Starting with the fact of the Second Law of Kepler, and using the result of the Orbital Angular Momentum as a constant value in terms so the reduced mass and the eccentricity of the ellipse, the Third Law of Kepler can be shown. The square of the period of a planet is directly proportional to the semimajor axis to the third power. But this law may be used also for moons in orbit around their planets.
All is in this video
http://www.showme.com/sh?h=RfJjM5A
