these photographs involves an enormous
amount of labour, and is even yet far from com-
pleted, but we know enough to expect a result
of the greatest value. More than this we have
not time to say here about this great problem,
but it will have been made clear that just when
astronomers were beginning to wonder whether
it was worth while continuing the monotonous
discovery of new minor planets by the handful,
the 433rd discovery also turned out to be one of
the greatest importance.

To canons for the advantageous prosecution
of research, if we care to make them, we may
therefore add this — that there is no line of re-
search, however apparently unimportant or mono-
tonous, which we can afford to neglect. Just when

URANUS AND EROS 37

we are on the point of relinquishing it under the
impression that the mine is exhausted, we may
be about to find a nugget worth all our previous
and future labour. This rule will not, perhaps,
help us very much in choosing what to work at ;
indeed, it is no rule at all, for it leaves us the
whole field of choice unlimited. But this negative
result will recur again and again as we examine
the lessons taught by discoveries : there seem to
be no rules at all. Whenever we seem to be
able to deduce one from an experience, some
other experience will flatly contradict it. Thus
we might think that the discovery of Eros taught
us to proceed patiently with a monotonous duty,
and not turn aside to more novel and attractive
work ; yet it is often by leaving what is in hand
and apparently has first claim on our attention
that we shall do best, and we shall learn in the
next chapter how a failure thus to turn flexibly
aside was repented.

CHAPTER II
THE DISCOVERY OF NEPTUNE

Search IN the last chapter we saw that the circumstances
finite6 under which planets were discovered varied con-
objects. siderably. Sometimes the discoveries were not
previously expected, occurring during a general
examination of the heavens, or a search for other
objects ; and, on one occasion at -least, the dis-
covery may be said to have been even contrary
to expectation, though, as the existence of a
number of minor planets began to be realised,
there have also been many cases where the dis-
covery has been made as the result of a definite
and deliberate search. But the search cannot be
said to have been inspired by any very clear or
certain principle : for the law of Bode, successful
though it has been in indicating the possible
existence of new planets, cannot, as yet, be said
to be founded upon a formulated law of nature.
We now come, however, to a discovery made in
direct interpretation of Newton's great law of
gravitation — the discovery of Neptune from its
observed disturbance of Uranus. I will first
briefly recall the main facts relating to the actual
discovery.

THE DISCOVERY OF NEPTUNE 39

After Uranus had been discovered and observed Disturb-
sufficiently long for its orbit to be calculated, it
was found that the subsequent position of the
planet did not always agree with this orbit ; and,
more serious than this, some early observations
were found which could not be reconciled with
the later ones at all. It is a wonderful testimony
to the care and sagacity of Sir William Herschel,
as was remarked in the last chapter, that Uranus
was found to have been observed, under the mis-
taken impression that it was an ordinary star, by
Flamsteed, Lemonnier, Bradley, and Mayer, all
observers of considerable ability. Flamsteed's
five observations dated as far back as 1690,
1712, and 1715 ; observations by others were
in 1748, 1750, 1753, 1756, and so on up to 1771,
and the body of testimony was so considerable
that there was no room for doubt as to the
irreconcilability of the observations with the
orbit, such as might have been the case had
there been only one or two, possibly affected
with some errors.

It is difficult to mention an exact date for the
conversion into certainty of the suspicion that
no single orbit could be found to satisfy all the
observations ; but we may certainly regard this
fact as established in 1821, when Alexis Bouvard
published some tables of the planet, and showed
fully in the introduction that when every correc-
tion for the disturbing action of other planets had
been applied, it was still impossible to reconcile

40 ASTRONOMICAL DISCOVERY

the old observations with the orbit calculated from
Suspicion the new ones. The idea accordingly grew up
turtog that there might be some other body or bodies
iet' attracting the planet and causing these dis-
crepancies. Here again it is not easy to say
exactly when this notion arose, but it was cer-
tainly existent in 1834, as the following letter
to the Astronomer Royal will show. I take it
from his well-known " Account of some Circum-
stances historically connected with the Discovery
of the Planet exterior to Uranus," which he gave to
the Royal Astronomical Society at its first meet-
ing after that famous discovery (Monthly Notices
of the R.A.S., vol. iii., and Memoirs, vol. xvi.).

No. i.— The REV. T. J. HUSSEY to G. B. AIRY.

[Extract.]
"'HAYES, KENT, ijih November 1834.

" ' With M. Alexis Bouvard I had some con-
versation upon a subject I had often meditated,
which will probably interest you, and your opinion
may determine mine. Having taken great pains
last year with some observations of Uranus, I
was led to examine closely Bouvard's tables of
that planet. The apparently inexplicable dis-
crepancies4 between the ancient and modern ob-
servations suggested to me the possibility of some
disturbing body beyond Uranus, not taken into
account because unknown. My first idea was
to ascertain some approximate place of this sup-

THE DISCOVERY OF NEPTUNE 41

posed body empirically, and then with my large
reflector set to work to examine all the minute
stars thereabouts : but I found myself totally
inadequate to the former part of the task. If
I could have done it formerly, it was beyond
me now, even supposing I had the time, which
was not the case. I therefore relinquished the
matter altogether ; but subsequently, in conversa-
tion with Bouvard, I inquired if the above might
not be the case : his answer was, that, as might
have been expected, it had occurred to him, and
some correspondence had taken place between
Hansen and himself respecting it. Hansen's
opinion was, that one disturbing body would
not satisfy the phenomena; but that he conjec-
tured there were two planets beyond Uranus
Upon my speaking of obtaining the places em-
pirically, and then sweeping closely for the bodies,
he fully acquiesced in the propriety of it, intimat-
ing that the previous calculations would be more
laborious than difficult ; that if he had leisure he
would undertake them and transmit the results
to me, as the basis of a very close and accurate
sweep. I have not heard from him since on the
subject, and have been too ill to write. What is
your opinion on the subject? If you consider the
idea as possible, can you give me the limits,
roughly, between which this body or those bodies
may probably be found during the ensuing winter ?
As wre might expect an eccentricity [inclination ?]
approaching rather to that of the old planets than

42 ASTRONOMICAL DISCOVERY

of the new, the breadth of the zone to be examined
will be comparatively inconsiderable. I may be
wrong, but I am disposed to think that, such is
the perfection of my equatoreal's object-glass, I
could distinguish, almost at once, the difference
of light of a small planet and a star. My plan
of proceeding, however, would be very different :
I should accurately map the whole space within
the required limits, down to the minutest star I
could discern ; the interval of a single week would
then enable me to ascertain any change. If the
whole of this matter do not appear to you a
chimaera, which, until my conversation with
Bouvard, I was afraid it might, I shall be very
glad of any sort of hint respecting it.'

" My answer was in the following terms : —
No. 2. — G. B. AIRY to the REV. T. J. HUSSEY.

[Extract.]

"'OBSERVATORY, CAMBRIDGE, 1834, Nov. 23.

Airy's " ' I have often thought of the irregularity of

cS1 Uranus, and since the receipt of your letter have
looked more carefully to it. It is a puzzling
subject, but I give it as my opinion, without
hesitation, that it is not yet in such a state as
to give .the smallest hope of making out the
nature of any external action on the planet . . .
if it were certain that there were any extraneous
action, I doubt much the possibility of determin-
ing the place of a planet which produced it. I
am sure it could not be done till the nature of

THE DISCOVERY OF NEPTUNE 43

the irregularity was well determined from several
successive revolutions.' "

Although only a sentence or two have been
selected from Airy's reply (he was not yet Astro-
nomer Royal), they are sufficient to show that the
problem of finding the place of such a possible
disturbing body was regarded at that time as one
of extreme difficulty ; and no one appears seriously
to have contemplated embarking upon its solu-
tion. It was not until many years later that the
solution was attempted. Of the first attempt we
shall speak presently, putting it aside for the
moment because it had no actual bearing on the
discovery of the planet, for reasons which form
an extraordinary episode of this history. The
attempt which led to success dates from Novem-
ber 1845. The great French astronomer Le Le
Verrier, on November 10, 1845, read to the papers! S
French Academy a paper on the Orbit of Uranus,
considering specially the disturbances produced
by Jupiter and Saturn, and showing clearly that
with no possible orbit could the observations be
satisfied. On June i, 1846, followed a second
paper by the same author, in which he considers
all the possible explanations of the discordance,
and concludes that none is admissible except that
of a disturbing planet exterior to Uranus. And
assuming, in accordance with Bode's Law, that
the distance of this new planet from the sun
would be about double that of Uranus (and it

44 ASTRONOMICAL DISCOVERY

is important to note this assumption), he proceeds

to investigate the orbit of such a planet, and to

calculate the place where it must be looked for

in the heavens. This was followed by a third

paper on August 3ist, giving a rather completer

discussion, and arriving at the conclusion that

Planet the planet should be recognisable from its disc.

detected This again is an important point. We remem-

bydisc. 1^ ^at |n ^e djgcoyer Of Uranus it needed

considerable skill on the part of Sir William
Herschel to detect the disc, to see in fact any
difference between it and surrounding stars ; and
that other observers, even when their attention
had been called to the planet, found it difficult
to see this difference. It might be expected,
therefore, that with a planet twice as far away
(as had been assumed for the new planet) the
disc would be practically unrecognisable, and as
we shall presently see, this assumption was made
in some searches for the planet which had been
commenced even before the publication of this
third paper. Le Verrier's courageous announce-
ment, which he deduced from a consideration of
the mass of the planet, that the disc should be
recognisable, led immediately to the discovery of
Gaiie's the suspected body. He wrote to a German
o?tChTry astronomer, Dr. Galle (still, I am glad to say,
planet, alive and well, though now a very old man),
telling him the spot in the heavens to search,
and stating that he might expect to detect the
planet by its appearance in this way; and the

THE DISCOVERY OF NEPTUNE 45

same night Dr. Galle, by comparing a star map
with the heavens, found the planet.

To two points to which I have specially called
attention in this brief summary — namely, the
preliminary assumption that the planet would be,
according to Bode's Law, twice as far away as
Uranus ; secondly, the confident assertion that it
would have a visible disc — I will ask you to add,
thirdly, that it was found by the aid of a star map,
for this map played an important part in the
further history to which we shall now proceed.
It may naturally be supposed that the announce-
ment of the finding of a planet in this way, the
calculation of its place from a belief in the uni-
versal action of the great Law of Gravitation, the
direction to an eminent observer to look in that
place for a particular thing, and his immediate
success, — this extraordinary combination of cir-
cumstances caused a profound sensation through-
out not only the astronomical, but the whole
world ; and this sensation was greatly enhanced
by the rumour which had begun to gather strength
that, but for some unfortunate circumstances, the
discovery might have been made even earlier and as
a consequence of totally independent calculations
made by a young Cambridge mathematician,
J. C. Adams. Some of you are doubtless already Adams'
familiar with the story in its abridged form, for it uciy an-
has been scattered broadcast through literature. nouced-
In England it generally takes the form of em-
phasising the wickedness or laziness of the

46 ASTRONOMICAL DISCOVERY

Astronomer Royal who, when told where to look
for a planet, neglected his obvious duty, so that
in consequence another astronomer who made the
calculation much later and gave a more virtuous
observer the same directions where to look,
obtained for France the glory of a discovery which
ought to have been retained in England. There
is no doubt that Airy's conduct received a large
amount of what he called " savage abuse." When
the facts are clearly stated I think it will be
evident that many of the harsh things said of him
were scarcely just, though at the same time it is
also difficult to understand his conduct at two or
three points of the history, even as explained by
himself.

Facts un- There is fortunately no doubt whatever about
doubted. anv of fae facts. Airy himself gave a very clear
and straightforward account of them at the time,
for which more credit is due to him than he
commonly receives ; and since the death of the
chief actors in this sensational drama they have
been naturally again ransacked, with the satis-
factory result that there is practically no doubt
about any of the facts. As to the proper interpre-
tations of them there certainly may be wide dif-
ferences of opinion, nor does this circumstance
detract from their interest. It is almost impossible
to make a perfectly colourless recital of them, nor
is it perhaps necessary to do so. I will therefore
ask you to remember in what I now say that there
is almost necessarily an element of personal bias,

THE DISCOVERY OF NEPTUNE 47

and that another writer would probably give a
different colouring. Having said this, I hope I
may speak quite freely as the matter appears in
my personal estimation.

Airy's account was, as above stated, given to the A
Royal Astronomical Society at their first meeting count."
(after the startling announcement of the discovery
of the new planet), on November 13, 1846, and
I have already quoted an extract from it. He
opens with a tribute to the sensational character
of the discovery, and then states that although
clearly due to two individuals (namely, Le Verrier
and Galle), it might also be regarded as to some
extent the consequence of a movement of the age. "Amove-
.His actual words are these : "The principal steps the age."
in the theoretical investigations have been made
by one individual, and the published discovery of
the planet was necessarily made by one individual.
To these persons the public attention has been
principally directed ; and well do they deserve the
honours which they have received, and which they
will continue to receive. Yet we should do wrong
if we considered that these two persons alone are
to be regarded as the authors of the discovery
of this planet. I am confident that it will
be found that the discovery is a consequence
of what may properly be called a movement of
the age ; that it has been urged by the feeling
of the scientific world in general, and has been
nearly perfected by the collateral, but independent
labours, of various persons possessing the talents

48 ASTRONOMICAL DISCOVERY

or powers best suited to the different parts of the
researches."

I have quoted these words as the first point at
which it is difficult to understand Airy's conduct
in excluding from them all specific mention of
Adams, knowing as he did the special claims
which entitled him to such mention ; claims
indeed which he proceeded immediately to make
Airy clear. It seems almost certain that Airy entirely
estimated under-estimated the value of Adams' work through-
work18' ouk But this will become clearer as we proceed.
The "account" takes the form of the publication
of a series of letters with occasional comments.
Airy was a most methodical person, and filed all
his correspondence with great regularity. It was
jestingly said of him once that if he wiped his
pen on a piece of blotting-paper, he would date
the blotting-paper and file it for reference. The
letters reproduced in this "account" are still in
the Observatory at Greenwich, pinned together
just as Airy left them ; and in preparing his
"account" it was necessary to do little else than
to have them copied out and interpolate comments.
From two of them I have already quoted to show
how difficult the enterprise of finding an exterior
planet from its action on Uranus was considered
in 1834. To these may be added the following
sentence from No. 4, dated 1837. "If it be the
effect of any unseen body," writes Airy to Bouvard,
" it will be nearly impossible ever to find out its
place." But the first letter which need concern

THE DISCOVERY OF NEPTUNE 49

us is No. 6, and it is only necessary to explain that
Professor Challis was the Professor of Astronomy
at Cambridge, and in charge of the Cambridge
Observatory, in which offices he had succeeded
Airy himself on his leaving Cambridge for Green-
wich some eight years earlier.

No. 6. — PROFESSOR CHALLIS to G. B. AIRY.

[Extract.}

"'CAMBRIDGE OBSERVATORY, Feb. 13, 1844.
" * A young friend of mine, Mr. Adams of St. chains
John's College, is working at the theory of
Uranus, and is desirous of obtaining errors of the Airy
tabular geocentric longitudes of this planet, when
near opposition, in the years 1818-1826, with the
factors for reducing them to errors of heliocentric
longitude. Are your reductions of the planetary
observations so far advanced that you could furnish
these data? and is the request one which you
have any objection to comply with ? If Mr.
Adams may be favoured in this respect, he is
further desirous of knowing, whether in the
calculation of the tabular errors any alterations
have been made in Bouvard's Tables of Uranus
besides that of Jupiter s mass/

" My answer to him was as follows : —
No. 7. — G. B. AIRY to PROFESSOR CHALLIS.

[Extract.']

"' ROYAL OBSERVATORY, GREENWICH, 1844, Feb. 15.
" * I send all the results of the observations of
Uranus made with both instruments (that is, the

50 ASTRONOMICAL DISCOVERY

heliocentric errors of Uranus in longitude and
latitude from 1754 to 1830, for all those days
on which there were observations, both of right
ascension and of polar distance). No alteration
is made in Bouvard's Tables of Uranus except
in increasing the two equations which depend on
Jupiter by -^ part. As constants have been
added (in the printed tables) to make the
equations positive, and as ^ part of the numbers
in the tables has been added, -^ part of the
constants has been subtracted from the final
results.'

" Professor Challis in acknowledging the receipt
of these, used the following expressions :—

No. 8. — PROFESSOR CHALLIS to G. B, AIRY.

[Extract.}

"'CAMBRIDGE OBSERVATORY, Feb. 16, 1844.
" ' I am exceedingly obliged by your sending so
complete a series of tabular errors of Uranus.
. . . The list you have sent will give Mr. Adams
the means of carrying on in the most effective
manner the inquiry in which he is engaged.'

" The next letter shows that Mr. Adams has
derived results from these errors.

No. 9. — PROFESSOR CHALLIS to G. B. AIRY.

"'CAMBRIDGE OBSERVATORY, Sept. 22, 1845.
" ' My friend Mr. Adams (who will probably
deliver this note to you) has completed his

THE DISCOVERY OF NEPTUNE 51

calculations respecting the perturbation of the and
orbit of Uranus by a supposed ulterior planet, and Adams'3
has arrived at results which he would be glad to Green-
communicate to you personally, if you could spare wlch>
him a few moments of your valuable time. His
calculations are founded on the observations you
were so good as to furnish him with some time
ago ; and from his character as a mathematician,
and his practice in calculation, I should consider
the deductions from his premises to be made in a
trustworthy manner. If he should not have the
good fortune to see you at Greenwich, he hopes
to be allowed to write to you on this subject/

" On the day on which this letter was dated, I
was present at a meeting of the French Institute.
I acknowledged it by the following letter : —

No. 10. — G. B. AIRY to PROFESSOR CHALLIS.

"' ROYAL OBSERVATORY, GREENWICH, 1845, Sept. 29.

" ' I was, I suppose, on my way from France,
when Mr. Adams called here ; at all events, I
had not reached home, and therefore, to my
regret, I have not seen him. Would you mention
to Mr. Adams that I am very much interested
with the subject of his investigations, and that
I should be delighted to hear of them by letter
from him ? '

"On one of the last days of October 1845, Mr.
Adams called at the Eoyal Observatory, Green-

52 ASTRONOMICAL DISCOVERY

wich, in my absence and left the following
important paper : —

No. ii. — J. C. ADAMS, Esq., to G. B. AIRY.

Adams' "'According to my calculations, the observed
irregularities in the motion of Uranus may be
accounted for by supposing the existence of an
exterior planet, the mass and orbit of which are
as follows : —

Mean distance (assumed nearly in accord-
ance with Bode's Law) . . . 38.4
Mean sidereal motion in 365.25 days . i°3o'-9
Mean longitude, ist October 1845 • • 323 34
Longitude of perihelion . . . 315 55

Eccentricity . . . . . .0.1610.

Mass (that of the sun being unity) . 0.0001656.

For the modern observations I have used the
method of normal places, taking the mean of the
tabular errors, as given by observations near three
consecutive oppositions, to correspond with the
mean of the times ; and the Greenwich observa-
tions have been used down to 1830: since which,
the Cambridge and Greenwich observations, and
those given in the Astronomische Nachrichten,
have been made use of. The following are the
remaining errors of mean longitude : —
Observation — Theory.

1780

+ 0.27

1801

- 0.04

l822

+ 0.30

1783

-0.23

1804

+ 1.76

1825

+ 1.92

1786

- 0.96

1807

-0.21

1828

+ 2.25

1789

+ 1.82

1810

+ 0.56

1831

- 1. 06

1792

-0.91

1813

-0.94

1834

- i-44

1795

+ 0.09

1816

-0.3I

1837

-1.62

1798

-0.99

1819

- 2.00

1840

+ 1-73

THE DISCOVERY OF NEPTUNE 53

The error for 1780 is concluded from that for
1781 given by observation, compared with those
of four or five following years, and also with
Lemonnier's observations in 1769 and 1771.

" ' For the ancient observations, the following
are the remaining errors : —

Observation — Theory.

1690 +44-4 175° -1-6 1763 - s'.'i
1712 + 6.7 1753 +5.7 1769 + 0.6
1715 - 6.8 1756 -4.0 1771 +n. 8

The errors are small, except for Flamsteed's
observation of 1690. This being an isolated
observation, very distant from the rest, I thought
it best not to use it in forming the equations of
condition. It is not improbable, however, that
this error might be destroyed by a small change
in the assumed mean motion of the planet.'

"I acknowledged the receipt of this paper in
the following terms : —

No. 12. — G. B. AIRY to J, C. ADAMS, Esq.

tte EOYAL OBSERVATORY, GREENWICH, 1845, Nov. 5.

" * I am very much obliged by the paper of
results which you left here a few days since,
showing the perturbations on the place of
Uranus produced by a planet with certain
assumed elements. The latter numbers are all
extremely satisfactory : I am not enough
acquainted with Flamsteed's observations about

54 ASTRONOMICAL DISCOVERY

1690 to say whether they bear such an error,
but I think it extremely probable.

" * But I should be very glad to know whether
this assumed perturbation will explain the error
of the radius vector of Uranus. This error is
now very considerable, as you will be able to
ascertain by comparing the normal equations,
given in the Greenwich observations for each
year, for the times before opposition with the
times after opposition/

" I have before stated that I considered the
establishment of this error of the radius vector
of Uranus to be a very important determination.
I therefore considered that the trial, whether
the error of radius vector would be explained
by the same theory which explained the error of
longitude, would be truly an experimentum crucis.
And I waited with much anxiety for Mr. Adams'
answer to my query. Had it been in the affir-
mative, I should at once have exerted all the
influence which I might possess, either directly,
or indirectly through my friend Professor Challis,
to procure the publication of Mr. Adams' theory.

"From some cause with which I am un-
acquainted, probably an accidental one, I received
no immediate answer to this inquiry. I regret
this deeply, for many reasons."

Adams' Here we may leave Airy's "account" for a few
moments to consider the reason why he received
no answer. Adams was a very shy and retiring

THE DISCOVERY OF NEPTUNE 55

young man, and very sensitive ; though capable of
a great resolution, and of enormous perseverance
in carrying it out. We know (what is not in-
dicated in the above account), how steadily he
had kept in view the idea of solving this great
problem. It was characteristic of him that as
early as 1841 he had formed a resolution to under-
take it, although at the time he was not able to
enter upon its accomplishment. The following
memorandum, which is still in existence, having
been found among his papers after his death,
records these facts :

" 1841, July 3. Formed a design, in the
beginning of this week, of investigating, as
soon as possible after taking my degree, the
irregularities in the motion of Uranus, which
were as yet unaccounted for : in order to find
whether they may be attributed to the action of
an undiscovered planet beyond it, and if possible
thence to determine the elements of its orbit,
&c., approximately, which would probably lead
to its discovery."

Accordingly, " as soon asj possible after taking
his degree" he embarked upon the enterprise, and
the first solution was made in the long vacation of
1843, assuming the orbit of the unknown planet
to be a circle with a radius equal to twice the
mean distance of Uranus from the sun (an as-
sumption which, as we have seen, was also made
by Le Verrier). Having satisfied himself that

56 ASTRONOMICAL DISCOVERY

there was a good general agreement between his
results and the observations, Adams began a more
complete solution ; indeed from first to last he
made no less than six separate solutions, the one
which he announced to Airy in the above letter
being the fourth. Hence he had already done an
enormous amount of work on the problem, and
was in his own mind so justly convinced of the
correctness and value of his results that he was
liable to forget that others had not had the same
opportunity of judging of their completeness ;
and he was grievously disappointed when his
announcement was not received with full
confidence.

But perhaps it should first be stated that by a
series of mischances Adams had been already much
disappointed at the failure of his attempts to see
the Astronomer Royal on his visits to Greenwich.
This does not seem to have been exactly Airy's
fault ; he was, as may well be supposed, an
extremely busy man, and was much occupied at
the time on a question of great practical im-
portance, at the direct request of the Government,
namely, the settling of the proper gauge for rail-
ways throughout the country. The first time
Adams cajled to see him, he was actually in
London sitting on the Committee which dealt
with this question, and Adams was asked to call
later ; when the visit was repeated, Airy was un-
fortunately at dinner (and it may be added that
his hours for dinner were somewhat peculiar), and

THE DISCOVERY OF NEPTUNE 57

the butler, acting somewhat in the manner of his
kind, protected his master's dinner by sending
away one whom he doubtless regarded as a trouble-
some visitor. There is, as I have said, little doubt
about any of the facts, and it seems well established
that Airy himself did not learn of Adams' visits
until afterwards, and it would scarcely be just to
blame him for a servant's oversight. But Adams
had left the paper above reproduced, and Airy with
his business-like habits ultimately proceeded to
deal with it ; he wrote the answer given above
asking Adams a definite question, filed a copy of
it with the original letter, and then dismissed the
matter from his thoughts until the reply from
Adams, which he confidently expected should
again bring it under notice.

This further disappointment was, however, too and at
much for Adams ; he regarded the question put question.
by Airy as having so obvious an answer that it
was intended as an evasion, though this was far
from being the case. Airy was thoroughly in
earnest about his question, though it must be
admitted that a more careful study of the problem
would have shown him that it was unnecessary.
Later, when he learnt of Le Verrier's researches,
he put the same question to him, and received a
polite but very clear answer, showing that the sug-
gested test was not an experimentum crucis as he
supposed. But Adams did not feel equal to
making this reply; he shrank into his shell
and solaced himself only by commencing afresh

58 ASTRONOMICAL DISCOVERY

another solution of the problem which had so
engrossed his life at that time.

The I have heard severe or contemptuous things said

Afcy?of about this question by those who most blame Airy.

question. Some of them have no hesitation in accusing him
of intellectual incompetence : they say that it was
the question of a stupid man. I think that in the
first place they forget the difference between a
deliberate error of judgement and a mere conse-
quence of insufficient attention. But there is
even more than this to be said in defence of
the question. The " error of radius vector " came
before Airy in an entirely independent way, and
as an entirely independent phenomenon, from the
" error of longitude," and there was nothing un-
natural in regarding it as requiring independent
explanation. It is true that, as the event proved,
a mere readjustment of the orbit of Uranus got rid
of this error of radius vector (this was substantially
Le Verrier's answer to Airy's question) ; but we
must not judge of what was possible before the
event in the light of what we now know. The

The range original possibilities were far wider, though we
have forgotten their former extent now that they
have been narrowed down by the discovery. If a
sentry during war time hears a noise in a certain
direction, he may be compelled to make the as-
sumption that it is the movement of an enemy ;
and if he fires in that direction and kills him, and
thus saves his own army from destruction, he is
deservedly applauded for the success which attends

THE DISCOVERY OF NEPTUNE 59

his action. But it does not follow that the as-
sumption on which he acted was the only possible
one. Or, to take a more peaceful illustration, in
playing whist it sometimes becomes apparent that
the game can only be won if the cards lie in a
certain way ; and a good player will thereupon
assume that this is the fact, and play accordingly.
Adams and Le Verrier played to win the game on
the particular assumption that the disturbance of
Uranus was due to an external planet revolving
at a distance from the sun about twice that of
Uranus ; and won it ; and we applaud them for
doing so. But it is easy to imagine a rearrange-
ment of the cards with which they would have
lost it ; and Airy's question simply meant that he
was alive to these wider possibilities, and did not
see the need for attempting to win the game in
that particular way.

One such alternative possibility has already been
mentioned. " Hansen's opinion was, that one dis-
turbing body would not satisfy the phenomena; but
he conjectured that there were two planets beyond
Uranus" Another conceivable alternative is that
there was some change in the law of gravitation at
the distance of Uranus, which, it must be remem-
bered, is twice as great as that of any planet pre-
viously known. Or some wandering body might
have passed close enough to Uranus to change
its orbit somewhat suddenly. We now know, for
instance, that the swarm of meteorites which

60 ASTRONOMICAL DISCOVERY

gives rise to the well-known " November meteors "
must have passed very close to Uranus in A.D. 126,
assuming that neither the planet nor the swarm
have been disturbed in any unknown manner in
the meantime. It is to this encounter that we
owe the introduction of this swarm to our solar
system : wandering through space, they met
Uranus, and were swept by his attraction into an
orbit round the sun. Was there no reaction upon
Uranus himself? The probabilities are that the
total mass of the swarm was so small as to affect
the huge planet inappreciably ; but who was to
say that some other swarm of larger mass, or other
body, might not have approached near Uranus at
some date between 1690 and 1845, an(^ been
responsible at any rate in part for the observed
errors ? These are two or three suppositions from
our familiar experience ; and there are, of course,
limitless possibilities beyond. Which is the true
scientific attitude, to be alive to them all, or to
concentrate attention upon one ?

But we are perhaps wandering too far from
the main theme. It is easy to do so in review-
ing this extraordinary piece of history, for at
almost every point new possibilities are suggested.

We must return, however, to Airy's " account."
We reached the point where he had written to
Adams (on November 5, 1845), asking his ques-
tion about the radius vector, and received no
reply; and there the matter remained, so far as

III.— U. J. LK VERRIER.

(From a print in the possession of the R-tyal Astronomical Society.)

IV — J. G. GALLE.

WHO FIRST SAW THE PLANET NEPTUXE.

THE DISCOVERY OF NEPTUNE 61

he was concerned, until the following June,
when Le Verrier's memoir reached him; and Airy re-
we will let him give his own version of the
result.

"This memoir reached me about the 23rd or
24th of June. I cannot sufficiently express the
feeling of delight and satisfaction which I received
from it. The place which it assigned to the
disturbing planet was the same, to one degree,
as that given by Mr. Adams' calculations, which
I had perused seven months earlier. To this
time I had considered that there was still room
for doubt of the accuracy of Mr. Adams' in-
vestigations ; for I think that the results of
algebraic and numerical computations, so long
and so complicated as those of an inverse
problem of perturbations, are liable to many
risks of error in the details of the process : I
know that there are important numerical errors
in the Mecanique Celeste of Laplace ; in the
Theorie de la Lune of Plan a ; above all, in
Bouvard's first tables of Jupiter and Saturn;
and to express it in a word, I have always
considered the correctness of a distant mathe-
matical result to be a subject rather of moral
than of mathematical evidence. But now I felt
no doubt of the accuracy of both calculations, as
applied to the perturbation in longitude. I was,
however, still desirous, as before, of learning
whether the perturbation in radius vector was

62 ASTRONOMICAL DISCOVERY

fully explained. I therefore addressed to M. Le
Verrier the following letter: —

No. 13. — G. B. AIRY to M. LE VERRIER.

"' ROYAL OBSERVATORY, GREENWICH, 1846, June 26.

He puts "'I have read, with very great interest, the
"radius- account of your investigations on the probable
question place of a planet disturbing the motions of
Verrier Uranus, which is contained in the Compte
Rendu de TAcademie of June i ; and I now
beg leave to trouble you with the following
question. It appears, from all the later obser-
vations of Uranus made at Greenwich (which
are most completely reduced in the Greenwich
Observations of each year, so as to exhibit the
effect of an error either in the tabular helio-
centric longitude, or the tabular radius vector),
that the tabular radius vector is considerably too
small. And I wish to inquire of you whether
this would be a consequence of the disturbance
produced by an exterior planet, now in the
position which you have indicated?"

There is more of the letter, but this will suffice
to show that he wrote to Le Verrier in the same
way as to Adams, and, as already stated, received
a reply dated three or four days later. But the
rest of the letter contains no mention of Adams,
and thus arises a second difficulty in understand-
ing Airy's conduct. It seems extraordinary that

THE DISCOVERY OF NEPTUNE 63

when he wrote to Le Verrier he made no mention but makes
of the computations which he had previously Sor^o?"
received from Adams ; or that he should not Adams-
have written to Adams, and made some attempt
to understand his long silence, now that, as he
himself states, he " felt no doubt of the accuracy
of both calculations." The omission may have
been, and probably was, mere carelessness or for-
getfulness ; but he could hardly be surprised if
others mistook it for deliberate action.

However, attention had now been thoroughly Airy an-
attracted to the near possibility of finding the J^e Si-
planet. On June 29, 1846, there was a special *™^
meeting of the Board of Visitors of Greenwich Planet»
Observatory, and Airy incidentally mentioned to
them this possibility. The impression produced
must have been definite and deep; for Sir John
Herschel, who was present, was bold enough to
say on September loth following to the British
Association assembled at Southampton: "We
see it (the probable new planet) as Columbus
saw America from the shores of Spain. Its
movements have been felt trembling along the
far-reaching line of our analysis with a certainty
hardly inferior to that of ocular demonstration."
Airy discussed the matter with Professor Challis and
(who, it will be remembered, had originally
written to him on behalf of Adams), suggesting
that he should immediately commence a search bridge
for the supposed planet at Cambridge. It may
be asked \vhy Airy did not commence this search

64 ASTRONOMICAL DISCOVERY

himself at Greenwich, and the answer is that he
had no telescope which he regarded as large
enough for the purpose. The Royal Observatory
at Greenwich has always been, and is now, better
equipped in some respects than any other observa-
tory, as might be expected from its deservedly
great reputation ; but to possess the largest exist-
ing telescope has never been one of its ambitions.
The instruments in which it takes most pride
are remarkable for their steadiness and accuracy
rather than for their size ; and at that time the
nothav- best telescope possessed by the observatory was
abLftete not> in Airy's opinion, large enough to detect the
scope at planet with certainty. In this opinion we now
wich know that he was mistaken ; but, again, we must
not judge his conduct before the event in the
light of what we have since discovered. It may
be recalled here that it was not until Le Verrier's
third paper, published on August 31, that he (Le
Verrier) emphatically pointed out that the new
planet might be of such a size as to have a
sensible disc ; and it was this remark which led
immediately to its discovery. Until this was so
decisively stated, it must have seemed exception-
ally improbable ; for we saw in the last chapter
how diligently the Zodiac had been swept in
the search for minor planets, — how, for instance,
Hencke had searched for fifteen years without
success ; and it might fairly be considered that
if there were a fairly bright object (such as
Neptune has since been found to be) it would

THE DISCOVERY OF NEPTUNE 65

have been discovered earlier. Hence Airy not
unreasonably considered it necessary to spread
his net for very small objects. On July 9 he
wrote to Professor Challis as follows : —

No. 15. — G. B. AIRY to PROFESSOR CHALLIS.

" THE DEANERY, ELY, 1846, July 9.

" You know that I attach importance to the
examination of that part of the heavens in which
there is ... reason for suspecting the existence
of a planet exterior to Uranus. I have thought
about the way of making such examination, but I
am convinced that (for various reasons, of declina-
tion, latitude of place, feebleness of light, and
regularity of superintendence) there is no prospect
whatever of its being made with any chance of
success, except with the Northumberland tele-
scope.

"Now, I should be glad to ask you, in the
first place, whether you could make such an
examination ?

" Presuming that your answer would be in the
negative, I would ask, secondly, whether, suppos-
ing that an assistant were supplied to you for this
purpose, you would superintend the examination?

" You will readily perceive that all this is in
a most unformed state at present, and that I am
asking these questions almost at a venture, in the
hope of rescuing the matter from a state which is,

without the assistance that you and your instru-

66 ASTRONOMICAL DISCOVERY

ments can give, almost desperate. Therefore I
should be glad to have your answer, not only
responding simply to my questions, but also enter-
ing into any other considerations which you think
likely to bear on the matter.

" The time for the said examination is approach-
ing near."

chains Professor Challis did not require an assistant,
tairesthe ^ut determined to undertake the work himself,
search. an(j ^evised his own plan of procedure ; but he
also set out on the undertaking with the expecta-
tion of a long and arduous search. No such idea
as that of finding the planet on the first night ever
entered his head. For one thing, he had no map
of the region to be examined, for although the
map used by Galle had been published, no copy
of it had as yet reached Cambridge, and Professor
Challis had practically to construct a map for
himself. In these days of photography to make
such a map is a simple matter, but at that time
the process was terribly laborious. " I get over
the ground very slowly," he wrote on September
2nd to Airy, " thinking it right to include all stars
to 10-1 1 magnitude ; and I find that to scrutinise
thoroughly in this way the proposed portion of
the heavens will require many more observations
than I can take this year." With such a prospect,
it is not surprising that one night's observations
were not even compared with the next ; there
would be a certain economy in waiting until a

THE DISCOVERY OF NEPTUNE 67

large amount of material had been accumulated,
and then making the comparisons all together,
and this was the course adopted. But when Le
Verrier's third paper, with the decided opinion
that the planet would be bright enough to be seen
by its disc, ultimately reached Professor Challis, it
naturally gave him an entirely different view of the
possibilities ; he immediately began to compare
the observations already made, and found that he He finds
had observed the planet early in August. But it that ^e
was now too late to be first in the field, for Galle
had already made his announcement of discovery. Planet
Writing to Airy on October 12, Challis could ,
only lament that after four days' observing the
planet was in his grasp, {/"only he had examined
or mapped the observations, and if he had not
delayed doing so until he had more observations
to reduce, and if he had not been very busy with
some comet observations. Oh ! these terrible ifs
which come so often between a man and success !
The third of them is a peculiarly distressing one,
for it represents that eternal conflict between one
duty and another, which is so constantly recurring
in scientific work. Shall we finish one piece of
work now well under way, or shall we attend to
something more novel and more attractive ?
Challis thought his duty lay in steadily com-
pleting the comet observations already begun.
We saw in the last lecture how the steady pursuit
of the discovery of minor planets, a duty which
had become tedious and apparently led nowhere,

68 ASTRONOMICAL DISCOVERY

suddenly resulted in the important discovery of
Eros. But Challis was not so fortunate in elect-
ing to plod along the beaten track ; he would
have done better to leave it. There is no golden
rule for the answer; we must be guided in
each case by the special circumstances, and the
dilemma is" consequently a new one on every
occasion, and perhaps the more trying with each
repetition.

Such are briefly the events which led to the
discovery of Neptune, which was made in Ger-
many by direction from France, when it might
have been made in Cambridge alone. The in-
sensation cidents created a great stir at the time. The
byThe "Account" of them, as read by Airy to the Royal
discovery. Astronomical Society on November 13, 1846,
straightforward and interesting though it was,
making clear where he had himself been at fault,
nevertheless stirred up angry passions in many
quarters, and chiefly directed against Airy himself.
Cambridge was furious at Airy's negligence,
which it considered responsible for costing the
University a great discovery ; and others were
equally irate at his attempting to claim for Adams
some of that glory which they considered should
go wholly to Le Verrier. But it may be remarked
Not ail that feeling was not purely national. Some
jealousy, foreigners were cordial in their recognition of the
work of Adams, while some of those most eager to
oppose his claims were found in this country. In
their anxiety to show that they were free from

THE DISCOVERY OF NEPTUNE 69

national jealousy, scientific men went almost too
far in the opposite direction.

Airy's conduct was certainly strange at several
points, as has already been remarked. One cannot
understand his writing to Le Verrier in June 1846
without any mention of Adams. He could not
even momentarily have forgotten Adams' work ;
for he tells us himself how he noticed the close .,
correspondence of his result with that of Le
Verrier : and had he even casually mentioned
this fact in writing to the latter, it would have
prepared the way for his later statement. But we
can easily understand the unfavourable impression
produced by this statement after the discovery had
been made, when there had been no previous hint
on the subject at all. Of those who abused him The
Cambridge had the least excuse ; for there is no
doubt that with a reasonably competent Professor
of Astronomy in Cambridge, she need not have matter.
referred to Airy at all. It would not seem to
require any great amount of intelligence to under-
take to look in a certain region for a strange object
if one is in possession of a proper instrument.
We have seen that Challis had the instrument,
and when urged to do so was equal to the task of
finding the planet ; but he was a man of no initia-
tive, and the idea of doing so unless directed by
some authority never entered his head. He had
been accustomed for many years to lean rather
helplessly upon Airy, who had preceded him in
office at Cambridge. For instance, when appointed

70 ASTRONOMICAL DISCOVERY

to succeed him, and confronted with the necessity
of lecturing to students, he was so helpless that
he wrote to implore Airy to come back to Cam-
bridge and lecture for him ; and this was actually
done, Airy obtaining leave from the Government
to leave his duties at Greenwich for a time in
order to return to Cambridge, and show Challis
chains how to lecture. Now it seems to me that this
weakest helplessness was the very root of all the mischief
of which Cambridge so bitterly complained. I
claimed at the outset the privilege of stating my
own views, with which others may not agree : and
of all the mistakes and omissions made in this
little piece of history, the most unpardonable and
the one which had most serious consequences
seems to me to be this : that Challis never made
the most casual inquiry as to the result of the
visit to Greenwich which he himself had directed
Adams to make. I am judging him to some
extent by default ; because I assume the facts
from lack of evidence to the contrary : but it
seems practically certain that after sending this
young man to see Airy on this important topic,
Challis thereupon washed his hands of all respon-
sibility so completely that he never even took the
trouble to inquire on his return, " Well ! how did
you get on ? What did the Astronomer Royal
say ? " Had he put this simple question, which
scarcely required the initiative of a machine, and
learnt in consequence, as he must have done, that
the sensitive young man thought Airy's question

THE DISCOVERY OF NEPTUNE 71

trivial, and did not propose to answer it, I think
we might have trusted events to right themselves.
Even Challis might have been trusted to reply,
" Oh ! but you must answer the Astronomer
Royal's question : you may think it stupid, but
you had better answer it politely, and show him
that you know what you are about." It is un-
profitable to pursue speculation further ; this did
not happen, and something else did. But I have
always felt that my old University made a scape-
goat of the wrong man in venting its fury upon
Airy, when the real culprit was among themselves,
and was the man they had themselves chosen to
represent astronomy. He was presumably the
best they had ; but if they had no one better than
this, they should not have been surprised, and
must not complain, if things went wrong. If a
University is ambitious of doing great things, it
must take care to see that there are men of ability
and initiative in the right places. This is a most
difficult task in any case, and we require all pos-
sible incentives towards it. To blink the facts
when a weak spot is mercilessly exposed by the
loss of a great opportunity is to lose one kind of
incentive, and perhaps not the least valuable.

Let us now turn to some curious circumstances Curious
attending this remarkable discovery of a planet between

by mathematical investigation, of which there are
several. The first is, that although Neptune was Planefc-
found so near the place where it was predicted,
its orbit, after discovery, proved to be very dif-

72 ASTRONOMICAL DISCOVERY

ferent from that which Adams and Le Verrier
had supposed. You will remember that both
calculators assumed the distance from the sun,
in accordance with Bode's Law, to be nearly twice
that of Uranus. The actual planet was found to
have a mean distance less than this by 25 per
cent, an enormous quantity in such a case. For
instance, if the supposed planet and the real were
started round the sun together, the real planet
would soon be a long way ahead of the other,
and the ultimate disturbing effect of the two on
Uranus would be very different. To explain the
difference, we must first recall a curious pro-
perty of such disturbances. When two planets
are revolving, so that one takes just twice or
three times, or any exact number of times, as long
to revolve round the sun as the other, the usual
mathematical expressions for the disturbing action
of one planet on the other would assign an infinite
disturbance, which, translated into ordinary lan-
guage, means that we must start with a fresh
assumption, for this state of things cannot persist.
If the period of one were a little longer than this
critical value, some of the mathematical expres-
sions would be of contrary sign from those corre-
sponding to a period a little shorter. Now it is
curious that the supposed planet and the real had
orbits on opposite sides of a critical value of this
kind, namely, that which would assign a period
of revolution for Neptune exactly half that of
Uranus ; and it was pointed out in America by

THE DISCOVERY OF NEPTUNE 73

Professor Peirce that the effect of the planet Professor
imagined by Adams and Le Verrier was thus
totally different from that of Neptune. He there-
fore declared that the mathematical work had not coveir

WtlS Si

really led to the discovery at all ; but that it had mere

• 11- • • accident.

resulted irom mere coincidence, and this opinion —
somewhat paradoxical though it was — found con-
siderable support. It was not replied to by Adams
until some thirty years later, when a short reply
was printed in Liouville's Journal. The explana- The ex-

,. • „ i • ,i • • i -i 1 -r» planation.

tion is this : the expressions considered by Pro-
fessor Peirce are those representing the action of
the planet throughout an indefinite past, and did
not enter into the problem, which would have
been precisely the same if Neptune had been
suddenly created in 1690; while, on the other
hand, if Neptune had existed up till 1690 (the
time when Uranus was first observed, although
unknowingly), and then had been destroyed,
there would have been no means of tracing its
previous existence. In past ages it had no doubt
been perturbing the orbit of Uranus, and had
effected large changes in it ; but if it had then
been suddenly destroyed, we should have had no
means of identifying these changes. There might
have been instead of Neptune another planet, such
as .that supposed by Adams and Le Verrier ; and
its action in all past time would have been very
different from that of Neptune, as is properly re-
presented in the mathematical expressions which
Professor Peirce considered. In consequence the

74 ASTRONOMICAL DISCOVERY

orbit of Uranus in 1690 would have been very
different from the orbit as it was actually found ;
but in either case the mathematicians Adams and
Le Verrier would have had to take it as they
found it ; and the disturbing action which they
considered in their calculations was the compara-
tively small disturbance which began in 1690 and
ended in 1846. During this limited number of
years the disturbance of the planet they imagined,
although not precisely the same as that of Nep-
tune, was sufficiently like it to give them the
approximate place of the planet.

Still it is somewhat bewildering to look at the
mathematical expressions for the disturbances as
used by Adams and Le Verrier, when we can now
compare with them the actual expressions to which
they ought to correspond; and one may say frankly
that there seems to be no sort of resemblance.
Recently a memorial of Adams' work has been
published by the Royal Astronomical Society ;
they have reproduced in their Memoirs a facsimile
of Adams' MS. containing the " first solution,"
which he made in 1843 in the Long Vacation
after he had taken his degree, and which would
have given the place of Neptune at that time with
an error of 15°. In an introduction describing
the whole 'of the MSS., written by Professor R. A.
Sampson of Durham, it is shown how different
the actual expressions for Neptune's influence are
from those used by Adams, and it is one of the
curiosities of this remarkable piece of history that

THE DISCOVERY OF NEPTUNE 75

some of them seem to be actually in the wrong
direction; and others are so little alike that it is
only by fixing our attention resolutely on the
considerations above mentioned that we can
realise that the analytical work did indeed lead
to the discovery of the planet.

A second curiosity is that a mistaken idea Suggested
should have been held by at least one eminent tary
man (Sir J. Herschel), to the effect that it would
have been possible to find the place of the planet
by a much simpler mathematical calculation than
that actually employed by Adams or Le Verrier.
In his famous '* Outlines of Astronomy " Sir John
Herschel describes a simple graphical method,
which he declares would have indicated the place
of the planet without much trouble. Concerning
it I will here merely quote Professor Sampson's
words : —

" The conclusion is drawn that Uranus arrived
at a conjunction with the disturbing planet about
1822 ; and this was the case. Plausible as this
argument may seem, it is entirely baseless. For
the maximum of perturbations depending on the
eccentricities has no relation to conjunction, and
the others which depend upon the differences of
the mean motions alone are of the nature of forced
oscillations, and conjunction is not their maximum
or stationary position, but their position of most
rapid change."

Professor Sampson goes on to show that a more

76 ASTRONOMICAL DISCOVERY

elaborate discussion seems quite as unpromising ;
and he concludes that the refinements employed
were not superfluous, although it seems now clear
that a different mode of procedure might have led
more certainly to the required conclusion.
The evil For the third curious point is that both calcula-
of^Bode's t°rs should have adhered so closely to Bode's Law.
If they had not had this guiding principle it seems
almost certain that they would have made a better
approximation to the place of the planet, for
instead of helping them it really led them astray.
We have already remarked that if two planets are
at different distances from the sun, however slight,
and if they are started in their revolution together,
they must inevitably separate in course of time,
and the amount of separation will ultimately
become serious. Thus by assuming a distance for
the planet which was in error, however slight, the
calculators immediately rendered it impossible for
themselves to obtain a place for the planet which
should be correct for more than a very brief period.
Professor Sampson has given the following inter-
esting lists of the dates at which Adams' six solu-
tions gave the true place of the planet and the
intervals during which the error was within 5°
either way.

I. II. III. IV. v. VI.

Correct .... 1820 1835 1872 1830 1861 1856

Within + c° J l812 I827 I865 I8l3 1815 1826
(1827 1842 1877 1866 1871 1868

Now the date at which it was most important to
obtain the correct place was 1845 or thereabouts

THE DISCOVERY OF NEPTUNE 77

when it was proposed to look for the planet ; but
no special precaution seems to have been taken by
either investigator to secure any advantage for
this particular date. Criticising the procedure
after the event (and of course this is a very
unsatisfactory method of criticism), we should say
that it would have been better to make several
assumptions as regards the distance instead of
relying upon Bode's Law ; but no one, so far as I
know, has ever taken the trouble to write out a
satisfactory solution of the problem as it might
have been conducted. Such a solution would be
full of interest, though it could only have a small"
weight in forming our estimation of the skill
with which the problem was solved in the first
instance.

Fourthly, we may notice a very curious point. Le
Le Verrier went to some trouble not only to point erroneous
out the most likely place for the planet, but to
indicate limits outside which it was not necessary
to look. This part of his work is specially com-
mented upon with enthusiasm by Airy, and I will
reproduce what he says. It is rather technical
perhaps, but those who cannot follow the mathe*
matics will be able to appreciate the tone of
admiration.

" M. Le Verrier then enters into a most ingenious
computation of the limits between which the
planet must be sought. The principle is this :
assuming a time of revolution, all the other un-

78 ASTRONOMICAL DISCOVERY

known quantities may be varied in such a manner
that though the observations will not be so well
represented as before, yet the errors of observation
will be tolerable. At last, on continuing the
variation of elements, one error of observation will
be intolerably great. Then, by varying the ele-
ments in another way, we may at length make
another error of observation intolerably great ; and
so on. If we compute, for all these different
varieties of elements, the place of the planetfor 1847,
its locus will evidently be a discontinuous curve
or curvilinear polygon. If we do the same thing
with different periodic times, we shall get different
polygons ; and the extreme periodic times that
can be allowed will be indicated by the polygons
becoming points. These extreme periodic times
are 207 and 233 years. If now we draw one
grand curve, circumscribing all the polygons, it is
certain that the planet must be within that curve.
In one direction, M. Le Verrier found no difficulty
in assigning a limit ; in the other he was obliged
to restrict it, by assuming a limit to the eccentri-
city. Thus he found that the longitude of the
planet was certainly not less than 321°, and not
greater than 335° or 345°, according as we limit
the eccentricity to 0.125 or 0.2. And if we adopt
0.125 as the limit, then the mass will be included
between the limits 0.00007 and 0.00021 ; either
of which exceeds that of Uranus. From this cir-
cumstance, combined with a probable hypothesis
as to the density, M. Le Verrier concluded that

THE DISCOVERY OF NEPTUNE 79

the planet would have a visible disk, and sufficient The
light to make it conspicuous in ordinary tele-
scopes.

" M. Le Verrier then remarks, as one of the strong
proofs of the correctness of the general theory,
that the error of radius vector is explained as
accurately as the error of longitude. And finally,
he gives his opinion that the latitude of the dis-
turbing planet must be small.

"My analysis of this paper has necessarily been
exceedingly imperfect, as regards the astronomical
and mathematical parts of it ; but I am sensible
that, in regard to another part, it fails totally. I
cannot attempt to convey to you the impression
which was made on me by the author's undoubt-
ing confidence in the general truth of his theory,
by the calmness and clearness with which he
limited the field of observation, and by the firm-
ness with which he proclaimed to observing
astronomers, 'Look in the place which I have
indicated, and you will see the planet well.'
Since Copernicus declared that, when means
should be discovered for improving the vision,
it would be found that Venus had phases like
the moon, nothing (in my opinion) so bold,
and so justifiably bold, has been uttered in
astronomical prediction. It is here, if I mis-
take not, that we see a character far superior
to that of the able, or enterprising, or indus-
trious mathematician ; it is here that we see the
philosopher."

8o ASTRONOMICAL DISCOVERY

Peirce's But now this process of limitation was faulty
the limits, and actually misleading. Let us compare what

is said about it by Professor Peirce a little

later.

" Guided by this principle, well established,
and legitimate, if confined within proper limits,
M. Le Verrier narrowed with consummate skill the
field of research, and arrived at two fundamental
propositions, namely : —

" i st. That the mean distance of the planet
cannot be less than 35 or more than 37.9. The
corresponding, limits of the time of sidereal revolu-
tion are about 207 and 233 years.

" 2nd. * That there is only one region in which
the disturbing planet can be placed in order to
account for the motions of Uranus ; that the mean
longitude of this planet must have been, on
January i, 1800, between 243° and 252°.'

" ' Neither of these propositions is of itself
necessarily opposed to the observations which
have been made upon Neptune, but the two com-
bined are decidedly inconsistent with observation.
It is impossible to find an orbit, which, satisfying
the observed distance and motion, is subject to
them. If, for instance, a mean longitude and
time of revolution are adopted according with the
first, the corresponding mean longitude in 1800
must have been at least 40° distant from the
limits of the second proposition. And again, if
the planet is assumed to have had in 1800 a

THE DISCOVERY OF NEPTUNE 81

mean longitude near the limits of the second
proposition, the corresponding time of revolution
with which its motions satisfy the present observa-
tions cannot exceed 1 70 years, and must therefore
be about 40 years less than the limits of the first
proposition.'

"Neptune cannot, then, be the planet of M.
Le Verrier's theory, and cannot account for the
observed perturbations of Uranus under the form
of the inequalities involved in his analysis" —
(Proc. Amer. Acad. /., 1846-1848, p. 66).

At the time when Professor Peirce wrote, the
orbit of Neptune was not sufficiently well deter-
mined to decide whether one of the two limita-
tions might not be correct, though he could see
that they could not both be right, and we now
know that they are both wrong. The mean dis-
tance of Neptune is 30, which does not lie between
35 and 37.9; and the longitude in 1800 was 225°,
which does not lie between 243° and 252°. The
ingenious process which Airy admired and which
Peirce himself calls "consummately skilful" was
wrong in principle. As Professor Newcomb has New-
said, "the error was the elementary one that, criticism,
instead of considering all the elements simultane-
ously variable, Le Verrier took them one at
a time, considering the others as fixed, and
determining the limits between which each could
be contained on this hypothesis. No solver of
least square equations at the present day ought to

82 ASTRONOMICAL DISCOVERY

make such a blunder. Of course one trouble in
Le Verrier's demonstration, had he attempted a
rigorous one, would have been the impossibility
of forming the simultaneous equations expressive
of possible variations of all the elements."

The account of Le Verrier's limits by Professor
Peirce, though it exhibits the error with special
clearness, is a little unfair to Le Verrier in one
point. If, instead of taking the limits for the
date 1 800, we take them for 1846 (when the
search for Neptune was actually made), we shall
find that they do include the actual place of
the planet, as Airy found. The erroneous mean
motion of Le Verrier's planet allowed of his
being right at one time and wrong at another;
and Airy examined the limits under favourable
conditions, which explains his enthusiasm. But
we can scarcely wonder that Professor Pairce
came to the conclusion that the planet discovered
was not the one pointed out by Le Verrier, and
had been found by mere accident. And all these
circumstances inevitably contribute to a general
impression that the calculators had a large
Element element of good fortune to thank for their
fortune, success. Nor need we hesitate to make this
admission, for there is an element of good
fortune in all discoveries. To look no further
than this — if a man had not been doing a par-
ticular thing at a particular time, as he might
easily not have been, most discoveries would
never have been made. If Sir William Herschel

15" *

V.— CORNER OF '/HE BERLIN MAP, BY THE USE OF WHICH GALLE FOUND NEPTUNE

THE DISCOVERY OF NEPTUNE 83

had not been looking at certain small stars for
a totally different purpose he would never have
found Uranus ; and no one need hesitate to admit
the element of chance in the finding of Neptune.
It is well illustrated by a glance at the map The map
which, as has been remarked, Galle used to com- ™^7
pare with the sky on the night when he made the
actual discovery. The planet was found down
near the bottom corner of the map, and since the
limits assigned for its place might easily have
varied a few degrees one way or the other, it
might easily have been off the map ; in which case
it is probable that the search would not have been
successful, or at any rate that success would have
been delayed.

Thus, it is a most remarkable feature of the Everyone
discovery of Neptune that mistakes were made mistakes,
by almost every one concerned, however eminent.
Airy made a mistake in regarding the question of
the Radius Vector as of fundamental importance ;
Sir J. Herschel was wrong in describing an ele-
mentary method which he considered might have
found the planet ; Professor Peirce was wrong
in supposing that the actual and the supposed
planet were essentially different in their action
on Uranus ; Le Verrier was wrong in assigning
limits outside which it was not necessary to look
when the actual planet was outside them ; Adams
was more or less wrong in thinking that the
eccentricity of the new planet could be found
from the material already at disposal of man.

84 ASTRONOMICAL DISCOVERY

Both Adams and Le Verrier gave far too much
importance to Bode's Law.

To review a piece of history of this kind and
note the mistakes of such men is certainly
comforting, and need not in any way lessen our
admiration. In the case of the inve'sti-gaftors
themselves, much may be set down to1 excitement
in the presence of a possible discovery. Professor
Sampson has provided us with a small but typical
instance of this fact. When Adams had carried
through all his computations for finding Neptune,
and was approaching the actual place of the
planet, he, "who could carry through fabulous
computations without error," for the first time
wrote down a wrong figure. The mistake was
corrected upon the MS., "probably as soon as
made," but no doubt betrays the excitement
which the great worker could not repress at this
critical moment. There is a tradition that, simi-
larly, when the mighty Newton was approaching
the completion of his calculations to verify the
Law of Gravitation, his excitement was so great
that he was compelled to assign to a friend the
task of finishing them.

Finally, we may remark how the history of
the discovery of Neptune again illustrates the
difficulty of formulating any general principles
for guiding scientific work. Sometimes it is
well to follow the slightest clue, however im-
perfectly understood ; at other times we shall
do better to refuse such guidance. Bode's Law

THE DISCOVERY OF NEPTUNE 85

pointed to the existence of minor planets, and
might conceivably have helped in finding Uranus :
but by trusting to it in the case of Neptune, the
investigators were perilously near going astray.
Sometimes it is better to follow resolutely the
work in hand whatever it may be, shutting one's
ears to other calls ; but Airy and Challis lost their
opportunities by just this course of action. The
history of science is full of such contradictory
experiences ; and the only safe conclusion seems
to be that there are no general rules of conduct
for discovery.

CHAPTEK III

BRADLEY'S DISCOVERIES OF THE ABERRATION OF
LIGHT AND OF THE NUTATION OF THE EARTH'S
AXIS

Bio- IN examining different types of astronomical dis-
meSwcT covery, we shall find certain advantages in varying
adopted. j.Q some extent the method of presentation. In the
two previous chapters our opportunities for learn-
ing anything of the life and character of those
who made the discoveries have been slight ; but I
propose to adopt a more directly biographical
method in dealing with Bradley' s discoveries,
which are so bound up with the simple earnest-
ness of his character that we could scarcely
appreciate their essential features properly with-
out some biographical study. But the record of
his life apart from his astronomical work is not
in any way sensational ; indeed it is singularly
devoid of incident. He had not even a scientific
quarrel. There was scarcely a man of science of
that periool who had not at least one violent
quarrel with some one, save only Bradley, whose
gentle nature seems to have kept him clear of
them all. Judged by ordinary standards his life
was uneventful : and yet it may be doubted
whether, to him who lived it, that life contained

BRADLEY'S DISCOVERIES 87

one dull moment. Incident came for him in his
scientific work : in the preparation of apparatus,
the making of observations, above all in the hard-
thinking which he did to get at the clue which
would explain them ; and after reviewing his
biography,1 I think we shall be inclined to admit
that if ever there was a happy life, albeit one of
unremitting toil, it was that of James Bradley.

He was born at Sherbourn, in Gloucestershire, Bradiey's
in 1693. We know little of his boyhood except
that he went to the Grammar School at North-
leach, and that the memory of this fact was
preserved at the school in 1832 when Rigaud was
writing his memoir. [The school is at present
shut up for want of funds to carry it on ; and all
inquiries I have made have failed to elicit any
trace of this memory.] Similarly we know little
of his undergraduate days at Oxford, except that
he entered as a commoner at Balliol in 1710, took
his B.A. in the regular course in 1714, and his
M.A. in 1717. As a career he chose the Church,
being ordained in 1719, and presented to the
vicarage of Bridstow in Monmouthshire ; but he
only discharged the duties of vicar for a couple of
years, for in 1721 he returned to Oxford as Pro-
fessor of Astronomy, an appointment which
involved the resignation of his livings ; and so
slight was this interruption to his career as an

1 The facts were collected with great care and ability by S. P.
Rigaud, and published by the Oxford University Press in 1832 as
" Miscellaneous Works and Correspondence of the Rev. James
Bradley."

88 ASTRONOMICAL DISCOVERY

astronomer that we may almost disregard it, and
consider him as an astronomer from the first.
But to guard against a possible misconception, let
Brief me say that Bradley entered on a clerical career
career! in a thoroughly earnest spirit ; to do otherwise
would have been quite foreign to his nature. When
vicar of Bridstow he discharged his duties faith-
fully towards that tiny parish, and moreover was
so active in his uncle's parish of Wansted that he
left the reputation of having been curate there,
although he held no actual appointment. And
thirty years later, when he was Astronomer Royal
and resident at Greenwich, and when the valuable
vicarage of Greenwich was offered to him by
the Chancellor of the Exchequer, he honourably
refused the preferment, "because the duty of a
pastor was incompatible with his other studies
and necessary engagements."

Learnt But now let us turn to Bradley' s astronomical
nomynoi education. I must admit, with deep regret, that
at Oxford, we cannot allow any of the credit of it to
Oxford. There was a great astronomer in Oxford
when Bradley was an undergraduate, for Edmund
Halley had been appointed Savilian Professor of
Geometry in 1703, and had immediately set to
work to compute the orbits of comets, which led
to hisimrn'ortal discovery that some of these bodies
return to us again and again, especially the one
which bears his name — Halley's Comet — and
returns every seventy-five years, being next ex-
pected about 1910. But there is no record that

BRADLEY'S DISCOVERIES 89

Bradley came under Halley's teaching or influ-
ence as an undergraduate. In later years the two
men knew each other well, and it was Halley's
one desire towards the close of his life that
Bradley should succeed him as Astronomer Royal
at Greenwich ; a desire which was fulfilled in
rather melancholy fashion, for Halley died with-
out any assurance that his wish would be gratified.
But Bradley got no astronomical teaching at
Oxford either from Halley or others. The art
of astronomical observation he learnt from his
maternal uncle, the Rev. James Pound, Rector of but from
Wansted, in Essex. He is the man to whom we Slm^s016'
owe Bradley' s training and the great discoveries Pound-
which came out of it. He was, I am glad to say,
an Oxford man too ; very much an Oxford man ;
for he seems to have spent some thirteen years
there migrating from one Hall to another. His
record indeed was such as good tutors of colleges
frown upon ; for it was seven years before he
managed to take a degree at all ; and he could
not settle to anything. After ten years at Oxford
he thought he would try medicine ; after three
years more he gave it up and went out in 1700
as chaplain to the East Indies. But he seems to
have been a thoroughly lovable man, for news
was brought of him four years later that he had
a mind to come home, but was dissuaded by the
Governor saying that " if Dr. Pound goes, I and
the rest of the Company will not stay behind."
Soon afterwards the settlement was attacked in an

90 ASTRONOMICAL DISCOVERY

insurrection, and Pound was one of the few who
escaped with his life, losing however all the pro-
perty he had gradually acquired. He returned to
England in 1 706, and was presented to the living
of Wansted ; married twice, and ended his days
in peace and fair prosperity in 1724. Such are
briefly the facts about Bradley's uncle, James
Pound ; but the most important of all remains
to be told — that somehow or other he had learnt
Pound a to make first-rate astronomical observations, how

first-rate

observer, or when is not recorded; but m 1719 he was
already so skilled that Sir Isaac Newton made
him a present of fifty guineas for some observa-
tions ; and repeated the gift in the following
year ; and even three years before this we find
Halley writing to ask for certain observations
from Mr. Pound.

With this excellent man Bradley used fre-
quently to stay. To his nephew he seems to
have been more like a father than an uncle.
When his nephew had smallpox in 1717, he
nursed him through it ; and he supplemented
from his own pocket the scanty allowance which
was all that Bradley's own father could afford.
But what concerns us most is that he fostered,
if he did not actually implant, a love of astro-
nomical observation in his nephew. The two
workeT wor^e(^ together, entering their observations one
with him. after the other on the same paper; and it was
to the pair of them together, rather than to the
uncle alone, that Newton made his princely pre-

BRADLEY'S DISCOVERIES 91

sents, and Halley wrote for help in his observa-
tions. There seems to be no doubt that the uncle
and nephew were about this time the best astro-
nomical observers in the world. There was no
rivalry between them, and therefore there is no
need to discuss whether the partnership was one
of equal merit on both sides ; but it is interesting
to note that it probably was. The ability of
Pound was undoubted; many were keenly de-
sirous that he, and not his nephew, should be
elected to the Oxford Chair in 1721, but he felt
unequal to the duties at his advanced age. On
the other hand, when Bradley lost his uncle's
help, there was no trace of faltering in his steps
to betray previous dependence on a supporting
or guiding hand. He walked erect and firm, and
trod paths where even his uncle might not have
been able to follow.

A few instances will suffice to show the kind The work
of observations made by this notable firm of Pound7
Pound and Bradley. They observed the positions Bradley.
of the fixed stars and nebulse : these being gene-
rally the results required by Halley and Newton.
They also observed the places of the planets
among the stars, and especially the planet Mars,
and determined its distance from the Earth by
the method of parallax, thus anticipating the
modern standard method of finding the Sun's
distance ; and though with their imperfect instru-
ments they did not obtain a greater accuracy than
i in 10, still this was a great advance on what

92 ASTRONOMICAL DISCOVERY

had been done before, and excited the wonder
and admiration of Halley. They also paid some
attention to double stars, and did a great deal of
work on Jupiter's satellites. We might profitably
linger over the records of these early years, which
are full of interest, but we must press on to the
time of the great discoveries, and we will dismiss
them with brief illustrations of three points :
Bradley's assiduity, his skill in calculation, and
his wonderful skill in the management of instru-
ments. Of his assiduity an example is afforded
by his calculations of the orbits of two comets
which are still extant. One of them fills thirty-
two pages of foolscap, and the other sixty; and
it must be remembered that the calculations them-
selves were quite novel at that time. Of his skill
in calculation, apart from his assiduity, we have
a proof in a paper communicated to the Royal
Society rather later (1726), where he determines
the longitudes of Lisbon and New York from the
eclipses of Jupiter's satellites, using observations
which were not simultaneous, and had therefore
to be corrected by an ingenious process which
Bradley devised expressly for this purpose. And
Use of finally, his skill in the management of instruments
rexy long -g s]lown by nis measuring the diameter of the
scopes, planet Venus with a telescope actually 2 1 2 J feet
in length. It is difficult for us to realise in these
days what this means ; even the longest telescope
of modern times does not exceed 100 feet in
length, and it is mounted so conveniently with

BRADLEY'S DISCOVERIES 93

all the resources of modern engineering, in the
shape of rising floors, &c., that the management
of it is no more difficult than that of a iofoot
telescope. But Bradley had no engineering appli-
ances beyond a pole to hold up one end of the
telescope and his own clever fingers to work the
other; and he managed to point the unwieldy
weapon accurately to the planet, and measure the
diameter with an exactness which would do credit
to modern times. A few words of explanation Keason
may be given why such long telescopes were used
at all. The reason lay in the difficulty of getting
rid of coloured images, due to the composite
character of white light. Whenever we use a
single lens to form an image, coloured fringes
appear. Nowadays we know that by making
two lenses of different kinds of glass and putting
them together, we can practically get rid of these
coloured fringes ; but this discovery had not been
made in Bradley's time. The only known ways
of dealing with the evil then were to use a reflect-
ing telescope like Newton and Gregory, or if a
lens was used, to make one of very great focal
length ; and hence the primary necessity for these
very long telescopes. They had another advan-
tage in producing a large image, or they would
probably have given way to the reflector. This
advantage is gradually bringing them back into
use, and perhaps in the eclipse of 1905 we may
use a telescope as long as Bradley's ; but we shall
not use it as he did in any case. It will be laid

94 ASTRONOMICAL DISCOVERY

comfortably flat on the ground, and the rays of
light reflected into it by a coelostat.

Bradley In 1721 Bradley was appointed to the Savilian
at^xford, Professorship of Astronomy at Oxford, vacant by
the death of Dr. John Keill. Once it became
clear that there was no chance of securing his
uncle for this position, Bradley himself was sup-
ported enthusiastically by all those whose support
was worth having, especially by the Earl of
Macclesfield, who was then Lord Chancellor;
by Martin Foulkes, who was afterwards the
President of the Royal Society ; and by Sir
Isaac Newton himself. He was accordingly
elected on October 31, 1721, and forthwith re-
signed his livings. His resignation of the livings
was necessitated by a definite statute of the Uni-
versity relating to the Professorship, and not by
the existence of any very onerous duties attach-
ing to it ; indeed such duties seem to have been
conspicuously absent, and after Bradley's election
but con- he passed more time than ever with his uncle in
work at Wansted, making the astronomical observations
Wansted. ^^ both loved ; for there was not the vestige
of an observatory in Oxford. His uncle's death
in 1724 interrupted the continuity of these joint
observations, and by an odd accident prepared the
way for Bradley's great discovery. He was fain
to seek elsewhere that companionship in his work
which had become so essential to him, and his
new friend gave a new bent to his observations.
Samuel Molyneux was a gentleman of fortune

BRADLEY'S DISCOVERIES 95

much attached to science, and particularly to Samuel
astronomy, who was living about this time at jjux.
Kew. He was one of the few, moreover, who
are not content merely to amuse themselves
with a telescope, but had the ambition to do
some real earnest work, and the courage to
choose a problem which had baffled the human
race for more than a century. The theory
of Copernicus, that the earth moved round
the sun, necessitated a corresponding apparent
change in the places of the stars, one relatively
to another; and it was a standing difficulty in
the way of accepting this theory that no such
change could be detected. In the old days
before the telescope it was perhaps easy to
understand that the change might be too small
to be noticed, but the telescope had made it
possible to measure changes of position at least
a hundred times as small as before, and still no
" parallax/' as the astronomical term goes, could
be found for the stars. The observations of
Galileo, and the measures of Tycho Brans', as
reduced to systematic laws by Kepler, and finally
by the great Newton, made it clear that the
Copernican theory was true : but no one had
succeeded in proving its truth in this particular
way. Samuel Molyneux must have been a man
of great courage to set himself to try to crack
this hard nut ; and we can understand the attrac- Attempts
tion which his enterprise must have had for s£eiiar
Bradley, who had just lost the beloved colleague of Parallax-

96 ASTRONOMICAL DISCOVERY

many courageous astronomical undertakings. His
co-operation seems to have been welcomed from
the first ; his help was invited and freely given
in setting up the instrument, and he fortunately
had the leisure to spend considerable time at Kew
making the observations with Molyneux, just as
he had been wont to observe with his uncle.

I must now briefly explain what these observa-
tions were. There is a bright star y Draconis,
which passes almost directly overhead in the lati-
tude of London. Its position is slowly changing
owing to the precession of the equinoxes, but for
two centuries it has been, and is still, under con-
stant observation by London astronomers owing
to this circumstance, that it passes directly over-
head, and so its position is practically undisturbed
by the refraction of our atmosphere.

It was therefore thought at the time that, there
being no disturbance from refraction, the disturb-
ance from precession being accurately known, and
there being nothing else to disturb the position
but "parallax" (the apparent shift due to the
earth's motion which it was desirable to find),
this star ought to be a specially favourable object
for the determination of parallax. Indeed it had
been announced many years before by Hooke that
its parallax had been found ; but his observations
were not altogether satisfactory, and it was with
a view of either confirming them or seeing what
was wrong with them that Molyneux and Bradley
started their search. They set up a much more

BRADLEY'S DISCOVERIES 97

delicate piece of apparatus than Hooke had em-
ployed. It was a telescope 24 feet long pointed The m-
vertically upwards to the star, and firmly attached
to a large stack of brick chimneys within the
house. The telescope was not absolutely fixed, for
the lower end could be moved by a screw so as to
make it point accurately to the star, and a plumb-
line showed how far it was from the vertical when
so pointing. Hence if the star changed its posi-

Fio. 2.

tion, however slightly, the reading of this screw
would show the change. Now, before setting out Expected
On the observations, the observers knew what to r(
expect if the star had a real parallax ; that is to
say, they knew that the star would seem to be
farthest south in December, farthest north in
June, and at intermediate positions in March and
September ; though they did not know how much
farther south it would appear in December than
in June — this was exactly the point to be decided.

98 ASTRONOMICAL DISCOVERY

The reason of this will be clear from Fig. 2.
[Remark, however, that this figure and the cor-
responding figure 4 do not represent the case of
Bradley's star, 7 Draconis : another star has been
chosen which simplifies the diagram, though the
principle is essentially the same.] Let A B O D
represent the earth's orbit, the earth being at
A in June, at B in September, and so on,
and let K represent the position of the star on
the line D B. Then in March and September
it will be seen from the earth in the same
direction, namely, D B K ; but the directions
in which it is seen in June and December, viz.
A K and C K, are inclined in opposite ways
to this line. The farther away the star is, the
less will this inclination or " parallax " be ; and
the star is actually so far away that the inclina-
tion can only be detected with the utmost diffi-
culty : the lines C K and A K are sensibly
parallel to D B K. But Bradley did not know
this; it was just:this point which he was to
examine, and he expected the greatest inclina-
tion in one direction to be in December. Accord-
ingly when a few observations had been made
on December 3, 5, n, and 12 it was thought that
the star had been caught at its most southerly
apparent' position, and might be expected there-
after to move northwards, if at all. But when
Unex- Bradley repeated the observation on December
resuhl :7» ne found to his great surprise that the star
was still moving southwards. Here was some-

BRADLEY'S DISCOVERIES 99

thing quite new and unexpected, and such a
keen observer as Bradley was at once on the
alert. He soon found that the changes in the
position of the star were of a totally unex-
pected character. Instead of the extreme posi-
tions being occupied in June and December,
they were occupied in March and September,
just midway between these. And the range
in position was quite large, about 40" — not a
quantity which could have been detected in
the days before telescopes, but one which was
unmistakable with an instrument of the most
moderate measuring capacity.

What, then, was the cause of this quite unfore-
seen behaviour on the part of the star ? The first Tentative
thought of the observers was that something might tk>n&]na
be wrong with their instrument, and it was care-
fully examined, but without result The next was
that the apparent movement was in the plumb-
line, the line of reference. If the whole earth,
instead of carrying its axis round the sun in a
constant direction, were to be executing an oscil-
lation, then all our plumb-lines would oscillate,
and when the direction of a star like 7 Draconis
was compared with that of the plumb-line it
would seem to vary, owing actually to the varia-
tion in the plumb-line. The earth might have
a motion of this kind in two ways, which it will
be necessary for us to distinguish, and the adopted
names for them are " nutation of the axis " and
" variation of latitude " respectively. In the case

ioo ASTRONOMICAL DISCOVERY

of nutation the North Pole remains in the same
geographical position, but points to a different
part of the heavens. The "variation of lati-
tude," on the other hand, means that the North
Pole wanders about on the earth itself. We
shall refer to the second phenomenon more par-
ticularly in the sixth chapter.

Nutation ? But it was the first kind of change, the nutation,
which Bradley suspected ; and very early in the
series of observations he had already begun to
test this hypothesis. If it was not the star, but
the earth and the plumb-line, which were in
motion, then other stars ought to be affected.
The telescope had been deliberately restricted in
its position to suit 7 Draconis ; but since the stars
circle round the Pole, if we draw a narrow belt in
the heavens with the Pole as centre, and includ-
ing 7 Draconis, the other stars included would
make the same circuit, preceding or following
7 Draconis by a constant interval. Most of them
would be too faint for observation with Bradley's
telescope ; but there was one bright enough to
be observed, which also came within its limited
range, and it was promptly put under surveillance
when a nutation of the earth's axis was suspected.
Careful watching showed that it was not affected
in the saine way as 7 Draconis, and hence the
movement could not be in the plumb-line. Was
there, then, after all, some effect of the earth's
atmosphere which had been overlooked ? We
have already remarked that since the star passes

BRADLEY'S DISCOVERIES

101

directly overhead there should be practically no
refraction ; and this assumption was made by
Molyneux and Bradley in choosing this parti-
cular star for observation. It follows at once, if
we assume that the atmosphere surrounds the
earth in spherical layers. But perhaps this was Anoma-
not so ? Perhaps, on the contrary, the atmos- fraction.
phere was deformed by the motion of the earth,
streaming out behind her like the smoke of a
moving engine ? No possibility must be over-

Atmospfcere/

Earth*

FIG. 3.

looked if the explanation of this puzzling fact
was to be got at.

The way in which a deformation of the atmos-
phere might explain the phenomenon is best seen
by a diagram. First, it must be remarked that
rays of light are only bent by the earth's atmos-
phere, or ''refracted," if they enter it obliquely.

If the atmosphere were of the same density
throughout, like a piece of glass, then a vertical
ray of light, A B (see Fig. 3), entering the
atmosphere at B would suffer no bending or

102 ASTRONOMICAL DISCOVERY

refraction, and a star shining from the direction
A B would be seen truly in that direction from
C. But an oblique ray, D E, would be bent on
entering the atmosphere at E along the path
EF, and a star shining along D E would appear
from F to be shining along the dotted line G E F.
The atmosphere is not of the same density
throughout, but thins out as we go upwards from

Apparent

the earth ; and in consequence there is no clear-
cut surface, B E, and no sudden bending of the
rays as at E : they are gradually bent at an in-
finite succession of imaginary surfaces. But it
still remains true that there is no bending at all
for vertical rays ; and of oblique rays those most
oblique are most bent.

Now, suppose the atmosphere of the earth took
up, owing to its revolution round the sun, an
elongated shape like that indicated in diagram 4,

BRADLEY'S DISCOVERIES 103

and suppose the star to be at a great distance away
to the right of the diagram. When the earth is in
the position labelled "June," the light would fall
vertically on the nose of the atmosphere at A,
and there would be no refraction. Similarly in
"December" the light would fall at C on the
stern, also vertically, and there would be no
refraction. [The rays from the distant star in
December are to be taken as sensibly parallel to
those received in June, notwithstanding that the
earth is on the opposite side of the sun, as was
remarked on p. 98.] But in March and Sep-
tember the rays would strike obliquely on the
sides of the supposed figure, and thus be bent in
opposite directions, as indicated by the dotted
lines ; and the extreme positions would thus
occur in March and September, as had been
observed. The explanation thus far seems satis-
factory enough.

But we have assumed the star to lie in the
plane of the earth's orbit; and the stars under
observation by Bradley did not lie in this plane,
nor did they lie in directions equally inclined to
it. Making the proper allowance for their direc-
tions, it was found impossible to fit in the facts
with this hypothesis, which had ultimately to be
abandoned.

It is remarkable to find that two or three years Delay m
went by before the real explanation of this new reai^f-
phenomenon occurred to Bradley, and during this Planatlon-
time he must have done some hard thinking.

io4 ASTRONOMICAL DISCOVERY

We have all had experience of the kind of think-
ing if only in the guessing of conundrums. We
know the apparent hopelessness of the quest at
the outset : the racking of our brains for a clue,
the too frequent despair and "giving it up," and
the simplicity of the answer when once it is
declared. But with scientific conundrums the
expedient of " giving it up " is not available. We
must find the answer for ourselves or remain in
ignorance ; and though we may feel sure that the
answer when found will be as simple as that to
the best conundrum, this expected simplicity does
not seem to aid us in the search. Bradley was
not content with sitting down to think : he set to
work to accumulate more facts. Molyneux's
instrument only allowed of the observation of
two stars, 7 Draconis and the small star above
Bradley mentioned. Bradley determined to have an
another instrument of his own which should command
menTat a w^er range of stars ; and by this time he was
Wansted. able to return to his uncle's house at Wansted for
this purpose. His uncle had been dead for two
or three years, and the memory of the loss was
becoming mellowed with time. His uncle's widow
was only too glad to welcome back her nephew,
though no longer to the old rectory, and she
allowed him to set up a long telescope, even
though he cut holes in her floor to pass it through.
The object-glass end was out on the roof and the
eye end down in the coal cellar ; and accordingly in
this coal cellar Bradley made the observations which

BRADLEY'S DISCOVERIES 105

led to his immortal discovery. He had a list of
seventy stars to observe, fifty of which he observed
pretty regularly. It may seem odd that he did
not set up this new instrument at Oxford, but
we find from an old memorandum that his pro-
fessorship was not bringing him in quite ^140 a
year, and probably he was glad to accept his
aunt's hospitality for reasons of economy. By
watching these different stars he gradually got
a clear conception of the laws of aberration.
The real solution of the problem, according to
a well-authenticated account, occurred to him
almost accidentally. We all know the story of the Finds the
apple falling and setting Newton to think about "
the causes of gravitation. It was a similarly
trivial circumstance which suggested to Bradley
the explanation which he had been seeking for
two or three years in vain. In his own words,
" at last, when he despaired of being able to
account for the phenomena which he had observed,
a satisfactory explanation of them occurred to
him all at once when he was not in search of
it." He accompanied a pleasure party in a sail
upon the river Thames. The boat in which they
were was provided with a mast which had a vane
at the top of it. It blew a moderate wind, and
the party sailed up and down the river for a con-
siderable time. Dr. Bradley remarked that every
time the boat put about the vane at the top of the A wind-
boat's mast shifted a little, as if there had been a
slight change in the direction of the wind. He

io6 ASTRONOMICAL DISCOVERY

observed this three or four times without speak-
ing ; at last he mentioned it to the sailors, and
expressed his surprise that the wind should shift
so regularly every time they put about. The
sailors told him that the wind had not shifted, but
that the apparent change was owing to the change
in the direction of the boat, and assured him that
the same thing invariably happened in all cases.
This accidental observation led him to conclude

->0

FIG. 5.

that the phenomenon which had puzzled him so
much was owing to the combined motion of light
and of the earth. To explain exactly what is
meant we must again have recourse to a diagram ;
and we may also make use of an illustration which
has become classical.

If rain is falling vertically, as represented by
the direction A B ; and if a pedestrian is walking
horizontally, in the direction C D, the rain will
appear to him to be coming in an inclined
direction, E F, and he will find it better to tilt his
umbrella forwards. The quicker his pace the
more he will find it advisable to tilt the umbrella.
This analogy was stated by Lalande before the

BRADLEY'S DISCOVERIES 107

days of umbrellas in the following words: "Je
suppose que, dans un temps calme, la pluie tombe
perpend iculairement, et qu'on soit dans une
voiture ouverte sur le devant ; si la voiture est en
repos, on ne re§oit pas la moindre goutte de
pluie ; si la voiture avance avec rapidite, la pluie
entre sensiblement, comme si elle avoit pris une
direction oblique." Lalande's example, modified
to suit modern conditions, has been generally
adopted by teachers, and in examinations candi-
dates produce graphic pictures of the stationary,
the moderate-paced, and the flying, possessors of
umbrellas.

Applying it to the phenomenon which it is
intended to illustrate, if light is being received
from a star by an earth, travelling across the
direction of the ray, the telescope (which in this
case represents the umbrella) must be tilted for-
ward to catch the light. Now on reference to
Fig. 4 it will be seen that the earth is travelling
across the direction of rays from the star in
March and September ; and in opposite directions
in the two cases. Hence the telescope must be
tilted a little, in opposite directions, to catch the
light ; or, in other words, the star will appear to
be farthest south in March, farthest north in
September. And so at last the puzzle was solved,
and the solution was found, as so often happens,
to be of the simplest kind ; so simple when once
we know, and so terribly hard to imagine when
we don't ! It may comfort us in our struggles

1 08 ASTRONOMICAL DISCOVERY

with minor problems to reflect that Bradley man-
fully stuck to his problem for two or three years.
It was probably never out of his thoughts,
waking or sleeping ; when at work it was the
chief object of his labours, and when on a
pleasure party he was ready to catch at the
slightest clue, in the motion of a wind-vane on
a boat, which might help him to the solution.
Results of The discovery of aberration made Bradley
' famous at a bound. Oxford might well be proud
of her two Savilian Professors at this time, for
they had both made world-famous discoveries—
Halley that of the periodicity of comets, and
Bradley of the aberration of light. How dif-
ferent their tastes were and how difficult it
would have been for either to do the work of
the other ! Bradley was no great mathematician,
and though he was quite able to calculate the
orbit of a comet, and carried on such work when
Halley left it, it was probably not congenial to
him. Halley, on the other hand, almost despised
accurate observations as finicking. " Be sure you
are correct to a minute," he was wont to say,
" and the fractions do not so much matter."
With such a precept Bradley would never have
made his discoveries. No quantity was too small
in his eyes, and no sooner was the explanation
of aberration satisfactorily established than he
perceived that though it would account for the
main facts, it would not explain all. There was
something left. This is often the case in the

BRADLEY'S DISCOVERIES 109

history of science. A few years ago it was thought
that we knew the constitution of our air com-
pletely — oxygen, nitrogen, water vapour, and
carbonic acid gas; but a great physicist, Lord
Rayleigh, found that after extracting all the
water and carbonic acid gas, all the oxygen and
all the nitrogen, there was something left — a
very minute residuum, which a careless experi-
menter would have overlooked or neglected, but
which a true investigator like Lord Rayleigh saw
the immense importance of. He kept his eye on
that something left, and presently discovered a
new gas which we now know as argon. Had he
repeated the process, extracting all the argon
after the nitrogen, he might have found by a
scrutiny much more accurate still yet another gas,
helium, which we now know to exist in extremely
minute quantities in the air. But meantime this
discovery was made in another way.

When Bradley had extracted all the aberration stiiisome-
from his observations he found that there was be JL °
something left, another problem to be solved and pla
some more thinking to be done to solve it. But
he was now able to profit by his previous labours,
and the second step was made more easily than
the first. The residuum was not the parallax
of which he had originally been in search, for
it did not complete a cycle within the year; it
was rather a progressive change from year to
year. But there was an important clue of another
kind. He saw that the apparent movements of

no ASTRONOMICAL DISCOVERY

all stars were in this case the same ; and he
knew that a movement of this kind can be
referred, not to the stars themselves, but to
the plumb-line from which their directions are
measured. He had thought out the possible
causes of such a movement of the plumb-line or
of the earth itself, and had realised that there
Probably might be a nutation which would go through a
cycle in about nineteen years, the period in which
the moon's nodes revolve. He was not mathe-
matician enough to work out the cause completely,
but he saw clearly that to trace the whole effect
he must continue the observations for nineteen
years ; and accordingly he entered on this long
campaign without any hesitation. His instru-
ment was still that in his aunt's house atWansted,
where he continued to live and make the obser-
vations for a few years, but in 1732 he removed
to Oxford, as we shall see, and he must have
made many journeys between Wansted and
Oxford in the course of the remaining fifteen
years during which he continued to trace out the
effects of nutation. His aunt too left Wansted
to accompany Bradley to Oxford, and the house
His nine- passed into other hands. It is to the lasting
campaign, credit of the new occupant, Mrs. Elizabeth
Williams,- that the great astronomer was allowed
to go on and complete the valuable series of
observations which he had commenced. Bradley
was not lodged in her house ; he stayed with a
friend close by on his visits to Wansted, but

BRADLEY'S DISCOVERIES in

came freely in and out of his aunt's old home
to make his observations. How many of us are
there who would cheerfully allow an astronomer
to enter our house at any hour of the night to
make observations in the coal-cellar ! It says much,
not only for Bradley's fame, but for his personal
attractiveness, that he should have secured this
permission, and that there should be no record
of any friction during these fifteen years. At the
end of the whole series of nineteen years his
conclusions were abundantly verified, and his
second great discovery of nutation was established.
Honours were showered upon him, and no doubt
the gentle heart of Mrs. Elizabeth Williams was
uplifted at the glorious outcome of her long for-
bearance.

But we may now turn for a few moments from
Bradley's scientific work to his daily life. We
have said that in 1732, after holding his profes-
sorship for eleven years, he first went definitely to
reside in Oxford. He actually had not been able Residence

re , . • i IT- • i at Oxford.

to anord it previously. His income was only
^140 a year, and the statutes prevented him from
holding a living : so that he was fain to accept
Mrs. Pound's hospitable shelter. But in 1729 an
opportunity of adding to his income presented
itself, by giving lectures in " experimental philo-
sophy." The observations on nutation were not
like those on aberration : he was not occupied
day and night trying to find the solution : he had
practically made up his mind about the solution,

ii2 ASTRONOMICAL DISCOVERY

and the actual observations were to go on in a
quiet methodical manner for nineteen years, so
that he now had leisure to look about him for

other employment. Dr. Keill, who had been Pro-
fessor of Astronomy before Bradley, had attracted
large classes to lectures, not on astronomy, but on
experimental philosophy : but had sold his ap-
paratus and goodwill to Mr. Whiteside, of Christ
Church, one of the candidates who were disap-
pointed by Bradley's election. In 1729 Bradley
purchased the apparatus from Whiteside, and
began to give lectures in experimental philosophy.
His discovery of aberration had made him famous,
so that his classes were large from the first, and
paid him considerable fees. Suddenly therefore
he changed his poverty for a comfortable income,
and he was able to live in Oxford in one of two
red brick houses in New College Lane, which
were in those days assigned to the Savilian Pro-
fessors (now inhabited by New College under-
graduates). His aunt, Mrs. Pound, to whom he
was devotedly attached, came with him, and two
of her nephews. In his time of prosperity Bradley
was thus able to return the hospitality which had
been so generously afforded him in times of stress.
Astro- Before he completed his observations for nuta-

tion another great change in his fortunes took
place. In 1742 he was elected to succeed Halley
as Astronomer Royal. It was Halley's dying
wish that Bradley should succeed him, and it is
said that he was even willing to resign in his

BRADLEY'S DISCOVERIES 113

•— *

favour, for his right hand had been attacked by
paralysis, and the disease was gradually spreading.
But he died without any positive assurance that
his wish would be fulfilled. The chief difficulty
in securing the appointment of Bradley seems to
have been that he was the obvious man for the
post in universal opinion. " It is not only my
friendship for Mr. Bradley that makes me so Letter
ardently wish to see him possessed of the posi- Earl of
tion," wrote the Earl of Macclesfield to the Lord
Chancellor ; " it is my real concern for the
honour of the nation with regard to science. For
as our credit and reputation have hitherto not
been inconsiderable amongst the astronomical
part of the world, I should be extremely sorry we
should forfeit it all at once by bestowing upon a
man of inferior skill and abilities the most hon-
ourable, though not the most lucrative, post in
the profession (a post so well filled by Dr. Halley
and his predecessor), when at the same time we
have amongst us a man known by all the foreign,
as well as our own astronomers, not to be inferior
to either of them, and one whom Sir Isaac Newton
was pleased to call the best astronomer in Europe."
And again, " As Mr. Bradley' s abilities in astro-
nomical learning are allowed and confessed by
all, so his character in every respect is so well
established, and so unblemished, that I may defy
the worst of his enemies (if so good and worthy a
man have any) to make even the lowest or most

trifling objection to it."

ii4 ASTRONOMICAL DISCOVERY

" After all," the letter goes on, " it may be said
if Mr. Bradley's skill is so universally acknow-
ledged, and his character so established, there is
little danger of opposition, since no competitor
can entertain the least hope of success against
him. But, my lord, we live in an age when most
men how little soever their merit may be, seem to
think themselves fit for whatever they can get,
and often meet with some people, who by their
recommendations of them appear to entertain the
same opinion of them, and it is for this reason
that I am so pressing with your lordship not to
lose any time."

Such recommendations had, however, their
effect : the dreaded possibility of a miscarriage of
justice was averted, and Bradley became the third
Astronomer Royal, though he did not resign his
professorship at Oxford. Halley, Bradley, and
Bliss, who were Astronomers Royal in succession,
all held the appointment along with one of the
Savilian professorships at Oxford ; but since the
death of Bliss in 1761, the appointment has
always gone to a Cambridge man.

instru- When Bradley went to Greenwich, in June

veryde- 1742> ne was at first unable to do much from the

3tive. wretched, state in which he found the instruments.

Halley was not a good observer : his heart was

not in the work, and he had not taken the trouble

to set the instruments right when they went wrong.

The counterpoises of that instrument which ought

to have been the best in the world at the time

BRADLEY'S DISCOVERIES 115

rubbed against the roof so that the telescope could
scarcely be moved in some positions : and some
of the screws were broken. There was no proper
means of illuminating the cross-wires, and so on.
With care and patience Bradley set all this right,
and began observations. He had the good fortune
to secure the help of his nephew, John Bradley,
as assistant, and the companionship seems to have
been as happy as that previous one of James Bradley
and his uncle Pound. John Bradley was able to
carry on the observations when his uncle was absent
in Oxford, and the work the two got through
together in the first year is (in the words of
Bradley's biographer Rigaud) " scarcely to be
credited." The transit observations occupy 177
folio pages, and no less than 255 observations
were taken on one night. And at the same time,
it must be remembered, Bradley was still carrying
on his nutation observations at Wansted, still
lecturing at Oxford, and not content with all this,
began a course of experiments on the length of
the seconds' pendulum. Truly a giant for hard
work!

But, in spite of his care in setting them right,
the instruments in the Observatory were found to
be hopelessly defective. The history of the in-
struments at the Royal Observatory is a curious
one. When Flamsteed was appointed the first
Astronomer Royal he was given the magnificent
salary of £100 a year, and no instruments to
observe with. He purchased some instruments

n6 ASTRONOMICAL DISCOVERY

with his own money, and at his death they were
claimed by his executors. Hence Halley, the
second Astronomer Royal, found the Observatory
totally unprovided in this respect. He managed
to persuade the nation to furnish the funds for an
equipment ; but Halley, though a man of great
ability in other ways, did not know a good instru-
ment from a bad one ; so that Bradley's first few
years at the Observatory were wasted owing to the
imperfection of the equipment. When this was
New in- fully realised he asked for funds to buy new

struments. . -i i i n i c t

instruments, and such was the confidence felt
in him that he got what he asked for without
much difficulty. More than ^1000, a large sum
for those days, was spent under his direction,
the principal purchases being two quadrants for
observation of the position of the stars, one to
the north and the other to the south. With
these quadrants, which represented the perfection
of such apparatus at that time, Bradley made
that long and wonderful series of observations
which is the starting-point of our knowledge of
the movements of the stars. The instruments
are still in the Royal Observatory, the more
important of the two in its original position as
Bradley mounted it and left it.

Work at It seems needless to mention his work as
Astronomer Royal, but I will give quite briefly
a summary of what he accomplished, and then
recall a particular incident, which shows how far
ahead of his generation his genius for observa-

BRADLEY'S DISCOVERIES 117

tion placed him. The summary may be given as
follows. We owe to Bradley —

1. A better knowledge of the movements of
Jupiter's satellites.

2. The orbits of several comets calculated
directly from his own observations, when such
work was new and difficult.

3. Experiments on the length of the pendulum.

4. The foundation of our knowledge of the
refraction of our atmosphere.

5. Considerable improvements in the tables of
the moon, and the promotion of the method for
finding the longitude by lunar distances.

6. The proper equipment of our national Ob-
servatory with instruments, and the use of these
to form the basis of our present knowledge of
the positions and motions of the stars.

Many men would consider any one of these six
achievements by itself a sufficient title to fame.
Bradley accomplished them all in addition to his
great discoveries of aberration and nutation.

And with a little more opportunity he might Might
have added another great discovery which has found

shed lustre on the work of the last decade. We
said earlier in this chapter that the axis of the tude>
earth may move in one or two ways. Either it
may point to a different star, remaining fixed
relatively to the earth, as in the nutation which
Bradley discovered ; or it may actually change its
position in the earth. This second kind of move-
ment was believed until twenty years ago not to

nS ASTRONOMICAL DISCOVERY

exist appreciably ; but the work of Kiistner and
Chandler led to the discovery that it did exist,
and its complexities have been unravelled, and
will be considered in the sixth chapter. Now a
century and a half ago Bradley was on the track
of this "variation of latitude." His careful obser-
vations actually showed the motion of the pole,
as Mr. Chandler has recently demonstrated ; and,
moreover, Bradley himself noticed that there was
something unexplained. Once again there was
a residuum after (first) aberration and (next)
nutation had been extracted from the observa-
tions ; and with longer life he might have ex-
plained this residuum, and added a third great
discovery to the previous two. Or another coming
after him might have found it ; but after the giant
came men who could not tread in his footsteps,
and the world waited 150 years before the dis-
crepancy was explained.

The attitude of our leading universities towards
science and scientific men is of sufficient import-
ance to justify another glance at the relations
Oxford's between Bradley and Oxford. We have seen that
recogni- Oxford's treatment of Bradley was not altogether
Bradley, satisfactory. She left him to learn astronomy as
he best cQuld, and he owes no teaching to her.
She made him Professor of Astronomy, but gave
him no observatory and a beggarly income which
he had to supplement by giving lectures on a
different subject. But when he had disregarded
these discouragements and made a name for him-

BRADLEY'S DISCOVERIES 119

self, Oxford took her share in recognition. He
was created D.D. by diploma in 1742 ; and when
his discovery of nutation was announced in 1 748,
and produced distinctions and honours of all kinds
from over the world, we are are told that " amidst
all these distinctions, wide as the range of modern
science, and permanent as its history, there was
one which probably came nearer his heart, and
was still more gratifying to his feeling than all.
Lowth (afterwards Bishop of London), a popular
man, an elegant scholar, and possessed of con-
siderable eloquence, had in 1751 to make his last
speech in the Sheldonian Theatre at Oxford as
Professor of Poetry. In recording the benefits
for which the University was indebted to its
benefactors, he mentioned the names of those
whom Sir Henry Savile's foundation had estab-
lished there : * What men of learning ! what
mathematicians ! we owe to Savile, Briggs,
Wallis, Halley ; to Savile we owe Greaves,
Ward, Wren, Gregory, Keill, and one whom I
will not name, for posterity will ever have his
name on its lips.' Bradley was himself present ;
there was no one in the crowded assembly on
whom the allusion was lost, or who did not feel
the truth and justice of it ; all eyes were turned
to him, while the walls rung with shouts of
heartfelt affection and admiration ; it was like the
triumph of Themistocles at the Olympic games."

These words of Eigaud indicate the fame
deservedly acquired by an earnest and simple-

120 ASTRONOMICAL DISCOVERY

minded devotion to science : but can we learn
The study anything from the study of Bradley's work to
siduai guide us in further research ? The chief lessons
mena°" would seem to be that if we make a series of
careful observations, we shall probably find some
deviation from expectation : that we must follow
up this clue until we have found some explana-
tion which fits the facts, not being discouraged if
we cannot hit upon the explanation at once, since
Bradley himself was puzzled for several years :
that after finding one vera causa, and allowing
for the effect of it, the observations may show
traces of another which must again be patiently
hunted, even though we spend nineteen years in
the chase : and that again we may have to leave
the complete rectification of the observations to
posterity. But though we may admit the general
helpfulness of these directions, and that this
patient dealing with residual phenomena seems
to be a method capable of frequent application,
we cannot deduce any universal principle of pro-
cedure from them : witness the difficulty of deal-
ing with meteorological observations, for instance.
It is not always possible to find any orderly
arrangement of the residuals which will give us
a clue to start with. When such an arrangement
is manifested, we must certainly follow up the
clue ; it would almost seem that no expense
should be prohibitive, since it is impossible to
foresee the importance of the result.

CHAPTER IV
ACCIDENTAL DISCOVERIES

IN reviewing various types of astronomical dis-
covery I have laid some stress upon the fact that
they are, generally speaking, far from being acci-
dental in character. A new planet does not
" swim into our ken," at any rate not usually, but
is found only after diligent search, and then only
by an investigator of acute vision, or other special
qualifications. But this is, of course, not always
the case. Some discoveries are made by the
merest accident, as we have had occasion to
remark incidentally in the case of the minor
planets ; and for the sake of completeness it is
desirable to include among our types at least one
case of such accidental discovery. As, however,
the selection is a little invidious, I may perhaps
be pardoned for taking the instance from my own
experience, which happens to include a case where
one of those remarkable objects called " new stars "
walked deliberately into a net spread for totally The Ox-
different objects. There is the further reason for star D
choosing this instance : that it will afford me the
opportunity of saying something about the special
research in which we were actually engaged, the
work of mapping out the heavens by photography,

122 ASTRONOMICAL DISCOVERY

found or, as it has been called, the Astrographic Chart —
workon a great scheme of international co-operation by
graphic which it is hoped to leave as a legacy for future
chart. centuries a record of the state of the sky in our
age. Such a record cannot be complete ; for how-
ever faint the stars included, we know that there
are fainter stars, which might have been included
had we given longer exposures to the plates. Nor
can it be in other ways final or perfect ; however
large the scale, for instance, on which the map is
made, we can imagine the scale doubled or increased
many-fold. But the map will be a great advance
on anything that has hitherto been made, and some
account of it will therefore no doubt be of interest.
Origin of We may perhaps begin with a brief historical
lrt< account of the enterprise. Photographs of the
stars were taken many years ago, but only by a few
enthusiasts, and with no serious hope of competing
with eye observations of the sky. The old wet-
plate photography was, in fact, somewhat unsuited
to astronomical purposes ; to photograph faint
objects a long exposure is necessary, and the wet
plate may dry up before the exposure is concluded
— nay, even before it is commenced, if the observer
has to wait for passing clouds — and therefore it
may be said that the successful application of
photography to astronomy dates ,from the time
when the dry plate was invented ; when it became
possible to expose a plate in the telescope for
hours, or by accumulation even for days. The
dry plate remains sensitive for a long period, and
if it is desired to extend an exposure beyond the

VII.— GREAT COMET OF Nov. 7TH, 1882.

(From a photograph taken at the Royal Observatory ; Cape of Good Hope.)

ACCIDENTAL DISCOVERIES 123

limits of one night, it is quite easy to close up the
telescope and return to the operations again on
the next fine night ; and so on, if not perhaps
indefinitely, at any rate so long as to transcend
the limits of human patience up to the present.

But to consider our particular project. We
may assign, perhaps, the date 1882 as that incometof
which it first began to take shape. In that year I(
there was a magnificent bright comet, the last
really large comet which we, in the Northern
Hemisphere, have had the good fortune to see.
Some of us, of course, were not born at that time,
and perhaps others who were alive may neverthe-
less not have seen that comet ; for it kept somewhat
uncomfortably early morning hours, and I can well
remember myself feeling rather more resentment
than gratitude to the man who waked me up
about four o'clock to see it. Many observations
were of course made of this interesting visitor,
and what specially concerns us is that at the Cape
of Good Hope some enterprising photographers
tried to photograph it. They tried in the first
instance with ordinary cameras, and soon found —
what any astronomer could have told them — that
the movement of the earth, causing an apparent
movement of the comet and the stars in the
opposite direction, frustrated their efforts. The
difficulties of obtaining pictures of moving objects
are familiar to all photographers. A " snap-shot"
might have met the difficulty, but the comet was
scarcely bright enough to affect the plate with a
short exposure. Ultimately Dr. David Gill, the

i24 ASTRONOMICAL DISCOVERY

astronomer at the Cape Observatory, invited one
of the photographers to strap his camera to one
of the telescopes at the Observatory, a telescope
which could be carried round by clockwork in the
usual way, so as to counteract the earth's motion,
and in effect to keep the comet steadily in view,
as though it were at rest. As a consequence,
stars some very beautiful and successful pictures of the
thepie-n cornet were obtained, and on them a large number
tures. o£ s^ars were also shown. They were, as I have
said, not by any means the first pictures of stars
obtained by photography, but they represented in
facility and in success so great an advance upon
what had been formerly obtained that they at-
tracted considerable attention. They were sent to
Europe and stimulated various workers to further
experiments.

The late Dr. Common in England, an amateur
astronomer, began that magnificent pioneer work
in astronomical photography which soon brought
him the Gold Medal of the Eoyal Astronomical
Society for his photographs of nebulae. But the
most important result for our purpose was pro-
duced in France. There had been started many
years before by the French astronomer Chacornac
a series 9f star maps round the Zodiac similar
in intention to the Berlin maps which figured in
the history of the discovery of Neptune. Chacornac
died before his enterprise was very far advanced,
and the work was taken up by two brothers,
Paul and Prosper Henry, who followed Cha-
cornac in adopting for the work the laborious

ACCIDENTAL DISCOVERIES

125

method of eye observation of each individual
star. They proceeded patiently with the work on
these lines ; but when they came to the region
where the Zodiac is crossed by the Milky Way,
and the number of stars in a given area increases
enormously, they found the labour so great as to
be practically prohibitive, and were in doubt how
to deal with the difficulty. It was at this critical The
moment that these comet photographs, showing
the stars so beautifully, suggested the alternative gin work-
of mapping the stars photographically. They
immediately set to work with a trial lens, and
obtained such encouraging results that they pro-
ceeded themselves to make a larger lens of the
same type ; this again was satisfactory, and the
idea naturally arose of extending to the whole
heavens the scheme which they had hitherto
intended only for the Zodiac, a mere belt of the
heavens. But this rendered the enterprise too
large for a single observatory. It became
necessary to obtain the co-operation of other
observatories, and with this end in view an confer-
International Conference was summoned to meet 1887. °J
in Paris in 1887 to consider the whole project.
There were delegates from, if not all nations, at
any rate a considerable number : —

France . 20
British Em-

U.S. America

Austria .

Spain . . i
Switzerland . i

pire . 8
Germany . 6
Russia . 3
Holland . 3

Sweden .
Denmark
Belgium .
Italy

2
2
I
I

Portugal . i
Brazil . . i
Argentine Re-
public . i

i'26 ASTRONOMICAL DISCOVERY

The Conference had a number of very impor-
tant questions to discuss, for knowledge of the
photographic method and its possibilities was at
that time in its infancy. There was, for instance,
the question whether all the instruments need be
of the same pattern, and if so what that pattern
should be. The first of these questions was
settled in the affirmative, as we might expect ;
in the interests of uniformity it was desirable
that the maps should be as nearly similar as

choice of possible. The second question was not so easy;

ment. there were at least three different types of instru-
ments which might be used. First of all, there
was the photographic lens, such as is familiar to
all who have used an ordinary camera, consisting
of two lenses with a space between ; though since
each of these lenses is itself made up of two, we
should more correctly say four lenses in all. It
was with a lens of this kind that the comet
pictures had been taken at the Cape of Good
Hope, and it might seem the safest plan to adopt
what had been shown to be capable of such good
work. But there was this difficulty ; the pictures
of the comet were on a very small scale, and taken
with a small lens ; a much larger lens was re-
quired for the scheme now under contemplation,
and when' there are four separate lenses to be
made, each with two surfaces to polish, and each
requiring a perfectly sound clear piece of glass,
it will be obvious that the difficulties of making
a large compound lens of this kind are much

ACCIDENTAL DISCOVERIES 127

greater, and the expense much more serious than Expense

,1 /. . •, i f. of'doub-

in the case of a single lens, or even a pair. It iet."
was this question of expense which had led the
brothers Henry to experiment with a different
kind of instrument, in which only one pair of
lenses was used instead of two. Their instru-
ment was, in fact, very similar to the ordinary
telescope, excepting that they were bound to make
their lenses somewhat different in shape in order
to bring to focus the rays of light suitable for
photography, which are not the same as those
suitable for eye observation with the ordinary
telescope. Dr. Common, again, had used a third
kind of instrument, mainly with the view of re-
ducing the necessary expense still further, or,
perhaps, of increasing the size of the instrument
for the same expense. His telescope had no lens
at all, but a curved mirror instead, the mirror
being made of glass silvered on the face (not on
the back as in the ordinary looking-glass). In Advan-
this case there is only one surface to polish reflector.
instead of four, as in the Henrys' telescope, or
eight, as in the case of the photographic doublet ;
and, moreover, since the rays of light are reflected
from the surface of the glass, and do not pass
through it at all, the internal structure of the
glass is not so strictly important as in the other
cases. Hence the reflector is a very cheap instru-
ment, and it is, moreover, quite free from some
difficulties attached to the other instruments. No
correction for rays of light of different colours is

128 ASTRONOMICAL DISCOVERY

required, since all rays of whatever colour come
to the same focus automatically. These advan-
tages of the reflector were so considerable as to
almost outweigh one well-known disadvantage,
which is, however, not very easily expressed in
words. The reflector might be described as an
instrument with a temper ; sometimes it gives
excellent results, but at others something seems
to be wrong, though the worried observer does
not exactly know what. Long experience and
patience are requisite to humour the instrument
and get the best results from it, and it was felt
that this uncertainty was sufficient to disqualify
the instrument for the serious piece of routine
work contemplated in mapping the heavens.
Refractor Accordingly the handier and more amiable in-
strument with which the brothers Henry had
done such good work was selected as the pattern
to be adopted.

It is curious that at the Conference of 1887
nothing at all was said about the type of in-
strument first mentioned (the "doublet lens"),
although a letter was written in its favour by
Professor Pickering of Harvard College Observa-
tory. Since that time we have learnt much of its
advantages, and it is probable that if the Con-
ference we're to meet now they might arrive at a
different decision ; but at that time they were, to
put it briefly, somewhat afraid of an instrument
which seemed to promise, if anything, too well,
especially in one respect. With the reflector and

ACCIDENTAL DISCOVERIES 129

the refractor it had been found that the field of
good images was strictly limited. The Henrys'
telescope would not photograph an area of the
sky greater in extent than 2° in diameter at any
one time, and the reflector was more limited still ;
within this area the images of the stars were
good, and it had been found that their places
were accurately represented. Now the "doublet" Doublet
seemed to be able to show much larger areas than ha
this with accuracy, but no one had been able to better-
test the accuracy to see whether it was sufficient
for astronomical purposes ; and although no such
feeling was openly expressed or is on record, I
think there is no doubt that a feeling existed of
general mistrust of an instrument which seemed
to offer such specious promises. Whatever the
reason, its claims were passed over in silence at
the Conference, and the safer line (as it was then
thought) of adopting as the type the Henrys'
instrument, was taken.

This was perhaps the most important question
settled at the Conference, and the answers to
many of the others naturally followed. The size
of the plates, for instance, was settled automatic-
ally. The question down to what degree of faint-
ness should stars be included, resolved itself
into the equivalent question, What should be
the length of time during which the plates
were exposed ? Then, again, the question, What
observatories should take part in the work ? be-
came simply this: What observatories could afford

130

ASTRONOMICAL DISCOVERY

The
otServ

Sky

covered

twice.

to acquire the instruments of this new pattern
and get other funds for carrying out the work
specified ? It was ultimately found that eighteen
observatories were able to obtain the apparatus
and funds, though unfortunately three of the
eighteen have since found it impossible to pro-
ceed. The following is the original list, and
in brackets are added the names of three other
observatories which in 1900 undertook to fill the
places of the defaulters. ' ,. ru

OBSERVATORIES CO-OPERATING FOR THE ASTROGRAPHIC
CHART.

Observatory.

Zones of
Declination.

Number
of Plates.

Greenwich

+ 90° to + 65°

1149

Rome

+ 64° „ +55°

1140

Catania

+ 54° „ +47°

1008

Helsingfors

+ 46° „ +40°

100$

Potsdam

+ 39° » +32°

1232

Oxford

+ 3i° » +25°

1180

Paris .

+ 24° „ +18°

1260

Bordeaux

+ 17° „ +11°

1260

Toulouse

+ 10° „ + 5°

1080

Algiers

+ 4° „ - 2°

1260

San Fernando

- 3° „ - 9°

1260

Tacubaya .

- 10° „ - 16°

1260

Santiago (Monte Video)
La Plata (Cordoba) .

-17° „ -23°
-24° „ -31°

1260
1360

Rio (Perth, Australia)

-32° „ -40°

1376

Cape of Good Hope ,
Sydney
Melbourne .

-4i°,, -5i°
-52°,, -64°
-65° „ -90°

1512
1400
1149

In the list is also shown the total number of
plates that were to be taken by each observatory.
-When, once the size of the plates had been settled,

ACCIDENTAL DISCOVERIES 131

it was a straightforward matter to divide up the
sky into the proper number of regions necessary
to cover it completely, not only without gaps be-
tween the plates, but with actually a small over-
lap of contiguous plates. And more than this,
it was decided that the whole sky should be com-
pletely covered twice over. It was conceivable
that a question might arise whether an apparent
star image on a plate was, on the one hand, a
dust speck, or, on the other hand, a planet, or
perhaps a variable or new star. By taking two
different plates at slightly different times, ques-
tions of this kind could be settled ; and to make
the check more independent it was decided that
the plates should not be exactly repeated on the
same portion of sky, but that in the second series
the centre of a plate should occupy the point
assigned to the corner of a plate in the first
series.

Then there came the important question of Times of
time of exposure, which involved a long debate ^
between those who desired the most modest pro-
gramme possible consistent with efficiency, and
those enthusiasts who were anxious to strain the
programme to the utmost limits attainable. Ulti-
mately it was resolved to take two series of
plates ; one series of long exposure which was
set in the first instance at 10 minutes, then
became 15, then 30, then 40, and has by some
enterprising observers been extended to i J hours ;
the other a series of short exposures which have

132 ASTRONOMICAL DISCOVERY

been generally fixed at 6 minutes. Thus instead
of covering the sky twice, it was decided to cover
it in all four times, and the number of plates
assigned to each observatory in the above list
must be regarded as doubled by this new deci-
sion. And further still, on the series of short-
exposure plates it was decided to add to the ex-
posure of six minutes another one of three minutes,
having slightly shifted the telescope between the
two so that they should not be superimposed ;
and later still, a third exposure of twenty seconds
was added to these. It would take too long to
explain here the reasons for these details, but it
will be clear that the general result of the discus-
sion was to extend the original programme con-
siderably, and render the work even more laborious
than it had appeared at the outset.

When all these plates have been taken, the
work is by no means finished ; indeed, it is only
Measure- just commencing. There remains the task of mea-
suring accurately on each of the short-exposure
plates the positions of the stars which it represents,
numbering on the average some 300 or 400 ; so that
for instance at Oxford the total number of stars
measured on the twelve hundred plates is nearly
half a million. These are not all separate stars ;
for the sky is represented twice over, and there is
also the slight overlap of contiguous plates ; but
the number of actual separate stars measured at
this one observatory is not far short of a quarter
of a million, and it has taken nearly ten years to

ACCIDENTAL DISCOVERIES 133

make the measurements, with the help of three
or four measurers trained for the purpose. To The
render the measures easy, a network or r£seau of n
cross lines is photographed on each plate by
artificial light after it has been exposed to the
stars, so that on development these cross lines
and the stars both appear. We can see at a
glance the approximate position of a star by
counting the number of the space from left to
right and from top to bottom in which it occurs ;
and we can also estimate the fraction of a space
in addition to the whole number ; but it is neces-
sary for astronomical purposes to estimate this
fraction with the greatest exactness. The whole
numbers are already given with great exactness
by the careful ruling of the cross lines<> which
can be spaced with extraordinary perfection. To
measure the fraction, we place the plate under a
microscope in the eye-piece of which there is a The
finally divided cross scale ; the centre of the cross
is placed over a star image, and then it is noted
where the lines of the rdseau cut the cross scale.
In this way the position of the image of a star is
read off with accuracy, and after a little practice
with considerable rapidity. It has been found
at Oxford that under favourable conditions the
places of nearly 200 stars* can be recorded in
this way by a single measurer, if he has some
one to write down for him the numbers he calls
out. This is only one form of measuring appa-
ratus ; there are others in which, instead of a

134 ASTRONOMICAL DISCOVERY

scale in the eye-piece, micrometer screws are used
to measure the fractions ; but the general principle
in all these instruments is much the same, and
the rate of work is not very different; while to
the minor advantages and disadvantages of the
different types there seems no need here to
refer. One particular point, however, is worth
noting. After a plate has been measured, it is

Keversai turned round completely, so that left is now right,
and top is now bottom, and the measurements
are repeated. This repetition has the advantage
first of all of checking any mistakes. When a
long piece of measuring or numerical work of any
kind is undertaken there are invariably moments
when the attention seems to wander, and some
small error is the result. But there are also
certain errors of a systematic character similar to
those denoted by the term "personal equation,"
which has found its way into other walks of life.

Personal In the operation of placing a cross exactly over
on' the image of a star, different observers would
show slight differences of habit ; one might place
it a little more to the right than another. But
when the plate is turned round the effect of this
habit on the measure is exactly reversed, and
hence if we take the mean of the two measures
any personal habit of this kind is eliminated. It
has been found by experience that such personal
habits are much smaller for measures of this kind
than for those to which we have long been accus-
tomed in observations made by eye on the stars

ACCIDENTAL DISCOVERIES ' fa 5

themselves. The troubles from " personal equa-
tion " have been much diminished by the photo-
graphic method, and certain peculiarities of the
former method have been clearly exhibited by the
comparison. For instance, it has gradually become
clear that with eye observations personal equation
is not a constant quantity, but is different for
stars of different brightness. When observing
the transit of a bright star the observer apparently
records an instant definitely earlier than in re-
cording the transit of a faint one ; and this pecu-
liarity seems to be common to the large majority
of observers, which is perhaps the reason why it
was not noticed earlier. But when positions of
the stars determined in this way are compared
with their positions measured on the photographic
plates, the peculiarity is made clearly manifest.
For example, at Oxford, our first business after
making measurements is to compare them with
visual observations on a limited number of the
brighter stars made at Cambridge about twenty
years ago. (About 14,000 stars were observed at
Cambridge, and we are dealing with ten times
that number.) The comparison shows that the
Cambridge observations are affected with the
following systematic errors : —

If stars of magnitude 10 are observed correctly,
then „ „ 9 „ o.io sees, too early

„ ,, 8 „ 0.16 „

11 11 7 » 0-T9 . »

„ ,, 6 „ 0.21 „

11 11 5 11 °-23 »

136 ASTRONOMICAL DISCOVERY

This may serve as an illustration of various
incidental results which are already flowing from
the enormous and laborious piece of work which,
as far as the University Observatory at Oxford is
concerned, we have just completed, though some
of the other colleagues are not so far advanced.
- But the main results will not appear just yet.
work? * e The work must be repeated, and the positions of
the stars just obtained must be compared with
those which they will be found to occupy at some
future date, in order to see what kind of changes
are going on in the heavens. Whether this future
date shall be one hundred years hence, or fifty, or
ten, or whether we should begin immediately to
repeat what has been done, is a matter not yet
decided, and one which requires some little con-
sideration.

I have said perhaps enough to give you a
general idea of the work on which we have been
engaged at Oxford for the last ten years. Ten
years ago it seemed to stretch out in front of us
rather hopelessly ; the pace we were able to
make seemed so slow in view of the distance to
be covered. We felt rather like the schoolboy
who has just returned to school and sees the
next holidays as a very remote prospect, and we
solaced ourselves much in the same way as he
does, by making a diagram representing the total
number of plates to be dealt with and crossing off
each one as it was finished, just as he sometimes
crosses off the days still remaining between him

ACCIDENTAL DISCOVERIES 137

and the prospective holidays. It was pleasant to
watch the growth of the number of crosses on this
diagram, and by the end of the year 1902 we had
the satisfaction of seeing very little blank space The con-
remaining. Now, up to this point it had not
much mattered whether any particular plate was
secured in any particular year, or in a subsequent
year, so long as there were always sufficient plates
to keep us occupied in measuring them. But it
then became a matter of importance to secure each
plate at the proper time of year; for the sun, as
we know, travels round the Zodiac among the
stars, obliterating by his radiance a large section
of the sky for a period of some months, and in
this way a particular region of the heavens is apt
to "run into daylight," as the observatory phrase
goes, and ceases to be available for photography
during several months, until the sun is again far
enough away to allow of the particular region
being seen at night.

Roughly speaking then, if a plate which should
be taken in February is not secured in this month
owing to bad weather, the proper time for taking
it will not occur again until the following
February ; and when there was a fair prospect of
finishing our work in 1903, it became important
to secure each plate at the proper time in that
year. Hence we were making special efforts to
utilise to the full any fine night that Providence
sent in our way, and on such occasions it is clearly
an economy, if not exactly to "make hay while
the sun shines," at any rate to take plates

1 3 8 ASTRONOMICAL DISCOVERY

vigorously while the sun is not shining and the
night is fine ; leaving the development of them
until the daytime. There is, of course, the risk
that the whole night's work may in this way be
lost owing to some fault in the plates, which
might have been detected if some of them were
immediately developed. Perhaps in the early days
of our work it would have been reckless or foolish
to neglect this little precaution ; but we had for
years been accustomed to rely upon the excellence
of the plates without finding our trust betrayed ;
and the sensitiveness of the plates had increased
A disap- rather than diminished as time went on. Hence
it will be readily understood that when one fatal
morning we developed a series of some thirty
plates, and found that owing to some unexplained
lack of sensitiveness they were all unsuitable for
our purpose, it came as a most unwelcome and
startling surprise. It was, of course, necessary to
make certain that there was no oversight, that the
developer was not at fault, and that the weather
had not been treacherous. All such possibilities
were carefully considered before communication
with the makers of the plates, but it ultimately
became clear that there had been some unfortunate
failure in sensitiveness, and that it would be
necessary to repeat the work with opportunities
restricted by the intervening lapse of time. How-
ever, disappointments from this or similar causes
are not unknown in astronomical work ; and we set
about this repetition with as little loss of time and
cheerfulness as was possible. Under the circum-

ACCIDENTAL DISCOVERIES 139

stances, however, it seemed desirable to examine
carefully whether anything could be saved from
the wreck — whether any of the plates could be
admitted as just coming up to the minimum
requirements. And I devoted a morning to this
inquiry. In the course of it I came across one A curious
plate which certainly seemed worth an inclusion p a
among our series from the point of view of the
number of stars shown upon it. It seemed quite
rich in stars, perhaps even a little richer than
might have been expected. On inquiry I was
told that this was not one of the originally con-
demned plates, but one which had been taken
since the failure in sensitiveness of the plates
had been detected ; was from a new and specially
sensitive batch with which the courteous makers
had supplied us ; but though there were cer-
tainly a sufficient number of stars upon the plate,
owing to some unexplained cause the telescope
had been erroneously pointed, and the region
taken did not correspond to the region required.
To investigate the cause of the discrepancy I
thereupon took down from our store of plates the
other one of the same region which had been
rejected for insufficiency of stars, and on comparing
the two it was at once evident that there was a
strange object on the plate taken later of the two, Astrauge
a bright star or other heavenly body, which was ° ]ec
not on the former plate. I have explained that
by repeating the exposure more than once, it is
easily possible to recognise whether a mark upon
the plate is really a celestial body or is an ac-

i4o ASTRONOMICAL DISCOVERY

cidental blot or dust speck, and there was no
doubt that this was the image of some strange
celestial body. It might, of course, be a new
planet, or even an old one which had wandered
into the region ; but a few measures soon showed
that it was not in movement. The measures con-
sisted in comparing the separation of the three
exposures with the separation of the corresponding
exposures of obvious stars, for the exposures were
not, of course, simultaneous, and if the body were
a planet and had moved in the interval between
them, this would be made manifest on measuring
the separations. No such movements could be
detected ; and the possibilities were thus restricted
to two. So far as we knew the object was a star,
but might be either a star of the class known as
variable or of that known as new. In the former
case it would become bright and faint at more or
less regular intervals, and might possibly have
been already catalogued ; for the number of these
bodies already known amounts to some hundreds.
Search being made in the catalogues, no entry of it
was found, though it still might be one of this
class which had hitherto escaped detection. Or it
A new might be a " new star," one of those curious bodies
which blaze up quite suddenly to brightness and
then die away gradually until they become practi-
cally invisible. The most famous perhaps of these
is the star which appeared in 1572, and was so
carefully observed by Tycho Brahe ; but such
apparitions are rare, and altogether we have not
records as yet of a score altogether; so that in

ACCIDENTAL DISCOVERIES 141

this latter case the discovery would be of much
greater interest than in the former. In either
event it was desirable to inform other observers as
soon as possible of the existence of a strange
body ; already some time had elapsed since the
plate had been taken, March i6th, for the ex-
amination of which I have spoken was not made
until March 24th. Accordingly, a telegram was at
once despatched to the Central Office at Kiel,
which undertakes to distribute such information
all over the world, and a few post-cards were sent
to observers close at hand who might be able to
observe the star the same night. Certain observa-
tions with the spectroscope soon made it clear that
the object was really a " new star."

This, therefore, is the discovery wjiich we
made at Oxford : as you will see, in an entirely
accidental manner, during the course of a piece of
work in which it was certainly never contemplated.
Its purely accidental nature is sufficiently illus- The
trated by the fact that if the plates originally
supplied by the makers had been of the proper
quality, the plate which led to the discovery would
never have been taken. If the plates exposed in
February had been satisfactory, we should have
been content, and should not have repeated the
exposure on March i6th. Again I can testify per-
sonally how purely accidental it was that the ex-
amination was made on March 24th to see whether
anything could be saved, as I have said, from the
wreck. The idea came casually into my mind as
I was walking through the room and saw the neat

142

ASTRONOMICAL DISCOVERY

pile of rejected plates ; and one may fairly call it
an accidental impulse. This new star is not, how-
ever, the first of such objects to have been dis-
covered "accidentally"; many of the others were
found just as much by chance, though a notable
exception must be made of those discovered at the
Harvard Observatory, which are the result of a
deliberate search for such bodies by the careful
Mrs. examination of photographic plates. Mrs. Fleming,
disemmg s who spends her life in such work, has had the
ies> good fortune to detect no less than six of these
wonderful objects as the reward of her laborious
scrutiny ; and she is the only person who has thus
found new stars by photography until this ac-
cidental discovery at Oxford. The following is a
complete list of new stars discovered to date :—

LIST OF NEW STARS.

Ref. No.

Constellation.

Year.

Discoverer.

Cassiopeia

1572

Tycho Brahe.

Cygnus .
Ophiuchus

1600
1604

Janson.
Kepler.

Vulpecula

1670

Anthelm.

Ophiuchus

1848

Hind.

Scorpio .

1860

Auwers.

Corona Boreal s

1866

Birmingham.

Cygnus .

1876

Schmidt.

Andromeda

1885

Hartwig.

Perseus .

1887 .

Fleming.

Auriga .

1891

Anderson.

Norma ,

1893

Fleming.

Carina •

1895

Fleming.

Centaurus

1895

Fleming.

Sagittarius

1898

Fleming.

Aquila .

1899

Fleming.

Perseus .

1901

Anderson.

Gemini .

1903

At Oxford.

• .

•

t.
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w
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Z ' *•

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^2 "*"
•*. o

?* j *~

'S ^

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H K

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• •

1 •

Z i

•^r ^ ^

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's 5

z 5^

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"^- r-"

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. * ^ SS X

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. * •*

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- -

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ACCIDENTAL DISCOVERIES 143

Generally these stars have been noted by eye Dr. An-
observation, as in the case of the two found by d
Dr. Anderson of Edinburgh. In these cases also
we may say that deliberate search was rewarded ;
for Dr. Anderson is probably the most assiduous
" watcher of the skies " living, though he seldom
uses a telescope ; sometimes he uses an opera-
glass, but usually the naked eye. He describes
himself as an "Astrophil" rather than as an
astronomer. "I love the stars," he says; "and
whenever they are shining, I must be looking."
And so on every fine night he stands or sits at
his open study window gazing at the heavens. I
believe he was just about to leave them for his
bed, near 3 A.M. on the night of February 21,
1901, when, throwing a last glance upward, he
suddenly saw a brilliant star in the constellation
Perseus. His first feeling was actually one of Nova
disappointment, for he felt sure that this object P
must have been there for some time past without
his knowing of it, and he grudged the time lost
when he might have been regarding it. More in
a spirit of complaint than of inquiry, he made his
way to the Royal Observatory at Edinburgh next
day to hear what they had to say about it,
though he found it difficult to approach the sub-
ject. He first talked about the weather, and the
crops, and similar topics of general interest ; and
only after some time dared he venture a casual
reference to the " new portent in the heavens."
Seeing his interlocutor look somewhat blank, he

144 ASTRONOMICAL DISCOVERY

ventured a little farther, and made a direct refer-
ence to the nevv star in Perseus ; and then found
to his astonishment, as also to his great delight,
that he was the first to bring news of it. The
news was soon communicated to other observers ;
all the telescopes of the world were soon trained
upon it; and this wonderful "new star of the
new century " has taught us more of the nature
of these extraordinary bodies than all we knew
before.
Records Perhaps I may add a few remarks on one or

previous „ /»i IT -n« i i

to dis- two features 01 these bodies. r irstly, let us note
that Professor Pickering of Harvard is now able
to make a most important contribution to the
former history of these objects — that is to say,
their history preceding their actual detection.
We remember that, after Uranus had been dis-
covered, it was found that several observers had
long before recorded its place unknowingly ; and
similarly Professor Pickering and his staff have
usually photographed other new objects unknow-
ingly. There are on the shelves at Harvard vast
stores of photographs, so many that they are
unable to examine them when they have been
taken; but once any object of interest has been
discovered, it is easy to turn over the store and
examine the particular plates which may possibly
show it at an earlier date. In this way it was
found that Dr. Anderson's new star had been
visible only for a few days before its discovery,
there being no trace of it on earlier plates, Simi-

ACCIDENTAL DISCOVERIES 145

larly, in the case of the new star found at Oxford,
plates taken on March ist and 6th, fifteen days
and ten days respectively before the discovery-
plate of March i6th, showed the star. But, in
this particular instance, greater interest attaches
to two still earlier plates taken elsewhere, and
with exposures much longer than any available
at Harvard. One had been obtained at Heidel-
berg by Dr. Max Wolf, and another at the
Yerkes Observatory of Chicago University, by Mr.
Parkhurst ; and on both there appeared to be a
faint star of about the fourteenth or fifteenth mag-
nitude, in the place subsequently occupied by the
Nova; and the question naturally arose, Was Was Nova
this the object which ultimately blazed up and
became the new star? To settle this point, it
was necessary to measure its position, with refer- faintly*
ence to neighbouring stars, with extreme preci-
sion ; and here it was unfortunate that the
photographs did not help us as much as they
might, for they were scarcely capable of being
measured with the requisite precision. The point
was an important one, because if the identity of
the Nova with this faint star could be established,
it would be the second instance of the kind ; but so
far as they went, measurements of the photographs
were distinctly against the identity. Such was
the conclusion of Mr. Parkhurst from his photo-
graph alone ; and it was confirmed by measures
made at Oxford on copies of both plates, which

146 ASTRONOMICAL DISCOVERY

were kindly sent there for the purpose. The con-
clusion seemed to be that there was a faint star
very near, but not at, the place of the new star ;
and it was therefore probable that, although this
faint star was temporarily invisible from the bright-
ness of the adjacent Nova, as the latter became
fainter (in the way with which we have become
familiar in the case of new stars), it might be
The possible to see the two stars alongside each other.

suspicion :_ . . .

negatived. Ihis critical observation was ultimately made by
the sharp eyes of Professor Barnard, aided by the
giant telescope of the Yerkes Observatory ; and it
seems clear therefore that the object which blazed
up to become the Nova of 1903 could not have
previously been so bright as a faint star of the
fourteenth magnitude. Although this is merely a
negative conclusion, it is an important one in
the history of these bodies.

The second point to which I will draw your

attention is from the history of the other Nova

just mentioned — Dr. Anderson's New Star of

1901. In this instance it is not the history pre-

vious to discovery, but what followed many months

after discovery, that was of engrossing interest ;

and again Yerkes Observatory, with its magnificent

equipment, played an important part in the drama.

When, with its giant reflecting telescope, photo-

Nebula graphs were taken of the region of Nova Persei

Nova after it had become comparatively faint, it was

sel< found that there was an extraordinarily faint nebu-

losity surrounding the star.* Repeating the photo-

SEPT. 20, IQ01.

NOV. 13, 1901.

IX. — NEBULOSITY ROUND NOVA PERSEI.

(From photographs taken at the Yerkcs Observatory by G. W. Ritchey

>' or THE

UNIVERSITY

ACCIDENTAL DISCOVERIES 147

graphs at intervals, it was found that this nebulosity its
was rapidly changing in shape. " Rapidly" is, of changes<
course, a relative term, and a casual inspection
of two of the photographs might not convey any
impression of rapidity ; it is only when we come
to consider the enormous distance at which the
movements, or apparent movements, of the nebulae
must be taking place that it becomes clear how
rapid the changes must be. It was not possible
to determine this distance with any exactness,
but limits to it could be set, and it seemed pro-
bable that the velocity of the movement was
comparable with that of light. The conclusion
suggested itself that the velocity might actually
be identical with that of light, in which case what
we saw was not the movement of actual matter, Due to
but merely that of illumination, travelling from
point to point of matter already existing. tion-

An analogy from the familiar case of sound
may make clearer what is meant. If a loud noise
is made in a large hall, we hear echoes from the
walls. The sound travels with a velocity of about
1 100 feet per second, reaches the walls, is re-
flected back from them, and returns to us with
the same velocity. From the interval occupied
in going and returning we could calculate the
distance of the walls. The velocity of light is so
enormous compared with that of sound that we
are usually quite unable to observe any similar
phenomenon in the case of light. If we strike a
match in the largest hall, all parts of it are

148 ASTRONOMICAL DISCOVERY

illuminated so immediately that we cannot pos-
sibly realise that there was really an interval
between the striking of the match, the travelling
of the light to the walls, and its return to our
eyes. The scale of our terrestrial phenomenon is
far too small to render this interval perceptible.
But those who accept the theory above mentioned
regarding the appearances round Nova Persei
(although there are some who discredit it) believe
that we have in this case an illustration of just
this phenomenon of light echoes, on a scale large
enough to be easily visible. They think that,
surrounding the central star which blazed up so
brightly in February 1901, there was a vast dark
nebula, of which we had no previous knowledge,
because it was not shining with any light of its
own. When the star blazed up, the illumination
travelled from point to point of this dark nebula
and lighted it up ; but the size of the nebula was
so vast that, although the light was travelling with
the enormous velocity of 200,000 miles per second,
it was not until months afterwards that it reached
different portions of this nebula ; and we accord-
ingly got news of the existence of this nebula
some months after the news reached us of the
When did central conflagration, whatever it was. Eemark
happen? that all we can say is that the news of the nebula
reached us some months later than that of the
outburst. The actual date when either of the
actual things happened, we have as yet no means
of knowing ; it may have been hundreds or even

ACCIDENTAL DISCOVERIES 149

thousands of years ago that the conflagration
actually occurred of which we got news in Feb-
ruary 1901, the light having taken all that time
to reach us from that distant part of space ; and
the light reflected from the nebula was following
it on its way to us all these years at that same
interval of a few months.

Now, let me refer before leaving this point to An objec-
the chief objection which has been urged against ta'
this theory. It has been maintained that the
illumination would necessarily appear to travel
outwards from the centre with an approach to
uniformity, whereas the observed rate of travel is
not uniform, and has been even towards the
centre instead of away from it ; which would
seem as though portions of the nebula more
distant from the centre were lighted up sooner
than those closer to it. By a simple illustration
from our solar system, we shall see that these
curious anomalies may easily be explained. Let
us consider for simplicity two planets only, say
the Earth and Saturn. We know that Saturn
travels round the sun in an orbit which is ten
times larger than the orbit of the earth. Suppose
now that the sun were suddenly to be extinguished ;
light takes about eight minutes to travel from
the sun to the earth, and consequently we should
not get news of the extinction for some eight
minutes ; the sun would appear to us to still go on
shining for eight minutes after he had really been
extinguished. Saturn being about ten times as

ISO ASTRONOMICAL DISCOVERY

far away from the sun, the news would take
eighty minutes to reach Saturn ; and from the earth
we should see Saturn shining more1 than eighty
minutes after the sun had been extinguished,
although we ourselves should have lost the sun's
light after eight minutes. I think we already
begin to see possibilities of curious anomalies ;
but they can be made clearer than this. Instead
of imagining an observer on the earth, let us
suppose him removed to a great distance away
in the plane of the two orbits ; and let us sup-
pose that the sun is now lighted up again as
suddenly as the new star blazed up in February
1901. Then such an observer would first see this
blaze in the centre ; eight minutes afterwards the
illumination would reach the earth, a little speck
of light near the sun would be illuminated, just
as we saw a portion of the dark nebula round
Nova Persei illuminated ; eighty minutes later
another speck, namely, Saturn, would begin to
shine. But now, would Saturn necessarily appear
to the distant observer to be farther away from
the sun than the earth was? Looking at the
diagram, we can see that if Saturn were at S1? then
it would present this natural appearance of being
farther away from the sun than the earth ; but it
might be at S2 or S3, in which case it would seem
to be nearer the sun, and the illumination would
seem to travel inwards towards the central body

1 Since the light must travel from the sun to Saturn and back
again to the earth, the interval would be more nearly 150 minutes.

ACCIDENTAL DISCOVERIES 151

instead of outwards. Without considering other
cases in detail, it will be tolerably clear that almost
any anomalous appearance might be explained by
choosing a suitable arrangement of the nebulous
matter which we suppose lighted up by the ex-
plosion of Nova Persei. Another objection urged
against the theory I have sketched is that the
light reflected from such a nebula would be so
feeble that it would not affect our photographic

plates. This depends upon various assumptions
which we have no time to notice here ; but I
think we may say that there is certainly room for
the acceptance of the theory.

Now, if this dark nebula was previously existing Did the
in this way all round the star which blazed up, cause^he
the question naturally arises whether the nebula outbl
had anything to do with the conflagration. Was
there previously a star, either so cold or so distant
as not to be shining with appreciable light, which,
travelling through space, encountered this vast

152 ASTRONOMICAL DISCOVERY

nebula, and by the friction of the encounter was
suddenly rendered so luminous as to outshine a
star of the first magnitude ? The case of meteoric
stones striking our own atmosphere seems to
suggest such a possibility. These little stones
are previously quite cold and invisible, and are
travelling in some way through space, many of
them probably circling round our sun. If they
happen in their journey to encounter our earth,
even the extremely tenuous atmosphere, so thin
that it will scarcely bend the rays of light
appreciably, even this is sufficient by its friction
to raise the stones to a white heat, so that they
blaze up into the falling stars with which we are
familiar. This analogy is suggested, but we
must be cautious in accepting it; for we know
so very little of the nature of nebula? such as
that of which we have been speaking. But in
any case, a totally new series of phenomena have
been laid open to our study by those wonderful
photographs taken at the Yerkes Observatory and
the Lick Observatory in the few years which the
present century has as yet run.

impor- One thing is quite certain : we must lose no
opportunity of studying such stars as may appear,
and no diligence spent in discovering them at the
earliest possible moment is thrown away. We
have only known up to the present, as already
stated, less than a score of them, and of these
many have told us but little ; partly because they
were only discovered too late (after they had

ACCIDENTAL DISCOVERIES 153

become faint), and partly because the earlier ones
could not be observed with the spectroscope,
which had not then been invented. It seems
clear that in the future we must not allow acci-
dent to play so large a part in the discovery of
these objects ; more must be done in the way of
deliberate search. Although we know beforehand
that this will involve a vast amount of apparently
useless labour, that months and years must be
spent in comparing photographic plates, or por-
tions of the sky itself, with one another without
detecting anything remarkable, it will not be the
first time that years have been cheerfully spent in
such searches without result. We need only
recall Hencke's fifteen years of fruitless search,
before finding a minor planet, to realise this fact.

One thing of importance may be done ; we
may improve our methods of making the search,
so as to economise labour, and several suc-
cessful attempts have already been made in this
direction. The simplest plan is to superpose two Super-
photographs taken at different dates, so that the of plates,
stars on one lie very close to those on the other ;
then if an image is seen to be unpaired we may
have found a new star, though of course the object
may be merely a planet or a variable. The super-
position of the plates may be either actual or
virtual. A beautiful instrument has been devised
on the principle of the stereoscope for examining
two plates placed side by side, one with each eye.
We know that in this way two photographs of

154 ASTRONOMICAL DISCOVERY

the same object from different points of view will
appear to coalesce, and at the same time to give
an appearance of solidity to the object or land-
scape, portions of which will seem to stand out in
The front of the background. Applying this principle
compara- to two photographs of stars, what happens is this :
if the stars have all remained in the same posi-
tions exactly, the two pictures will seem to us to
coalesce, and the images all to lie on a flat
background ; but if in the interval between the
exposures of the two plates one of the stars has
appreciably moved or disappeared, it will seem,
when looked at with this instrument, to stand out
in front of this background, and is accordingly
detected with comparatively little trouble. This
new instrument, to which the name Stereo-com-
parator has been given, promises to be of immense
value in dredging the sky for strange bodies in
the future. I am glad to say that a generous
friend has kindly presented the University Obser-
vatory at Oxford with one of these beautiful instru-
ments, which have been constructed by Messrs.
Zeiss of Jena after the skilful designs of Dr.
Pulfrich. Whether we shall be able to repeat by
deliberate search the success which mere accident
threw in our way remains to be seen.

CHAPTER V

SCHWABE AND THE SUN-SPOT PERIOD

IN preceding chapters we have reviewed dis- Discove-
coveries, some of which have been made as a tiary to

result of a deliberate search, and others acciden- l
tally in the course of work directed to a totally
different end ; but so far we have not considered
a case in which the discoverer entered upon an
enterprise from which he was positively dissuaded.
In the next chapter we shall come across a
very striking instance of this type ; but even
in the discovery that there was a periodicity in
the solar spots, with which I propose to deal
now, Herr Schwabe began his work in the face
of deterrent opinions from eminent men. His
definite announcement was first made in 1843,
though he had himself been convinced some years
earlier. In 1857 the Royal Astronomical Society
awarded him their gold medal for the discovery ;
and in the address delivered on the occasion
the President commenced by drawing atten-
tion to this very fact, that astronomers who had
expressed any opinions on the subject had been
uniformly and decidedly against the likelihood Nothing
of there being anything profitable in the study f^° €
of the solar spots. I will quote the exact words spots'

155

156 ASTRONOMICAL DISCOVERY

of the President, Mr. Manuel Johnson, then Rad-
clifie Observer at Oxford.

"It was in 1826 that Heinrich Schwabe, a
gentleman resident in Dessau, entered upon those
researches which are now to engage our attention.
I am not aware of the motive that induced him —
whether any particular views had suggested them-
selves to his own mind — or whether it was a
general desire of investigating, more thoroughly
than his predecessors had done, the laws of a
remarkable phenomenon, which it had long been
the fashion to neglect. He could hardly have
anticipated the kind of result at which he has
arrived ; at the same time we cannot imagine a
course of proceeding better calculated for its
detection, even if his mind had been prepared
for it, than that which he has pursued from the
very commencement of his career. Assuredly
if he entertained such an idea, it was not borrowed
from the authorities of the last century, to whom
the solar spots were objects of more attention
than they have been of late years.

" ' Nulla constanti temporum lege apparent aut
evanescunt,' says Keill in 1739. — Introduct. ad
Physic. Astronom., p. 253.

" ' II est manifest par ce que nous venons de
rapporter qu'il n'y a point de regie certaine de
leur formation, ni de leur nombre et de leur
figure,' says Cassini II. in 1740. — EUm d'Astron.,
vol. i. p. 82.

SCHWABE AND THE SUN-SPOT PERIOD 157

" ' II semble qu'elles ne suivent aucune loi dans
leur apparitions/ says Le Monnier in 1746. —
Instit. Astron., p. 83.

" ' Solar spots observe no regularity in their shape,
magnitude, number, or in the time of their appear-
ance or continuance/ says Long in 1 764. — Astron.,
vol. ii. p. 472.

" ' Les apparitions des taches du soleil n'ont
rien de regulier/ says Lalande in 1771. — Astron.,
vol. iii. § 3131, 2nd edit.

" And Delambre's opinion may be inferred from
a well-known passage in the third volume of his
'Astronomy' (p. 20), published in 1814, where treat-
ing of the solar spots he says, * II est vrai qu'elles
sont plus curieuses que vraiment utiles.'"1

It will thus be evident that Herr Schwabe had
the courage to enter upon a line of investigation
which others had practically condemned as likely
to lead nowhere, and that his discovery was quite
contrary to expectation. It is a lesson to us that
not even the most unlikely line of work is to be
despised ; for the outcome of Schwabe's work was
the first step in the whole series of discoveries
which have gradually built up the modern science
of Solar Physics, which occupies so deservedly large
a part of the energies of, for instance, the great
observatory attached to the University of Chicago.

It has been our practice to recall the actual

1 Monthly Notices of the Royal Astronomical Society, vol. xvii.
p. 126.

158 ASTRONOMICAL DISCOVERY

Schwabe's words in which the discoverer himself stated his
discovery, and I will give the original modest
announcement of Schwabe, though for convenience
of those who do not read German I will attempt
a rough translation. He had communicated year
by year the results of his daily counting of the
solar spots to the Astronomische Nachrichten,
and after he had given ten years' results in this
way he collected them together, but he made
no remark on the curious sequence which they
undoubtedly showed at that time. Waiting
patiently six years for further material, in 1843
he ventured to make his definite announcement
as follows : — " From my earlier observations,
which I have communicated annually to this
journal, there was manifest already a certain
periodicity of sun-spots ; and the probability of
this being really the case is confirmed by this
year's results. Although I gave in volume 15
the total numbers of groups for the years
1826-1837, nevertheless I will repeat here a
complete series of all my observations of sun-
spots, giving not only the number of groups,
but also the number of days of observation, and
further the days when the sun was free from
spots. The number of groups alone will not in
itself give 'sufficient accuracy for determination of
a period, since 1 have convinced myself that when
there are a large number of sun-spots the number
will be reckoned somewhat too small, and when
few sun- spots, the number somewhat too large ;

FEB. l8, 1894.

FEB. IQ, 1094.

X.— PHOTOGRAPHS OF THE Sux TAKEN AT THE ROYAL OBSERVATORY
GREENWICH, SHEWING SUNSPOTS,

f UNIVERSITY

FEB. 20, 1894.

FEB. 21, 1894.

XI. — PHOTOGRAPHS ov THE SUN TAKEN AT THE ROYAL OBSERVATORY,
GREENWICH, SHEWING SUNSPOTS

Jf THF \

UNIVERSITY

SCHWABE AND THE SUN-SPOT PERIOD 159

in the first case several groups are often counted
together in one, and in the second it is easy to
divide a group made up of two component parts
into two separate groups. This must be my
excuse for repeating the early catalogue, as
follows : —

Year.

Number of
Groups.

Days free
from Spots.

Days of
Observation.

1826
1827
1828
1829
1830

118
161
225
199
190

22
2
0
0
I

277

273
282
244
217

1831
1832
1833
1834
1835

149
84

33
51
173

3
49
139

120

239

270
267

273
244

1836

1837
1838

1839
1840

272

333
282
162
152

0
0

o
3

200

1 68

202
205
263

1841
1842

1843
(1844)

1 02

64
149
(in)

283
307

324
(320)

" If we now compare together the number of
groups, and the days free from spots, we find that
the sun-spots have a period of about ten years,
and that for about five years they are so numerous
that during this period few days, if any, are free
from spots. The sequel must show whether this
period is constant, whether the minimum activity

160 ASTRONOMICAL DISCOVERY

of the sun in producing spots lasts for one or two
years, and whether this activity increases more
quickly than it decreases."

Attracted This brief announcement is all that the dis-
tention, coverer says upon the subject ; and it is perhaps
not remarkable that it attracted very little atten-
tion, especially when we remember that it related
to a matter which the astronomical world had
agreed to put aside as unprofitable and not worth
attention. Next year, in giving his usual paper
on the spots for 1844 he recurs to the subject in
the following sentence : " The periodicity of spots
of about ten years which was indicated in my
summary published last year, is confirmed by this
year's observations." I have added in brackets
to the table above reproduced the numbers for
1 844 subsequently given, and it will be seen how
nearly they might have been predicted.

Still the subject attracted little attention.
Turning over the leaves of the journal at random,
I came across the annual report of the Astronomer
Royal of England, printed at length. But in it
there is no reference to this discovery, which
opened up a line of work now strongly repre-
sented in the annual programme of the Royal
Observatory at Greenwich. Mr. Johnson remarks
that the only person who had taken it up was
Julius Schmidt, who then resided near Hamburg.

until

eight But Schwabe went on patiently accumulating

fater! facts ; and in 1851 the great Von Humboldt in

the third volume of his Cosmos, drew attention to

SCHWABE AND THE SUN-SPOT PERIOD 161

the discovery, which was accordiDgly for the first
time brought into general notice. It will be seen
that there are not many facts of general interest
relating to the actual discovery beyond the courage
with which the work was commenced in a totally
unpromising direction, and the scant attention it
received after being made for us. We may admit
that interest centres chiefly in the tremendous
consequences which flowed from it. We now
recognise that many other phenomena are bound
up with this waxing and waning of the solar spots.
We might be prepared for a sympathy in pheno- other
mena obviously connected with the sun itself; but
it was an unexpected and startling discovery that
magnetic phenomena on the earth had also a
sympathetic relation with the changes in, sun-
spots, and it is perhaps not surprising that when
once this connection of solar and terrestrial pheno-
mena was realised, various attempts have been
made to extend it into regions where we cannot as
yet allow that it has earned a legitimate right of
entry. We have heard of the weather and of
Indian famines occurring in cycles identical with
the sun-spot cycle ; and it is obvious how tremend-
ously important it would be for us if this were
found to be actually the case. For we might in
this way predict years of possible famine and
guard against them ; or if we could even partially
foretell the kind of weather likely to occur some
years hence, we might take agricultural measures
accordingly. The importance of the connection,

1 62 ASTRONOMICAL DISCOVERY

if only it could be established, is no doubt the
reason which has misled investigators into laying
undue stress on evidence which will not bear
and others close scrutiny. For the present we must say
decidedly that no case has been made out for
paying serious attention to the influence of sun-
spots on weather. Nevertheless, putting all this
aside, there is quite enough of first-rate import-
ance in the sequel to Schwabe's discovery.

Let us review the facts in order. Most of us,
though we may not have had the advantage of
seeing an actual sun-spot through a telescope,
have seen drawings or photographs of spots.
There is a famous drawing made by James
Nasmyth (of steam-hammer fame), in July, 1864,
which is of particular interest, because at that
time Nasmyth was convinced — and he convinced
many others with him — that the solar surface was
made up of a miscellaneous heap of solid bodies,,
in shape like willow leaves, or grains of rice,
thrown together almost at random, and the draw-
ing was made by him to illustrate this idea. Com-
paring a modern photograph with it, we see that
there is something to be said for Nasmyth's view,
which attracted much attention at the time and
occasioned a somewhat heated controversy. But
since the invention of the spectroscope it has
become quite obsolete; it probably does not
Green- correspond in any way to the real facts. But
records, instead of looking at pictures which have been
enlarged to show the detailed structure in and

SCHWABE AND THE SUN-SPOT PERIOD 163

near a spot, we will look at a series of pictures
of the whole sun taken on successive days at
Greenwich in which the spots are necessarily much
smaller, but which show the behaviour of the spots
from day to day. (See Plates X. and XI.) From
the date at the foot of each it will be seen that
they gradually cross the disc of the sun (a fact
first discovered by Galileo in 1610), showing that
the sun rotates on an axis once in about every The sun's

r> i mi • , , • rotation.

twenty-five days. ihere are many interesting
facts connected with this rotation ; especially
that the sun does not rotate as a solid body,
the parts near the (Sun's) Equator flowing quicker
than those nearer the Poles ; but for the present
we cannot stop to dwell upon them. What
interests us particularly is the history, not from
day to day, but from year to year, as Schwabe has
already given it for a series of years.

When it became generally established that this Wolfs
periodicity existed, Rudolf Wolf of Zurich col- n
lected the facts about sun-spots from the earliest
possible date, and represented this history by a
series of numbers which are still called Wolf's
Sun-Spot Numbers. You will see from the dia-
gram the obvious rise and fall for eleven years, —
not ten years, as Schwabe thought, but just a little
over eleven years. The facts are, however, given
more completely by the work done at the Eoyal
Observatory at Greenwich. It is part of the
regular daily work of that Observatory to photo-
graph the sun at least twice. Many days are of

1 64 ASTRONOMICAL DISCOVERY

course cloudy or wet, so that photographs cannot
be obtained ; but there are available photographs
similarly taken in India or in Mauritius, where
the weather is more favourable, and from these
the gaps are so well filled up that very few days,
if any, during the whole year are left without
Green- some photograph of the sun's surface. On these
areas. photographs the positions and the areas of the
spots are carefully measured under a microscope,
and the results when submitted to certain neces-t
sary calculations are published year by year. It is
clearly a more accurate estimate of the spottedness
of the sun to take the total area of all the spots
rather than their mere number, for in the latter case-
a large spot and a small one count equally. Hence
the Greenwich records will perhaps give us an
even better idea of the periodicity than Wolfs
numbers. Now, at the same observatory magnetic
observations are also made continuously. If a
magnet be suspended freely we are accustomed to1
say that it will point to the North Pole ; but this
is only very roughly true. In the first placej it is'
probably well known to you that there is a con-
siderable deviation from due north owing to the
fact that the magnetic North Pole is not the same
as the geographical North Pole ; but this for the
Magnetic present need not concern us. What does concern
Sons1*" us ig> that if the needle is hung up and left long
enough to come to rest, it does not then remain
steadily at rest, but executes slow and small
oscillations backwards and forwards, up and down,

NUMBER OF SUNSPOTS (Wolf)

DAILY RANGE <f MAGNETIC DECLINATION

DAILY RANCXE of MAGNET/ C HOR2.L, FORCE
observed at QreenuricK)

1840 50 60 yp 80 90

|l"'r»f«lfirffrffff|ffii|iiii|t,|,|im|llvl|lll||||M

X
100

6'-

LS&

IOO

HORIZONTAL FORCE

PLATE XII.

GREENWICH MAGNETIC CURVES

1859-60

CRE£NWICH MAGNETIC CURVES FOR APRIL

SCHWABE AND THE SUN-SPOT PERIOD 165

throughout the day ; repeating nearly the same
oscillations on the following day, but at the same
time gradually changing its behaviour so as to
oscillate differently in summer and winter. These
changes are very small, and would pass unnoticed
by the naked eye ; but when carefully watched
through a telescope, or better still, when photo-
graphed by some apparatus which will at the same
time magnify them, they can be rendered easily
visible. When the history of these changes is
traced it is seen at once that there is a manifest
connection with the cycle of sun-spot changes ; for
instance, if we measure the range of swing back-
wards and forwards during the day and take
the average for all the days in the year, and then
compare this with the average number of sun-
spots, we shall see that the averages rise and fall
together. Similarly we may take the up and
down swing, find the average amount of it
throughout the year, and again we shall find that
this corresponds very closely with the average
number of sun-spots.

But perhaps the most striking way to exhibit
the sympathy is to combine different variations of
the needle into one picture. And first we must
remark that there is another important variation
of the earth's magnetic action which we have not
yet considered. We have mentioned the swing of
the needle to and fro, and the swing up and down,
and these correspond to changes in the direction
of the force of attraction on the needle. But

1 66 ASTRONOMICAL DISCOVERY

there may be also changes in intensity of this
action ; the pull may be a little stronger or a
little weaker than before, and these variations are
not represented by any actual movement of the
needle, though they can be measured by proper
experiments. We can, however, imagine them
represented by a movement of the end of the
needle if we suppose it made of elastic material, so
that it would lengthen when the force was greater
Daily and contract slightly when the force was less. If
a pencil were attached to the end of such an
elastic needle so as to make a mark on a sheet of
paper, and if for a moment we exclude the up
and down movements, the pencil would describe
during the day a curve on the paper, as the end
of the needle swung backwards and forwards with
the change in direction, and moved across the
direction of swing with the change in intensity.
Now when curves of this kind are described for a
day in each month of the year, there is a striking
difference between the forms of them. During
Difference the summer months they are, generally speaking,
summer comparatively large and open, and during the
and win- wjnter months they are small and close. This
change in form is seen by a glance at Plate XIII. ,
which gives the curves throughout the whole of
one year. Let us now, instead of forming a curve
of this kind for each month, form a single average
curve for the whole year ; and let us further do
this for a series of years. (Plate XIV.) We
see that the curves change from year to year in a

SCHWABE AND THE SUN-SPOT PERIOD 167

manner very similar to that in which they change
from month to month in any particular year, and
the law of change is such that in years when there andbe-

J tween

are many sun-spots we get a large open curve sun-spot
similar to those found in the summer, while for and mini
years when there are few sun-spots we get small mum'
close curves very like those in the winter. Hence
we have two definite conclusions suggested : firstly,
that the changes of force are sympathetic with
the changes in the sun-spots ; and secondly, that
times of maximum sun-spots correspond to summer,
and times of minimum to winter. And here I must
admit that this is about as far as we have got at
present in the investigation of this relationship. Cause un-
Why the needle behaves in this way we have as
yet only the very vaguest ideas ; suggestions of
different kinds have certainly been put forward,
but none of them as yet can be said to have much
evidence in favour of its being the true one. For
our present purpose, however, it is sufficient to
note that there is this very real connection, and
that consequently Schwabe's sun-spot period may
have a very real importance with regard to changes
in our earth itself.

But I may perhaps repeat the word of caution
already uttered against extending without suf-
ficient evidence this notion of the influence of
sun-spots to other phenomena, such as weather.
A simple illustration will perhaps serve better
than a long argument to show both the way in
which mistakes have been made and the way in

1 68 ASTRONOMICAL DISCOVERY

which they can be seen to be mistakes. There

is at the Royal Observatory at Greenwich an in-

strument for noting the direction of the wind,

the essential part being an ordinary wind-vane,

the movements of which are automatically re-

niustra- corded on a sheet of paper. As the wind shifts

spurious from north to east the pencil moves in one

160 direction, and when it shifts back aain towards

the north the pencil moves in the reverse way.
But sometimes the wind shifts continuously from
north to east, south, west, vand back to north
again, the vane making a complete revolution ;
and this causes the pencil to move continuously
in one direction, until when the vane has come to
north again, the pencil is far away from the con-
venient place of record ; on such occasions it is
often necessary to replace it by hand. Then
again, the vane may turn in the opposite direction,
sending the pencil inconveniently to the other
side of the record. During the year it is easy to
count the number of complete changes of wind in
either direction, and subtracting one number from
the other, we get the excess of complete revolutions
of the vane in one direction over that in the other.
Now if these rather arbitrary numbers are set
down year by year, or plotted in the shape of a
diagram, we get a curve which may be compared
with the sun-spot curve, and during a period of
no less than sixteen years — from 1858 to 1874 —
there was a remarkable similarity between the two
diagrams. From this evidence alone it might

SCHWABE AND THE SUN-SPOT PERIOD 169

fairly be inferred that the sun-spots had some
curious effect upon the weather at Greenwich,
traceable in this extraordinary way in the changes
of the wind. But the particular way in which
these changes are recorded is so arbitrary that we
should naturally feel surprise if there was a real
connection between the two phenomena ; and
fortunately there were other records preceding
• these years and following them which enabled us
to test the connection further, and it was found, as
we might naturally expect, that it was not con-
firmed. On looking at diagrams (Plate XV.) for the
periods before and after, no similarity can be traced
between the sun-spot curve and the wind-vane
curve, and we infer that the similarity during the
period first mentioned was entirely accidental.
This shows that we must be cautious in accepting,
from a limited amount of evidence, a connection
between two phenomena as real and established ;
for it may be purely fortuitous. We may
particularly remark that it is desirable to have
repetitions through several complete periods in-
stead of one alone. It is possible to reduce to
mathematical laws the rules for caution in this
matter ; and much useful work has already been
done in this direction by Professor Schuster of
Manchester and others, though as yet too little
attention has been paid to their rules by investi-
gators naturally eager to discover some hitherto
unthought-of connection between phenomena.
With this example of the need for caution, we

1 70 ASTRONOMICAL DISCOVERY

may return to phenomena of which we can cer-
tainly say that they vary sympathetically with the
sun-spots. Roughly speaking, the whole history
of the sun seems to be bound up with them.
Besides these dark patches which we call spots
(which, by the way, are not really dark but only
less bright than the surrounding part of the disc),
there are patches brighter than the rest which
have been called faculse. With ordinary tele-
scopes, either visual or photographic, these can
generally only be detected near the edge of the
sun's disc ; but even with this limitation it can
easily be established that the faculae vary in
number and size from year to year much in the
same way as the spots, and this conclusion is
amply confirmed by the beautiful method of
observing the, faculse with the new instrument
designed by Professor Hale of the Yerkes Obser-
vatory. With this instrument, called a spectro-
heliograph, it is possible to photograph the
facula3 in all parts of the sun's disc, and thus to
obtain a much more complete history of them,
and there is no doubt whatever of their variation
sympathetically with the spots. Nor is there any
doubt about similar variations in other parts of
and the the sun which we cannot see at all with ordinary
sphere0 telescopes, except on the occasions when the, sun
is totally eclipsed. Roughly speaking, these out-
lying portions of the sun consist of two kinds, the
chromosphere and the corona, the former looking
like an irregular close coating of the ordinary sun,

SMOOTHED SUNSPOT CURVE (WOLF) COMPARED WITH THE NUMBER
OF TURNS MADE IN EACH YEAR BY THE Osi.ER ANEMOMETER VANE
OF THE ROYAL OBSERVATORY, GREENWICH (THE EXCESS OF THE

DIRECT TURNS (D) OVER THE RETROGRADE TURNS (R) OR VICE VERSA.)

1842

1846

36 D H

THE UPPER CURVE IS IN' EACH CASE THE SuNSPOT CURVE, THE LOWER THE VANE CURVE.

THE BREAK IN l832 IN THE VANE CURVE IS DUE TO THE OMISSION OK EVIDENTLY

ACCIDENTAL TURNS FROM THAT DATE.

PLATE XV.

SCHWABE AND THE SUN-SPOT PERIOD 171

and the latter like a pearly halo of light extending
to many diameters of the sun's disc, but not with
any very regular form.

The chromosphere, from which shoot out the
prominences or " red flames," can now be observed
without an eclipse if we employ the beautiful
instrument above-mentioned, the spectrohelio-
graph ; and Professor Hale has succeeded in pho-
tographing spots, faculae, and prominences all on
the same plate. But although many have made
the attempt (and Professor Hale, perhaps, a more
determined attempt than any man living), no one
has yet succeeded in obtaining any picture or
evidence of the existence of the corona excepting
on the occasion of a total solar eclipse.

Now these occasions are very rare. There are Eclipses
two or three eclipses of the sun every year, but
they are generally of the kind known as partial ;
when the moon does indeed come between us and
the sun to some extent, but only cuts off a portion
of his light — a clean-cut black disc is seen to en-
croach more or less on the surface of the sun.
Most of us have had an opportunity of seeing a
partial eclipse, probably more than once ; but few
have seen a total eclipse. For this the moon
must come with great exactness centrally be-
tween us and the sun ; and the spot where this
condition is fulfilled completely only covers a few
hundred miles of the earth's surface at one moment.
As the earth turns round, and as the moon revolves
in its orbit, this patch from which the sun is totally

ASTRONOMICAL DISCOVERY

eclipsed travels over the earth's surface, marking
out a track some thousands of miles in length
possibly, but still not more than 200 miles wide ;
Total and in order to see the sun totally eclipsed even
rare. on the rare occasions when it is possible at all
(for, as already remarked, in the majority of cases
the eclipse is only partial), we must occupy some
station in this narrow belt or track, which often
tantalisingly passes over either the ocean or some
regions not easily accessible to civilised man.
Moreover, if we travel to such favoured spots
the whole time during which the sun is totally
eclipsed cannot exceed a few minutes, and hence
observations are made under rather hurried and
trying conditions. In these modern days of pho-
tography it is easier to take advantage of these
precious moments than it used to be when there
was only the eye and memory of an excited
observer to rely upon. It is perhaps not sur-
prising that some of the evidence collected on
these earlier occasions was conflicting ; but nowa-
days the observers, generally speaking, direct
their energies in the first place to mounting
accurately in position photographic apparatus of
different kinds, each item of it specially designed
to settle t some particular problem in the most
feasible way ; secondly, to rehearsing very care-
fully the exact programme of exposures necessary
during the critical few minutes ; and finally, to
securing these photographs with as few mistakes
as possible when the precious moments actually

SCHWABE AND THE SUN-SPOT PERIOD 173

arrive. Even then the whole of their efforts are
quite likely to be rendered unavailing by a passing
cloud; and bitter is the disappointment when,
after travelling thousands of miles, and spending
months in preparation, the whole enterprise ends
in nothing owing to some caprice of the weather.
Hence it will easily be imagined that our know-
ledge of the corona, the part of the sun which we
can still only study on occasions of a total solar
eclipse, advances but slowly. During the last
twenty years there has been altogether scarcely
half-an-hour available for this research, though it
may fairly be said that the very best possible use
has been made of that half-hour. And, what is
of importance for our immediate purpose, it has
gradually been established by comparing the pho-
tographs of one eclipse with those of another, that
the corona itself undergoes distinct changes in Corona
form in the same period which governs the changes
of sun-spots. When there are many sun-spots
the corona spreads out in all directions from the
edge of the sun's disc ; when there are few sun-
spots the corona extends very much further in
the direction of the sun's equator, so that at sun-
spot minimum there is an appearance of two huge
wings. Although the evidence is necessarily
collected in a scrappy manner, by this time there
is sufficient to remove this relationship out of the
region of mere suspicion, and to give it a well-
established place in our knowledge of the sun's
surroundings.

174 ASTRONOMICAL DISCOVERY

Now the corona of the sun may be compared
to some rare animal which we only see by pay-
ing a visit to some distant land, and may con-
sider ourselves even then fortunate to get a
glimpse of; and it might be thought that the
habits of such an animal are not likely to be of
any great importance in our everyday life. But
so far from this being the case in regard to the
corona, it is more than possible that the know-
ledge of its changes may be of vital interest to
us. I have already said that, as yet, we have no
satisfactory account of the reason why changes
in sun-spots seem to influence changes in our
magnets on the earth ; but one of the theories
put forward in explanation, and one by no means
the least plausible, is that this influence may come,
not from the sun-spots themselves, but from some
other solar phenomenon which varies in sympathy
with them ; and in particular that it may come
Corona from the corona. These wings which reach out
fuTence" at sun-spot minimum can be seen to extend a
magnets, considerable distance, and there is no reason to
suppose that they actually cease at the point
where they become too faint for us to detect
them further; they may extend quite as far as
the earth itself and even beyond ; and they may
be of such' a nature as to influence our magnets.
As the earth revolves round the sun it may some-
time plunge into them, to emerge later and pass
above or below them ; as again the wings spread
themselves at sun-spot minimum and seem to

SCHWABE AND THE SUN-SPOT PERIOD 175

shrink at maximum, so our magnets may respond
by sympathetic though very small vibrations.
Hence it is quite possible that the corona is
directly influencing the magnetic changes on the
earth.

But it may be urged that these changes are Possible
so slight as to be merely of scientific interest,
That may be true to-day, but who will be bold
enough to say that it will be true to-morrow ? If
we are thinking of practical utility alone, we may
remember that two great forces of Nature which
we have chained into the service of man, steam
and electricity, put forth originally the most
feeble manifestations, which might readily have
been despised as valueless ; but by careful atten-
tion to proper conditions results of overwhelming
practical importance have been obtained from
these forces, which might have been, and for
many centuries were, neglected as too trivial to be
worth attention. Recently the world has been
startled by the discovery of new elements, such as
radium, whose very existence was only detected
by a triumph of scientific acuteness in investiga-
tion, and yet which promise to yield influences
on our lives which may overwhelm in importance
all that has gone before. And similarly it may
be that these magnetic changes, when properly
interpreted or developed, may become of an im-
portance in the future out of all proportion to
the attention which they have hitherto attracted.
Hence, although perhaps sufficient has already

i;6 ASTRONOMICAL DISCOVERY

been established to show the immense con-
sequences which flow from Schwabe's remarkable
discovery of the periodicity in solar spots, we
may be as yet only on the threshold of its real
value.

From what little causes great events spring !
How little can Schwabe have realised, when he
began to point his modest little telescope at the
sun, and to count the number of spots — the
despised spots which he had been assured were
of no interest and exhibited no laws, and were
generally unprofitable — that he was taking the
first step in the invention of the great science of
Solar Physics ! — a science which is, I am glad to
say, occupying at the present moment so much
of the attention, not only of the great Yerkes
Observatory, but of many other observatories
scattered over the globe.

CHAPTER VI
THE VARIATION OF LATITUDE

IF we should desire to classify discoveries in
order of merit, we must undoubtedly give a high
place to those which are made under direct dis-
couragements. In the last chapter we saw that
Schwabe entered upon his work under conditions
of this kind, it being the opinion of experienced
astronomers who had looked at the facts that
there was nothing of interest to be got by watch-
ing sun-spots. In the present chapter I propose
to deal with a discovery made in the very teeth
of the unanimous opinion of the astronomical
world by an American amateur, Mr. S. C. Chandler
of Cambridge (Massachusetts). It is my purpose
to allow him to himself explain the steps of this
discovery by giving extracts from the magni-
ficent series of papers which he contributed to
the Astronomical Journal on the subject in the
years 1891-94, but it may help in the under-
standing of these extracts if I give a brief
summary of the facts. And I will first explain
what is meant by the "Variation of Latitude."

We are all familiar with the existence of a
certain star in the heavens called the Pole' Star,
and we know that at any particular place it is

177 M

1 78 ASTRONOMICAL DISCOVERY

seen constantly in the north at a definite height
Latitude, above the horizon, which is the latitude of the
place. When watched carefully with a telescope
it is found to be not absolutely stationary, but
to describe a small circle in the heavens day
by day, or rather night by night. These simple
facts are bound up with the phenomenon of the
earth's rotation in this way : the axis about which
it is rotating points to the centre of that little
circle, and any change in the position of the axis
can therefore be determined by observing these
motions of the Pole Star. Such changes may be
of two kinds : firstly, we might find that the size
of the circle increased or diminished, and this
would mean that the earth's axis was pointing
farther away from the Pole Star or nearer to it —
pointing, that is to say, in a different direction
in space. This actually happens (as has been
known for some thousands of years) owing to
Preces- the phenomenon called " precession " ; the circle
described by our Pole Star is at present getting a
little smaller, but it will ultimately increase in
size, and after thousands of years become so large
that the Pole Star will entirely lose its character
as a steady guide to the North.

Secondly (and this is what more immediately
change of concerns us), the centre of the circle may alter
its position and be no longer at the same height
above the horizon of any given place. This would
mean that the earth's axis was shifting in the earth
itself— that the North Pole which our explorers

THE VARIATION OF LATITUDE 179

go to seek is not remaining in the same place.
That it does not change appreciably in position
we know from familiar experience ; our climates,
for instance, would suffer considerably if there
were any large changes. But astronomers are
concerned with minute changes which would not
have any appreciable effect on climate, and the
question has long been before them whether, put-
ting aside large movements, there were any minute
variations in position of the North Pole. Twenty Twenty
years ago the answer to this question would have aS*8
been given decidedly in the negative ; it was b<
considered as certain that the North Pole did
not move at all within the limits of our most
refined astronomical observations. Accepted
theory seemed to indicate that any movements
must in any case recur after a period of ten
months, and careful discussion of the observa-
tions showed that there was no oscillation in
such a period. Now we know that the theory
itself was wrong, or rather was founded upon a
mistaken assumption ; and that the facts when
properly examined show clearly a distinct move-
ment of the North Pole, not a very large one, for
all its movements take place within the area
occupied by a moderate-sized room, but still a
movement easily measurable by astronomical ob-
servations, and Mr. Chandler was the first to
point out the law of these movements, and very
possibly the first to suspect them.

With these few words of explanation I will

i8o ASTRONOMICAL DISCOVERY

chand- let Mr. Chandler tell his own story. His first
papers, paper appeared in the Astronomical Journal in
November 1891, and is courageously headed, " On
the Variation of Latitude " — I say courageously,
because at that time it was believed that the
latitude did not vary, and Mr. Chandler him-
self was only in possession of a small portion
of the facts. They unravelled themselves as
he went forward ; but he felt that he had firm
hold of the end of the thread, and he faced the
world confidently in that belief. He begins
thus : —

First "In the determination of the latitude of Cam-

change, bridge * with the Almucantar, about six years and
a half ago, it was shown that the observed
values, arranged according to nights of observa-
tion, exhibited a decided and curious progression
throughout the series, the earlier values being
small, the later ones large, and the range from
November 1884 to April 1885 being about
four-tenths of a second. There was no known
or imaginable instrumental or personal cause
for this phenomenon, yet the only alternative
seemed to be an inference that the latitude had
actually changed. This seemed at the time too
bold an inference to place upon record, and I
therefore left the results to speak for themselves.
The subsequent continuation of the series of
observations to the end of June 1885 gave a

1 This should be Cambridge, Mass.

THE VARIATION OF LATITUDE 181

maximum about May i, while the discussion
of the previous observations from May to
November 1884 gave a minimum about Sep-
tember i, indicating a range of 0^.7 within a
half-period of about seven months."

Mr. Chandler then gives some figures in support
of these statements, presenting them with the
clearness which is so well marked a feature of
the whole series of papers, and concludes this
introductory paper as follows : —

"It thus appears that the apparent change in
the latitude of Cambridge is verified by this
discussion of more abundant material. The
presumption that it is real, on this determina-
tion alone, would justify further inquiry.

" Curiously enough Dr. Klistner, in his deter- Confirmed
mination of the observation from a series of"
observations coincident in time with those of
the Almucantar, came upon similar anomalies,
and his results, published in 1888, furnish a
counterpart to those which I had pointed out
in 1885. The verification afforded by the recent
parallel determinations at Berlin, Prague, Pots-
dam, and Pulkowa, which show a most surprising
and satisfactory accordance, as to the character
of the change, in range and periodicity, with
the Almucantar results, has led me to make
further investigations on the subject. They
seem to establish the nature of the law of those

1 82 ASTRONOMICAL DISCOVERY

changes, and I will proceed to present them in
due order."

The second paper appeared on November 23,
and opens with the following brief statement of
his general results at that time : —

"Before entering upon the details of the
investigations spoken of in the preceding
number, it is convenient to say that the general
result of a preliminary discussion is to show a
427 days' revolution of the earth's pole in a period of 427
od' days, from west to east, with a radius of thirty
feet, measured at the earth's surface. Assuming
provisionally, for the purpose of statement, that
this is a motion of the north pole of the principal
axis of inertia about that of the axis of rotation,
the direction of the former from the latter lay
towards the Greenwich meridian about the
beginning of the year 1890. This, with the
period of 427 days, will serve to fix ap-
proximately the relative positions of these axes
at any other time, for any given meridian. It
is not possible at this stage of the investigation
to be more precise, as there are facts which
appear to show that the rotation is not a
perfectly uniform one, but is subject to secular
change, and perhaps irregularities within brief
spaces of time."

It is almost impossible, now that we have
become familiar with the ideas conveyed in this

THE VARIATION OF LATITUDE 183

paragraph, to understand, or even fully to re-
member, the impression produced by them at the
time ; the sensation caused in some quarters, and
the ridicule excited in others. They were in flat Contrary
contradiction to all accepted views ; and it was ceived
believed that these views were not only theoreti- v]
cally sound, but had been matured by a thorough
examination of observational evidence. The only
period in which the earth's pole could revolve was
believed to be ten mouths ; and here was Mr.
Chandler proclaiming, apparently without any
idea that he was contradicting the laws of
dynamics, that it was revolving in fourteen
months ! The radius of its path had been found
to be insensible by careful discussion of observa-
tions, and now he proclaimed a sensible radius of
thirty feet. Finally, he had the audacity to
announce a variable period, to which there was
nothing at all corresponding in the mathematical
possibilities. This was the bitterest pill of all.
Even after Professor Newcomb had shown us how
to swallow the other two, he could not recommend
any attempt at the third, as we shall presently
see ; and Mr. Chandler was fain ultimately to gild
it a little before it could be gulped.

But this is anticipating, and it is our intention
to follow patiently the evidence adduced in support
of the above statements, made with such splendid
confidence to a totally disbelieving world. Mr.
Chandler first examines the observations of Dr.
Kiistner of Berlin, quoted at the end of his last

i84 ASTRONOMICAL DISCOVERY

paper, and shows how well they are suited by the
existence of a variation in the latitude of 427
days ; and that this new fact is added — when the
Cambridge (U.S.A.) latitudes were the smallest
those of Berlin were the largest, and vice versa, as
would clearly be the case if the phenomenon was
due to a motion of the earth's pole ; for if it moved
nearer America it must move further from Europe.
Puikowa He then examines a long series of observations
solved, made in the years 1864-1873 at Puikowa, near
St. Petersburg, and again finds satisfactory con-
firmation of his law of variation. Now it had long
been known that there was something curious
about these observations, but no one could tell
what it was. The key offered by Mr. Chandler
fitted the lock exactly, and the anomalies which
had been a puzzle were removed. This was in
itself a great triumph ; but there was another to
come, which we may let Mr. Chandler describe in
his own words : —

also "In 1862 Professor Hubbard began a series of

ton. observations of « Lyree at the Washington Obser-
vatory with the prime vertical transit instrument,
for the purpose of determining the constants of
aberration and nutation and the parallax of the
star. The 'methods of observation and reduction
were conformed to those used with such success
by W. Struve. After Hubbard's death the series
was continued by Professors Newcomb, Hall, and
Harkness until the beginning of 1867. Professor

THE VARIATION OF LATITUDE 185

Hall describes these observations as the most
accurate determinations of declination ever made
at the Naval Observatory. The probable error of
a declination from a single transit was +o".i4i,
and judging from the accidental errors, the series
ought to give trustworthy results. Upon reduc-
ing them, however, it was found that some ab-
normal source of error existed, which resulted in
anomalous values of the aberration-constant in
the different years, and a negative parallax in
all. A careful verification of the processes of
reduction failed to discover the cause of the
trouble, and Professor Hall says that the results
must stand as printed, and that probably some
annual disturbance in the observations or the
instrument occurred, which will never be ex-
plained, and which renders all deductions from
them uncertain. The trouble could not be con-
nected with personal equation, the anomalies
remaining when the observations of the four
observers who took part were separately treated.
Nor, as Professor Hall points out. will the theo-
retical ten-month period in the latitude furnish
the explanation.

"It is manifest, however, that if the 427-day
period exists, its effect ought to appear distinctly
in declination-measurements of such high degree
of excellence as these presumably were, and, as I
hope satisfactorily to show, actually are. When
this variation is taken into account the observa-
tions will unquestionably vindicate the high ex-

1 86 ASTRONOMICAL DISCOVERY

pectations entertained with regard to them by
the accomplished and skilful astronomers who
designed and carried them out."

From this general account I am excluding
technical details and figures, and unfortunately
a great deal is thereby lost. We lose the sense
of conviction which the long rows of accordant
figures force upon us, and we lose the oppor-
tunities of admiring both the astonishing amount
of work done and the beautiful way in which the
material is handled by a master. But I am
tempted to give one very small illustration of
the numerical results from near the end of the
paper. After discussing the Washington results,
and amply fulfilling the promise made in the pre-
Direction ceding extract, Mr. Chandler compares them with
tionof the Pulkowa results, and shows that the Earth's
Pole must be revolving from west to east, and not
from east to west. And then he writes down a
simple formula representing this motion, and com-
pares his formula with the observations. He
gives the results in seconds of arc, but for the
benefit of those not familiar with astronomical
measurements we may readily convert these into
feet ; and in the following tables are shown the
Example distances of the Earth's Pole in feet from its

of results. ... i , n ,TT . . ,

average position/ as observed at Washington and

1 The distances do not represent the total displacement, but only
the displacement towards Washington in one case and towards
Pulkowa in the other.

THE VARIATION OF LATITUDE 187

at Pulkowa, and the same distances calculated
according to the formula which Mr. Chandler
was able to write down at this early stage. The
signs 4- and — of course indicate opposite direc-
tions of displacement : —

WASHINGTON.

Deviation of Pole.

Date.

Observed.

Formula.

1864, Dec. 28

- 28 feet

- 23 feet

1865, Mar. 19

- I »

-12 „

„ June i

+ 15 „

+ 12 „

„ Aug. ii

+ 22 „

+ 23 »

„ Oct. 9

+ H »

+ 15 „

„ Dec. 13

-17 „

- 6 „

PULKOWA.

Deviation of Pole.

Date.

Observed.

Formula.

1865, July 25 . .

-18 feet

-12 feet

„ Sept. 9.

+ 3 »

+ 3 »,

„ NOV. 22 .

+ 26 „

+ 22 „

1 866, Feb. 22 .

+ 18 „

+ 13 »

„ June 4.

-ii „

-18 „

„ July 17.

-16 „

-23 „

Of course the figures are not exact in every case,
but they are never many feet wrong ; and it may

1 88 ASTRONOMICAL DISCOVERY

well be imagined that it is a difficult thing to
deduce, even from the most refined observations,
the position of the earth's pole to within a foot.
The difficulty is exactly the same as that of
measuring the length of an object 300 miles
away to within an inch !

Mr. Chandler winds up his second paper
thus : —

"We thus find that the comparison of the
simultaneous series at Pulkowa and Washington,
1863-1867, leads to the same conclusion as that
already drawn from the simultaneous series at
Berlin and Cambridge, 1884-1885. The direc-
tion of the polar motion may therefore be looked
upon as established with a large degree of pro-
bability.

" In the next paper I will present the results
derived from PETERS, STRUVE, BRADLEY, and
various other series of observations, after which
the results of all will be brought to bear upon
the determination of the best numerical values
of the constants involved."

Bradiey's The results were not, however, presented in
tk>ns.va this order. In the next paper, which appeared
on December 23, 1891, Mr. Chandler begins, with
the work of Bradley, the very series of observa-
tions at Kew and Wansted which led to the
discoveries of aberration and nutation, and which
we considered in the third chapter. He first

THE VARIATION OF LATITUDE 189

shows that, notwithstanding the obvious accuracy
of the observations, there is some unexplained
discordance. The very constant of aberration
which Bradley discovered from them differs by
half-a-second of arc from our best modern deter-
minations. Attempts have been made to ascribe
the discordance to changes in the instrument, but
Mr. Chandler shows that such changes, setting
aside the fact that Bradley would almost certainly
have discovered them, will not fit in with the
facts. The facts, when analysed with the skill
to which we have become accustomed, are that
there is a periodic swing in the results with a Latitude
period of about a year, and not fourteen months, twelve
as before, "a result so curious," as he admits,
that "if we found no further support, it might
lead us to distrust the above reasoning, and throw
us back to the possibility that, after all, BRADLEY'S
observations may have been vitiated by some kind
of annual instrumental error. But it will abun-
dantly appear, when I have had the opportunity
to print the deductions from all the other series
of observations down to the present time, that the
inference of an increase in the period of polar
revolution is firmly established by their concur-
rent testimony." We shall presently return to
this curious result, which might well have dis-
mayed a less determined researcher than Mr.
Chandler, but which only led him on to re-
newed exertions.

The results obtained from Bradley's obser-

1 90 ASTRONOMICAL DISCOVERY

vations may be put in the form of a diagram
thus : —

VARIATION OF LATITUDE
Bradley 's Observations,
<A*2**Hf.a*)

V V

-t'-O APRIL, APRIL, APRIL, -1"O

1728. 1729. 1730.

FIG. 7.

It will be seen that the maxima and minima
fall in the spring and autumn, and this fact alone
seemed to show that the effect could not be due
to temperature, for we should expect the greatest
effect in that case in winter and summer. It
could not be due to the parallax of the stars
for which Bradley began his search, for stars in
different quarters of the heavens would then be
differently affected, and this was not the case.
"There remains," concluded Mr. Chandler after
full discussion, " the only natural conclusion of
an actual displacement of the zenith, in other
words, a change of latitude." And he concludes
this paper with the following fine passage : —

u So far, then, as the results of this incompar-
able series of observations at Kew and Wansted,

THE VARIATION OF LATITUDE 191

considered by themselves alone, can now be
stated, the period of the polar rotation at that
epoch appears to have been probably somewhat
over a year, and certainly shorter by about two
months than it is at the present time. The
range of the variation was apparently in the
neighbourhood of a second of arc, or consider-
ably larger than that shown by the best modern
observations.

" Before taking leave of these observations for Bradiey's
the present I cannot forbear to speak of the pro- gre<
found impression which a study of them leaves
upon the mind, and the satisfaction which all
astronomers must feel in recognising that, besides
its first fruits of the phenomena of aberration and
nutation, we now owe also our first knowledge of
the polar motion to this same immortal work of
Bradley. Its excellence, highly appreciated as it
has been, has still been hitherto obscured by the
presence of this unsuspected phenomenon. When
divested of its effects, the wonderful accuracy of
this work must appear in a finer light, and our
admiration must be raised to higher pitch. Going
back to it after one hundred and sixty years seems
indeed like advancing into an era of practical
astronomy more refined than that from which we
pass. And this leads to a suggestion worthy of
serious practical consideration — whether we can
do better in the future study of the polar rotation,
than again to avail ourselves of Bradiey's method,

1 92 ASTRONOMICAL DISCOVERY

without endangering its elegant simplicity and
effectiveness by attempts at improvement, other
than supplying certain means of instrumental
control which would without doubt commend
themselves to his sagacious mind.

" In the next article Bradley's later observations
at Greenwich, the results of which are not so
distinct, will be discussed ; and also those of
Brinkley at Dublin, 1808-13 and 1818-22.
This will bring again to the surface one of the
most interesting episodes in astronomical history,
other the spirited and almost acrimonious dispute
explained, between Brinkley and Pond with regard to stellar
parallaxes. I hope to show that the hitherto
unsolved enigma of Brinkley's singular results
finds its easy solution in the fact of the polar
motion. The period of his epoch appears to have
been about a year, and, its range more than a
second. Afterwards will follow various dis-
cussions already more or less advanced towards
completion. These include Bessel's observations
at Konigsberg, 1820-24, with the Reichenbach
circle, and in 1842-44 with the Repsold circle;
the latitudes derived from the polar-point deter-
minations of Struve and Madler with the Dorpat
circle, 1822-38; Struve's observations for the
determination of the aberration ; Peters' observa-
tions of Polaris, 1841-43, with the vertical-circle ;
the results obtained from the reflex zenith-tube
at Greenwich, 1837-75, whose singular anomalies

THE VARIATION OF LATITUDE 193

can be referred in large part to our present
phenomenon, complicated with instrumental
error, to which until now they have been ex-
clusively attributed ; the Greenwich transit-circle
results, 1851-65, in which case, however, a similar
complication and the large accidental errors of
observation seem to frustrate efforts to get any
pertinent results ; the Berlin prime-vertical obser-
vations ofWeyerand Brtinnow, 1845-46, in which
I hope to show that the parallax of /3 Draconis
derived from them is simply a record of the
change of latitude ; the conflicting latitude deter-
minations at Cambridge, England ; the Washing-
ton observation of Polaris and other close Polars,
1866-87, with the transit-circle; also those at
Melbourne, 1863-84, a portion of which have
already been drawn upon in the last number of
the Journal, and some others. While the list is
a considerable one, I shall be able to compress
the statement of results for many of the series
into a short space.

" In connection with this synopsis of the scope
of the investigations, one or two particulars may
be of interest, which at the present writing seem
to foreshadow the probable outcome. I beg, how- P

ever, that the statement will be regarded merely nature of
as a provisional one. First, while the period is results*
manifestly subject to change, as has already once
or twice been intimated, I have hitherto failed in
tracing the variations to any regular law, expres-
sible in a numerical formula. Indeed, the general

194 ASTRONOMICAL DISCOVERY

impression produced by a study of these changes
in the length of the period is that the cause which
produces them operates capriciously to a certain
degree, although the average; effect for a century
•has been to diminish the velocity of the revolution
of the pole. How far this impression is due to
the uncertainty of the observations, and to the
.complication of the phenomenon with other
periodical changes of a purely instrumental kind,
I cannot say. , Almost all of the series of any
extent which have been examined, have the
peculiarity that ' they manifest the periodicity
quite uniformly and distinctly for a number of
years, then for a while obscurely. In some cases,
however, what at first .appears to be an objective
irregularity proves - not to be so by comparison
with overlapping series at other observatories. '

"Another characteristic which has struck my
attention, although somewhat vaguely, is that the
variations in, the length of the period seem to go
hand in hand with simultaneous alterations in the
amplitude of the rotation ; the shorter periods
being apparently associated with the larger cor
efficients for the latter. The verification of these
surmises awaits a closer comparative scrutiny, the
opportunity for which will come when the com-
putations are in a more forward state. If con-
firmed, these observations will afford a valuable
touchstone, in seeking for the cause of a pheno-
menon which now seems to be at variance with
the accepted laws of terrestrial rotation."

THE VARIATION OF LATITUDE 195

Let us now for a few moments turn aside from Reception
the actual research to see how the announcement
was received. It would be ungracious to reprint
here any of the early statements of incredulity
which found their way into print, especially in
Germany. But the first note of welcome came
from Simon Newcomb, in the same number of
the Astronomical Journal as the paper just dealt
with, and the following extract will indicate both
the difficulties felt in receiving Mr. Chandler's
results and the way in which Newcomb struck at
the root of them.

" Mr. Chandler's remarkable discovery, that the
apparent variations in terrestrial latitudes may be
accounted for by supposing a revolution of the
axis of rotation of the earth around that of figure,
in a period of 427 days, is in such disaccord with
the received theory of the earth's rotation that at
£rst I was disposed to doubt its possibility. But I
am now able to point out a vera causa which
affords a complete explanation of this period. Up New-
to the present time the treatment of this subject
has been this : The ratio of the moment of inertia
of the earth around its principal axis to the mean
of the other two principal moments, admits of very
accurate determination from the amount of pre-
cession and nutation. This ratio involves what
we might call, in a general way, the solid
ellipticity of the earth, or the ellipticity of a

196 ASTRONOMICAL DISCOVERY

homogeneous spheroid having the same moments
of inertia as the earth.

" When the differential equations of the earth's
rotation are integrated, there appear two arbitrary
constants, representing the position of any as-
signed epoch of the axis of rotation relative to
that of figure. Theory then shows that the axis
of rotation will revolve round that of figure, in a
period of 306 days, and in a direction from west
toward east. The attempts to determine the
value of these constants have seemed to show
that both are zero, or that the axes of rotation
and figure are coincident. Several years since,
Sir William Thomson published the result of
a brief computation from the Washington Prime-
Vertical observations of a Lyrae which I made at
his request and which showed a coefficient of o".O5.
This coefficient did not exceed the possible error of
the result ; I therefore regarded it as unreal.
The " The question now arises whether Mr. Chand-

forgotten ler's result can be reconciled with dynamic theory.

assump-

tion. I answer that it can, because the theory which
assigns 306 days as the time of revolution is based
on the hypothesis that the earth is an absolutely
rigid body. But, as a matter of fact, the fluidity
of the ocean plays an important part in the
phenomenon, as does also the elasticity of the
earth. The combined effect of this fluidity and
elasticity is that if the axis of rotation is displaced
by a certain amount, the axis of figure will, by the

THE VARIATION OF LATITUDE 197

changed action of the centrifugal force, be moved
toward coincidence with the new axis of rotation.
The result is, that the motion of the latter will be
diminished in a corresponding ratio, and thus
the time of revolution will be lengthened. An
exact computation of the effect is not possible
without a knowledge of the earth's modulus of
elasticity. But I think the result of investiga-
tion will be that the rigidity derived from Mr.
Chandler's period is as great as that claimed by
Sir William Thomson from the phenomena of the
tides."

This was very satisfactory. Professor New-
comb put his finger on the assumption which
had been made so long ago that it had been for-
gotten : and the lesson is well worth taking to
heart, for it is not the first time that mistaken
confidence in a supposed fact has been traced to
some forgotten preliminary assumption: and we
must be ever ready to cast our eyes backward
over all our assumptions, when some new fact
seems to challenge our conclusions. It might
further be expected that this discovery of the way
in which theory had been defective would as a
secondary consequence inspire confidence in the
other conclusions which Mr. Chandler had But
arrived at in apparent contradiction to theory ; i^s work
or at least suggest the suspension of judgment,
But Professor Newcomb did not feel that this
was possible in respect of the change of period,

198 ASTRONOMICAL DISCOVERY

from about twelve months in Bradley's time to
fourteen months in ours. We have seen that
Mr. Chandler himself regarded this as a " curious
result " requiring confirmation : but since the
confirmation was forthcoming, he stated it with
full confidence, and drew the following remarks
from Professor Newcomb in July 22, 1892 : —

" The fact of a periodic variation of terrestrial
latitudes, and the general law of that variation,
have been established beyond reasonable doubt
by the observations collected by Mr. Chandler.
But two of his minor conclusions, as enumerated
in No. 3 of this volume, do not seem to me well
founded. They are —

" i. That the period of the inequality is a vari-
able quantity.

" 2. That the amplitude of the inequality has
remained constant for the last half century."

Professor Newcomb proceeds to give his reasons
for scepticism, which are too technical in character
to reproduce here. But I will quote the following
further sentence from his paper :—

"The question now arises how far we are
entitled to assume that the period must be in-
variable. I reply that, perturbations aside, any
variation of the period is in such direct conflict
with the laws of dynamics that we are entitled to
pronounce it impossible. But we know that there
are perturbations, and I do not see how one can

THE VARIATION OF LATITUDE 199

doubt that they have so acted as to increase the
amplitude of the variation since 1840."

In other words, while recognising that there may
be a way of reconciling one of the "minor" con-
clusions with theory, Professor Newcomb con-
siders that in this case the other must go. Mr.
Chandler's answer will speak for itself. It was chand-
delayed a little in order that he might present ler'sreply'
an immense mass of evidence in support of his
conclusions, and was ultimately printed on August
23, 1892.

" The material utilised in the foregoing forty-
five series aggregates more than thirty-three thou-
sand observations. Of these more than one-third
were made in the southern hemisphere, a fact
which we owe principally to Cordoba. It com-
prises the work of seventeen observatories (four
of them in the southern hemisphere) with twenty-
one different instruments, and by nine distinct
methods of observation. Only three of the series
(XXL, XXV., and XXXV.), and these among
the least precise intrinsically, give results con-
tradictory of the general law developed in
No. 267. This degree of general harmony is
indeed surprising when the evanescent char-
acter of the phenomenon under investigation
is considered.

"The reader has now before him the means for
independent scrutiny of the material on which the
conclusions already drawn, and those which are

200 ASTRONOMICAL DISCOVERY

to follow, are based. The space taken in the
printing may seem unconscionable, but I hope
this will be charged to the extent of the evidence
collected, and not to diffuseness or the presenta-
tion of needless detail ; for I have studiously
sought to compress the form of statement with-
out omitting anything essential for searching
criticism. That it was important to do this is
manifest, since the conclusions, if established,
overthrow the existing theory of the earth's
rotation, as I have pointed out on p. 21. I am
neither surprised nor disconcerted, therefore, that
Professor Newcomb should hesitate to accept
some of these conclusions on the ground (A. J.,
No. 271) that they are in such conflict with the
laws of dynamics that we are entitled to pro-
nounce them impossible. He has been so
considerate and courteous in his treatment
of my work thus far, that I am sure he will
not deem presumptuous the following argument
in rebuttal.

He "put " It should be said, first, that in beginning these
teachings investigations last year, I deliberately put aside
j^eo „ all teachings of theory, because it seemed to me
high time that the facts should be examined by
a purely inductive process ; that the nugatory
results of' all attempts to detect the existence of
the Eulerian period probably arose from a defect
of the theory itself ; and that the entangled con-
dition of the whole subject required that it should
be examined afresh by processes unfettered by

THE VARIATION OF LATITUDE 201

any preconceived notions whatever. The problem
which I therefore proposed to myself was to see
whether it would not be possible to lay the
numerous ghosts — in the shape of numerous dis-
cordant residual phenomena pertaining to deter-
minations of aberration, parallaxes, latitudes, and
the like — which had heretofore flitted elusively
about the astronomy of precision during the
century ; or to reduce them to tangible form
by some simple consistent hypothesis. It was
thought that if this could be done, a study of
the nature of the forces, as thus indicated, by
which the earth's rotation is influenced, might
lead to a physical explanation of them.

"Naturally, then, I am not much dismayed by and "is
the argument of conflict with dynamic laws, since ^ayed."
all that such a phrase means must refer merely
to the existent state of the theory at any given
time. When the 427-day period was propounded,
it was as inconsistent with known dynamic law as
the variation of it now appears to be. Professor
Newcomb's own happy explanation has already
set aside the first difficulty, as it would appear,
and advanced the theory by an important step.
Are we so sure yet of a complete knowledge of
all the forces at work as to exclude the chance
of a vera causa for the second ? "

There is a splendid ring of resolution about Faraday's
these words. Let us compare them with a notable ™
utterance of Faraday : —

202 ASTRONOMICAL DISCOVERY

"The philosopher should be a man willing to
listen to every suggestion, but determined to judge
for himself. He should not be biassed by appear-
ances ; have no favourite hypothesis ; be of no
school ; and in doctrine have no master. He
should not be a respecter of persons, but of
things. Truth should be his primary object. If
to these qualities be added industry, he may in-
deed hope to walk within the veil of the temple
of Nature."

Tested by this severe standard, Mr. Chandler
fails in no particular, least of all in that of
industry. The amount of work he got through
about this time was enormous, for besides the
main line of investigation, of which we have
chandler's onlv had after all a mere glimpse, he had been

other work * ... n. i » •«•

at this able to turn aside to discuss a subsidiary question
with Professor Comstock ; he had examined with
great care some puzzling characteristics in the
variability of stars ; he computed some comet
ephemerides ; and he was preparing a new cata-
logue of variable stars — a piece of work involving
the collection and arrangement of great masses
of miscellaneous material. Yet within a few
months after replying as above to Professor New-
comb's criticism, he was able to announce that
he had found the key to the new puzzle, and that

Hisuiti- "theory and observation were again brought into

Sctory 1S~ complete accord." We will as before listen to
elation. ^g accounj- of fafe new g^ep jn kjs own wor(JSj

THE VARIATION OF LATITUDE 203

but a slight preliminary explanation may help
those unaccustomed to the terminology. The
polar motion was found to be compounded of two
independent motions, both periodic, but having
different periods. Now, the general results of
such a composition are well known in several
different branches of physics, especially in the
theory of sound. If two notes of nearly .the same
pitch be struck at the same time, we hear the
resultant sound alternately swell and die away,
because the vibrations caused by the two notes interfer-
are sometimes going in the same direction, and twowaves.
after an interval are going exactly in opposite
directions. Diagrammatically we should repre-
sent the vibrations by two waves, as below; the

FIG. 8.

upper wave goes through its period seven and
a half times between A and D, the lower only
six times ; and it is easily seen that at A and C
the waves are sympathetic, at B and D anti-
pathetic. At A and C the compound vibration
would be doubled ; at B and D reduced to insensi-
bility. The point is. so important that perhaps
a numerical illustration of it will not be super-
fluous. The waves are now represented by rows

204 ASTRONOMICAL DISCOVERY

of figures as below. The first series recurs after
every 6, the second after every 7.

First Wave .. . 1234321234321234321234321234321
Second Wave . 1234432123443212344321234432123

Combined Effect . 2468753357764446665555555666444
Great disturbance. Calm.

First Wave . . 2343212343212343212343212343212
Second Wave . 4432123443212344321234432123443

Combined Effect . 6775335786424687533577644466655

Great disturbance.

Adding the two rows together, the oscillations
at first reinforce one another and we get numbers
ranging from 2 to 8 instead of from i to 4 ; but
one wave gains on the other, until it is rising
when the other is falling, and the numbers add
up to a steady series of 5's. It will be seen that
there are no less than seven consecutive 5's, and
all the variation seems to have disappeared. But
presently the waves separate again, and the period
of great disturbance recurs ; it will be seen that in
the " combined effect" the numbers repeat exactly
after the 42nd term. Now those unfamiliar with
the subject may not be prepared for the addition of
one physical wave to another, as though they were
iiiustra- numbers, but the analogy is perfect. Travellers
tion from ky some Of the fast twin-screw steamers have had

OCGclll *

travel. unpleasant occasion to notice this phenomenon,
when the engineer does not run the two screws
precisely at the same speed; there come times
when the ship vibrates violently, separated by

THE VARIATION OF LATITUDE 205

periods of comparative stillness. Instances from
other walks of life may recur to the memory
when once attention is called to the general facts ;
but enough has been said to explain the point
numbered (2) in the subjoined statement. To
understand the rest, we must remember that if the
two waves are not equal in " amplitude," i.e. if the
backward and forward motion is not the same in
both, they cannot annul one another, but the
greater will always predominate. Those interested
in following the matter further should have no
difficulty in constructing simple examples to illus-
trate such points. We will proceed to give Mr.
Chandler's statements : —

" We now come upon a new line of investiga- chand-
tion. Heretofore, as has been seen, the method formulae.
has been to condense the results of each series
of observations into the interval comprised by a
single period, then to determine the mean epoch
of minimum and the mean range for each series,
and, finally, by a discussion of these quantities,
to establish the general character of the law of
the rotation of the pole. It is now requisite to
analyse the observations in a different way, and
discover whether the deviations from the general
provisional law, in the last column of Table II. ,
are real, and also in what manner the variation
of the period is brought about. The outcome
of this discussion, which is to be presented in
the [present paper, is extremely satisfactory. The

206 ASTRONOMICAL DISCOVERY

real nature of the phenomenon is most distinctly
revealed, and may be described as follows :- —

" i . The observed variation of the latitude is the
resultant curve arising from two periodic fluctua-
tions superposed upon each other. The first of
these, and in general the more considerable, has
a period of about 427 days, and a semi-amplitude
of about o". 12. The second has an annual period
with a range variable between 0^.04 and o".2o
during the last half-century. During the middle
portion of this interval, roughly characterised as
between 1860 and 1880, the value represented by
the lower limit has prevailed, but before and after
those dates, the higher one. The minimum and
maximum of this annual component of the varia-
tion occur at the meridian of Greenwich, about
ten days before the vernal and autumnal equinoxes
respectively, and it becomes zero just before the
solstices.

"2. As the resultant of these two motions, the
effective variation of the latitude is subject to a
systematic alternation in a cycle of seven years'
duration, resulting from the commensurability of
the two terms. According as they conspire or
interfere, the total range varies between two*
thirds of a second as a maximum, to but a few
hundredths of a second, generally speaking, as
a minimum.

"3. In consequence of the variability of the co-
efficient of the annual term above mentioned, the
apparent average period between 1840 and 1855

THE VARIATION OF LATITUDE 207

approximated to 380 or 390 days ; widely fluctu-
ated from 1855 to 1865 ; from 1865 to about 1885
was very nearly 427 days, with minor, fluctuations ;
afterwards increased to near 440 days, and very
recently fell to somewhat below 400 days. The
general course of these fluctuations is quite faith-
fully represented by the law of eq. (3), (No. 267),
and accurately, even down to the minor oscilla-
tions of individual periods, by the law of eq. (15),
hereafter given, and verbally interpreted above.
This law also gives a similarly accurate account
of the corresponding oscillations in the amplitude.
The closeness of the accordance between observa-
tion and the numerical theory, in both particulars,
places the reality of the law beyond reasonable
doubt."

Those who cannot follow the details of the
above statement will nevertheless catch the general
purport — that the difficulties felt by Professor
Newcomb have been surmounted; and this, is
made clearer by a later extract : —

"A very important conclusion necessarily fol-
lows from the agreement of the values of the 427^
day term, deduced from the intervals between the
consecutive values of T in Table XII. , namely,
that there has been no discontinuity in the revo:
lution, such as Professor Newcomb regarded as so
probable that he doubted the possibility of draw-
ing any conclusions from the comparison of obser-
vations before and after 1860 (A. J. 271, p. 50).

2o8 ASTRONOMICAL DISCOVERY

''The present investigation demonstrates that
the way out of the apparently irreconcilable con-
Theory tradiction of theory and observation in this matter
it win not does not lie in the direction of discrediting the
vatkm?1" observations, as he is inclined to do. On the
contrary, the result is a beautiful vindication of
the trustworthiness of the latter, and, at the same
time, of the theory that demands an invariable
rate of motion ; providing a perfectly fitting key
to the riddle by showing that another cause has
intervened to produce the variability of the period.
I feel confident that Professor Newcomb will agree
with the reality of the explanation here set forth,
and will reconsider his view that the perturbations
in the position of the Pole must be of the nature
of chance accumulations of motion, a view which
he then considered necessary to the maintenance
of the constancy in the period of latitude-varia-
tion."

The paper from which these words are taken
appeared on November 4, 1892. The next paper
on the main theme did not appear till a year later,
though much work was being done in the mean-
time on the constant of aberration and other
matters arising immediately after the discovery.
On November 14, 1893, Mr. Chandler winds up
The final the series of eight papers " On the Variation of
Latitude," which he had commenced just two
years before. His work was by no means done ;
rather was it only beginning, for the torch he had

THE VARIATION OF LATITUDE 209

lit illuminated many dark corners. But he rightly
regarded his discovery as now so firmly estab-
lished that the series of papers dealing with it as
still under consideration might be terminated.
In this final paper he first devotes the most care-
ful attention to one point of detail. He had shown
earlier in the series that the North Pole must be
revolving from West to East, and not from East to
West ; but this was when the motion was supposed
to be simple and not complex, and it was neces-
sary to re-examine the question of direction for
each of the components. After establishing con-
clusively that the original direction holds for each
of the components, he almost apologises for the
trouble he has taken, thus : —

" It is therefore proved beyond reasonable doubt
that the directions of the rotations ri« from West
to East in both elements ; whence the general
form of the equation for the variation of latitude
adopted in A. </., 284, p. 154, eq. (19). It may be
thought that too much pains have been here be-
stowed upon a point which might be trusted to
theory to decide. I cannot think so. One of
the most salient results of these articles has been
the proof of the fact that theory has been a blind
guide with regard to the velocity of the Polar
rotation, obscuring truth and misleading investi-
gators for a half a century. And even if we were
certain, which we are not, that the fourteen
months' term is the Eulerian period in a modi-

210 ASTRONOMICAL DISCOVERY
i

fied forin, It would still be necessary to settle

by observation the direction of the annual motion,
with regard to which theory is powerless to in-
form us. To save repetition of argument, I must
refer to the statement in A. J., 273, pp. 68, 70, of
the principles adopted in beginning these inquiries
in 1891."

Finally, he answers one of the few objectors of
eminence who still lingered, the great French
physicist Cornu : —

" The ground is now cleared for examination of
ed* the only topic remaining to be covered, to estab-
lish, upon the foundation of fact, every point in
the present theory of these remarkable movements
of the earth's axis. This is the question of the
possibility that these movements are not real, but
merely misinterpretations of the observed pheno-
mena ; being in whole or in part an illusory effect
of instrumental error due to the influence of tem-
perature. Such a possibility has been a night-
mare in practical astronomy from the first,
frightening us in every series of unexplained
residuals, brought to light continually in nearly
all attempts at delicate instrumental research. A
source of danger so subtile could not fail to be
ever present in the mind of every astronomer and
physicist who has given even a superficial atten-
tion to the question of the latitude variations,
and there is no doubt that some are even now
thus deterred from accepting these variations as

THE VARIATION OF LATITUDE 211

proved facts. Perhaps the most explicit and for-
cible statement of the doubts that may arise on
this subject has been given very recently by Mr.
Cornu. The views of so distinguished a physicist,
and of others who are inclined to agree with him,
call for careful attention, and cannot be neglected
in the present closing argument upon the theory
presented in these articles. It is unnecessary,
for the purpose of disposing of objections of the
sort raised by Cornu, to insist that it is not suffi-
cient to show that the observed variations, attri-
buted to the unsteadiness of the Earth's Pole, are
near the limit of precision attainable in linear
differential measures, and in the indication of the
direction of gravity by means of the air bubble of
the level ; or to show that there are known varia-
tions in divided circles and in levels, dependent
on temperature and seasons. Nor need we re-
quire of objectors the difficult, although essential,
task — which they have not distinctly attempted —
of showing that these errors are not eliminated,
as they appear to be, by the modes in which
astronomers use their instruments. Neither need
we even urge the fact that a large portion of the
data which have been utilised in the present re-
searches on the latitude were derived by methods
which dispense with levels, or with circles, a part
of them indeed with both, and yet that the results
of all are harmonious. On the contrary, let us
admit, although merely for argument's sake, that
all the known means of determining the direction

212 ASTRONOMICAL DISCOVERY

of gravity — including the plumb-line, the level,
and a fluid at rest, whether used for a reflecting
surface or as a support for a floating instrument —
are subject to a common law of periodical error
which vitiates the result of astronomical observa-
tion, obtained by whatever methods, and in pre-
cisely the same manner. Now, the observed law
of latitude variation includes two terms, with
periods of fourteen and twelve months respec-
tively. Since the phases of the first term are
repeated at intervals of two months in successive
years, and hence in a series of years come into all
possible relations to conditions of temperature
dependent on season, the argument against the
reality of this term, on this ground, absolutely
fails, and needs no further notice. As to the
second, or annual term, while the phases, as
observed in any given longitude, are indeed
synchronical with the seasons, they are not so
as regards different longitudes. If, therefore, the
times of any given phase, as observed in the same
latitude, but in successively increasing longitudes,
occurred at the same date in all of them, there
would be a fatal presumption against the existence
of an annual period in the polar motion. If, on
the contrary, they occur at times successively
corresponding to the differences of longitude, the
presumption is equally fatal to the hypothesis
that they can possibly be due to temperature
variation as affecting instrumental measurement.
But the facts given in the foregoing section cor-

THE VARIATION OF LATITUDE 213

respond most distinctly to the latter condition.
Therefore, unless additional facts can be brought
to disprove successively these observed results, we
may dismiss for ever the bugbear which has un-
doubtedly led many to distrust the reality of the
annual component of the latitude-variation, while
they admit the existence of the 427-day term."

At this point we must leave the fascinating
account of the manner in which this great dis-
covery was established, in the teeth of opposition
such as might have dismayed and dissuaded a less
clear-sighted or courageous man. It is my purpose
to lay more stress upon the method of making the
discovery than upon its results ; but we may afford
a brief glance at some of the consequences which Conse-
have already begun to flow from this step in
advance. Some of them have indeed already come C(
before us, especially that large class represented
by the explanation of anomalies in series of obser-
vations which had been put aside as inexplicable.
We have seen how the observations made in
Russia, or in Washington, or at Greenwich, in
all of which there was some puzzling error, were
immediately straightened out when Chandler
applied his new rule to them. We in England suspected
have special cause to be grateful to Chandler ; not acquitted.
only has he demonstrated more clearly than ever
the greatness of Bradley, but he has rehabilitated
Pond, the Astronomer Royal of the beginning of
the nineteenth century ; showing that his obser-

214 ASTRONOMICAL DISCOVERY

vations, which had been condemned as in some
way erroneous, were really far more accurate than
might have been expected ; and further he has
shown that the beautiful instrument designed by
Airy, and called the Eeflex Zenith Tube, which
seemed to have unaccountably failed in the purpose
for which it was designed, was really all the time
accumulating observations of this new phenomenon,
the Variation of Latitude. Instead of Airy having
failed in his design, he had in Chandler's words
" builded better than he knew."
Constant Secondly, there is the modifying influence of

of Aberra- . -, . i ,1 ' i i

tionim- this new phenomenon on other phenomena al-
proved. rea(jy known, such, for instance, as that of
" aberration." We saw in the third chapter
how Bradley discovered this effect of the velo-
city of light, and how the measure of it is
obtained by comparing the velocity of light with
that of the earth. This comparison can be effected
in a variety of ways, and we should expect all the
results to agree within certain limits ; but this
agreement was not obtained, and Chandler has
been able to show one reason why, and to remove
some of the more troublesome differences. It is
impossible to give here an idea of the far-reaching
consequences which such work as this may have ;
so long as there are differences of this kind we
cannot trust any part of the chain of evidence,
and there is in prospect the enormous labour of
examining each separate link until the error is
found. The velocity of light, for instance, may be

THE VARIATION OF LATITUDE 215

measured by a terrestrial experiment ; was there
anything wrong in the apparatus ? The velocity of
the earth in its journey round the sun depends
directly upon the distance of the sun : have we
measured this distance wrongly, and if so what
was the error in the observations made? These
are some of the questions which may arise so long
as the values for the Constant of Aberration are
still conflicting ; but it requires considerable know-
ledge of astronomy to appreciate them fully.

Another example will, perhaps, be of more Latitude
general interest. If the axis of the earth is Tide,
executing small oscillations of this kind, there
should be an effect upon the tides ; the liquid
ocean should feel the wobble of the earth's axis
in some way ; and an examination of tidal registers
showed that there was in fact a distinct effect. It
may cause some amusement when I say that the
rise and fall are only a few inches in any case ;
but they are unmistakable evidences that the
earth is not spinning smoothly, but has this kind
of unbalanced vibration, which I have compared
to the vibrations felt by passengers on an im-
perfectly engineered twin-screw steamer. A more
sensational effect is that apparently earthquakes
are more numerous at the time when the vibration
is greatest. We remarked that the vibration
waxes and wanes, much as that of the steamer Earth-
waxes and wanes if the twin-screws are not qua
running quite together. Now the passengers on
the steamer would be prepared to find that break-

216 ASTRONOMICAL DISCOVERY

ages would be more numerous during the times
of vigorous oscillation ; and it seems probable that
in a similar way the little cracks of the earth's
skin which we call great earthquakes are more
numerous when these unbalanced vibrations are at
their maximum ; that is to say, about once every
seven years. This result is scarcely yet worthy of
complete confidence, for our observations of earth-
quakes have only very recently been reduced to
proper order ; but if it should turn out to be true,
it is scarcely necessary to add any words of mine
to demonstrate the importance of this rather un-
expected result of the Latitude Variation.

Finally I will mention another phenomenon
which seems to be at present more of a curiosity
than anything else, but which may lead to some
future great discovery. It is the outcome of obser-
vations which have been recently made to watch
these motions of the Pole ; for although there
seems good reason to accept Mr. Chandler's laws
of variation as accurate, it is necessary to establish
their accuracy and complete the details by making
observations for some time yet to come ; and there
could be no better proof of this necessity than the
The discovery recently made by Mr. Kimura, one of
^heno-a those engaged in this watch of the Pole in Japan,
menon. Perhaps f can give the best idea of it by men-
tioning one possible explanation, which, however,
I must caution you may not be by any means the
right one. We are accustomed to think of this
great earth as being sufficiently constant in shape;

THE VARIATION OF LATITUDE 217

if asked, for instance, whether its centre of gravity
remains constantly in the same place inside it, we
should almost certainly answer in the affirmative,
just as only twenty years ago we thought that the
North Pole remained in the same place. But it
seems possible that the centre of gravity moves
a few feet backwards and forwards each year —
this would at any rate explain certain curious
features in the observations to which Mr. Kimura
has drawn attention. Whatever the explanation
of them may be, or to settle whether this expla-
nation is correct, we want more observations,
especially observations in the Southern Hemis-
phere ; and it is a project under consideration
by astronomers at the present moment whether
three stations can be established in the Southern
Hemisphere for the further observation of this
curious phenomenon. The question resolves itself
chiefly into a question of money ; indeed, most
astronomical projects do ultimately resolve them-
selves into questions of money ; and I fear the
world looks upon scientific men as insatiable in
this respect. One can only hope that on the
whole the money is expended so as to give a
satisfactory return. In this instance I have no
hesitation in saying that an immediate return of
value for a comparatively modest expenditure is
practically certain, if only in some way we can get
the means of making the observations.

^ 4*-**

218 ASTRONOMICAL DISCOVERY

It would be natural, at the conclusion of this
brief review of some types of astronomical dis-
covery, to summarise the lessons indicated : but
there is the important difficulty that there appear
to be none. It has been pointed out as we pro-
ceeded that what seemed to be a safe deduction
from one piece of history has been flatly contra-
dicted by another ; no sooner have we learnt that
important results may be obtained by pursuing
steadily a line of work in spite of the fact that it
seems to have become tedious and unprofitable
(as in the search for minor planets) than we are
confronted with the possibility that by such
simple devotion to the day's work we may be
losing a great opportunity, as Challis did. We
can scarcely go wrong in following up the study
of residual phenomena in the wake of Bradley ;
but there is the important difficulty that we may
be wholly unable to find a clue for the arrange-
ment of our residuals, as is at present largely the
case in meteorology. And, in general, human
expectations are likely to be quite misleading, as
has been shown in the last two chapters ; the
discoveries we desire may lie in the direction
precisely opposite to that indicated by the best
opinion at present available. There is no royal
road to discovery, and though this statement may
meet with such ready acceptance that it seems
scarcely worth making, it is hoped that there may
be sufficient of interest in the illustrations of its
truth.

THE VARIATION OF LATITUDE 219

The one positive conclusion which we may
derive from the examples studied is that dis-
coveries are seldom made without both hard work
and conspicuous ability. A new planet, even
as large as Uranus, does not reveal itself to a
passive observer : thirteen times it may appear
to such a one without fear of detection, until at
last it encounters an alert Herschel, who suspects,
tests, and verifies, and even then announces a
comet — so little did he realise the whole truth.
Fifteen years of unrequited labour before Astrsea
was found, nineteen years of observation before
the discovery of nutation could be announced :
how seldom do these years of toil present them-
selves to our imaginations when we glibly say
that ** Bradley discovered nutation," or " Hencke
discovered Astrsea " ! That the necessary labour
is so often forgotten must be my excuse for re-
calling attention to it somewhat persistently in
these examples.

But beyond the fact that he must work hard,
it would seem as though there were little of value
to tell the would-be discoverer. The situation
has been well summarised by Jevons in his
chapter on Induction in the " Principles of
Science ; " and his words will form a fitting con-
clusion to these chapters : —

"It would seem as if the mind of the great
discoverer must combine contradictory attributes.
He must be fertile in theories and hypotheses,

220 ASTRONOMICAL DISCOVERY

and yet full of facts and precise results of ex-
perience. He must entertain the feeblest analo-
gies, and the merest guesses at truth, and yet he
must hold them as worthless till they are verified
in experiment. When there are any grounds
of probability he must hold tenaciously to an
old opinion, and yet he must be prepared at any
moment to relinquish it when a clearly contra-
dictory fact is encountered."

INDEX

ABERRATION, 105-109, in, 112,

117, 118, 185, 188, 192, 214,

215
Accidental discovery, 15, 73,

121-154

Adams, 12, 45-85; resolution, 55
Airy, 32, 40-85, 214
Algiers, 130
Alleghenia, 26
Almucantar, 180, 181
Alphabet used for planets, 27
Anderson, Dr. T. C., 8, 142, 143,

144, 146
Anthelm, 142
Apollo, 9
Argon, 109
Ascension, 34

Assumption, forgotten, 196
Astraea, 22, 23,219
Astrographic chart, 122, 125, 130
AstronomicalJournal, 177-217
Astronomische Nachrichten, 52,

158

Astrophil, 143
Auwers, 142

BALL, Sir R, 24
Balliol College, 87
Banks, Sir J., 9
Barnard, E. E., 146, 220
Berlin, 181, 183, 184, 188, 193
Berlin star-map, 45, 66, 83, 124
Bessel, 192
Bettina, 26, 27
Birmingham, 142

321

"Black Drop" (in transit of

Venus), 30
Bliss, 114
Board of Visitors of Greenwich

Observatory, 63
Bode, u, 14, 15, 22
Bode's Law, 12, 13, 38, 43, 45,

52, 72, 76, 77, 84
Bourdeaux, 130
Bouvard, 39, 40, 42, 48, 49, 50,

61
Bradley, 39, 86-120, 188-192,

213,214, 218, 219
Bradley, John, 1 1 5
Bremen, 20
Bridstow, 87, 88, 94
Briggs, 119
Brinkley, 192
British Association, 63
Briinnow, 193

CALIFORNIA, 26

Cambridge (Mass.), 180, 184,

1 88
Cambridge Observatory, 23, 42,

49,52,63,65,66, 135, 193
Cambridge University, 68-71,

114

Cape Observatory, 123, 124, 130
Cards, n
Cassini II., 1 56
Catania, 130
Ceres, 14-22
Chacornac, 124
Challis, 49-54, 63-68, 71, 85, 218

222

INDEX

Chandler, S. C., 118, 177-217

Chapman's " Homer," 2

Chicago, 157

Chromosphere, 170

Clarke, C. C., 2

Coelostat, 94

Columbus, 63

Comet, 4-8, 88, 108, 117, 123,

125 •

Commission, planetary, 27
Common, A. A., 124, 127
Compte Rendu, 62
Comstock, 202
Conference, Astrographic, 125-

136

Copernicus, 79, 95
Cordoba, 130, 199
Cornu, 210-213
Corona, 170-175
Cosmos (Humboldfs), 160

DELAMBRE, 157

Deviation of Pole, 187

Disc of Neptune, 44, 64, 79

Disc of Uranus, 4-7

Dorpat, 192

Doublet (photographic), 127-129

Draconis, -y, 96-104

Draconis, |8, 193

Driessen, 23

Dry plate, 122

Dublin, 192

EARTHQUAKES, 215
Earth's Pole, 177-217
Eccentricity, 41, 83
Eclipses, 1707176
Edinburgh, 143
Eduarda, 26
Egeria, 22
Endymion, 25
Eriphyla, 26

Eros, 25, 26, 28, 35, 37, 68
Eulerian, 200, 209

Evelyn, 26

Exposure, times of, 122, 131

FACUL^E, 170

Faraday, 201

Flamsteed, 39, 53, 115

Fleming, Mrs., 142

Flora, 22

Foulkes, Martin, 94

French Academy, 43, 51, 62

GALILEO, 95, 163

Galle, 44, 45, 47, 66, 67, 83

Gasparis, 22

Gauge (railways), 56

Gauss, 17-20

Geminorum, H., 4

George III., 8, 10

"Georgian," 11

Georgium Sidus, 8, 10, II

Gill, Sir D., 32, 34, 35, 123

Gilliss, 32

Gotha, 20

Gould, 32

Graham, 22, 23

Gravitation, law of, 38, 45, 59,
84, 105

Greaves, 119

Greenwich Observatory, 48-64,
88, 89, 114-117, 130, 160-169,
182, 192, 193, 206, 213

Gregory, 93, 119

HALE, G. E., 170, 171

Hall, A., 184, 185

Halley, 88-92, 1 08, 112-116, 119

Hansen, 41, 59

Harkness, 184

Hartwig, 142

Harvard College Observatory,

128, 142, 144, 145
Hebe, 22
Hegel, 15
Heidelberg, 145
Heliometer, 32, 34

INDEX

223

Helium, 109

Helsingfors, 130

Hencke, 22. 23, 64, 153, 219

Henry brothers, 124-129

Herschel, Sir John, 63, 75, 83

Herschel, Sir William, 2-11, 39,

44,82,219

Herschel (Uranus), 11, 12
Hind, 22, 23, 25, 142
Hooke, 96, 97
Hubbard, 184
Humboldt, 160
Hussey, Rev. T. J., 40, 42
Hygeia, 22

ILMATA, 26

Industria, 26

Ingeborg, 26

Instruments at Greenwich, 114-

116
Iris. 22, 23, 32, 35

JANSON, 142
Jevons, 219
Johnson, M., 156, 1 60
Juno, 9, 21, 22

Jupiter, 9, 28, 43, 49> 5°> 6l '•>
satellites, 92, 117

KEATS, 1-3, 7, 8
Keill, 94, 112, 119, I56
Kelvin, Lord, 196, 197
Kepler, 95, 142
Kew, 95, 96, 1 88, 190
Kiel, 141
Kimura, 2 1 6
Konigsberg, 192
Kiistner, 118, 181, 183

LALANDE, 7, 11, 107, 157
Lameia 26
Laplace, 61
La Plata, 130

Latitude variation, 99, 100, 117
118, 177-217

Lemonnier, 39, 53, 157
Le Verrier, 12, 43-85
Libussa, 26
Lick Observatory, 152
Liouville's Journal, 73
Lisbon, longitude of, 92
London, 23, 25, 96
Long, 157
Longitude, 92, 117
Lowth, Bishop, 119
Lyrae, a, 184, 196

MACCLESFIELD, Earl of, 94, 113

Madler, 192

Magnetic observations, 161, 164,

174

Magnitude equation, 135 •
Markree, 23

Mars, 9, 28, 32, 34, 35, 91
Mayer. 39

Measurement of plates, 132-135
M&anique Celeste, 61
Melbourne, 130, 193
Memorandum (Adams), 55
Mercury, 9
Messier, 7
Meteorites, 59
Meteors (November), 60
Metis, 22, 23
Micrometer, 5, 133
Milky Way, 125
Minerva, 9
Minor planets, 1 3-28
Minor planets tables, 22, 24, 26
Mistakes, 71-83
Molyneux, Samuel, 94-96, 101,

104

Monte Video, 130
Moon, tables of, 117

NAMES of minor planets, 22-28
Nasmyth, 162
" Nautieal Almanac," 1 1
Nebula, 124, 146-152

224

INDEX

Neptune, n, 12, 38-85, 124
New College Lane, 112
Newcomb, Simon, 81, 183, 184,

195-202, 207, 208
New stars, 121, 140-154
Newton, 38, 84, 90-95, 105, 113
New York, longitude, 92
Ninina, 26
Northleach, 87
Northumberland, 65
Nova Geminorum, 141, 145, 146
Nova Persei, 143, 146-152
Nutation, 99, 100, no, 115, 117,

118, 188, 219

Observatory (magazine), 26

Ocllo, 26

Olbers, 20-22

Olympic games, 119

Oriani, 15

Ornamenta, 26

Oxford University, 87-89, 94,

105-119
Oxford University Observatory,

121,130,132,136,142,145,154

PALERMO, Observatory of, 18

Palisa, 26

Pallas, 9, 21, 22

Parallax, 34, 91, 95-98, 109, 185

Paris, 130

Parkhurst, J. A., 145

Parthenope, 22

Peirce, 73, 80-83

Pendulum, 117

Perseus, 8, 143

Personal equation, 31, 134, 135,

185

Perth, 130
Perturbations of Uranus, 1 2, 42,

51,54, 55,6i,75
Peters, 188, 192
Phaetusa, 26
Philosopher, 201, 219

Philosophical Transactions, 3, 4, 9
Photographica, 26
Photographic methods, 24, 33,
36, 121-139; lenses, 125, 126
Photographs of sun, 163, 170-

173

Piazzi, 13-18, 22

Pickering, E. 0., 128, 144

Pittsburghia, 26

Plana, 61

Planetary distances, 13; com-
mission, 27 ; numbering, 27

Planets by photography, 24

Pole Star (Polaris), 177, 178,
192, 193

Pond, 192, 213

Potsdam, 130, 181

Pound, Mrs., 104, 110-112

Pound, Rev. James, 89-94, 104,

Prague, 181
Precession, 96, 178
Prymno, 26
Puiseux, 32
Pulfrich, 154
Pulkowa, 181-188, 213

QUADRANTS at Greenwich, 116

RADIUM, 175

Radius vector, 52-58, 60-62, 79,

Rayleigh, Lord, 109
Records before discovery, 144
Reflector, 93, 127, 128
Reflex zenith tube, 192, 214
Refraction, 96, 101-103, IJ7
Refractor, 93, 128
Roseau, 133
Residual phenomena, 108-110,

118, 120, 218

Rigaud, S. P., 87, 115, 119
Rome, 130
Rothschild, 27

INDEX

Royal Astronomical Society, 40, ' Toulouse Observatory, 1 30

225

47, 68, 74, 124, 155, 157
Royal Society, 4, 9, 10, 92, 94

SAMPSON, R. A., 74-76, 84

San Fernando, 130

Santiago, 130

Sappho, 32, 35

Saturn, 9, 43,61, 149, 150

Savile, Sir H., 119

Savilian professorship, 87-94,

108-119

Schmidt, Julius, 142, 160
Schuster, A., 169
Schwabe, 155-163, 176, 177
Sheldonian Theatre, 119
Sherbourn, 87
Solar eclipse, 26, 170-176
Spectro-heliograph, 170, 171
Star-maps, 45, 65, 83, 124
" Star-trap," 24
Stereo-comparator, 154
Stone, E. J., 32
Struve, 184, 1 88, 192
Sun's distance, 28-37
Sun-spots, 155-176
Sydney Observatory, 130

TACUBAYA OBSERVATORY, 130
Telescopes, 92, 124-129
Thames River, 105
Themistocles, 119
Theoria Motus, 17
Theory and observation, 208
Thomson, Sir W., 196, 197
Tides, 215
Titius, 13

Tycho Brahe, 95, 140, 142

URANUS, 2-14, 25, 38-85, 144,
219

VARIABLE stars, 140
Variation of latitude, 99, 100,

117, 118, 177-217
Venus, 9, 79 ; diameter of, 92 ;

transit of, 28-32, 34
Vesta, 21, 22
Victoria, 22, 25, 32, 35
Von Zach, 20

WALLACE, 119

Wansted, 88-94, 104, no, 115.

1 88, 190
Ward, 119
Washington Observatory, 184-

188, 193, 196,213
Weather and sun-spots, 161,

167-169
Weyer, 193
Whiteside, 112
Williams, Mrs. E., no, in
Wind -vane, revolutions, 167-

169

Winnecke, 32
Wolf, Dr. Max, 145
Wolf, Rudolf, 163
Wren, Sir 0., 119

YERKES Observatory, 145, 146,
152, 157, 170, 176

ZEISS, 154

Zodiac, 64, 124, 137

THE END

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