Britannica
Sources used in this answer
What is a deferent in the Ptolemaic system?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
Ptolemy
Ptolemy's theory of the solar system.
In the Ptolemaic system, a deferent is a large circular path centered near Earth along which the center of a smaller circle, known as an epicycle, revolves.12
Because the Ptolemaic system was a geocentric model that required planets to move in uniform circular motions, this combination of the deferent and the epicycle allowed astronomers to account for the complex, variable movements of planets as observed from Earth, including the phenomenon of retrograde motion. By coordinating the rotation of the planet on its epicycle with the movement of the epicycle's center along the deferent, the model could accurately predict planetary positions.12