Kepler's Laws

1. The law of ellipses: The orbit of a planet is an ellipse with the sun located at one focus.
2. The law of areas: The line joining a planet to the sun sweeps out equal areas in equal times.
3. The harmonic law: The square of the orbital period of a planet is proportional to the cube of its distance from the sun.

The original source of this document is: http://liftoff.msfc.nasa.gov/academy/rocket_sci/orbmech/kepler.html