Copernicus

The Copernican heliocentric system

The great breakthrough that ultimately led to modern astronomy and cosmology came with Copernicus’ heliocentric system for describing the heavenly motions. But practically nothing one can say about Copernicus, either about his system or about its origins, is simple.

For one thing, the Copernican Universe is not the modern one, but the ancient cosmology in many of its most important respects. It is still a finite world bounded by a sphere of fixed stars. For another, Copernicus was not started on his path to his great discoveries by seeking a system that could use the Earth's motion to provide a more unified account of the apparent motions of the Sun and planets. Rather, he was dismayed by the appearance of the device of the equant point in Ptolemy's system and sought a means of eliminating it from an account of the heavens. Copernicus was even more devoted than the Greeks to the view that all the heavenly motions must be described in terms of pure circular motions in which everything moved uniformly about the circle with respect to the actual central point of the circle. Of course, hierarchies of circles, for example epicycles whose centers moved uniformly on deferent circles, were permissible.