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Great Astronomers by R. S. Ball

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This etext was prepared by
Chris Brennen cbrennen@freenet.co.uk
Jill R. Diffendal
Barb Grow pebareka@iexpress.net.au
Christine L. Hall Goleta, CA. USA
Pamela L. Hall pamhall@www.edu



Lowndean Professor of Astronomy and Geometry in the
University of Cambridge

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



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.

The Observatory, Cambridge.
October, 1895









































By Permission of Messrs. A. & C. Black.
























From a Photograph by John Poole, Bath.



From a Photograph by Hill & Saunders, Eton.

From a Photograph by Hill & Saunders, Eton.

From a Photograph by Hill & Saunders, Eton.

From a Photograph by Hill & Saunders, Eton.


From a Photograph by W. Lawrence, Dublin.








From a Photograph by W. Lawrence, Dublin.

From a Photograph by W. Lawrence, Dublin.

From a Photograph by W. Lawrence, Dublin.

From a Photograph by W. Lawrence, Dublin.

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





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.



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

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

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

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

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

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.


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.


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 arrowhead. 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.



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.

Copernicus, the astronomer, whose discoveries make him the great
predecessor of Kepler and Newton, did not come from a noble family,
as certain other early astronomers have done, for his father was a
tradesman. Chroniclers are, however, careful to tell us that one of
his uncles was a bishop. We are not acquainted with any of those
details of his childhood or youth which are often of such interest in
other cases where men have risen to exalted fame. It would appear
that the young Nicolaus, for such was his Christian name, received
his education at home until such time as he was deemed sufficiently
advanced to be sent to the University at Cracow. The education that
he there obtained must have been in those days of a very primitive
description, but Copernicus seems to have availed himself of it to
the utmost. He devoted himself more particularly to the study of
medicine, with the view of adopting its practice as the profession of
his life. The tendencies of the future astronomer were, however,
revealed in the fact that he worked hard at mathematics, and, like
one of his illustrious successors, Galileo, the practice of the art
of painting had for him a very great interest, and in it he obtained
some measure of success.

By the time he was twenty-seven years old, it would seem that
Copernicus had given up the notion of becoming a medical
practitioner, and had resolved to devote himself to science. He was
engaged in teaching mathematics, and appears to have acquired some
reputation. His growing fame attracted the notice of his uncle the
bishop, at whose suggestion Copernicus took holy orders, and he was
presently appointed to a canonry in the cathedral of Frauenburg, near
the mouth of the Vistula.

To Frauenburg, accordingly, this man of varied gifts retired.
Possessing somewhat of the ascetic spirit, he resolved to devote his
life to work of the most serious description. He eschewed all
ordinary society, restricting his intimacies to very grave and
learned companions, and refusing to engage in conversation of any
useless kind. It would seem as if his gifts for painting were
condemned as frivolous; at all events, we do not learn that he
continued to practise them. In addition to the discharge of his
theological duties, his life was occupied partly in ministering
medically to the wants of the poor, and partly with his researches in
astronomy and mathematics. His equipment in the matter of
instruments for the study of the heavens seems to have been of a very
meagre description. He arranged apertures in the walls of his house
at Allenstein, so that he could observe in some fashion the passage
of the stars across the meridian. That he possessed some talent for
practical mechanics is proved by his construction of a contrivance
for raising water from a stream, for the use of the inhabitants of
Frauenburg. Relics of this machine are still to be seen.


The intellectual slumber of the Middle Ages was destined to be
awakened by the revolutionary doctrines of Copernicus. It may be
noted, as an interesting circumstance, that the time at which he
discovered the scheme of the solar system has coincided with a
remarkable epoch in the world's history. The great astronomer had
just reached manhood at the time when Columbus discovered the new

Before the publication of the researches of Copernicus, the orthodox
scientific creed averred that the earth was stationary, and that the
apparent movements of the heavenly bodies were indeed real
movements. Ptolemy had laid down this doctrine 1,400 years before.
In his theory this huge error was associated with so much important
truth, and the whole presented such a coherent scheme for the
explanation of the heavenly movements, that the Ptolemaic theory was
not seriously questioned until the great work of Copernicus
appeared. No doubt others, before Copernicus, had from time to time
in some vague fashion surmised, with more or less plausibility, that
the sun, and not the earth, was the centre about which the system
really revolved. It is, however, one thing to state a scientific
fact; it is quite another thing to be in possession of the train of
reasoning, founded on observation or experiment, by which that fact
may be established. Pythagoras, it appears, had indeed told his
disciples that it was the sun, and not the earth, which was the
centre of movement, but it does not seem at all certain that
Pythagoras had any grounds which science could recognise for the
belief which is attributed to him. So far as information is
available to us, it would seem that Pythagoras associated his scheme
of things celestial with a number of preposterous notions in natural
philosophy. He may certainly have made a correct statement as to
which was the most important body in the solar system, but he
certainly did not provide any rational demonstration of the fact.
Copernicus, by a strict train of reasoning, convinced those who would
listen to him that the sun was the centre of the system. It is
useful for us to consider the arguments which he urged, and by which
he effected that intellectual revolution which is always connected
with his name.

The first of the great discoveries which Copernicus made relates to
the rotation of the earth on its axis. That general diurnal
movement, by which the stars and all other celestial bodies appear to
be carried completely round the heavens once every twenty-four hours,
had been accounted for by Ptolemy on the supposition that the
apparent movements were the real movements. As we have already seen,
Ptolemy himself felt the extraordinary difficulty involved in the
supposition that so stupendous a fabric as the celestial sphere
should spin in the way supposed. Such movements required that many
of the stars should travel with almost inconceivable velocity.
Copernicus also saw that the daily rising and setting of the heavenly
bodies could be accounted for either by the supposition that the
celestial sphere moved round and that the earth remained at rest, or
by the supposition that the celestial sphere was at rest while the
earth turned round in the opposite direction. He weighed the
arguments on both sides as Ptolemy had done, and, as the result of
his deliberations, Copernicus came to an opposite conclusion from
Ptolemy. To Copernicus it appeared that the difficulties attending
the supposition that the celestial sphere revolved, were vastly
greater than those which appeared so weighty to Ptolemy as to force
him to deny the earth's rotation.

Copernicus shows clearly how the observed phenomena could be
accounted for just as completely by a rotation of the earth as by a
rotation of the heavens. He alludes to the fact that, to those on
board a vessel which is moving through smooth water, the vessel
itself appears to be at rest, while the objects on shore seem to be
moving past. If, therefore, the earth were rotating uniformly, we
dwellers upon the earth, oblivious of our own movement, would wrongly
attribute to the stars the displacement which was actually the
consequence of our own motion.

Copernicus saw the futility of the arguments by which Ptolemy had
endeavoured to demonstrate that a revolution of the earth was
impossible. It was plain to him that there was nothing whatever to
warrant refusal to believe in the rotation of the earth. In his
clear-sightedness on this matter we have specially to admire the
sagacity of Copernicus as a natural philosopher. It had been urged
that, if the earth moved round, its motion would not be imparted to
the air, and that therefore the earth would be uninhabitable by the
terrific winds which would be the result of our being carried through
the air. Copernicus convinced himself that this deduction was
preposterous. He proved that the air must accompany the earth, just
as his coat remains round him, notwithstanding the fact that he is
walking down the street. In this way he was able to show that all a
priori objections to the earth's movements were absurd, and therefore
he was able to compare together the plausibilities of the two rival
schemes for explaining the diurnal movement.


Once the issue had been placed in this form, the result could not be
long in doubt. Here is the question: Which is it more likely--that
the earth, like a grain of sand at the centre of a mighty globe,
should turn round once in twenty-four hours, or that the whole of
that vast globe should complete a rotation in the opposite direction
in the same time? Obviously, the former is far the more simple
supposition. But the case is really much stronger than this. Ptolemy
had supposed that all the stars were attached to the surface of a
sphere. He had no ground whatever for this supposition, except that
otherwise it would have been well-nigh impossible to have devised a
scheme by which the rotation of the heavens around a fixed earth
could have been arranged. Copernicus, however, with the just
instinct of a philosopher, considered that the celestial sphere,
however convenient from a geometrical point of view, as a means of
representing apparent phenomena, could not actually have a material
existence. In the first place, the existence of a material celestial
sphere would require that all the myriad stars should be at exactly
the same distances from the earth. Of course, no one will say that
this or any other arbitrary disposition of the stars is actually
impossible, but as there was no conceivable physical reason why the
distances of all the stars from the earth should be identical, it
seemed in the very highest degree improbable that the stars should be
so placed.

Doubtless, also, Copernicus felt a considerable difficulty as to the
nature of the materials from which Ptolemy's wonderful sphere was to
be constructed. Nor could a philosopher of his penetration have
failed to observe that, unless that sphere were infinitely large,
there must have been space outside it, a consideration which would
open up other difficult questions. Whether infinite or not, it was
obvious that the celestial sphere must have a diameter at least many
thousands of times as great as that of the earth. From these
considerations Copernicus deduced the important fact that the stars
and the other celestial bodies must all be vast objects. He was thus
enabled to put the question in such a form that it could hardly
receive any answer but the correct one. Which is it more rational to
suppose, that the earth should turn round on its axis once in
twenty-four hours, or that thousands of mighty stars should circle
round the earth in the same time, many of them having to describe
circles many thousands of times greater in circumference than the
circuit of the earth at the equator? The obvious answer pressed upon
Copernicus with so much force that he was compelled to reject
Ptolemy's theory of the stationary earth, and to attribute the
diurnal rotation of the heavens to the revolution of the earth on its

Once this tremendous step had been taken, the great difficulties
which beset the monstrous conception of the celestial sphere
vanished, for the stars need no longer be regarded as situated at
equal distances from the earth. Copernicus saw that they might lie
at the most varied degrees of remoteness, some being hundreds or
thousands of times farther away than others. The complicated
structure of the celestial sphere as a material object disappeared
altogether; it remained only as a geometrical conception, whereon we
find it convenient to indicate the places of the stars. Once the
Copernican doctrine had been fully set forth, it was impossible for
anyone, who had both the inclination and the capacity to understand
it, to withhold acceptance of its truth. The doctrine of a
stationary earth had gone for ever.

Copernicus having established a theory of the celestial movements
which deliberately set aside the stability of the earth, it seemed
natural that he should inquire whether the doctrine of a moving earth
might not remove the difficulties presented in other celestial
phenomena. It had been universally admitted that the earth lay
unsupported in space. Copernicus had further shown that it possessed
a movement of rotation. Its want of stability being thus recognised,
it seemed reasonable to suppose that the earth might also have some
other kinds of movements as well. In this, Copernicus essayed to
solve a problem far more difficult than that which had hitherto
occupied his attention. It was a comparatively easy task to show how
the diurnal rising and setting could be accounted for by the rotation
of the earth. It was a much more difficult undertaking to
demonstrate that the planetary movements, which Ptolemy had
represented with so much success, could be completely explained by
the supposition that each of those planets revolved uniformly round
the sun, and that the earth was also a planet, accomplishing a
complete circuit of the sun once in the course of a year.


It would be impossible in a sketch like the present to enter into any
detail as to the geometrical propositions on which this beautiful
investigation of Copernicus depended. We can only mention a few of
the leading principles. It may be laid down in general that, if an
observer is in movement, he will, if unconscious of the fact,
attribute to the fixed objects around him a movement equal and
opposite to that which he actually possesses. A passenger on a
canal-boat sees the objects on the banks apparently moving backward
with a speed equal to that by which he is himself advancing
forwards. By an application of this principle, we can account for
all the phenomena of the movements of the planets, which Ptolemy had
so ingeniously represented by his circles. Let us take, for
instance, the most characteristic feature in the irregularities of
the outer planets. We have already remarked that Mars, though
generally advancing from west to east among the stars, occasionally
pauses, retraces his steps for awhile, again pauses, and then resumes
his ordinary onward progress. Copernicus showed clearly how this
effect was produced by the real motion of the earth, combined with
the real motion of Mars. In the adjoining figure we represent a
portion of the circular tracks in which the earth and Mars move in
accordance with the Copernican doctrine. I show particularly the
case where the earth comes directly between the planet and the sun,
because it is on such occasions that the retrograde movement (for so
this backward movement of Mars is termed) is at its highest. Mars is
then advancing in the direction shown by the arrow-head, and the
earth is also advancing in the same direction. We, on the earth,
however, being unconscious of our own motion, attribute, by the
principle I have already explained, an equal and opposite motion to
Mars. The visible effect upon the planet is, that Mars has two
movements, a real onward movement in one direction, and an apparent
movement in the opposite direction. If it so happened that the earth
was moving with the same speed as Mars, then the apparent movement
would exactly neutralise the real movement, and Mars would seem to be
at rest relatively to the surrounding stars. Under the actual
circumstances represented, however, the earth is moving faster than
Mars, and the consequence is, that the apparent movement of the
planet backwards exceeds the real movement forwards, the net result
being an apparent retrograde movement.

With consummate skill, Copernicus showed how the applications of the
same principles could account for the characteristic movements of the
planets. His reasoning in due time bore down all opposition. The
supreme importance of the earth in the system vanished. It had now
merely to take rank as one of the planets.

The same great astronomer now, for the first time, rendered something
like a rational account of the changes of the seasons. Nor did
certain of the more obscure astronomical phenomena escape his

He delayed publishing his wonderful discoveries to the world until he
was quite an old man. He had a well-founded apprehension of the
storm of opposition which they would arouse. However, he yielded at
last to the entreaties of his friends, and his book was sent to the
press. But ere it made its appearance to the world, Copernicus was
seized by mortal illness. A copy of the book was brought to him on
May 23, 1543. We are told that he was able to see it and to touch
it, but no more, and he died a few hours afterwards. He was buried
in that Cathedral of Frauenburg, with which his life had been so
closely associated.


The most picturesque figure in the history of astronomy is
undoubtedly that of the famous old Danish astronomer whose name
stands at the head of this chapter. Tycho Brahe was alike notable
for his astronomical genius and for the extraordinary vehemence of a
character which was by no means perfect. His romantic career as a
philosopher, and his taste for splendour as a Danish noble, his
ardent friendships and his furious quarrels, make him an ideal
subject for a biographer, while the magnificent astronomical work
which he accomplished, has given him imperishable fame.

The history of Tycho Brahe has been admirably told by Dr. Dreyer, the
accomplished astronomer who now directs the observatory at Armagh,
though himself a countryman of Tycho. Every student of the career of
the great Dane must necessarily look on Dr. Dreyer's work as the
chief authority on the subject. Tycho sprang from an illustrious
stock. His family had flourished for centuries, both in Sweden and
in Denmark, where his descendants are to be met with at the present
day. The astronomer's father was a privy councillor, and having
filled important positions in the Danish government, he was
ultimately promoted to be governor of Helsingborg Castle, where he
spent the last years of his life. His illustrious son Tycho was born
in 1546, and was the second child and eldest boy in a family of ten.

It appears that Otto, the father of Tycho, had a brother named
George, who was childless. George, however, desired to adopt a boy
on whom he could lavish his affection and to whom he could bequeath
his wealth. A somewhat singular arrangement was accordingly entered
into by the brothers at the time when Otto was married. It was
agreed that the first son who might be born to Otto should be
forthwith handed over by the parents to George to be reared and
adopted by him. In due time little Tycho appeared, and was
immediately claimed by George in pursuance of the compact. But it
was not unnatural that the parental instinct, which had been dormant
when the agreement was made, should here interpose. Tycho's father
and mother receded from the bargain, and refused to part with their
son. George thought he was badly treated. However, he took no
violent steps until a year later, when a brother was born to Tycho.
The uncle then felt no scruple in asserting what he believed to be
his rights by the simple process of stealing the first-born nephew,
which the original bargain had promised him. After a little time it
would seem that the parents acquiesced in the loss, and thus it was
in Uncle George's home that the future astronomer passed his

When we read that Tycho was no more than thirteen years old at the
time he entered the University of Copenhagen, it might be at first
supposed that even in his boyish years he must have exhibited some of
those remarkable talents with which he was afterwards to astonish the
world. Such an inference should not, however, be drawn. The fact is
that in those days it was customary for students to enter the
universities at a much earlier age than is now the case. Not,
indeed, that the boys of thirteen knew more then than the boys of
thirteen know now. But the education imparted in the universities at
that time was of a much more rudimentary kind than that which we
understand by university education at present. In illustration of
this Dr. Dreyer tells us how, in the University of Wittenberg, one of
the professors, in his opening address, was accustomed to point out
that even the processes of multiplication and division in arithmetic
might be learned by any student who possessed the necessary

It was the wish and the intention of his uncle that Tycho's education
should be specially directed to those branches of rhetoric and
philosophy which were then supposed to be a necessary preparation for
the career of a statesman. Tycho, however, speedily made it plain to
his teachers that though he was an ardent student, yet the things
which interested him were the movements of the heavenly bodies and
not the subtleties of metaphysics.


On the 21st October, 1560, an eclipse of the sun occurred, which was
partially visible at Copenhagen. Tycho, boy though he was, took the
utmost interest in this event. His ardour and astonishment in
connection with the circumstance were chiefly excited by the fact
that the time of the occurrence of the phenomenon could be predicted
with so much accuracy. Urged by his desire to understand the matter
thoroughly, Tycho sought to procure some book which might explain
what he so greatly wanted to know. In those days books of any kind
were but few and scarce, and scientific books were especially
unattainable. It so happened, however, that a Latin version of
Ptolemy's astronomical works had appeared a few years before the
eclipse took place, and Tycho managed to buy a copy of this book,
which was then the chief authority on celestial matters. Young as
the boy astronomer was, he studied hard, although perhaps not always
successfully, to understand Ptolemy, and to this day his copy of the
great work, copiously annotated and marked by the schoolboy hand, is
preserved as one of the chief treasures in the library of the
University at Prague.

After Tycho had studied for about three years at the University of
Copenhagen, his uncle thought it would be better to send him, as was
usual in those days, to complete his education by a course of study
in some foreign university. The uncle cherished the hope that in
this way the attention of the young astronomer might be withdrawn
from the study of the stars and directed in what appeared to him a
more useful way. Indeed, to the wise heads of those days, the
pursuit of natural science seemed so much waste of good time which
might otherwise be devoted to logic or rhetoric or some other branch
of study more in vogue at that time. To assist in this attempt to
wean Tycho from his scientific tastes, his uncle chose as a tutor to
accompany him an intelligent and upright young man named Vedel, who
was four years senior to his pupil, and accordingly, in 1562, we find
the pair taking up their abode at the University of Leipzig.

The tutor, however, soon found that he had undertaken a most hopeless
task. He could not succeed in imbuing Tycho with the slightest taste
for the study of the law or the other branches of knowledge which
were then thought so desirable. The stars, and nothing but the
stars, engrossed the attention of his pupil. We are told that all
the money he could obtain was spent secretly in buying astronomical
books and instruments. He learned the name of the stars from a
little globe, which he kept hidden from Vedel, and only ventured to
use during the latter's absence. No little friction was at first
caused by all this, but in after years a fast and enduring friendship
grew up between Tycho and his tutor, each of whom learned to respect
and to love the other.

Before Tycho was seventeen he had commenced the difficult task of
calculating the movements of the planets and the places which they
occupied on the sky from time to time. He was not a little surprised
to find that the actual positions of the planets differed very widely
from those which were assigned to them by calculations from the best
existing works of astronomers. With the insight of genius he saw
that the only true method of investigating the movements of the
heavenly bodies would be to carry on a protracted series of
measurements of their places. This, which now seems to us so
obvious, was then entirely new doctrine. Tycho at once commenced
regular observations in such fashion as he could. His first
instrument was, indeed, a very primitive one, consisting of a simple
pair of compasses, which he used in this way. He placed his eye at
the hinge, and then opened the legs of the compass so that one leg
pointed to one star and the other leg to the other star. The compass
was then brought down to a divided circle, by which means the number
of degrees in the apparent angular distance of the two stars was

His next advance in instrumental equipment was to provide himself
with the contrivance known as the "cross-staff," which he used to
observe the stars whenever opportunity offered. It must, of course,
be remembered that in those days there were no telescopes. In the
absence of optical aid, such as lenses afford the modern observers,
astronomers had to rely on mechanical appliances alone to measure the
places of the stars. Of such appliances, perhaps the most ingenious
was one known before Tycho's time, which we have represented in the
adjoining figure.


Let us suppose that it be desired to measure the angle between two
stars, then if the angle be not too large it can be determined in the
following manner. Let the rod AB be divided into inches and parts of
an inch, and let another rod, CD, slide up and down along AB in such
a way that the two always remain perpendicular to each other.
"Sights," like those on a rifle, are placed at A and C, and there is
a pin at D. It will easily be seen that, by sliding the movable bar
along the fixed one, it must always be possible when the stars are
not too far apart to bring the sights into such positions that one
star can be seen along DC and the other along DA. This having been
accomplished, the length from A to the cross-bar is read off on the
scale, and then, by means of a table previously prepared, the value
of the required angular distance is obtained. If the angle between
the two stars were greater than it would be possible to measure in
the way already described, then there was a provision by which the
pin at D might be moved along CD into some other position, so as to
bring the angular distance of the stars within the range of the

(The arms, of walnut wood, are about 5 1/2 ft. long.)]

No doubt the cross-staff is a very primitive contrivance, but when
handled by one so skilful as Tycho it afforded results of
considerable accuracy. I would recommend any reader who may have a
taste for such pursuits to construct a cross-staff for himself, and
see what measurements he can accomplish with its aid.

To employ this little instrument Tycho had to evade the vigilance of
his conscientious tutor, who felt it his duty to interdict all such
occupations as being a frivolous waste of time. It was when Vedel
was asleep that Tycho managed to escape with his cross staff and
measure the places of the heavenly bodies. Even at this early age
Tycho used to conduct his observations on those thoroughly sound
principles which lie at the foundation of all accurate modern
astronomy. Recognising the inevitable errors of workmanship in his
little instrument, he ascertained their amount and allowed for their
influence on the results which he deduced. This principle, employed
by the boy with his cross-staff in 1564, is employed at the present
day by the Astronomer Royal at Greenwich with the most superb
instruments that the skill of modern opticians has been able to

(The arms, AB and AC, are about 5 1/2 ft. long.)]

After the death of his uncle, when Tycho was nineteen years of age,
it appears that the young philosopher was no longer interfered with
in so far as the line which his studies were to take was concerned.
Always of a somewhat restless temperament, we now find that he
shifted his abode to the University of Rostock, where he speedily
made himself notable in connection with an eclipse of the moon on
28th October, 1566. Like every other astronomer of those days, Tycho
had always associated astronomy with astrology. He considered that
the phenomena of the heavenly bodies always had some significance in
connection with human affairs. Tycho was also a poet, and in the
united capacity of poet, astrologer, and astronomer, he posted up
some verses in the college at Rostock announcing that the lunar
eclipse was a prognostication of the death of the great Turkish
Sultan, whose mighty deeds at that time filled men's minds. Presently
news did arrive of the death of the Sultan, and Tycho was accordingly
triumphant; but a little later it appeared that the decease had taken
place BEFORE the eclipse, a circumstance which caused many a laugh at
Tycho's expense.

(Made of steel: the arms, AB, AC, measure 4 ft.)

(The meridian circle, E B C A D, made of solid steel,
is nearly 6 ft. in diameter.)]

Tycho being of a somewhat turbulent disposition, it appears that,
while at the University of Rostock, he had a serious quarrel with
another Danish nobleman. We are not told for certain what was the
cause of the dispute. It does not, however, seem to have had any
more romantic origin than a difference of opinion as to which of them
knew the more mathematics. They fought, as perhaps it was becoming
for two astronomers to fight, under the canopy of heaven in utter
darkness at the dead of night, and the duel was honourably terminated
when a slice was taken off Tycho's nose by the insinuating sword of
his antagonist. For the repair of this injury the ingenuity of the
great instrument-maker was here again useful, and he made a
substitute for his nose "with a composition of gold and silver." The
imitation was so good that it is declared to have been quite equal to
the original. Dr. Lodge, however, pointedly observes that it does
not appear whether this remark was made by a friend or an enemy.

(Built of heart of oak; the radii about 19 ft.)


The next few years Tycho spent in various places ardently pursuing
somewhat varied branches of scientific study. At one time we hear of
him assisting an astronomical alderman, in the ancient city of
Augsburg, to erect a tremendous wooden machine--a quadrant of 19-feet
radius--to be used in observing the heavens. At another time we
learn that the King of Denmark had recognised the talents of his
illustrious subject, and promised to confer on him a pleasant
sinecure in the shape of a canonry, which would assist him with the
means for indulging his scientific pursuits. Again we are told that
Tycho is pursuing experiments in chemistry with the greatest energy,
nor is this so incompatible as might at first be thought with his
devotion to astronomy. In those early days of knowledge the
different sciences seemed bound together by mysterious bonds.
Alchemists and astrologers taught that the several planets were
correlated in some mysterious manner with the several metals. It
was, therefore hardly surprising that Tycho should have included a
study of the properties of the metals in the programme of his
astronomical work.



An event, however, occurred in 1572 which stimulated Tycho's
astronomical labours, and started him on his life's work. On the
11th of November in that year, he was returning home to supper after
a day's work in his laboratory, when he happened to lift his face to
the sky, and there he beheld a brilliant new star. It was in the
constellation of Cassiopeia, and occupied a position in which there
had certainly been no bright star visible when his attention had last
been directed to that part of the heavens. Such a phenomenon was so
startling that he found it hard to trust the evidence of his senses.
He thought he must be the subject of some hallucination. He
therefore called to the servants who were accompanying him, and asked
them whether they, too, could see a brilliant object in the direction
in which he pointed. They certainly could, and thus he became
convinced that this marvellous object was no mere creation of the
fancy, but a veritable celestial body--a new star of surpassing
splendour which had suddenly burst forth. In these days of careful
scrutiny of the heavens, we are accustomed to the occasional outbreak
of new stars. It is not, however, believed that any new star which
has ever appeared has displayed the same phenomenal brilliance as was
exhibited by the star of 1572.

This object has a value in astronomy far greater than it might at
first appear. It is true, in one sense, that Tycho discovered the
new star, but it is equally true, in a different sense, that it was
the new star which discovered Tycho. Had it not been for this
opportune apparition, it is quite possible that Tycho might have
found a career in some direction less beneficial to science than that
which he ultimately pursued.


When he reached his home on this memorable evening, Tycho immediately
applied his great quadrant to the measurement of the place of the new
star. His observations were specially directed to the determination
of the distance of the object. He rightly conjectured that if it
were very much nearer to us than the stars in its vicinity, the
distance of the brilliant body might be determined in a short time by
the apparent changes in its distance from the surrounding points. It
was speedily demonstrated that the new star could not be as near as
the moon, by the simple fact that its apparent place, as compared
with the stars in its neighbourhood, was not appreciably altered when
it was observed below the pole, and again above the pole at an
interval of twelve hours. Such observations were possible, inasmuch
as the star was bright enough to be seen in full daylight. Tycho
thus showed conclusively that the body was so remote that the
diameter of the earth bore an insignificant ratio to the star's
distance. His success in this respect is the more noteworthy when we
find that many other observers, who studied the same object, came to
the erroneous conclusion that the new star was quite as near as the
moon, or even much nearer. In fact, it may be said, that with regard
to this object Tycho discovered everything which could possibly have
been discovered in the days before telescopes were invented. He not
only proved that the star's distance was too great for measurement,
but he showed that it had no proper motion on the heavens. He
recorded the successive changes in its brightness from week to week,
as well as the fluctuations in hue with which the alterations in
lustre were accompanied.

It seems, nowadays, strange to find that such thoroughly scientific
observations of the new star as those which Tycho made, possessed,
even in the eyes of the great astronomer himself, a profound
astrological significance. We learn from Dr. Dreyer that, in Tycho's
opinion, "the star was at first like Venus and Jupiter, and its
effects will therefore, first, be pleasant; but as it then became
like Mars, there will next come a period of wars, seditions,
captivity, and death of princes, and destruction of cities, together
with dryness and fiery meteors in the air, pestilence, and venomous
snakes. Lastly, the star became like Saturn, and thus will finally
come a time of want, death, imprisonment, and all kinds of sad
things!" Ideas of this kind were, however, universally entertained.
It seemed, indeed, obvious to learned men of that period that such an
apparition must forebode startling events. One of the chief theories
then held was, that just as the Star of Bethlehem announced the first
coming of Christ, so the second coming, and the end of the world, was
heralded by the new star of 1572.

The researches of Tycho on this object were the occasion of his first
appearance as an author. The publication of his book was however,
for some time delayed by the urgent remonstrances of his friends, who
thought it was beneath the dignity of a nobleman to condescend to
write a book. Happily, Tycho determined to brave the opinion of his
order; the book appeared, and was the first of a series of great
astronomical productions from the same pen.


The fame of the noble Dane being now widespread, the King of Denmark
entreated him to return to his native country, and to deliver a
course of lectures on astronomy in the University of Copenhagen. With
some reluctance he consented, and his introductory oration has been
preserved. He dwells, in fervent language, upon the beauty and the
interest of the celestial phenomena. He points out the imperative
necessity of continuous and systematic observation of the heavenly
bodies in order to extend our knowledge. He appeals to the practical
utility of the science, for what civilised nation could exist without
having the means of measuring time? He sets forth how the study of
these beautiful objects "exalts the mind from earthly and trivial
things to heavenly ones;" and then he winds up by assuring them that
"a special use of astronomy is that it enables us to draw conclusions
from the movements in the celestial regions as to human fate."

An interesting event, which occurred in 1572, distracted Tycho's
attention from astronomical matters. He fell in love. The young
girl on whom his affections were set appears to have sprung from
humble origin. Here again his august family friends sought to
dissuade him from a match they thought unsuitable for a nobleman.
But Tycho never gave way in anything. It is suggested that he did
not seek a wife among the highborn dames of his own rank from the
dread that the demands of a fashionable lady would make too great an
inroad on the time that he wished to devote to science. At all
events, Tycho's union seems to have been a happy one, and he had a
large family of children; none of whom, however, inherited their
father's talents.


Tycho had many scientific friends in Germany, among whom his work was
held in high esteem. The treatment that he there met with seemed to
him so much more encouraging than that which he received in Denmark
that he formed the notion of emigrating to Basle and making it his
permanent abode. A whisper of this intention was conveyed to the
large-hearted King of Denmark, Frederick II. He wisely realised how
great would be the fame which would accrue to his realm if he could
induce Tycho to remain within Danish territory and carry on there the
great work of his life. A resolution to make a splendid proposal to
Tycho was immediately formed. A noble youth was forthwith despatched
as a messenger, and ordered to travel day and night until he reached
Tycho, whom he was to summon to the king. The astronomer was in bed
on the morning Of 11th February, 1576, when the message was
delivered. Tycho, of course, set off at once and had an audience of
the king at Copenhagen. The astronomer explained that what he wanted
was the means to pursue his studies unmolested, whereupon the king
offered him the Island of Hven, in the Sound near Elsinore. There he
would enjoy all the seclusion that he could desire. The king further
promised that he would provide the funds necessary for building a
house and for founding the greatest observatory that had ever yet
been reared for the study of the heavens. After due deliberation and
consultation with his friends, Tycho accepted the king's offer. He
was forthwith granted a pension, and a deed was drawn up formally
assigning the Island of Hven to his use all the days of his life.

The foundation of the famous castle of Uraniborg was laid on 30th
August, 1576. The ceremony was a formal and imposing one, in
accordance with Tycho's ideas of splendour. A party of scientific
friends had assembled, and the time had been chosen so that the
heavenly bodies were auspiciously placed. Libations of costly wines
were poured forth, and the stone was placed with due solemnity. The
picturesque character of this wonderful temple for the study of the
stars may be seen in the figures with which this chapter is

One of the most remarkable instruments that has ever been employed in
studying the heavens was the mural quadrant which Tycho erected in
one of the apartments of Uraniborg. By its means the altitudes of
the celestial bodies could be observed with much greater accuracy
than had been previously attainable. This wonderful contrivance is
represented on the preceding page. It will be observed that the
walls of the room are adorned by pictures with a lavishness of
decoration not usually to be found in scientific establishments.

A few years later, when the fame of the observatory at Hven became
more widely spread, a number of young men flocked to Tycho to study
under his direction. He therefore built another observatory for
their use in which the instruments were placed in subterranean rooms
of which only the roofs appeared above the ground. There was a
wonderful poetical inscription over the entrance to this underground
observatory, expressing the astonishment of Urania at finding, even
in the interior of the earth, a cavern devoted to the study of the
heavens. Tycho was indeed always fond of versifying, and he lost no
opportunity of indulging this taste whenever an occasion presented

Around the walls of the subterranean observatory were the pictures of
eight astronomers, each with a suitable inscription--one of these of
course represented Tycho himself, and beneath were written words to
the effect that posterity should judge of his work. The eighth
picture depicted an astronomer who has not yet come into existence.
Tychonides was his name, and the inscription presses the modest hope
that when he does appear he will be worthy of his great predecessor.
The vast expenses incurred in the erection and the maintenance of
this strange establishment were defrayed by a succession of grants
from the royal purse.

For twenty years Tycho laboured hard at Uraniborg in the pursuit of
science. His work mainly consisted in the determination of the
places of the moon, the planets, and the stars on the celestial
sphere. The extraordinary pains taken by Tycho to have his
observations as accurate as his instruments would permit, have justly
entitled him to the admiration of all succeeding astronomers. His
island home provided the means of recreation as well as a place for
work. He was surrounded by his family, troops of friends were not
wanting, and a pet dwarf seems to have been an inmate of his curious
residence. By way of change from his astronomical labours he used
frequently to work with his students in his chemical laboratory. It
is not indeed known what particular problems in chemistry occupied
his attention. We are told, however, that he engaged largely in the
production of medicines, and as these appear to have been dispensed
gratuitously there was no lack of patients.

Tycho's imperious and grasping character frequently brought him into
difficulties, which seem to have increased with his advancing years.
He had ill-treated one of his tenants on Hven, and an adverse
decision by the courts seems to have greatly exasperated the
astronomer. Serious changes also took place in his relations to the
court at Copenhagen. When the young king was crowned in 1596, he
reversed the policy of his predecessor with reference to Hven. The
liberal allowances to Tycho were one after another withdrawn, and
finally even his pension was stopped. Tycho accordingly abandoned
Hven in a tumult of rage and mortification. A few years later we
find him in Bohemia a prematurely aged man, and he died on the 24th
October, 1601.


Among the ranks of the great astronomers it would be difficult to
find one whose life presents more interesting features and remarkable
vicissitudes than does that of Galileo. We may consider him as the
patient investigator and brilliant discoverer. We may consider him
in his private relations, especially to his daughter, Sister Maria
Celeste, a woman of very remarkable character; and we have also the
pathetic drama at the close of Galileo's life, when the philosopher
drew down upon himself the thunders of the Inquisition.

The materials for the sketch of this astonishing man are sufficiently
abundant. We make special use in this place of those charming
letters which his daughter wrote to him from her convent home. More
than a hundred of these have been preserved, and it may well be
doubted whether any more beautiful and touching series of letters
addressed to a parent by a dearly loved child have ever been
written. An admirable account of this correspondence is contained in
a little book entitled "The Private Life of Galileo," published
anonymously by Messrs. Macmillan in 1870, and I have been much
indebted to the author of that volume for many of the facts contained
in this chapter.

Galileo was born at Pisa, on 18th February, 1564. He was the eldest
son of Vincenzo de' Bonajuti de' Galilei, a Florentine noble.
Notwithstanding his illustrious birth and descent, it would seem that
the home in which the great philosopher's childhood was spent was an
impoverished one. It was obvious at least that the young Galileo
would have to be provided with some profession by which he might earn
a livelihood. From his father he derived both by inheritance and by
precept a keen taste for music, and it appears that he became an
excellent performer on the lute. He was also endowed with
considerable artistic power, which he cultivated diligently. Indeed,
it would seem that for some time the future astronomer entertained
the idea of devoting himself to painting as a profession. His
father, however, decided that he should study medicine. Accordingly,
we find that when Galileo was seventeen years of age, and had added a
knowledge of Greek and Latin to his acquaintance with the fine arts,
he was duly entered at the University of Pisa.

Here the young philosopher obtained some inkling of mathematics,
whereupon he became so much interested in this branch of science,
that he begged to be allowed to study geometry. In compliance with
his request, his father permitted a tutor to be engaged for this
purpose; but he did so with reluctance, fearing that the attention of
the young student might thus be withdrawn from that medical work
which was regarded as his primary occupation. The event speedily
proved that these anxieties were not without some justification. The
propositions of Euclid proved so engrossing to Galileo that it was
thought wise to avoid further distraction by terminating the
mathematical tutor's engagement. But it was too late for the desired
end to be attained. Galileo had now made such progress that he was
able to continue his geometrical studies by himself. Presently he
advanced to that famous 47th proposition which won his lively
admiration, and on he went until he had mastered the six books of
Euclid, which was a considerable achievement for those days.

The diligence and brilliance of the young student at Pisa did not,
however, bring him much credit with the University authorities. In
those days the doctrines of Aristotle were regarded as the embodiment
of all human wisdom in natural science as well as in everything
else. It was regarded as the duty of every student to learn
Aristotle off by heart, and any disposition to doubt or even to
question the doctrines of the venerated teacher was regarded as
intolerable presumption. But young Galileo had the audacity to think
for himself about the laws of nature. He would not take any
assertion of fact on the authority of Aristotle when he had the means
of questioning nature directly as to its truth or falsehood. His
teachers thus came to regard him as a somewhat misguided youth,
though they could not but respect the unflagging industry with which
he amassed all the knowledge he could acquire.


We are so accustomed to the use of pendulums in our clocks that
perhaps we do not often realise that the introduction of this method
of regulating time-pieces was really a notable invention worthy the
fame of the great astronomer to whom it was due. It appears that
sitting one day in the Cathedral of Pisa, Galileo's attention became
concentrated on the swinging of a chandelier which hung from the
ceiling. It struck him as a significant point, that whether the arc
through which the pendulum oscillated was a long one or a short one,
the time occupied in each vibration was sensibly the same. This
suggested to the thoughtful observer that a pendulum would afford the
means by which a time-keeper might be controlled, and accordingly
Galileo constructed for the first time a clock on this principle. The
immediate object sought in this apparatus was to provide a means of
aiding physicians in counting the pulses of their patients.

The talents of Galileo having at length extorted due recognition from
the authorities, he was appointed, at the age of twenty-five,
Professor of Mathematics at the University of Pisa. Then came the
time when he felt himself strong enough to throw down the gauntlet to
the adherents of the old philosophy. As a necessary part of his
doctrine on the movement of bodies Aristotle had asserted that the
time occupied by a stone in falling depends upon its weight, so that
the heavier the stone the less time would it require to fall from a
certain height to the earth. It might have been thought that a
statement so easily confuted by the simplest experiments could never
have maintained its position in any accepted scheme of philosophy.
But Aristotle had said it, and to anyone who ventured to express a
doubt the ready sneer was forthcoming, "Do you think yourself a
cleverer man than Aristotle?" Galileo determined to demonstrate in
the most emphatic manner the absurdity of a doctrine which had for
centuries received the sanction of the learned. The summit of the
Leaning Tower of Pisa offered a highly dramatic site for the great
experiment. The youthful professor let fall from the overhanging top
a large heavy body and a small light body simultaneously. According
to Aristotle the large body ought to have reached the ground much
sooner than the small one, but such was found not to be the case. In
the sight of a large concourse of people the simple fact was
demonstrated that the two bodies fell side by side, and reached the
ground at the same time. Thus the first great step was taken in the
overthrow of that preposterous system of unquestioning adhesion to
dogma, which had impeded the development of the knowledge of nature
for nearly two thousand years.

This revolutionary attitude towards the ancient beliefs was not
calculated to render Galileo's relations with the University
authorities harmonious. He had also the misfortune to make enemies
in other quarters. Don Giovanni de Medici, who was then the Governor
of the Port of Leghorn, had designed some contrivance by which he
proposed to pump out a dock. But Galileo showed up the absurdity of
this enterprise in such an aggressive manner that Don Giovanni took
mortal offence, nor was he mollified when the truths of Galileo's
criticisms were abundantly verified by the total failure of his
ridiculous invention. In various ways Galileo was made to feel his
position at Pisa so unpleasant that he was at length compelled to
abandon his chair in the University. The active exertions of his
friends, of whom Galileo was so fortunate as to have had throughout
his life an abundant supply, then secured his election to the
Professorship of Mathematics at Padua, whither he went in 1592.


It was in this new position that Galileo entered on that marvellous
career of investigation which was destined to revolutionize science.
The zeal with which he discharged his professorial duties was indeed
of the most unremitting character. He speedily drew such crowds to
listen to his discourses on Natural Philosophy that his lecture-room
was filled to overflowing. He also received many private pupils in
his house for special instruction. Every moment that could be spared
from these labours was devoted to his private study and to his
incessant experiments.

Like many another philosopher who has greatly extended our knowledge
of nature, Galileo had a remarkable aptitude for the invention of
instruments designed for philosophical research. To facilitate his
practical work, we find that in 1599 he had engaged a skilled workman
who was to live in his house, and thus be constantly at hand to try
the devices for ever springing from Galileo's fertile brain. Among
the earliest of his inventions appears to have been the thermometer,
which he constructed in 1602. No doubt this apparatus in its
primitive form differed in some respects from the contrivance we call
by the same name. Galileo at first employed water as the agent, by
the expansion of which the temperature was to be measured. He
afterwards saw the advantage of using spirits for the same purpose.
It was not until about half a century later that mercury came to be
recognised as the liquid most generally suitable for the thermometer.

The time was now approaching when Galileo was to make that mighty
step in the advancement of human knowledge which followed on the
application of the telescope to astronomy. As to how his idea of
such an instrument originated, we had best let him tell us in his own
words. The passage is given in a letter which he writes to his
brother-in-law, Landucci.

"I write now because I have a piece of news for you, though whether
you will be glad or sorry to hear it I cannot say; for I have now no
hope of returning to my own country, though the occurrence which has
destroyed that hope has had results both useful and honourable. You
must know, then, that two months ago there was a report spread here
that in Flanders some one had presented to Count Maurice of Nassau a
glass manufactured in such a way as to make distant objects appear
very near, so that a man at the distance of two miles could be
clearly seen. This seemed to me so marvellous that I began to think
about it. As it appeared to me to have a foundation in the Theory of
Perspective, I set about contriving how to make it, and at length I
found out, and have succeeded so well that the one I have made is far
superior to the Dutch telescope. It was reported in Venice that I
had made one, and a week since I was commanded to show it to his
Serenity and to all the members of the senate, to their infinite
amazement. Many gentlemen and senators, even the oldest, have
ascended at various times the highest bell-towers in Venice to spy
out ships at sea making sail for the mouth of the harbour, and have
seen them clearly, though without my telescope they would have been
invisible for more than two hours. The effect of this instrument is
to show an object at a distance of say fifty miles, as if it were but
five miles."

The remarkable properties of the telescope at once commanded
universal attention among intellectual men. Galileo received
applications from several quarters for his new instrument, of which
it would seem that he manufactured a large number to be distributed
as gifts to various illustrious personages.

But it was reserved for Galileo himself to make that application of
the instrument to the celestial bodies by which its peculiar powers
were to inaugurate the new era in astronomy. The first discovery
that was made in this direction appears to have been connected with
the number of the stars. Galileo saw to his amazement that through
his little tube he could count ten times as many stars in the sky as
his unaided eye could detect. Here was, indeed, a surprise. We are
now so familiar with the elementary facts of astronomy that it is not
always easy to realise how the heavens were interpreted by the
observers in those ages prior to the invention of the telescope. We
can hardly, indeed, suppose that Galileo, like the majority of those
who ever thought of such matters, entertained the erroneous belief
that the stars were on the surface of a sphere at equal distances
from the observer. No one would be likely to have retained his
belief in such a doctrine when he saw how the number of visible stars
could be increased tenfold by means of Galileo's telescope. It would
have been almost impossible to refuse to draw the inference that the
stars thus brought into view were still more remote objects which the
telescope was able to reveal, just in the same way as it showed
certain ships to the astonished Venetians, when at the time these
ships were beyond the reach of unaided vision.

Galileo's celestial discoveries now succeeded each other rapidly.
That beautiful Milky Way, which has for ages been the object of
admiration to all lovers of nature, never disclosed its true nature
to the eye of man till the astronomer of Padua turned on it his magic
tube. The splendid zone of silvery light was then displayed as
star-dust scattered over the black background of the sky. It was
observed that though the individual stars were too small to be seen
severally without optical aid, yet such was their incredible number
that the celestial radiance produced that luminosity with which every
stargazer was so familiar.

But the greatest discovery made by the telescope in these early days,
perhaps, indeed, the greatest discovery that the telescope has ever
accomplished, was the detection of the system of four satellites
revolving around the great planet Jupiter. This phenomenon was so
wholly unexpected by Galileo that, at first, he could hardly believe
his eyes. However, the reality of the existence of a system of four
moons attending the great planet was soon established beyond all
question. Numbers of great personages crowded to Galileo to see for
themselves this beautiful miniature representing the sun with its
system of revolving planets.

Of course there were, as usual, a few incredulous people who refused
to believe the assertion that four more moving bodies had to be added
to the planetary system. They scoffed at the notion; they said the
satellites may have been in the telescope, but that they were not in
the sky. One sceptical philosopher is reported to have affirmed,
that even if he saw the moons of Jupiter himself he would not believe
in them, as their existence was contrary to the principles of

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