From Citizendium, the Citizens' Compendium
Kepler's laws of planetary motion, or simply Kepler's laws, are three laws stated by the German astronomer Johannes Kepler. These laws govern the motion of the planets around the sun. Stated briefly, the laws are:
- The orbit of a planet is elliptical, with the sun at one of the two foci of the ellipse.
- The line joining a planet and the sun sweeps out equal areas in equal intervals of time.
- The square of the periodic time of a planet is directly proportional to the cube of the semi-major axis of its elliptical orbit.