Page 14 - GALIET PHYSICS BLOSSOMS I+

II. THE EPICYCLE THEORY.
(i) Explain in detail how the epicycle theory, in the later form developed by Ptolemy, was supposed to work. Use pictures.
Before Ptolemy’s Almagest there was no satisfactory theoretical model to explain the complex apparent retrograde motions of the planets. To account for these apparent motions, Ptolemy combined the epicycle and eccentric methods of Apollonius of Perga (3rd c bce) as later innovated by Hipparchus of Rhodes. The epicycle (Gr. “on the circle”) was a geometric model used to explain the complex apparent retrograde planetary motions. Ptolemy expanded on Hipparchus’ epicycle theory by innovatively replacing the idea of deferents with the idea of equants.
BEFORE PTOLEMY
Following Aristotle, the earliest models were geocentric, that is, the Earth lay at the center of the universe and all other bodies move around it. These models also used the perfect form, the circle, as taught by Plato and Aristotle. Uniform motion around a circle in a geocentric system, however, could not account for the observed changes in the brightness of planets or the apparent retrograde motion. Hence a more complex model was ingeniously devised: the Epicycle Model.
Originally, the great Grecian geometer Apollonius of Perga’s epicycle theory began with the idea of deferents and epicycles.
In this model, each planet moved uniformly around a small circle, an epicycle, whose center moved uniformly around Earth on a second and larger circle, the deferent.
Deferents were centered (“C”) on the earth, and the earth did not spin.
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