Kepler’s Laws

First Law
"The orbit of every planet is an ellipse with the sun at one of the foci."

Second Law
"A line joining a planet and the sun sweeps out equal areas during equal intervals of time."

Third Law
"The squares of the orbital periods of planets are directly proportional to the cubes of the semi-major axis of the orbits."

P = orbital period of planet
a = semimajor axis of orbit
 

On to coordinate systems.

 Back to the science.

Source: http://en.wikipedia.org/wiki/Kepler's_laws_of_planetary_motion

               http://demonstrations.wolfram.com/KeplersSecondLaw/