Ptolemy to the Rescue?

In order to preserve the geocentric cosmology of the time and to account for retrograde motion, Ptolemy had to make a model of epicycles. This makes the orbits of planets very complicated and violates the scientific search for simplicity.

A visualation of epicycles:

Full Animated Epicycle

Correct Explanation Retrograde Applet ( but do you really need a computer animation to visualize this? )

Explanation of apparent retrograde motion in the heliocentric model is that its a simple consequence of the relative orbit of the earth overtaken that of mars as measured against a fixed background of reference stars. This is shown below:

The Ptolemaic Moment will stand for about 1500 years with this model for planetary orbits remains unchallenged, partly because Ptolemy's model did improve the precision to which planetary positions could be predicted. Remember, however, the precision at measuring planetary positions was quite poor and therefore there was a large random error involved when comparing predictions with observations.

The Ptolomaic model represents an excellent example of "cultural" bias in science. Most citizens of the cultural wanted to accept the idea of the Earth being the center of the Universe (i.e. the earth was special) and therefore observations were molded in such a way so as to preserve that cultural norm. (Things may not be very different today).

Recall, there was a competing model of the time, that of Aristarchus of Samos (280 BC):

( Aristarchus of Samos (c. 310-230 BC), Greek astronomer, first to maintain that the Earth rotates and revolves around the Sun. On this ground, Cleanthes the Stoic declared that he ought to be indicted for impiety.

Aristarchus' advanced ideas on the movement of the Earth are known from Archimedes and Plutarch; his only extant work is a short treatise, "On the Sizes and Distances of the Sun and Moon." The values he obtained, by using geometry, are inaccurate, because of faulty observations. Aristarchus found a more precise value for the length of the solar year.)

Aristarchus was extremely clever and also figured out a way to determine the ratio of distances between the earth and the moon and the earth and the sun. While wrong, at least he reasoned that the sun was much farther from the earth than the moon.