How did the ptolemaic model of the universe explain retrograde motion?

How did the ptolemaic model of the universe explain retrograde motion?How did the ptolemaic model of the universe explain retrograde motion?Straight from this link: http://www.lasalle.edu/~smithsc/Astronom…



The model of the solar system developed by Ptolemy (87 - 150 A.D.) was a refinement of Aristotle's (384 - 322 B.C.) universe. This model consisted of a series of concentric spheres, with the Earth at the center (geocentric). The motions of the Sun, Moon, and stars was based on perfect circles. To account for the observed retrograde motion of the planets, it was necessary to resort to a system of epicycles, whereby the planets moved around small circular paths that in turn moved around larger circular orbits around the Earth. This accounts for retrograde motion.



In its final form, the model was extremely complicated, requiring many nested levels of epicycles, and with even the major orbits offset so that they were no longer truly centered on the Earth. Despite all of this fine tuning, there remained significant discrepancies between the actual positions of the planets and those predicted by the model. Nevertheless, it was the most accurate model available, and it remained the accepted theory for over 13 centuries, before it was finally replaced by the model of Copernicus.