KEPLER'S LAWS

The three empirical laws describing the motions of planets in their orbits, discovered by Johannes Kepler (1571 - 1630). These are: (1) The orbits of the planets are ellipses, with the sun at a common focus. (2) As a planet moves in its orbit, the line joining the planet and sun sweeps over equal areas in equal intervals of time. Also called "law of equal areas". (3) The squares of the periods of revolution of any two planets are proportional to the cubes of their mean distances from the sun.