Harmony of the Spheres Slide 13 of 24

It is for this reason that they believed the motions of the heavenly bodies to be based on circles and spheres. Nothing less than perfection could account for the behaviour of objects so close to the Gods. The problem was that when they looked carefully at the motion of the planets, they observed some pretty irregular motion. Some planetes would actually retrace their steps in the sky at certain times, instead of moving along a smooth continuous arc. In order to retain the symmetry principle, Ptolemy figured out that by adding circles within circles (called ``epicycles'') it was possible to account for such ``retrograde'' motion, as it was called. However, as observations got better and better, it was necessary to add more and more circles within circles, until the perfection of the underlying geometrical shape (the circle) was completely overshadowed by the complexity of the model. Moreover, by adding enough circles, it is possible to reproduce any orbit whatsoever, so this model lacks the essential feature of being falsifiable, and does not qualify as a scientific theory. It therefore appears that this was a misguided application of symmetry. As it turns out the symmetry principle was not so bad. It was the geocentric viewpoint (i.e. the notion that the Earth was the center of the Universe) that messed them up. Once it was realized, by Copernicus, that the Sun was the center of planetary motion and not the Earth, then the orbits of the planets once again became very nearly circular.

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