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.
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