German astronomer Johannes Kepler (1571-1630) is most famous for his three laws of planetary motion which provided mathematical support to the Copernican sun centered model of the solar system. These three laws are:
1) Planets move around the Sun in ellipses, with the Sun at one focus.
2) The line connecting the Sun to a planet sweeps equal areas in equal times.
3) The square of the orbital period of a planet (it's year) is proportional to the cube (3rd power) of the mean distance from the Sun.
http://www-spof.gsfc.nasa.gov/stargaze/Kep3laws.htm
