Great Astronomers - Robert Stawell Ball (the little red hen ebook .TXT) 📗
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complete circuit of the heavens was accomplished. Hipparchus
traced out this phenomenon, and established it on an impregnable
basis, so that all astronomers have ever since recognised the
precession of the equinoxes as one of the fundamental facts of
astronomy. Not until nearly two thousand years after Hipparchus
had made this splendid discovery was the explanation of its cause
given by Newton.
From the days of Hipparchus down to the present hour the science
of astronomy has steadily grown. One great observer after another
has appeared from time to time, to reveal some new phenomenon with
regard to the celestial bodies or their movements, while from time
to time one commanding intellect after another has arisen to
explain the true import of the facts of observations. The history
of astronomy thus becomes inseparable from the history of the
great men to whose labours its development is due.
In the ensuing chapters we have endeavoured to sketch the lives
and the work of the great philosophers, by whose labours the
science of astronomy has been created. We shall commence with
Ptolemy, who, after the foundations of the science had been laid
by Hipparchus, gave to astronomy the form in which it was taught
throughout the Middle Ages. We shall next see the mighty
revolution in our conceptions of the universe which are associated
with the name of Copernicus. We then pass to those periods
illumined by the genius of Galileo and Newton, and afterwards we
shall trace the careers of other more recent discoverers, by
whose industry and genius the boundaries of human knowledge have
been so greatly extended. Our history will be brought down late
enough to include some of the illustrious astronomers who laboured
in the generation which has just passed away.
PTOLEMY.
[PLATE: PTOLEMY.]
The career of the famous man whose name stands at the head of this
chapter is one of the most remarkable in the history of human
learning. There may have been other discoverers who have done
more for science than ever Ptolemy accomplished, but there never
has been any other discoverer whose authority on the subject of
the movements of the heavenly bodies has held sway over the minds
of men for so long a period as the fourteen centuries during which
his opinions reigned supreme. The doctrines he laid down in his
famous book, “The Almagest,” prevailed throughout those ages. No
substantial addition was made in all that time to the undoubted
truths which this work contained. No important correction was
made of the serious errors with which Ptolemy’s theories were
contaminated. The authority of Ptolemy as to all things in
the heavens, and as to a good many things on the earth (for the
same illustrious man was also a diligent geographer), was
invariably final.
Though every child may now know more of the actual truths of the
celestial motions than ever Ptolemy knew, yet the fact that his
work exercised such an astonishing effect on the human intellect
for some sixty generations, shows that it must have been an
extraordinary production. We must look into the career of this
wonderful man to discover wherein lay the secret of that
marvellous success which made him the unchallenged instructor of
the human race for such a protracted period.
Unfortunately, we know very little as to the personal history of
Ptolemy. He was a native of Egypt, and though it has been
sometimes conjectured that he belonged to the royal families of
the same name, yet there is nothing to support such a belief.
The name, Ptolemy, appears to have been a common one in Egypt in
those days. The time at which he lived is fixed by the fact that
his first recorded observation was made in 127 AD, and his last in
151 AD. When we add that he seems to have lived in or near
Alexandria, or to use his own words, “on the parallel of
Alexandria,” we have said everything that can be said so far as
his individuality is concerned.
Ptolemy is, without doubt, the greatest figure in ancient
astronomy. He gathered up the wisdom of the philosophers who had
preceded him. He incorporated this with the results of his
own observations, and illumined it with his theories. His
speculations, even when they were, as we now know, quite
erroneous, had such an astonishing verisimilitude to the actual
facts of nature that they commanded universal assent. Even in
these modern days we not unfrequently find lovers of paradox who
maintain that Ptolemy’s doctrines not only seem true, but actually
are true.
In the absence of any accurate knowledge of the science of
mechanics, philosophers in early times were forced to fall back
on certain principles of more or less validity, which they derived
from their imagination as to what the natural fitness of things
ought to be. There was no geometrical figure so simple and so
symmetrical as a circle, and as it was apparent that the heavenly
bodies pursued tracks which were not straight lines, the
conclusion obviously followed that their movements ought to be
circular. There was no argument in favour of this notion, other
than the merely imaginary reflection that circular movement, and
circular movement alone, was “perfect,” whatever “perfect” may
have meant. It was further believed to be impossible that the
heavenly bodies could have any other movements save those which
were perfect. Assuming this, it followed, in Ptolemy’s opinion,
and in that of those who came after him for fourteen centuries,
that all the tracks of the heavenly bodies were in some way or
other to be reduced to circles.
Ptolemy succeeded in devising a scheme by which the apparent
changes that take place in the heavens could, so far as he knew
them, be explained by certain combinations of circular movement.
This seemed to reconcile so completely the scheme of things
celestial with the geometrical instincts which pointed to the
circle as the type of perfect movement, that we can hardly wonder
Ptolemy’s theory met with the astonishing success that attended
it. We shall, therefore, set forth with sufficient detail the
various steps of this famous doctrine.
Ptolemy commences with laying down the undoubted truth that the
shape of the earth is globular. The proofs which he gives of this
fundamental fact are quite satisfactory; they are indeed the same
proofs as we give today. There is, first of all, the well-known
circumstance of which our books on geography remind us, that when
an object is viewed at a distance across the sea, the lower part
of the object appears cut off by the interposing curved mass of
water.
The sagacity of Ptolemy enabled him to adduce another argument,
which, though not quite so obvious as that just mentioned,
demonstrates the curvature of the earth in a very impressive
manner to anyone who will take the trouble to understand it.
Ptolemy mentions that travellers who went to the south reported,
that, as they did so, the appearance of the heavens at night
underwent a gradual change. Stars that they were familiar with in
the northern skies gradually sank lower in the heavens. The
constellation of the Great Bear, which in our skies never sets
during its revolution round the pole, did set and rise when a
sufficient southern latitude had been attained. On the other
hand, constellations new to the inhabitants of northern climes
were seen to rise above the southern horizon. These
circumstances would be quite incompatible with the supposition
that the earth was a flat surface. Had this been so, a little
reflection will show that no such changes in the apparent
movements of the stars would be the consequence of a voyage to the
south. Ptolemy set forth with much insight the significance of
this reasoning, and even now, with the resources of modern
discoveries to help us, we can hardly improve upon his arguments.
Ptolemy, like a true philosopher disclosing a new truth to the
world, illustrated and enforced his subject by a variety of happy
demonstrations. I must add one of them, not only on account of
its striking nature, but also because it exemplifies Ptolemy’s
acuteness. If the earth were flat, said this ingenious reasoner,
sunset must necessarily take place at the same instant, no matter
in what country the observer may happen to be placed. Ptolemy,
however, proved that the time of sunset did vary greatly as
the observer’s longitude was altered. To us, of course, this is
quite obvious; everybody knows that the hour of sunset may have
been reached in Great Britain while it is still noon on the
western coast of America. Ptolemy had, however, few of those
sources of knowledge which are now accessible. How was he to show
that the sun actually did set earlier at Alexandria than it would
in a city which lay a hundred miles to the west? There was no
telegraph wire by which astronomers at the two Places could
communicate. There was no chronometer or watch which could be
transported from place to place; there was not any other reliable
contrivance for the keeping of time. Ptolemy’s ingenuity,
however, pointed out a thoroughly satisfactory method by which the
times of sunset at two places could be compared. He was
acquainted with the fact, which must indeed have been known from
the very earliest times, that the illumination of the moon is
derived entirely from the sun. He knew that an eclipse of the
moon was due to the interposition of the earth which cuts off the
light of the sun. It was, therefore, plain that an eclipse of the
moon must be a phenomenon which would begin at the same instant
from whatever part of the earth the moon could be seen at the
time. Ptolemy, therefore, brought together from various quarters
the local times at which different observers had recorded the
beginning of a lunar eclipse. He found that the observers to the
west made the time earlier and earlier the further away their
stations were from Alexandria. On the other hand, the eastern
observers set down the hour as later than that at which the
phenomenon appeared at Alexandria. As these observers all
recorded something which indeed appeared to them simultaneously,
the only interpretation was, that the more easterly a place the
later its time. Suppose there were a number of observers along a
parallel of latitude, and each noted the hour of sunset to be
six o’clock, then, since the eastern times are earlier than
western times, 6 p.m. at one station A will correspond to 5 p.m.
at a station B sufficiently to the west. If, therefore, it is
sunset to the observer at A, the hour of sunset will not yet be
reached for the observer at B. This proves conclusively that the
time of sunset is not the same all over the earth. We have,
however, already seen that the apparent time of sunset would be
the same from all stations if the earth were flat. When Ptolemy,
therefore, demonstrated that the time of sunset was not the same
at various places, he showed conclusively that the earth was not
flat.
As the same arguments applied to all parts of the earth where
Ptolemy had either been himself, or from which he could gain the
necessary information, it followed that the earth, instead of
being the flat plain, girdled with an illimitable ocean, as was
generally supposed, must be in reality globular. This led at once
to a startling consequence. It was obvious that there could be no
supports of any kind by which this globe was sustained; it
therefore followed that the mighty object must be simply poised in
space. This is indeed an astonishing doctrine to anyone who
relies on what merely seems the evidence of the senses, without
giving to that evidence its due intellectual interpretation.
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