[محمد ابراهيم]

Ptolemaic Explanation


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, as shown in the animation below:

Retrograde Motion in the Ptolemaic (Geocentric) System


(This animated gif was obtained from Dr. Stephen J. Daunt's Astronomy 161 web site at The University of Tennesee, Knoxville.)

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.

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Copernican Explanation


Copernicus replaced the geocentric universe of Ptolemy with one that was centered on the Sun (heliocentric), with only the Moon orbiting the Earth. His model was still based on circular orbits (and therefore still required further refinement), but it was able to achieve superior precision than the Ptolemaic model without the need for epicycles or other complications. The explanation for retrograde motion in this system arises from the fact that the planets further from the sun are moving more slowly in their orbits than those closer to the sun. The retrograde motion of Mars occurs when the Earth passes by the slower moving Mars, as shown in the following animation:


Retrograde Motion in the Copernican (Heliocentric) System