The motion of the planets

Hipparchus did not achieve a satisfactory theory for the motion of the five planets. Partly this is due to the fact that he, unlike many of his predecessors, had examined sufficiently many observations over a period of many years to realize that the motions were exceedingly complex, with, for example, retrograde arcs, the lengths of which vary according to the position of the planet in its orbit. As Ptolemy later put it:

... Hence it was, I think, that Hipparchus, being a great lover of truth, for all the above reasons, and especially because he did not yet have in his possession such a groundwork of resources in the form of accurate observations from earlier times as he himself has provided to us, although he investigated the theories of the sun and moon, and, to the best of his ability, demonstrated with every means at his command that they are represented by uniform circular motions, did not even make a beginning in establishing theories for the five planets, not at least in his writings which have come down to us. All that he did was to make a compilation of the planetary observations arranged in a more useful way, and to show by means of these that the phenomena were not in agreement with the hypotheses of the astronomers of that time.

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