Kepler's laws summarize Kepler's work on the motions of the planets: 1) The first law states that planets move in ellipses with the Sun at one focus. 2) The second law says that planets sweep out equal areas in equal times. 3) The third law relates the orbital periods of planets to the diameters of their orbits, establishing that a line connecting a planet and the Sun sweeps out equal areas in equal time intervals.