KEP'LER, JOI1ANN One of the world's greatest astronomers. He was born on December 27, 1571, at Weil der Stadt, in Wurt temberg, Germany. He was sickly in his early childhood, and his constitution remained weak throughout life. in 1584 he was sent to Adel• berg, and in 1586 to the cloister school in Adlaul brunt" On passing a brilliant maturity examina tion, he was admitted in 1589 to the University of Tiibingen. llere he studied chiefly theology and the classics. At the same time he became acqlliiinted with the teachings of Copernicus, which greatly influenced his later career. In 1594 lie accepted the chair of astronomy and mathematics at Gratz, which he held until 1600, when lie was compelled to leave in ac count of religious difficulties. Since 1599 Tycho Brahe had been mathematician and astron omer to Emperor Rudolph 11., and in 1600 Kep ler became his assistant in the observatory near Prague. On October 13, 1601, Tycho Brahe died, and Kepler succeeded him in both of his impor tant posts. Ills compensation was to be 500 florins a year, but, owing to the desperate con dition of the Imperial finances, it was never paid in full. While retaining this position, Kepler, in 1612, accepted the office of mathematician to the States of Upper Austria. In 1 626 lie moved to Ulm, where lie undertook the publication of the Ru dolphinian Tables. In July,1628,he left the service of the Emperor Ferdinand IL and entered that of Wallenstein, who promised to pay the amount of his former salary that still remained unpaid. Wal lenstein, however, did not keep his promise. With the intention of presenting his ease to the Impe rial Diet, Kepler undertook a journey to Ratis bon. But on his way he was attacked by fever, and shortly after reaching Ratisbon died, on November 15, 1630. While in Gratz, in 1697, he married Barbara von Miihleck, who died in 1611. Two years later he married Susanna Reutlinger, who survived him.
Kepler early conceived that there must be sonic intelligible reason for the actual disposition of the solar system; and it was mainly the develop ment of this idea that gained him a wide reputa tion and the friendship of Tycho Brahe and Gali leo. In the eapaeity of mathematician, lie completed the Rudolphinian Tables, which had been left unfinished by the death of his former patron, Tycho Brahe. But he was also compelled to discharge the duties of an astrologer, although lie limited his astrological work to the vague estimation of tendencies and probabilities. His chief title to fame is his discovery of the three laws of planetary motion, viz. the laws of elliptical orldis, of equal areas, and of the rela tions between periods and distances. (See ASTRON OMY; (inmITATiox.) The first two of these laws
appeared in his greatest work, Astronoinia Nora, etc. (1609). Other important features of this work were discoveries in regard to gravitation, and the explanation of the tide" by lunar attrac tion. In 1616, in Linz, Kepler calculated the first ephemerides based on his laws. In 1619, in his treatise llarmwrice Nandi, lie published his third law. In September, 1627, he finished the Rudolphinian Tables, the appendix of which con tained a catalogue of 1005 stars. In 1629 he called the attention of astronomers to the ap proaching transits. That of Mercury, which oc curred on November 7, 1631. was the first transit of a planet across the sun ever observed.
Kepler was also the founder of a theory of vortices. and did pioneer work in several impor tant scientific subjects. Having in 1604 given an approximation to the law of refraction, at the invention of the telescope he gave the theory of refraction by lenzes, and the principle of the in verting telescope. Ms theory of intinitesimals prepared the way for Cavalieri's theory of in divisibles and the invention of the calculus by Newton and Leibnitz. Ile was also very active in introducing logarithms into Germany. His principal writings, besides those already men tioned, include: Prodromus Dissertationum Cos mrographiearum Seu ill ystcriunc Gosmographi cum (1590 ; De Stella Nova in Peck Xerpcntarii (1606) ; Astronomiw Pars Optica (1604) ; Nova Stercometria Dolioruni (1613) ; De Corneas (1619) ; Ephemerides Nov• Notuum Cwlestium, ( 1616 ) ; Epilmaes Astronom Copernicame (1618-21 ) ; Chilies Logarithmorum. His extant manuscripts Were purchased by Empress Catharine IL of Russia, donated by her to the Academy of Saint Petersburg, and deposited in the observatory of Pulkowa, where they remained inaccessible for a long time. A complete edition of Kepler's works, in eight volumes, was prepared by Frisch under the title Johannis Kcpleri Opera Ontiria (1858-71).
Consult: Breitschwert, Johann Keplers Leben und Wirken (Stuttgart, 1831) ; Brewster, Lives of Galileo, Tycho Brake, and Kepler (London, 1874); Reitlinger, Johann Kepler (Stuttgart, 1868) ; Apelt. Johann Keplers astronomisehe Wellansicht (Leipzig, 1849) : duller, Die Kep lerschen Gesetze ( Brunswick, 1871) ; Reuschle, Kepler und die Astronomic (Frankfort., 187/1; (Vibe], Ueber Keplers astranomische Anseha u nyen (Halle, 1872) ; Hasner, ye/co Brahc und Kepler in Pray (Prague. 1872) ; Ofinthen "Kepler und der tellurisch-kosmische Magnetismus," in Pencles Geographische Abhandlungen (Vienna, 1888); Forster, Johann Kepler vial die !harmonic der Spharen (Berlin, 1802) ; \Volt, Geseltiehte der (Munich, 1877).