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.
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