
Johannes Kepler (15711630) took the basic heliocentric idea of Copernicus and turned it into a system that worked. Using the extremely accurate positions measured by the Danish astronomer Tycho Brahe, Kepler formulated three laws of planetary motion. First, the orbits of the planets are ellipses with the Sun at one focus. (An ellipse is a closed curve that can vary from a perfect circle to a very long, very thin oval.) The second law describes how a planet travels faster along its elliptical path when it gets closer to the Sun. And the third law gives a precise relation between the size of a planetís orbit and the length of time that planet takes to orbit the Sun. By abandoning the classical concept that motions had to be circular and at constant speeds, Kepler showed that the universe behaves by its own laws and doesnít necessarily follow our arbitrary preconceptions. 
