The ancient Greek mathematician Eudoxus developed a model for the motion of the Sun, Moon, and planets in which each body was carried around on a series of nested spheres that were all centered on Earth. Eudoxus’ geocentric model was incorporated into the highly successful cosmology of Aristotle. However, this model was unable to account accurately for the observed motions of the planets. Later astronomers such as Hipparchus and Ptolemy developed a new set of models in which each planet is carried around a circular epicycle, which in turn is carried around a circular deferent with its center near the Earth. Ptolemy even used these models to estimate distances to each planet. Although these models were quite accurate, they did suffer from some problems and were criticized or modified by medieval scholars.