Which Explanation is “Correct”?
Both theories “explain” the irregular motion of the planets.
- Ptolemy: Earth at center of universe. Motion of planets described by circles upon circles.. Earth still at center of universe.
- Copernicus: Earth just a planet just like other five planets. All go around sun. The strange motion of the planets (retrograde motion) is explained --- almost --- still must have circles on circles to describe detailed motion.
Which Agrees Better with the Data?
- At the time of Copernicus, there was NO BIG DIFFERENCE between the match between either theory and the data!