* Astronomy

In astronomy, Kepler's laws of planetary motion describe the orbit of two bodies when the mass of one is negligible compared to the other, such as a planet and its star. Although traditionally formulated in terms of the planets of the Solar System and the Sun, they apply equally to other gravitational systems. In their classic formulation they are:

1. The orbit of every planet is an ellipse with the Sun at a focus.
2. A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.
3. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

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