German astronomer, born at Weil, Württemberg, and educated at Tübingen, where he was influenced by Copernican Principles. Professor, Graz (1594); assistant to Tycho Brahe (q.v.) at observatory near Prague (1600); succeeded Brahe at observatory and as imperial mathematician and court astronomer (1601). Mathematician to the states of Upper Austria at Linz (1612); moved to Ulm (1626), and completed and published Brahe's Rudolphine Tables; moved to Sagan in Silesia (1628). Discovered the three important laws (Kepler's laws) of planetary motion, announcing the first two in his Astronomia Nova de Motebus Stellae Martis ex Observationibus Tychonis Brahe (1609) and the third in Harmonice Mundi (1619). Also wrote on optics, where he approximated the law of refraction. Did a pioneer work that led to the the invention of the Calculus.