# Kepler's laws

From Citizendium

**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.