At the left is the simple Ptolemaic model of the Sun and Mars as they orbit the Earth . Actually Copernicus did not object to the use of epicycles and, in fact, used them himself. (His main objection was to certain details of the more elaborate model, in particular, The Equant.) The Copernican model was not better in its predictions than that of Ptolemy, nor was it simpler. MORE
For clarity only the motion of Mars is shown. The change in the apparent size of Mars is approximate. Subject to the simple assumptions above, the apparent motion should be quite accurate and very similiar to that given by the Copernican model. The reasons the heliocentric model prevailed was because Kepler demonstrated that the orbits where Ellipses, and hence, removed the need for epicycles and thus greatly simplified the heliocentric model, and the rise of Newtonian Mechanics established that the mutual Sun-Earth gravitational force, while strong enough to sling the Earth around the Sun is not strong enough to do the converse.

The irony of the above is that Galileo, as much wedded to the sanctity of the heavenly circle as was Copernicus and, a millienium before him, Ptolemy, rejected Kepler's ellipses because they where not circles. Further, General Relativity (aka The Theory of Gravity) makes the question "Does the Earth orbit the Sun or the Sun orbit the Earth?" meaningless. Anything can orbit anything else. No problem! Every motion is relative. There is no prefered reference.

© 1999 Carl Adler mailto:Carl@Image-ination.com