Great Astronomers | Page 7

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