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.
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