Epicycle

Definition

An epicycle is a small circle whose center moves around the circumference of a larger circle (deferent). It was used in the Ptolemaic model to explain retrograde motion of planets.

5 Must Know Facts For Your Next Test

1. The concept of epicycles was introduced by ancient Greek astronomers.
2. Ptolemy's Almagest refined the idea of epicycles to explain planetary motion.
3. Epicycles were used to account for the observed irregularities in planetary orbits.
4. The use of epicycles was eventually replaced by Kepler's laws and heliocentric models.
5. Despite its inaccuracies, the epicycle model persisted for over a thousand years.

Review Questions

Related terms

Deferent:

A large circle around which the center of an epicycle moves.

Retrograde Motion:

The apparent backward movement of planets as observed from Earth.

Ptolemaic Model: An Earth-centered model of the universe that incorporates epicycles to explain planetary motions.