Great Astronomers

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GREAT ASTRONOMERS

By

SIR ROBERT S. BALL D.Sc. LL.D. F.R.S.

Lowndean Professor of Astronomy and Geometry in the
University of Cambridge

Author of "In Starry Realms" " In the High Heavens" etc.

[PLATE: GREENWICH OBSERVATORY.]

PREFACE.

It has been my object in these pages to present the life of each
astronomer in such detail as to enable the reader to realise in
some degree the man's character and surroundings; and I have
endeavoured to indicate as clearly as circumstances would permit
the main features of the discoveries by which he has become known.

There are many types of astronomers--from the stargazer who merely
watches the heavens, to the abstract mathematician who merely
works at his desk; it has, consequently, been necessary in the
case of some lives to adopt a very different treatment from that
which seemed suitable for others.

While the work was in progress, some of the sketches appeared in
"Good Words." The chapter on Brinkley has been chiefly derived from
an article on the "History of Dunsink Observatory," which was
published on the occasion of the tercentenary celebration of the
University of Dublin in 1892, and the life of Sir William Rowan
Hamilton is taken, with a few alterations and omissions, from an
article contributed to the "Quarterly Review" on Graves' life of
the great mathematician. The remaining chapters now appear for
the first time. For many of the facts contained in the sketch of
the late Professor Adams, I am indebted to the obituary notice
written by my friend Dr. J.W.L. Glaisher, for the Royal Astronomical
Society; while with regard to the late Sir George Airy, I have a
similar acknowledgment to make to Professor H.H. Turner. To my
friend Dr. Arthur A. Rambaut I owe my hearty thanks for his
kindness in aiding me in the revision of the work.

R.S.B.
The Observatory, Cambridge.
October, 1895

CONTENTS.

INTRODUCTION.

PTOLEMY.

COPERNICUS.

TYCHO BRAHE.

GALILEO.

KEPLER.

ISAAC NEWTON.

FLAMSTEED.

HALLEY.

BRADLEY.

WILLIAM HERSCHEL.

LAPLACE.

BRINKLEY.

JOHN HERSCHEL.

THE EARL OF ROSSE.

AIRY.

HAMILTON.

LE VERRIER.

ADAMS.

LIST OF ILLUSTRATIONS.

THE OBSERVATORY, GREENWICH.

PTOLEMY.

PTOLEMY'S PLANETARY SCHEME.

PTOLEMY'S THEORY OF THE MOVEMENT OF MARS.

THORN, FROM AN OLD PRINT.

COPERNICUS.

FRAUENBURG, FROM AN OLD PRINT.

EXPLANATION OF PLANETARY MOVEMENTS.

TYCHO BRAHE.

TYCHO'S CROSS STAFF.

TYCHO'S "NEW STAR" SEXTANT OF 1572.

TYCHO'S TRIGONIC SEXTANT.

TYCHO'S ASTRONOMIC SEXTANT.

TYCHO'S EQUATORIAL ARMILLARY.

THE GREAT AUGSBURG QUADRANT.

TYCHO'S "NEW SCHEME OF THE TERRESTRIAL SYSTEM," 1577.

URANIBORG AND ITS GROUNDS.

GROUND-PLAN OF THE OBSERVATORY.

THE OBSERVATORY OF URANIBORG, ISLAND OF HVEN.

EFFIGY ON TYCHO'S TOMB AT PRAGUE.
By Permission of Messrs. A. & C. Black.

TYCHO'S MURAL QUADRANT, URANIBORG.

GALILEO'S PENDULUM.

GALILEO.

THE VILLA ARCETRI.

FACSIMILE SKETCH OF LUNAR SURFACE BY GALILEO.

CREST OF GALILEO'S FAMILY.

KEPLER'S SYSTEM OF REGULAR SOLIDS.

KEPLER.

SYMBOLICAL REPRESENTATION OF THE PLANETARY SYSTEM.

THE COMMEMORATION OF THE RUDOLPHINE TABLES.

WOOLSTHORPE MANOR.

TRINITY COLLEGE, CAMBRIDGE.

DIAGRAM OF A SUNBEAM.

ISAAC NEWTON.

SIR ISAAC NEWTON'S LITTLE REFLECTOR.

SIR ISAAC NEWTON'S SUN-DIAL.

SIR ISAAC NEWTON'S TELESCOPE.

SIR ISAAC NEWTON'S ASTROLABE.

SIR ISAAC NEWTON'S SUN-DIAL IN THE ROYAL SOCIETY.

FLAMSTEED'S HOUSE.

FLAMSTEED.

HALLEY.

GREENWICH OBSERVATORY IN HALLEY'S TIME.

7, NEW KING STREET, BATH.
From a Photograph by John Poole, Bath.

WILLIAM HERSCHEL.

CAROLINE HERSCHEL.

STREET VIEW, HERSCHEL HOUSE, SLOUGH.
From a Photograph by Hill & Saunders, Eton.

GARDEN VIEW, HERSCHEL HOUSE, SLOUGH.
From a Photograph by Hill & Saunders, Eton.

OBSERVATORY, HERSCHEL HOUSE, SLOUGH.
From a Photograph by Hill & Saunders, Eton.

THE 40-FOOT TELESCOPE, HERSCHEL HOUSE, SLOUGH.
From a Photograph by Hill & Saunders, Eton.

LAPLACE.

THE OBSERVATORY, DUNSINK.
From a Photograph by W. Lawrence, Dublin.

ASTRONOMETER MADE BY SIR JOHN HERSCHEL.

SIR JOHN HERSCHEL.

NEBULA IN SOUTHERN HEMISPHERE.

THE CLUSTER IN THE CENTAUR.

OBSERVATORY AT FELDHAUSEN.

GRANITE COLUMN AT FELDHAUSEN.

THE EARL OF ROSSE.

BIRR CASTLE.
From a Photograph by W. Lawrence, Dublin.

THE MALL, PARSONSTOWN.
From a Photograph by W. Lawrence, Dublin.

LORD ROSSE'S TELESCOPE.
From a Photograph by W. Lawrence, Dublin.

ROMAN CATHOLIC CHURCH, PARSONSTOWN.
From a Photograph by W. Lawrence, Dublin.

AIRY.
From a Photograph by E.P. Adams, Greenwich.

HAMILTON.

ADAMS.

THE OBSERVATORY, CAMBRIDGE.

INTRODUCTION.

Of all the natural sciences there is not one which offers such
sublime objects to the attention of the inquirer as does the
science of astronomy. From the earliest ages the study of the
stars has exercised the same fascination as it possesses at the
present day. Among the most primitive peoples, the movements of
the sun, the moon, and the stars commanded attention from their
supposed influence on human affairs.

The practical utilities of astronomy were also obvious in primeval
times. Maxims of extreme antiquity show how the avocations of the
husbandman are to be guided by the movements of the heavenly
bodies. The positions of the stars indicated the time to plough,
and the time to sow. To the mariner who was seeking a way across
the trackless ocean, the heavenly bodies offered the only reliable
marks by which his path could be guided. There was, accordingly,
a stimulus both from intellectual curiosity and from practical
necessity to follow the movements of the stars. Thus began a
search for the causes of the ever-varying phenomena which the
heavens display.

Many of the earliest discoveries are indeed prehistoric. The
great diurnal movement of the heavens, and the annual revolution
of the sun, seem to have been known in times far more ancient than
those to which any human monuments can be referred. The acuteness
of the early observers enabled them to single out the more
important of the wanderers which we now call planets. They saw
that the star-like objects, Jupiter, Saturn, and Mars, with the
more conspicuous Venus, constituted a class of bodies wholly
distinct from the fixed stars among which their movements lay, and
to which they bear such a superficial resemblance. But the
penetration of the early astronomers went even further, for they
recognized that Mercury also belongs to the same group, though
this particular object is seen so rarely. It would seem that
eclipses and other phenomena were observed at Babylon from a very
remote period, while the most ancient records of celestial
observations that we possess are to be found in the Chinese
annals.

The study of astronomy, in the sense in which we understand the
word, may be said to have commenced under the reign of the
Ptolemies at Alexandria. The most famous name in the science of
this period is that of Hipparchus who lived and worked at Rhodes
about the year 160BC. It was his splendid investigations that
first wrought the observed facts into a coherent branch of
knowledge. He recognized the primary obligation which lies on the
student of the heavens to compile as complete an inventory as
possible of the objects which are there to be found. Hipparchus
accordingly commenced by undertaking, on a small scale, a task
exactly similar to that on which modern astronomers, with all
available appliances of meridian circles, and photographic
telescopes, are constantly engaged at the present day. He
compiled a catalogue of the principal fixed stars, which is of
special value to astronomers, as being the earliest work of its
kind which has been handed down. He also studied the movements of
the sun and the moon, and framed theories to account for the
incessant changes which he saw in progress. He found a much more
difficult problem in his attempt to interpret satisfactorily the
complicated movements of the planets. With the view of
constructing a theory which should give some coherent account of
the subject, he made many observations of the places of these
wandering stars. How great were the advances which Hipparchus
accomplished may be appreciated if we reflect that, as a
preliminary task to his more purely astronomical labours, he had
to invent that branch of mathematical science by which alone the
problems he proposed could be solved. It was for this purpose
that he devised the indispensable method of calculation which we
now know so well as trigonometry. Without the aid rendered by
this beautiful art it would have been impossible for any really
important advance in astronomical calculation to have been
effected.

But the discovery which shows, beyond all others, that Hipparchus
possessed one of the master-minds of all time was the detection of
that remarkable celestial movement known as the precession of the
equinoxes. The inquiry which conducted to this discovery involved
a most profound investigation, especially when it is remembered
that in the days of Hipparchus the means of observation of the
heavenly bodies were only of the rudest description, and the
available observations of earlier dates were extremely scanty.
We can but look with astonishment on the genius of the man who, in
spite of such difficulties, was able to detect such a phenomenon
as the precession, and to exhibit its actual magnitude. I shall
endeavour to explain the nature of this singular celestial
movement, for it may be said to offer the first instance in the
history of science in which we find that combination of accurate
observation with skilful interpretation, of which, in the
subsequent development of astronomy, we have so many splendid
examples.

The word equinox implies the condition that the night is equal to
the day. To a resident on the equator the night is no doubt equal
to the day at all times in the year, but to one who lives on any
other part of the earth, in either hemisphere, the night and the
day are not generally equal. There is, however, one occasion in
spring, and another in autumn, on which the day and the night are
each twelve hours at all places on the earth. When the night and
day are equal in spring, the point which the sun occupies on the
heavens is termed the vernal equinox. There is similarly another
point in which the sun is situated at the time of the autumnal
equinox. In any investigation of the celestial movements the
positions of these two equinoxes on the heavens are of primary
importance, and Hipparchus, with the instinct of genius, perceived
their significance, and commenced to study them. It will be
understood that we can always define the position of a point on
the sky with reference to the surrounding stars. No doubt we do
not see the stars near the sun when the sun is shining, but they
are there nevertheless. The ingenuity of Hipparchus enabled him
to determine the positions of each of the two equinoxes relatively
to the stars which lie in its immediate vicinity. After
examination of the celestial places of these points at different
periods, he was led to the conclusion that each equinox was moving
relatively to the stars, though that movement was so slow that
twenty five thousand years would necessarily elapse before a
complete circuit of the heavens was accomplished. Hipparchus
traced out this phenomenon, and established it on an impregnable
basis, so that all astronomers have ever since recognised the
precession of the equinoxes as one of the fundamental facts of
astronomy. Not until nearly two thousand years after Hipparchus
had made this splendid discovery was the explanation of its cause
given by Newton.

From the days of Hipparchus down to the present hour the science
of astronomy has steadily grown. One great observer after another
has appeared from time to time, to reveal some new phenomenon with
regard to the celestial bodies or their movements, while from time
to time one commanding intellect after another has arisen to
explain the true import of the facts of observations. The history
of astronomy thus becomes inseparable from the history of the
great men to whose labours its development is due.

In the ensuing chapters we have endeavoured to sketch the lives
and the work of the great philosophers, by whose labours the
science of astronomy has been created. We shall commence with
Ptolemy, who, after the foundations of the science had been laid
by Hipparchus, gave to astronomy the form in which it was taught
throughout the Middle Ages. We shall next see the mighty
revolution in our conceptions of the universe which are associated
with the name of Copernicus. We then pass to those periods
illumined by the genius of Galileo and Newton, and afterwards we
shall trace the careers of other more recent discoverers, by
whose industry and genius the boundaries of human knowledge have
been so greatly extended. Our history will be brought down late
enough to include some of the illustrious astronomers who laboured
in the generation which has just passed away.

PTOLEMY.

[PLATE: PTOLEMY.]

The career of the famous man whose name stands at the head of this
chapter is one of the most remarkable in the history of human
learning. There may have been other discoverers who have done
more for science than ever Ptolemy accomplished, but there never
has been any other discoverer whose authority on the subject of
the movements of the heavenly bodies has held sway over the minds
of men for so long a period as the fourteen centuries during which
his opinions reigned supreme. The doctrines he laid down in his
famous book, "The Almagest," prevailed throughout those ages. No
substantial addition was made in all that time to the undoubted
truths which this work contained. No important correction was
made of the serious errors with which Ptolemy's theories were
contaminated. The authority of Ptolemy as to all things in
the heavens, and as to a good many things on the earth (for the
same illustrious man was also a diligent geographer), was
invariably final.

Though every child may now know more of the actual truths of the
celestial motions than ever Ptolemy knew, yet the fact that his
work exercised such an astonishing effect on the human intellect
for some sixty generations, shows that it must have been an
extraordinary production. We must look into the career of this
wonderful man to discover wherein lay the secret of that
marvellous success which made him the unchallenged instructor of
the human race for such a protracted period.

Unfortunately, we know very little as to the personal history of
Ptolemy. He was a native of Egypt, and though it has been
sometimes conjectured that he belonged to the royal families of
the same name, yet there is nothing to support such a belief.
The name, Ptolemy, appears to have been a common one in Egypt in
those days. The time at which he lived is fixed by the fact that
his first recorded observation was made in 127 AD, and his last in
151 AD. When we add that he seems to have lived in or near
Alexandria, or to use his own words, "on the parallel of
Alexandria," we have said everything that can be said so far as
his individuality is concerned.

Ptolemy is, without doubt, the greatest figure in ancient
astronomy. He gathered up the wisdom of the philosophers who had
preceded him. He incorporated this with the results of his
own observations, and illumined it with his theories. His
speculations, even when they were, as we now know, quite
erroneous, had such an astonishing verisimilitude to the actual
facts of nature that they commanded universal assent. Even in
these modern days we not unfrequently find lovers of paradox who
maintain that Ptolemy's doctrines not only seem true, but actually
are true.

In the absence of any accurate knowledge of the science of
mechanics, philosophers in early times were forced to fall back
on certain principles of more or less validity, which they derived
from their imagination as to what the natural fitness of things
ought to be. There was no geometrical figure so simple and so
symmetrical as a circle, and as it was apparent that the heavenly
bodies pursued tracks which were not straight lines, the
conclusion obviously followed that their movements ought to be
circular. There was no argument in favour of this notion, other
than the merely imaginary reflection that circular movement, and
circular movement alone, was "perfect," whatever "perfect" may
have meant. It was further believed to be impossible that the
heavenly bodies could have any other movements save those which
were perfect. Assuming this, it followed, in Ptolemy's opinion,
and in that of those who came after him for fourteen centuries,
that all the tracks of the heavenly bodies were in some way or
other to be reduced to circles.

Ptolemy succeeded in devising a scheme by which the apparent
changes that take place in the heavens could, so far as he knew
them, be explained by certain combinations of circular movement.
This seemed to reconcile so completely the scheme of things
celestial with the geometrical instincts which pointed to the
circle as the type of perfect movement, that we can hardly wonder
Ptolemy's theory met with the astonishing success that attended
it. We shall, therefore, set forth with sufficient detail the
various steps of this famous doctrine.

Ptolemy commences with laying down the undoubted truth that the
shape of the earth is globular. The proofs which he gives of this
fundamental fact are quite satisfactory; they are indeed the same
proofs as we give today. There is, first of all, the well-known
circumstance of which our books on geography remind us, that when
an object is viewed at a distance across the sea, the lower part
of the object appears cut off by the interposing curved mass of
water.

The sagacity of Ptolemy enabled him to adduce another argument,
which, though not quite so obvious as that just mentioned,
demonstrates the curvature of the earth in a very impressive
manner to anyone who will take the trouble to understand it.
Ptolemy mentions that travellers who went to the south reported,
that, as they did so, the appearance of the heavens at night
underwent a gradual change. Stars that they were familiar with in
the northern skies gradually sank lower in the heavens. The
constellation of the Great Bear, which in our skies never sets
during its revolution round the pole, did set and rise when a
sufficient southern latitude had been attained. On the other
hand, constellations new to the inhabitants of northern climes
were seen to rise above the southern horizon. These
circumstances would be quite incompatible with the supposition
that the earth was a flat surface. Had this been so, a little
reflection will show that no such changes in the apparent
movements of the stars would be the consequence of a voyage to the
south. Ptolemy set forth with much insight the significance of
this reasoning, and even now, with the resources of modern
discoveries to help us, we can hardly improve upon his arguments.

Ptolemy, like a true philosopher disclosing a new truth to the
world, illustrated and enforced his subject by a variety of happy
demonstrations. I must add one of them, not only on account of
its striking nature, but also because it exemplifies Ptolemy's
acuteness. If the earth were flat, said this ingenious reasoner,
sunset must necessarily take place at the same instant, no matter
in what country the observer may happen to be placed. Ptolemy,
however, proved that the time of sunset did vary greatly as
the observer's longitude was altered. To us, of course, this is
quite obvious; everybody knows that the hour of sunset may have
been reached in Great Britain while it is still noon on the
western coast of America. Ptolemy had, however, few of those
sources of knowledge which are now accessible. How was he to show
that the sun actually did set earlier at Alexandria than it would
in a city which lay a hundred miles to the west? There was no
telegraph wire by which astronomers at the two Places could
communicate. There was no chronometer or watch which could be
transported from place to place; there was not any other reliable
contrivance for the keeping of time. Ptolemy's ingenuity,
however, pointed out a thoroughly satisfactory method by which the
times of sunset at two places could be compared. He was
acquainted with the fact, which must indeed have been known from
the very earliest times, that the illumination of the moon is
derived entirely from the sun. He knew that an eclipse of the
moon was due to the interposition of the earth which cuts off the
light of the sun. It was, therefore, plain that an eclipse of the
moon must be a phenomenon which would begin at the same instant
from whatever part of the earth the moon could be seen at the
time. Ptolemy, therefore, brought together from various quarters
the local times at which different observers had recorded the
beginning of a lunar eclipse. He found that the observers to the
west made the time earlier and earlier the further away their
stations were from Alexandria. On the other hand, the eastern
observers set down the hour as later than that at which the
phenomenon appeared at Alexandria. As these observers all
recorded something which indeed appeared to them simultaneously,
the only interpretation was, that the more easterly a place the
later its time. Suppose there were a number of observers along a
parallel of latitude, and each noted the hour of sunset to be
six o'clock, then, since the eastern times are earlier than
western times, 6 p.m. at one station A will correspond to 5 p.m.
at a station B sufficiently to the west. If, therefore, it is
sunset to the observer at A, the hour of sunset will not yet be
reached for the observer at B. This proves conclusively that the
time of sunset is not the same all over the earth. We have,
however, already seen that the apparent time of sunset would be
the same from all stations if the earth were flat. When Ptolemy,
therefore, demonstrated that the time of sunset was not the same
at various places, he showed conclusively that the earth was not
flat.

As the same arguments applied to all parts of the earth where
Ptolemy had either been himself, or from which he could gain the
necessary information, it followed that the earth, instead of
being the flat plain, girdled with an illimitable ocean, as was
generally supposed, must be in reality globular. This led at once
to a startling consequence. It was obvious that there could be no
supports of any kind by which this globe was sustained; it
therefore followed that the mighty object must be simply poised in
space. This is indeed an astonishing doctrine to anyone who
relies on what merely seems the evidence of the senses, without
giving to that evidence its due intellectual interpretation.
According to our ordinary experience, the very idea of an object
poised without support in space, appears preposterous. Would it
not fall? we are immediately asked. Yes, doubtless it could not
remain poised in any way in which we try the experiment.
We must, however, observe that there are no such ideas as upwards
or downwards in relation to open space. To say that a body falls
downwards, merely means that it tries to fall as nearly as
possible towards the centre of the earth. There is no one
direction along which a body will tend to move in space, in
preference to any other. This may be illustrated by the fact that
a stone let fall at New Zealand will, in its approach towards the
earth's centre, be actually moving upwards as far as any locality
in our hemisphere is concerned. Why, then, argued Ptolemy, may
not the earth remain poised in space, for as all directions are
equally upward or equally downward, there seems no reason why the
earth should require any support? By this reasoning he arrives at
the fundamental conclusion that the earth is a globular body
freely lying in space, and surrounded above, below, and on all
sides by the glittering stars of heaven.

The perception of this sublime truth marks a notable epoch in the
history of the gradual development of the human intellect. No
doubt, other philosophers, in groping after knowledge, may have
set forth certain assertions that are more or less equivalent to
this fundamental truth. It is to Ptolemy we must give credit,
however, not only for announcing this doctrine, but for
demonstrating it by clear and logical argument. We cannot easily
project our minds back to the conception of an intellectual state
in which this truth was unfamiliar. It may, however, be well
imagined that, to one who thought the earth was a flat plain of
indefinite extent, it would be nothing less than an intellectual
convulsion for him to be forced to believe that he stood upon a
spherical earth, forming merely a particle relatively to the
immense sphere of the heavens.

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