<P> Atmospheric explanations for many phenomena were preferred because the Eudoxan--Aristotelian model based on perfectly concentric spheres was not intended to explain changes in the brightness of the planets due to a change in distance . Eventually, perfectly concentric spheres were abandoned as it was impossible to develop a sufficiently accurate model under that ideal . However, while providing for similar explanations, the later deferent and epicycle model was flexible enough to accommodate observations for many centuries . </P> <P> Although the basic tenets of Greek geocentrism were established by the time of Aristotle, the details of his system did not become standard . The Ptolemaic system, developed by the Hellenistic astronomer Claudius Ptolemaeus in the 2nd century AD finally standardised geocentrism . His main astronomical work, the Almagest, was the culmination of centuries of work by Hellenic, Hellenistic and Babylonian astronomers . For over a millennium European and Islamic astronomers assumed it was the correct cosmological model . Because of its influence, people sometimes wrongly think the Ptolemaic system is identical with the geocentric model . </P> <P> Ptolemy argued that the Earth was a sphere in the center of the universe, from the simple observation that half the stars were above the horizon and half were below the horizon at any time (stars on rotating stellar sphere), and the assumption that the stars were all at some modest distance from the center of the universe . If the Earth was substantially displaced from the center, this division into visible and invisible stars would not be equal . </P> <P> In the Ptolemaic system, each planet is moved by a system of two spheres: one called its deferent; the other, its epicycle . The deferent is a circle whose center point, called the eccentric and marked in the diagram with an X, is removed from the Earth . The original purpose of the eccentric was to account for the differences of the lengths of the seasons (autumn is the shortest by a week or so), by placing the Earth away from the center of rotation of the rest of the universe . Another sphere, the epicycle, is embedded inside the deferent sphere and is represented by the smaller dotted line to the right . A given planet then moves around the epicycle at the same time the epicycle moves along the path marked by the deferent . These combined movements cause the given planet to move closer to and further away from the Earth at different points in its orbit, and explained the observation that planets slowed down, stopped, and moved backward in retrograde motion, and then again reversed to resume normal, or prograde, motion . </P>

Who created the model of the solar system