Kepler’s Laws of Planetary motion:

 

First:                The orbit of each planet is an ellipse with the sun at one of the foci.

Second:           The line joining the planet to the Sun, known as the radius vector, sweeps out equal area in equal time.

Third:               The square of the time the planet takes to go around the Sun (its ‘year’) is proportional to the cube of its mean distance from the Sun.