Planets move on elliptic orbits with the sun at a focal point.
The line connecting a planet with the sun sweeps out equal areas in equal periods of time. Comparing different planets, the squares of their orbital periods are proportional to the cubes of the long axes of their orbits. Biographic details Kepler's reconstruction of the orbit of Mars based on the data of Tycho Brahe. Kepler's laws can be derived from Newton's law of gravitation.