Kepler's three laws are geometric relationships that describe the motions of the planets in the solar system. German astronomer Johannes Kepler derived them in the early 1600s, with the help of more than two decades' worth of detailed observations by Danish astronomer Tycho Brahe. Kepler worked as Tycho's assistant for the last 18 months of Tycho's life. Much of Tycho's work described the position of Mars in its orbit around the Sun.

Kepler was one of the first people to attempt to explain why planets move the way they do around the Sun. When Kepler joined Tycho's crew of assistants, he used Tycho's data to confirm his intuitions about the motion of Earth--that our planet is not privileged, but rather, moves about the Sun much like all the other planets. While working with Tycho's stacks of records on Mars, Kepler devised...