epicycle

Epicycle Motion in a circle, the centre of which is in turn moving around a larger circle called the deferent. (see illustration). Epicyclic motion was a fundamental aspect of the geocentric model of the solar system propounded by Ptolemy in the second century AD. To improve the accuracy of predictions based on the model, Ptolemy had the centre of the epicycle revolve with uniform angular motion, not about the centre of the deferent, but about a point called the equant, which he displaced to one side. By also placing the Earth to the other side of the centre of the deferent and choosing the radii of the epicyle and deferent appropriately, the Ptolemaic theory could be used to predict planetary positions to an accuracy of one degree.