epicycle

An epicycle is a small circle whose center rides around the circumference of a larger circle, called the deferent. A planet placed on the epicycle traces out a looping path in the sky — sometimes moving forward, sometimes apparently backward — as seen from a central Earth.

Ptolemy used nested epicycles to reproduce the observed motions of the planets while preserving the ancient assumption that all celestial motion is built from uniform circles. The model was mathematically elaborate but predictively decent, and it dominated astronomy for more than a thousand years. Kepler finally discarded it when he showed that planetary orbits are ellipses.

The epicycle scheme was systematized by Ptolemy in the Almagest in the second century and remained the standard model of the heavens until Kepler's first law in 1609. Copernicus's 1543 heliocentric system still used epicycles — just fewer of them.