Kepler's laws

Enunciate the Kepler's Laws.

Kepler's first law: the orbit of every planet is an ellipse wth the Sun at one of the two foci.

Kepler's second law: a line joining a planet and the Sun sweeps out equal areas during equal intervals of time.

Kepler's third law: the square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.

$P^{2} \propto a^{3}$

$P = orbital \, period \, of \, a \, planet$
$a = semi-major \, axis \, of \, the \, orbit$

In the case of a circular orbit:

$4 \pi^{2} / P^{2} = G \cdot M / R^{3}$

$M \, is \, the \, mass \, of \, the \, larger \, body$
$R \, is \, the \, distance \, between \, the \, centres \, of \, the \, two \, masses$

Reference:
http://en.wikipedia.org/wiki/User:Gonfer