Kepler, Johannes 1571-1630
German astronomer

Born in Weilderstadt, W�rttemberg, he was educated at the University of T�bingen where he obtained a master's degree in theology in 1591. He became Professor of Mathematics at Graz in 1594. Among his duties at Graz was the publication of almanacs to forecast the weather and to predict favourable days for various undertakings with reference to the rules of astrology. For a time he was astrologer to Duke Albrecht of Wallenstein. He recorded in his first major publication, the Mysterium Cosmographium (1596), that the distances from the Sun of the six planets including the Earth could be related to the five regular solids of geometry, of which the cube is the simplest. He sent copies of his book to Galileo and Tycho Brahe, the greatest astronomers of the day, who responded favourably. When Kepler was later in difficulties in Graz, the latter invited him to join him at Prague. Kepler arrived in Prague in 1600, and when Brahe died in 1601 he was appointed to succeed him as imperial mathematician by the Emperor Rudolf II. His chief interest was the study of the planet Mars and he found that its movement could not be explained in terms of the customary cycles and epicycles. In this he broke with the tradition of more than 2,000 years by demonstrating that the planets do not move uniformly in circles, but in ellipses with the Sun at one focus and with the radius vector of each planet describing equal areas of the ellipse in equal times (Kepler's first and second laws). He completed his researches in dynamical astronomy 10 years later by formulating his third law, which connects the periods of revolution of the planets with their mean distances from the Sun. In 1627 he published the Tabulae Rudolphinae, which contained the ephemerides of the planets according to the new laws, and also an extended catalogue of 1,005 stars based on Tycho's observations.

Bibliography: Max Caspar, Kepler (1959)