Johannes Kepler. Caricature of the German astronomer Johannes Kepler (1571-1630), holding a model of a cube to represent his theories of the planetary orbits. Kepler first worked out the laws of planetary motion, which still bear his name. Using Copernicus' heliocentric model of the solar system, along with Tycho Brahe's data on the position of the planets, he found that planets orbit the Sun on elliptical paths with the Sun at one focus, that the area swept out by a planet is the same regardless of where it is in its orbit, and that a planet's orbital period is proportional to the long axis of its orbit to the power of 3/2. He thought the planetary distances were based on the sizes of nested Platonic solids, which include this cube.