What kind of questions did the Greeks ask themselves about the physical universe? We can paraphrase Plato: the stars move about the earth in circles, the perfect paths, and they move with uniform motion as befits divine and eternal beings. But five of these stars are planets (Greek for wanderers) which appear to have irregular motion, first moving forward, then actually stopping, and then moving backward for awhile. Since the heavens are incorruptible, the planets too must really be moving in uniform motion in circular paths. How then can we account for the apparently irregular motions? What uniform motions must be hypothesized to account for the observable wanderings? [excerpt]
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Bloom, Robert L. et al. "1. Greek and Medieval Science. Pt. VIII: The Development of Modern Science." Ideas and Institutions of Western Man (Gettysburg College, 1958), 1-11.