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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
