Kepler's laws of planetary motion Kepler (in 1609) discovered three laws of planetary motion through laborious analysis of data gathered by astronomer Tycho Brahe. These are stated as follows.

First law: The planets move around the sun in elliptical orbits with sun at one of the foci.

Second law: The line joining the sun to a planet sweeps equal areas in equal intervals of time.

Third law: The square of the time period of revolution of a planet is proportional to the cube of its mean distance from the sun.

The second law follows as a consequence of conservation of angular momentum. The proof of first and third laws are beyond the scope of this book. For a simple proof of third law see geostationary orbit.