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.