Celestial Mechanics: 

  Kepler's 3 Laws of Planetary Motion    

1. Ellipse Law: Each planet moves in an ellipse, with the Sun at one focus.

2. Law of Areas: The line between the Sun and the planet sweeps out equal areas in equal times.

3. Harmonic Law: The ratio of the cube of the semimajor axis to the square of the period is the same for each planet.