1571-1630. 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. Born in
Wurttemberg, Kepler became assistant to Tycho
Brahe 1600, and succeeded him as imperial
mathematician 1601. His analysis of Brahe's
observations of the planets led him to
discover his three laws, the first two of
which he published in Astronomia Nova 1609
and the third in Harmonices Mundi 1619.