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.