Ptolemy's model of the universe, known as the geocentric model, was based on the idea that the Earth was the center of the universe. He modified this model to include:

* Epicycles and Deferents: To explain the observed retrograde motion of planets (where planets appear to move backwards in the sky), Ptolemy introduced the concept of epicycles. He proposed that planets moved in smaller circles called epicycles, which in turn revolved around larger circles called deferents, centered on the Earth.

* Equants: To further improve the accuracy of the model, Ptolemy added the concept of an equant. An equant was a point that was offset from the center of the deferent, from which the planet appeared to move at a uniform speed. This helped to account for the fact that the planets didn't move at a constant speed in their orbits.

These modifications were intended to explain the observed motions of the planets and other celestial bodies, but they made the model increasingly complex. While the Ptolemaic model remained the dominant cosmological model for over 1400 years, it was eventually replaced by the heliocentric model proposed by Nicolaus Copernicus.