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LIBRARY 



ASTRONOMICAL DISCOVERY 



C T H F 




ASTRONOMERS ROYAL. 



ASTRONOMICAL 
DISCOVERY 



BY 



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

SAVILIAN PROFESSOR OF ASTRONOMY IN THE 
UNIVERSITY OF OXFORD 



OF THF 

UNWEPT 

Of 

.' iro 




WITH PLATES 



LONDON 
EDWARD ARNOLD 

41 & 43 MADDOX STREET, W. 
1904 

(All rights reserved) 



' 

ASTWfr 
LIBR> 



1 



TO 

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 


4+ o= 4 


Venus 


7 


4+ 3= 7 


The Earth 


10 


4+ 6 = 10 


Mars 


15 


4+ 12= 16 


( ) 


( ) 


4+ 24 = 28 


Jupiter 


52 


4+ 48= 52 


Saturn 


95 


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 P lanet - 
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 



i 4 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 H ypo - 

, , , * , , 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 



22 



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. 


I 


Ceres 


Pia/zi 


1801 


2 


Pallas 


Gibers 


1802 


3 


Juno 


Harding 


1804 


4 


Vesta 


Olbers 


1807 


5 


Astraea 


Hencke 


1845 


6 


Hebe 


Hencke 


1847 


7 


Iris 


Hind 


1847 


8 


Flora 


Hind 


1847 


9 


Metis 


Graham 


1848 


10 


Hygeia 


De Gasparis 


1849 


ii 


Parthenope 


De Gasp;iris 


1850 


12 


Victoria 


Hind 


1850 


13 


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 



i 




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 g ir R obert Ba u 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 

c 



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 planet 1 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 
finite 6 under which planets were discovered varied con- 
objects. s iderably. 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- 
crepancies 4 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 w r e 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 

cS 1 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 O f 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?t C hT ry 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 o f fa e 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- 
work 18 ' ou k 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 

D 



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 . i3o'-9 
Mean longitude, ist October 1845 3 2 3 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. 3 I 


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 a bout this question by those who most blame Airy. 

question. S ome 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 P lanet 
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 a nd 
(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 > i n Airy's opinion, large enough to detect the 
scope at pl an et 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- 

E 



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 ^ ev ised 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. P lanet 
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 P lanefc - 
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 covei r 

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 I82 7 I86 5 I8l 3 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 an d 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 to 1 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 

86 



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 canno t 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^s 016 ' 
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 Pound 7 
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 n i s 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 e iiar 
Bradley, who had just lost the beloved colleague of P arallax - 



9 6 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. 

G 



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 P lanatlon - 
time he must have done some hard thinking. 



io 4 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. a ble 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 



iE 



->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 reoit 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." 

H 



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- 1 74 2 > 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 



i2 4 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 . 


3 

2 


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 i e t." 
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 

i 



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 rseau 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 - T 9 . 

,, 6 0.21 

11 11 5 11 - 2 3 



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- 



i 4 o 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, 
di s emmg 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. 


2 

3 


Cygnus . 
Ophiuchus 


1600 
1604 


Janson. 
Kepler. 


4 


Vulpecula 


1670 


Anthelm. 


5 


Ophiuchus 


1848 


Hind. 


6 


Scorpio . 


1860 


Auwers. 


7 


Corona Boreal s 


1866 


Birmingham. 


8 


Cygnus . 


1876 


Schmidt. 


9 


Andromeda 


1885 


Hartwig. 


10 


Perseus . 


1887 . 


Fleming. 


ii 


Auriga . 


1891 


Anderson. 


12 


Norma , 


1893 


Fleming. 


*3 


Carina 


1895 


Fleming. 


14 


Centaurus 


1895 


Fleming. 


15 


Sagittarius 


1898 


Fleming. 


16 


Aquila . 


1899 


Fleming. 


17 


Perseus . 


1901 


Anderson. 


18 


Gemini . 


1903 


At Oxford. 



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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- faintl y* 
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 

K 



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 more 1 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 S 1? then 
it would present this natural appearance of being 
farther away from the sun than the earth ; but it 
might be at S 2 or S 3 , 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 

I* 

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 

OF 



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 


I 


277 

273 
282 
244 
217 


1831 
1832 
1833 
1834 
1835 


149 
84 

33 
51 
173 


3 
49 
139 

120 

18 


239 

270 
267 

273 
244 


1836 

1837 
1838 

1839 
1840 


272 

333 
282 
162 
152 


o 




o 
3 


200 

1 68 

202 
20 5 
263 


1841 
1842 

1843 
(1844) 


1 02 

68 

ft 


15 

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 6 4 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 to 1 
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 
Sons 1 *" us i g > that if the nee dle 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 flfirffrffff|ffii|iiii|t,|,| im | llvl | lll| | ||M 



HF 

X 
100 



6'- 




LS& 

IOO 



HORIZONTAL FORCE 

PLATE XII. 



GREENWICH MAGNETIC CURVES 

1859-60 




CRENWICH 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- w j n t e r 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, v and 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 
sphere 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 

N 



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 l e r'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 accoun j- o f fafe new g ^ e p j n kj s own wor( J Sj 



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 O f 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 r i 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- 

o 



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, I5 6 
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, 

"5 

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, 

83 

Rayleigh, Lord, 109 
Records before discovery, 144 
Reflector, 93, 127, 128 
Reflex zenith tube, 192, 214 
Refraction, 96, 101-103, IJ 7 
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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