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.
WITH NUMEROUS ILLUSTRATIONS
[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.
What Ptolemy saw in the movements of the stars led him to the
conclusion that they were bright points attached to the inside of a
tremendous globe. The movements of this globe which carried the
stars were only compatible with the supposition that the earth
occupied its centre. The imperceptible effect produced by a change
in the locality of the observer on the apparent brightness of the
stars made it plain that the dimensions of the terrestrial globe must
be quite insignificant in comparison with those of the celestial
sphere. The earth might, in fact, be regarded as a grain of sand
while the stars lay upon a globe many yards in diameter.
So tremendous was the revolution in human knowledge implied by this
discovery, that we can well imagine how Ptolemy, dazzled as it were
by the fame which had so justly accrued to him, failed to make one
further step. Had he made that step, it would have emancipated the
human intellect from the bondage of fourteen centuries of servitude
to a wholly monstrous notion of this earth's importance in the scheme
of the heavens. The obvious fact that the sun, the moon, and the
stars rose day by day, moved across the sky in a glorious
never-ending procession, and duly set when their appointed courses
had been run, demanded some explanation. The circumstance that the
fixed stars preserved their mutual distances from year to year, and
from age to age, appeared to Ptolemy to prove that the sphere which
contained those stars, and on whose surface they were believed by him
to be fixed, revolved completely around the earth once every day. He
would thus account for all the phenomena of rising and setting
consistently with the supposition that our globe was stationary.
Probably this supposition must have appeared monstrous, even to
Ptolemy. He knew that the earth was a gigantic object, but, large as
it may have been, he knew that it was only a particle in comparison
with the celestial sphere, yet he apparently believed, and certainly
succeeded in persuading other men to believe, that the celestial
sphere did actually perform these movements.
Ptolemy was an excellent geometer. He knew that the rising and the
setting of the sun, the moon, and the myriad stars, could have been
accounted for in a different way. If the earth turned round
uniformly once a day while poised at the centre of the sphere of the
heavens, all the phenomena of rising and setting could be completely
explained. This is, indeed, obvious after a moment's reflection.
Consider yourself to be standing on the earth at the centre of the
heavens. There are stars over your head, and half the contents of
the heavens are visible, while the other half are below your
horizon. As the earth turns round, the stars over your head will
change, and unless it should happen that you have taken up your
position at either of the poles, new stars will pass into your view,
and others will disappear, for at no time can you have more than half
of the whole sphere visible. The observer on the earth would,
therefore, say that some stars were rising, and that some stars were
setting. We have, therefore, two totally distinct methods, each of
which would completely explain all the observed facts of the diurnal
movement. One of these suppositions requires that the celestial
sphere, bearing with it the stars and other celestial bodies, turns
uniformly around an invisible axis, while the earth remains
stationary at the centre. The other supposition would be, that it is
the stupendous celestial sphere which remains stationary, while the
earth at the centre rotates about the same axis as the celestial
sphere did before, but in an opposite direction, and with a uniform
velocity which would enable it to complete one turn in twenty-four
hours. Ptolemy was mathematician enough to know that either of these
suppositions would suffice for the explanation of the observed
facts. Indeed, the phenomena of the movements of the stars, so far
as he could observe them, could not be called upon to pronounce which
of these views was true, and which was false.
Ptolemy had, therefore, to resort for guidance to indirect lines of
reasoning. One of these suppositions must be true, and yet it
appeared that the adoption of either was accompanied by a great
difficulty. It is one of his chief merits to have demonstrated that
the celestial sphere was so stupendous that the earth itself was
absolutely insignificant in comparison therewith. If, then, this
stupendous sphere rotated once in twenty-four hours, the speed with
which the movement of some of the stars must be executed would be so
portentous as to seem well-nigh impossible. It would, therefore,
seem much simpler on this ground to adopt the other alternative, and
to suppose the diurnal movements were due to the rotation of the
earth. Here Ptolemy saw, or at all events fancied he saw, objections
of the weightiest description. The evidence of the senses appeared
directly to controvert the supposition that this earth is anything
but stationary. Ptolemy might, perhaps, have dismissed this
objection on the ground that the testimony of the senses on such a
matter should be entirely subordinated to the interpretation which
our intelligence would place upon the facts to which the senses
deposed. Another objection, however, appeared to him to possess the
gravest moment. It was argued that if the earth were rotating, there
is nothing to make the air participate in this motion, mankind would
therefore be swept from the earth by the furious blasts which would
arise from the movement of the earth through an atmosphere at rest.
Even if we could imagine that the air were carried round with the
earth, the same would not apply, so thought Ptolemy, to any object
suspended in the air. So long as a bird was perched on a tree, he
might very well be carried onward by the moving earth, but the moment
he took wing, the ground would slip from under him at a frightful
pace, so that when he dropped down again he would find himself at a
distance perhaps ten times as great as that which a carrier-pigeon or
a swallow could have traversed in the same time. Some vague delusion
of this description seems even still to crop up occasionally. I
remember hearing of a proposition for balloon travelling of a very
remarkable kind. The voyager who wanted to reach any other place in
the same latitude was simply to ascend in a balloon, and wait there
till the rotation of the earth conveyed the locality which happened
to be his destination directly beneath him, whereupon he was to let
out the gas and drop down! Ptolemy knew quite enough natural
philosophy to be aware that such a proposal for locomotion would be
an utter absurdity; he knew that there was no such relative shift
between the air and the earth as this motion would imply. It
appeared to him to be necessary that the air should lag behind, if
the earth had been animated by a movement of rotation. In this he
was, as we know, entirely wrong. There were, however, in his days no
accurate notions on the subject of the laws of motion.
Assiduous as Ptolemy may have been in the study of the heavenly
bodies, it seems evident that he cannot have devoted much thought to
the phenomena of motion of terrestrial objects. Simple, indeed, are
the experiments which might have convinced a philosopher much less
acute than Ptolemy, that, if the earth did revolve, the air must
necessarily accompany it. If a rider galloping on horseback tosses a
ball into the air, it drops again into his hand, just as it would
have done had he been remaining at rest during the ball's flight; the
ball in fact participates in the horizontal motion, so that though it
really describes a curve as any passer-by would observe, yet it
appears to the rider himself merely to move up and down in a straight
line. This fact, and many others similar to it, demonstrate clearly
that if the earth were endowed with a movement of rotation, the
atmosphere surrounding it must participate in that movement. Ptolemy
did not know this, and consequently he came to the conclusion that
the earth did not rotate, and that, therefore, notwithstanding the
tremendous improbability of so mighty an object as the celestial
sphere spinning round once in every twenty-four hours, there was no
course open except to believe that this very improbable thing did
really happen. Thus it came to pass that Ptolemy adopted as the
cardinal doctrine of his system a stationary earth poised at the
centre of the celestial sphere, which stretched around on all sides
at a distance so vast that the diameter of the earth was an
inappreciable point in comparison therewith.
Ptolemy having thus deliberately rejected the doctrine of the earth's
rotation, had to make certain other entirely erroneous suppositions.
It was easily seen that each star required exactly the same period
for the performance of a complete revolution of the heavens. Ptolemy
knew that the stars were at enormous distances from the earth, though
no doubt his notions on this point came very far short of what we
know to be the reality. If the stars had been at very varied
distances, then it would be so wildly improbable that they should all
accomplish their revolutions in the same time, that Ptolemy came to
the conclusion that they must be all at the same distance, that is,
that they must be all on the surface of a sphere. This view, however
erroneous, was corroborated by the obvious fact that the stars in the
constellations preserved their relative places unaltered for
centuries. Thus it was that Ptolemy came to the conclusion that they
were all fixed on one spherical surface, though we are not informed
as to the material of this marvellous setting which sustained the
stars like jewels.
Nor should we hastily pronounce this doctrine to be absurd. The
stars do appear to lie on the surface of a sphere, of which the
observer is at the centre; not only is this the aspect which the
skies present to the untechnical observer, but it is the aspect in
which the skies are presented to the most experienced astronomer of
modern days. No doubt he knows well that the stars are at the most
varied distances from him; he knows that certain stars are ten times,
or a hundred times, or a thousand times, as far as other stars.
Nevertheless, to his eye the stars appear on the surface of the
sphere, it is on that surface that his measurements of the relative
places of the stars are made; indeed, it may be said that almost all
the accurate observations in the observatory relate to the places of
the stars, not as they really are, but as they appear to be projected
on that celestial sphere whose conception we owe to the genius of
Ptolemy.
This great philosopher shows very ingeniously that the earth must be
at the centre of the sphere. He proves that, unless this were the
case, each star would not appear to move with the absolute uniformity
which does, as a matter of fact, characterise it. In all these
reasonings we cannot but have the most profound admiration for the
genius of Ptolemy, even though he had made an error so enormous in
the fundamental point of the stability of the earth. Another error
of a somewhat similar kind seemed to Ptolemy to be demonstrated. He
had shown that the earth was an isolated object in space, and being
such was, of course, capable of movement. It could either be turned
round, or it could be moved from one place to another. We know that
Ptolemy deliberately adopted the view that the earth did not turn
round; he had then to investigate the other question, as to whether
the earth was animated by any movement of translation. He came to
the conclusion that to attribute any motion to the earth would be
incompatible with the truths at which he had already arrived. The
earth, argued Ptolemy, lies at the centre of the celestial sphere.
If the earth were to be endowed with movement, it would not lie
always at this point, it must, therefore, shift to some other part of
the sphere. The movements of the stars, however, preclude the
possibility of this; and, therefore, the earth must be as devoid of
any movement of translation as it is devoid of rotation. Thus it was
that Ptolemy convinced himself that the stability of the earth, as it
appeared to the ordinary senses, had a rational philosophical
foundation.
Not unfrequently it is the lot of the philosophers to contend against
the doctrines of the vulgar, but when it happens, as in the case of
Ptolemy's researches, that the doctrines of the vulgar are
corroborated by philosophical investigation which bear the stamp of
the highest authority, it is not to be wondered at that such
doctrines should be deemed well-nigh impregnable. In this way we
may, perhaps, account for the remarkable fact that the theories of
Ptolemy held unchallenged sway over the human intellect for the vast
period already mentioned.
Up to the present we have been speaking only of those primary motions
of the heavens, by which the whole sphere appeared to revolve once
every twenty-four hours. We have now to discuss the remarkable
theories by which Ptolemy endeavoured to account for the monthly
movement of the moon, for the annual movement of the sun, and for the
periodic movements of the planets which had gained for them the
titles of the wandering stars.
Possessed with the idea that these movements must be circular, or
must be capable, directly or indirectly, of being explained by
circular movements, it seemed obvious to Ptolemy, as indeed it had
done to previous astronomers, that the track of the moon through the
stars was a circle of which the earth is the centre. A similar
movement with a yearly period must also be attributed to the sun, for
the changes in the positions of the constellations in accordance with
the progress of the seasons, placed it beyond doubt that the sun made
a circuit of the celestial sphere, even though the bright light of
the sun prevented the stars in its vicinity, from being seen in
daylight. Thus the movements both of the sun and the moon, as well
as the diurnal rotation of the celestial sphere, seemed to justify
the notion that all celestial movements must be "perfect," that is to
say, described uniformly in those circles which were the only perfect
curves.
The simplest observations, however, show that the movements of the
planets cannot be explained in this simple fashion. Here the
geometrical genius of Ptolemy shone forth, and he devised a scheme by
which the apparent wanderings of the planets could be accounted for
without the introduction of aught save "perfect" movements.
To understand his reasoning, let us first set forth clearly those
facts of observation which require to be explained. I shall take, in
particular, two planets, Venus and Mars, as these illustrate, in the
most striking manner, the peculiarities of the inner and the outer
planets respectively. The simplest observations would show that
Venus did not move round the heavens in the same fashion as the sun
or the moon. Look at the evening star when brightest, as it appears
in the west after sunset. Instead of moving towards the east among
the stars, like the sun or the moon, we find, week after week, that
Venus is drawing in towards the sun, until it is lost in the
sunbeams. Then the planet emerges on the other side, not to be seen
as an evening star, but as a morning star. In fact, it was plain
that in some ways Venus accompanied the sun in its annual movement.
Now it is found advancing in front of the sun to a certain limited
distance, and now it is lagging to an equal extent behind the sun.
[FIG. 1. PTOLEMY'S PLANETARY SCHEME.]
These movements were wholly incompatible with the supposition that
the journeys of Venus were described by a single motion of the kind
regarded as perfect. It was obvious that the movement was connected
in some strange manner with the revolution of the sun, and here was
the ingenious method by which Ptolemy sought to render account of
it. Imagine a fixed arm to extend from the earth to the sun, as
shown in the accompanying figure (Fig. 1), then this arm will move
round uniformly, in consequence of the sun's movement. At a point P
on this arm let a small circle be described. Venus is supposed to
revolve uniformly in this small circle, while the circle itself is
carried round continuously by the movement of the sun. In this way
it was possible to account for the chief peculiarities in the
movement of Venus. It will be seen that, in consequence of the
revolution around P, the spectator on the earth will sometimes see
Venus on one side of the sun, and sometimes on the other side, so
that the planet always remains in the sun's vicinity. By properly
proportioning the movements, this little contrivance simulated the
transitions from the morning star to the evening star. Thus the
changes of Venus could be accounted for by a Combination of the
"perfect" movement of P in the circle which it described uniformly
round the earth, combined with the "perfect" motion of Venus in the
circle which it described uniformly around the moving centre.
In a precisely similar manner Ptolemy rendered an explanation of the
fitful apparitions of Mercury. Now just on one side of the sun, and
now just on the other, this rarely-seen planet moved like Venus on a
circle whereof the centre was also carried by the line joining the
sun and the earth. The circle, however, in which Mercury actually
revolved had to be smaller than that of Venus, in order to account
for the fact that Mercury lies always much closer to the sun than the
better-known planet.
[FIG. 2. PTOLEMY'S THEORY OF THE MOVEMENT OF MARS.]
The explanation of the movement of an outer planet like Mars could
also be deduced from the joint effect of two perfect motions. The
changes through which Mars goes are, however, so different from the
movements of Venus that quite a different disposition of the circles
is necessary. For consider the facts which characterise the
movements of an outer planet such as Mars. In the first place, Mars
accomplishes an entire circuit of the heaven. In this respect, no
doubt, it may be said to resemble the sun or the moon. A little
attention will, however, show that there are extraordinary
irregularities in the movement of the planet. Generally speaking, it
speeds its way from west to east among the stars, but sometimes the
attentive observer will note that the speed with which the planet
advances is slackening, and then it will seem to become stationary.
Some days later the direction of the planet's movement will be
reversed, and it will be found moving from the east towards the
west. At first it proceeds slowly and then quickens its pace, until
a certain speed is attained, which afterwards declines until a second
stationary position is reached. After a due pause the original
motion from west to east is resumed, and is continued until a similar
cycle of changes again commences. Such movements as these were
obviously quite at variance with any perfect movement in a single
circle round the earth. Here, again, the geometrical sagacity of
Ptolemy provided him with the means of representing the apparent
movements of Mars, and, at the same time, restricting the explanation
to those perfect movements which he deemed so essential. In Fig. 2
we exhibit Ptolemy's theory as to the movement of Mars. We have, as
before, the earth at the centre, and the sun describing its circular
orbit around that centre. The path of Mars is to be taken as
exterior to that of the sun. We are to suppose that at a point
marked M there is a fictitious planet, which revolves around the
earth uniformly, in a circle called the DEFERENT. This point M,
which is thus animated by a perfect movement, is the centre of a
circle which is carried onwards with M, and around the circumference
of which Mars revolves uniformly. It is easy to show that the
combined effect of these two perfect movements is to produce exactly
that displacement of Mars in the heavens which observation
discloses. In the position represented in the figure, Mars is
obviously pursuing a course which will appear to the observer as a
movement from west to east. When, however, the planet gets round to
such a position as R, it is then moving from east to west in
consequence of its revolution in the moving circle, as indicated by
the arrow-head. On the other hand, the whole circle is carried
forward in the opposite direction. If the latter movement be less
rapid than the former, then we shall have the backward movement of
Mars on the heavens which it was desired to explain. By a proper
adjustment of the relative lengths of these arms the movements of the
planet as actually observed could be completely accounted for.
The other outer planets with which Ptolemy was acquainted, namely,
Jupiter and Saturn, had movements of the same general character as
those of Mars. Ptolemy was equally successful in explaining the
movements they performed by the supposition that each planet had
perfect rotation in a circle of its own, which circle itself had
perfect movement around the earth in the centre.
It is somewhat strange that Ptolemy did not advance one step further,
as by so doing he would have given great simplicity to his system. He
might, for instance, have represented the movements of Venus equally
well by putting the centre of the moving circle at the sun itself,
and correspondingly enlarging the circle in which Venus revolved. He
might, too, have arranged that the several circles which the outer
planets traversed should also have had their centres at the sun. The
planetary system would then have consisted of an earth fixed at the
centre, of a sun revolving uniformly around it, and of a system of
planets each describing its own circle around a moving centre placed
in the sun. Perhaps Ptolemy had not thought of this, or perhaps he
may have seen arguments against it. This important step was,
however, taken by Tycho. He considered that all the planets revolved
around the sun in circles, and that the sun itself, bearing all these
orbits, described a mighty circle around the earth. This point
having been reached, only one more step would have been necessary to
reach the glorious truths that revealed the structure of the solar
system. That last step was taken by Copernicus.
COPERNICUS
[PLATE: THORN, FROM AN OLD PRINT.]
The quaint town of Thorn, on the Vistula, was more than two centuries
old when Copernicus was born there on the 19th of February, 1473. The
situation of this town on the frontier between Prussia and Poland,
with the commodious waterway offered by the river, made it a place of
considerable trade. A view of the town, as it was at the time of the
birth of Copernicus, is here given. The walls, with their
watch-towers, will be noted, and the strategic importance which the
situation of Thorn gave to it in the fifteenth century still belongs
thereto, so much so that the German Government recently constituted
the town a fortress of the first class.