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Johannes Kepler was a German Lutheran mathematician, astronomer and astrologer, and a key figure in the 17th century astronomical revolution. He is best known for his eponymous laws of planetary motion, codified by later astronomers based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican Astronomy. Before Kepler, planets' paths were computed by combinations of the circular motions of the celestial orbs. After Kepler, astronomers shifted their attention from orbs to orbits—paths that could be represented mathematically as an ellipse. Kepler's laws also provided one of the foundations for Isaac Newton's theory of universal gravitation. During his career Kepler was a mathematics teacher at a Graz seminary school, an assistant to Tycho Brahe, the court mathematician to Emperor Rudolf II, a mathematics teacher in Linz, Austria, and an adviser to General Wallenstein. He also did fundamental work in the field of optics and helped to legitimize the telescopic discoveries of his contemporary Galileo Galilei.