The idea of placing the sun at the center of the universe was not a particularly new one. But few either saw advantage to it and many considered it physically impossible (it was inconsistent with Aristotelian physics). That started to change with a polish physician, lawyer, artist and astronomer – Nicholas Copernicus (1473 - 1543).
Copernicus was aware of earlier writings which suggested a moving earth. He, however, seemed to be motivated by two main suggestions. One, he believed that the earth was not a particularly fit object to be the center of the universe but that the sun was a more divine object and thus more fit for the center. Second, Copernicus very much disliked the concept the equant. He thought it an abomination and a betrayal of the concept of circles. He appears to have been aware of works by Arabic mathematicians who, in an attempt to reconcile the Qu’ran (which suggested the earth moved) with the Ptolemaic system removed the equant in favor of additional epicycles. Copernicus, likewise, used epicycles in his calculations but no equant.
What really set Copernicus’s heliocentric model apart was its simplicity. It did no better than Ptolemy’s model at predicting the planets but it was easier to use and handle. While few actually read Copernicus’s deathbed publication of his work De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres), it did gain a few fans (such as Galileo).
Copernicus’s model handled the basic observations:
- Like the geocentric model, the earth was believed to be round.
- The earth rotated, and thus the stars, sun, and planets appeared to move around the earth
- Mercury and Venus were closer to the sun than the earth and so always appeared near the sun.
- As the earth passed Mars, Jupiter, and Saturn during opposition, the planets would appear to undergo retrograde motion.
It explained the same phenomena as the Ptolemaic model but did so more simply. This made it attractive to some. The next background page, Elongations and Configurations describes the model and the observations that provided data for it, in more detail.