Answer:
an epicycle
Explanation:
Ptolemy introduced in his geocentric model the concept of epicycle, devised by Apollonius of Perge in the third century B.C. This idea was to add a series of points outside the Earth around which the planets would describe circular orbits.
Thus, each planet would describe a circular orbit, or epicycle, around a point, P, that would, in turn, move in a circular orbit around the Earth, called a deferential. However, the deferential would be eccentric, that is, that the center of the deferential would not be the center of the Earth, but another point outside it, D.
It was still missing in this model to explain the reason that the speed with which the retrograde movement of the planets was observed was not uniform; Ptolemy solved this by introducing the idea of the equator, E, which would be a point outside the Earth from which it would appear that the planet is moving with constant speed.
As the observations were refined, especially after the invention of the telescope, the heliocentric system gained more acceptance, until it finally replaced the geocentric model in the sixteenth century.
