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 orbital period of a planet is directly proportional to the cube of the semi-major axis of its (elliptical) orbit.
Note that planetary orbits can be circular, as a circle is a special case of an ellipse. In this case, the sun is located at the center of the circle, and the semi-major axis is simply the circle's radius.