He knew that the earth was a gigantic object, but, large as
it may have been, he knew that it was only a particle in comparison
with the celestial sphere, yet he apparently believed, and certainly
succeeded in persuading other men to believe, that the celestial
sphere did actually perform these movements.
Ptolemy was an excellent geometer. He knew that the rising and the
setting of the sun, the moon, and the myriad stars, could have been
accounted for in a different way. If the earth turned round
uniformly once a day while poised at the centre of the sphere of the
heavens, all the phenomena of rising and setting could be completely
explained. This is, indeed, obvious after a moment's reflection.
Consider yourself to be standing on the earth at the centre of the
heavens. There are stars over your head, and half the contents of
the heavens are visible, while the other half are below your
horizon. As the earth turns round, the stars over your head will
change, and unless it should happen that you have taken up your
position at either of the poles, new stars will pass into your view,
and others will disappear, for at no time can you have more than half
of the whole sphere visible. The observer on the earth would,
therefore, say that some stars were rising, and that some stars were
setting.
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