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ASTRONOMICAL DISCOVERY
[Illustration: ASTRONOMERS ROYAL.]
ASTRONOMICAL
DISCOVERY
BY
HERBERT HALL TURNER, D.Sc., F.R.S.
SAVILIAN PROFESSOR OF ASTRONOMY IN THE
UNIVERSITY OF OXFORD
_WITH PLATES_
LONDON
EDWARD ARNOLD
41 & 43 MADDOX STREET, W.
1904
(All rights reserved)
TO
EDWARD EMERSON BARNARD
ASTRONOMICAL DISCOVERER
THESE PAGES ARE INSCRIBED IN MEMORY OF
NEVER-TO-BE-FORGOTTEN DAYS SPENT WITH HIM AT THE
YERKES OBSERVATORY OF
THE UNIVERSITY OF CHICAGO
PREFACE
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 instinctively feel that the finding of something valuable
was especially "fortunate." We should scarcely recommend such waste
material as the best hunting ground for gems.
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
acknowledged 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
permission to publish illustrations to the Royal Astronomical Society, the
Astronomer Royal, the editors of _The Observatory_, the Cambridge
University Press, the Harvard College Observatory, the Yerkes Observatory,
and the living representatives of two portraits.
H. H. TURNER.
UNIVERSITY OBSERVATORY, OXFORD,
_November 9, 1904_.
CONTENTS
PAGE
CHAPTER I
URANUS AND EROS 1
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 V
SCHWABE AND THE SUN-SPOT PERIOD 155
CHAPTER VI
THE VARIATION OF LATITUDE 177
INDEX 221
LIST OF PLATES
PLATE
I. PORTRAIT OF J. C. ADAMS _To face page_ 22
II. PORTRAIT OF A. GRAHAM " " 22
III. PORTRAIT OF U. J. LE VERRIER " " 60
IV. PORTRAIT OF J. G. GALLE " " 60
V. CORNER OF THE BERLIN MAP BY THE USE OF WHICH
GALLE FOUND NEPTUNE " " 82
VI. ASTRONOMERS ROYAL _Frontispiece_
VII. GREAT COMET OF NOV. 7, 1882 _To face page_ 122
VIII. THE OXFORD NEW STAR " " 142
IX. NEBULOSITY ROUND NOVA PERSEI " " 146
X. SUN-SPOTS AT GREENWICH, FEB. 18 AND 19, 1894 " " 158
XI. SUN-SPOTS AT GREENWICH, FEB. 20 AND 21, 1894 " " 162
XII. NUMBER OF SUN-SPOTS COMPARED WITH DAILY RANGE
OF MAGNETIC DECLINATION AND DAILY RANGE OF
MAGNETIC HORIZONTAL FORCE " " 164
XIII. GREENWICH MAGNETIC CURVES, 1859-60 " " 166
XIV. GREENWICH MAGNETIC CURVES, 1841-1860 " " 166
XV. SUN-SPOTS AND TURNS OF VANE " " 170
ERRATA
Page 133, line 27, _for_ "200 stars" _read_ "200 stars per hour."
" 145, See note on page 220.
" 146, bottom of page. This nebulosity was first discovered by Dr.
Max Wolf of Heidelberg. See _Astr. Nachr._ 3736.
" 181, line 17, _for_ "observation" _read_ "aberration."
ASTRONOMICAL DISCOVERY
CHAPTER I
URANUS AND EROS
[Sidenote: Popular view of discovery.]
Discovery is expected from an astronomer. The lay mind scarcely thinks of
a naturalist nowadays discovering new animals, or of a chemist as finding
new elements save on rare occasions; but it does think of the astronomer
as making discoveries. The popular imagination pictures him spending the
whole night in watching the skies from a high tower through a long
telescope, occasionally 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.
[Sidenote: Keats' lines.]
The popular conception finds expression in the 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 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.
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.
[Sidenote: Comparison with discovery of Uranus.]
Let us then, as our first example of the way in which astronomical
discoveries are made, turn to 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 Transactions for 1781.
PHILOSOPHICAL TRANSACTIONS, 1781
XXXII.--ACCOUNT OF A COMET
BY MR. HERSCHEL, F.R.S.
(Communicated by Dr. Watson, jun., of Bath, F.R.S.)
_Read April 26, 1781_
[Sidenote: Original announcement.]
"On Tuesday the 13th of March, between ten and eleven in the evening,
while I was examining 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 Royal 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 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."
[Sidenote: Called first a comet.]
[Sidenote: Other observers would not have found it at all.]
At first sight this seems to be the wrong reference, for it speaks of a
new comet, not a new planet. But it is indeed of Uranus that Herschel is
speaking; and so little did he realise the full 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 suspicion in a keen-eyed
Herschel. Would any other observer have noticed the difference at all?
Certainly several good observers had looked at the object before, and
looked at it with the care necessary to record its position, without
noting any peculiarity. Their observations were recovered 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
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."
[Sidenote: No "swimming into ken."]
We cannot, therefore, fit the facts to Keats' version of them. The planet
did not majestically reveal itself to a merely passive observer: 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 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?
[Sidenote: Though this may happen at times.]
[Sidenote: Name of new planet.]
I must guard against a possible misconception. I do not mean to convey
that astronomical discoveries 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 perhaps 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 opportunity. Other cases of planetary discovery may
be attributed more completely to diligence alone, as we shall presently
see. But before leaving 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
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 unbounded expanse of the starry heavens, must often be the
subject of conversation, not only of astronomers, 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.
[Sidenote: _Georgium Sidus._]
"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 remarkable incident, seems to be its chronology:
if in any future age it should be asked, _when_ this last 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 GEORGIUM 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 expressing my sense of
gratitude by giving the name _Georgium 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 President 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."
[Sidenote: Herschel.]
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 Herschel for the new planet, in honour of the
discoverer, and this name was used in France; but Bode, on the other hand,
was in favour of retaining 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 Almanac" the name "the Georgian"
(apparently preferred to Herschel's _Georgium Sidus_) was being used
officially after 1791, and did not disappear from that work until 1851
(published in 1847.)
[Sidenote: Uranus finally adopted.]
It would appear to have been the discovery of Neptune, with which we shall
deal in the next chapter, which led to this official change; for in the
volume for 1851 is included Adams' account of his discovery with the
title--
"ON THE PERTURBATIONS OF URANUS,"
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.
[Sidenote: Bode's law.]
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 methodical 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 as a constant quantity without
serious error. Now if we arrange the planetary distances in order, we
shall notice a remarkable connection between the terms of the series. Here
is a table showing this connection.
TABLE OF THE DISTANCES OF THE PLANETS FROM THE SUN, SHOWING "BODE'S LAW."
+----------------------------------------------------+
| Name of | Distance from | "Bode's Law" |
| Planet. | Sun, taking | (originally formulated |
| | that of Earth | by Titius, but brought |
| | as 10. | into notice by Bode). |
|----------------------------------------------------|
| Mercury | 4 | 4 + 0= 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 |
+----------------------------------------------------+
[Sidenote: Gap in the series suggesting unknown planet.]
[Sidenote: Search for it.]
[Sidenote: Accidental discovery.]
If we write down a series of 4's, 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 1-1/2, and not zero, to agree with the rest. Secondly,
there is a gap, or rather was a gap, after the discovery of Uranus,
between Mars and Jupiter; and thirdly, we see that when Uranus was
discovered, and its distance from the 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 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,
involving 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 succeeded in
organising a band of six German astronomers (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 constellation Taurus. The
discovery was made somewhat dramatically on the first evening of the
nineteenth century (January 1, 1801). Piazzi was not looking 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. Returning 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, although we must not forget that a careless observer might
easily have missed it. Piazzi was anything but careless, and watched the
new object assiduously till February 11th, when he became dangerously 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 20th respectively; and we can imagine the mixed feelings 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.
[Sidenote: Hegel's forecast.]
More curious still must have seemed the intelligence to a young
philosopher of Jena named Hegel, who has since become famous, but who had
just imperilled his future reputation by publishing 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.
[Sidenote: The planet lost again.]
The sensation caused by the news of the discovery 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 predicting 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 explain 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 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.
[Sidenote: Gauss shows how to find it.]
Briefly, the solution demanded a new mathematical method in astronomy. But
difficulties are sometimes the opportunities of great men, and this
particular difficulty attracted to astronomy the great mathematician
Gauss, who set himself to make the best of the observation available, and
produced his classical work, the _Theoria Motus_, which is the standard
work for such calculations to the present day. May we look for a few
moments at what he himself says in the preface 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 FROM THE PREFACE TO THE
_Theoria Motus_.
[Sidenote: The _Theoria Motus_.]
"Some ideas had occurred to me on this subject 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
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 1st of That Year at the Observatory Of Palermo; and
Shortly Afterwards the Actual Observations Which Had Been Made
Between January 1st And February 11th by the Renowned Philosopher
Piazzi Were Published. Nowhere in All The Annals of Astronomy Do We
Find Such an Important Occasion; and Scarcely Is It Possible To
Imagine a More Important Opportunity for Pointing Out, As
Emphatically As Possible, the Importance Of That Problem, As at the
Moment When Every Hope of Re-discovering, Among the Innumerable
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 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 efficacy and universality of this method.
"Now a good many astronomers, immediately after the rediscovery of
Ceres, desired me to publish the methods which had 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 continuing
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 admitted 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 which
I deal with in this work. Although indeed 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 suppressed. 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 connection of thought seemed to demand it."
[Sidenote: Rediscovery of Ceres.]
[Sidenote: Another planet found.]
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 familiar
with all the small stars along the track which was being searched for the
missing body, that he was at once struck by the appearance of a stranger
near the spot where he had just identified Ceres. 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 bewilderment of the
astronomical world, it proved to be another planet revolving round the sun
at a distance 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_!
[Sidenote: Hypothesis of many fragments.]
The thought occurred to Olbers that they were perhaps fragments of a
single body which had been blown to pieces by some explosion, and 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 periodically 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 profitably 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 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 accident; 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.|
|---------------------------------------|
| 1 | Ceres | Piazzi | 1801 |
| 2 | Pallas | Olbers | 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 |
| 11 | Parthenope| De Gasparis | 1850 |
| 12 | Victoria | Hind | 1850 |
| 13 | Egeria | De Gasparis | 1850 |
+---------------------------------------+
[Sidenote: Hencke's long search.]
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 abandoned; but being rewarded by
no success for some years, it was gradually dropped, and the belief gained
ground that the number of the planets 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, by name Hencke, who, in spite of the general 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 Observatory) 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.
[Illustration:
_By permission of Messrs. Macmillan & Co._
I.--J. C. ADAMS.]
[Illustration:
II.--A. GRAHAM.
DISCOVERER OF THE NINTH MINOR PLANET (METIS).]
[Sidenote: The photographic method.]
You will see the great variety characterising these discoveries; some of
them are the result of deliberate search, others have come accidentally,
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 what vast stores of patience and
diligence were required in carrying out the search. But of late years
photography has effected a great revolution in this respect. It is no
longer necessary to do more than set what Sir Robert Ball 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 sufficiently 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 photographs, 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.
1801 to 1850--altogether 13 discoveries.
1851 to 1860-- " 49 "
1861 to 1870-- " 49 "
1871 to 1880-- " 108 "
1881 to 1890-- " 83 "
1891 to 1900-- " 180 announcements
In 1901 " 36 "
" 1902 " 50 "
" 1903 " 41 "
---
Total 609
[_N.B._--Many of the more recent announcements turned out to refer to
old discoveries.]
[Sidenote: Scarcity of names.]
The known number of these bodies has accordingly increased so rapidly as
to become almost an embarrassment; and in one respect the embarrassment is
definite, for it has become quite difficult to find _names_ for the new
discoveries. 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 withdrawn. Later still
the restriction to feminine names has been broken through; one planet has
been named Endymion, and another, of which we shall presently speak more
particularly, has been called Eros. But before passing to him you 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
389 Industria
391 Ingeborg
433 Eros
443 Photographica
457 Alleghenia
462 Eriphyla
475 Ocllo
484 Pittsburghia
503 Evelyn
[Sidenote: Bettina.]
[Sidenote: The provisional letters.]
In connection with No. 250 there is an interesting 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 L50." 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 discovered 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
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 which was
adopted to fill up the interval required 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 discoveries 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, however, 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 AA, AB, AC, &c.; after
completing the alphabet again, the letter B was prefixed, and so on; and
astronomers began to fear that they had before them a monotonous prospect
of continually adding new planets, varied by no incident more exciting
than starting the alphabet over again after every score.
[Sidenote: Eros.]
Fortunately, however, on running through it for the fifth time, an object
of particular interest 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.
[Sidenote: Transit of Venus.]
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 measurement 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.
[Illustration: FIG. 1.]
[Sidenote: The "Black Drop."]
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 path
from that seen by another observer, as will be clear from the diagram. If
the size of the earth, the distance of the sun, and the _relative_
distance of Venus be known, it can be calculated 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,
it was hoped to determine this very important quantity, the sun's
distance. It was known from previous experience that there were certain
difficulties in observing very exactly the beginning and end of the
transit. There was an appearance called the "Black Drop," which had caused
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.
[Sidenote: Failure.]
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 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 appearances were found to be totally different
in character from those shown by the model; chiefly, perhaps, because it
had been impossible to imitate with a model the effect of the atmosphere
which surrounds the planet Venus. Observers trained beforehand, using
similar instruments, 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.
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 =93-1/4=
million (Airy, from British observations of 1874), =92-1/2= million
(Stone, from British observations of 1882), and =91-1/2= million
(Puiseux, from French observations), a range of 1-3/4 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.
[Sidenote: Modern method for sun's distance.]
[Sidenote: Photography.]
[Sidenote: Dr. Gill's expedition to Ascension.]
We have said that the _relative_ distance of Venus from the sun is
accurately known from observations of the exact time of revolution. It is
easy to see that these times of revolution can 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 distances
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 surrounding it on the same plate, and then by careful
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 photographs taken at stations a
known distance apart we can find the distance of the planet from the
earth; and hence, as above remarked, the distance 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 by a known interval. For in that interval the station
will have been carried by the earth's rotation 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 since in 1878,
when Dr. Gill set out on his enterprise of determining the sun's distance,
photography 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 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.
[Sidenote: Victoria, Iris, and Sappho.]
[Sidenote: Eros.]
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, somewhat difficult to allow for with certainty, is
introduced 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 of Mars, in spite of the fact that they were sensibly 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 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 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,
preparations 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 telescopes 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 completed, 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 research,
however apparently unimportant or monotonous, which we can afford to
neglect. Just when 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.