The following three laws about the motion of the planets round the Sun:

1. (i) The orbit of each planet is an ellipse with the Sun at one focus.

2. (ii) The line joining a planet and the Sun sweeps out equal areas in equal times.

3. (iii) The squares of the times taken by the planets to complete an orbit are proportional to the cubes of the lengths of the major axes of their orbits.

These laws were formulated by Kepler, based on observations, but each can be deduced from Newton’s inverse square law of gravitation.

http://astro.unl.edu/naap/pos/animations/kepler.swf Animation which illustrates all three laws.