How does the Ptolemaic system explain motion?

Ptolemy used eccentricity with an epicyclic model in order to explain the motion of the planets. Each planet in the Ptolemaic system circles evenly along a circular path (epicycle), with the epicycle's center rotating around Earth along a bigger circular path (deferent).

Explanation:

In order to explain the motion of the planets, Ptolemy combined eccentricity with an epicyclic model. In the Ptolemaic system each planet revolves uniformly along a circular path (epicycle), the centre of which revolves around Earth along a larger circular path (deferent). Because one half of an epicycle runs counter to the general motion of the deferent path, the combined motion will sometimes appear to slow down or even reverse direction (retrograde). By carefully coordinating these two cycles, the epicyclic model explained the observed phenomenon of planets retrograding when at perigee. Ptolemy enhanced the effect of eccentricity by making the epicycle’s centre sweep out equal angles along the deferent in equal times as seen from a point that he called the equant. The centre of the deferent was located midway between the equant and Earth, as can be seen in the figure.

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