Using the astronomical data collected by Tycho Brahe, Kepler developed three radical laws of planetary motion in which orbits were considered to be ellipses. The third law relates the period of orbit to the radius of the orbit. Newton later showed his work to be consistent with Kepler’s laws.
The slides provide a hand-waving derivation of Kepler’s third law and some sample questions. Low earth and geostationary orbits are also discussed.