What is an epicycle?

The Scientific Revolution Read more from
Chapter Skeptical and Natural Philosophy

An epicycle is a type of circular motion that is not observed but, rather, theoretically postulated. From the postulation, what could be observed became predictable, which was how it “saved the appearances,” or was consistent with what was observed. In the Ptolemaic system, the 80 epicycles were necessary to account for the different speeds and directions in the observed movements of the Moon, Sun, and five known planets. They also explained differences in how far the planets appeared to be from Earth at different times. The planets themselves were believed to move in small circles, which themselves moved along “deferents,” or large circles. Both the epicycles and deferents moved counter-clockwise in planes approximately parallel to the plane on which Earth was situated.


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