# Definition:Epicycle (Ptolemaic Astronomy)

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*This page is about Epicycle in the context of Ptolemaic Astronomy. For other uses, see Epicycle.*

## Definition

An **epicycle** is the orbit described by a body moving in a uniform circular motion around a point which is itself moving in a uniform circular motion around another point.

That point may itself also be moving in a uniform circular motion around yet another point.

## Also see

## Historical Note

Claudius Ptolemy's *Almagest* modelled the orbits of the planets of the solar system as a system of epicycles.

They modelled the actual behaviour of the planets to a high degree of accuracy.

However, when the heliocentric model of the solar system was proposed, it was realised that the latter model was considerably simpler.

## Sources

- 1989: Ephraim J. Borowski and Jonathan M. Borwein:
*Dictionary of Mathematics*... (previous) ... (next): Entry:**epicycle**:**2.**(*Ptolemaic astronomy*) - 1992: George F. Simmons:
*Calculus Gems*... (previous) ... (next): Chapter $\text {B}.25$: Kepler's Laws and Newton's Law of Gravitation - 2008: Ian Stewart:
*Taming the Infinite*... (previous) ... (next): Chapter $5$: Eternal Triangles: Base ten logarithms