German mathematician and astronomer. He formulated what are now called Kepler's laws of planetary motion: (1) the orbit of each planet is an ellipse with the Sun at one of the foci; (2) the radius vector of each planet sweeps out equal areas in equal times; (3) the squares of the periods of the planets are proportional to the cubes of their mean distances from the Sun.

Kepler became assistant to Danish astronomer Tycho Brahe 1600, and succeeded him 1601 as imperial mathematician to Holy Roman Emperor Rudolph II. Kepler observed in 1604 a supernova, the first visible since the one discovered by Brahe 1572. Kepler completed and published the Rudolphine Tables 1627, the first modern astronomical tables, based on Brahe's observations. His analysis of these data led to the discovery of his three laws, the first two of which he published in Astronomia nova 1609 and the third in Harmonices mundi 1619.
Kepler was born in Weil der Stadt in Baden-Württemberg, and studied at Tübingen. As a Lutheran Protestant, he was expelled twice from Graz, where he had been teaching; then from Prague 1612; then from Linz, Austria, from where he moved to Ulm. His other domestic problems included the unsuccessful prosecution in Wittenberg 1618 of his mother for witchcraft.
Kepler was one of the first advocates of Sun-centred cosmology, as put forward by Copernicus. Kepler's laws are the basis of our understanding of the Solar System, and such scientists as Isaac Newton built on his ideas.