The New International Encyclopædia/Kepler, Johann

KEP′LER, (1571–1630). One of the world's greatest astronomers. He was born on December 27, 1571, at Weil der Stadt, in Württemberg, Germany. He was sickly in his early

childhood, and his constitution remained weak throughout life. In 1584 he was sent to Adelberg, and in 1586 to the cloister school in Maulbronn. On passing a brilliant maturity examination, he was admitted in 1589 to the University of Tübingen. Here he studied chiefly theology and the classics. At the same time he became acquainted with the teachings of Copernicus, which greatly influenced his later career. In 1594 he accepted the chair of astronomy and mathematics at Gratz, which he held until 1600, when he was compelled to leave on account of religious difficulties. Since 1590 Tycho Brahe had been mathematician and astronomer to Emperor Rudolph II., and in 1600 Kepler became his assistant in the observatory near Prague. On October 13, 1601, Tycho Brahe died, and Kepler succeeded him in both of his important posts. His compensation was to be 500 florins a year, but, owing to the desperate condition 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 1626 he moved to Ulm, where he undertook the publication of the Rudolphinian Tables. In July, 1628, he left the service of the Emperor Ferdinand II. and entered that of Wallenstein, who promised to pay the amount of his former salary that still remained unpaid, Wallenstein, however, did not keep his promise. With the intention of presenting his case to the Imperial Diet, Kepler undertook a journey to Ratisbon. But on his way he was attacked by fever, and shortly after reaching Ratisbon died, on November 15, 1630. While in Gratz, in 1597, he married Barbara von Mühleek, who died in 1611. Two years later he married Susanna Reutlinger, who survived him.

Kepler early conceived that there must be some intelligible reason for the actual disposition of the solar system; and it was mainly the development of this idea that gained him a wide reputation and the friendship of Tycho Brahe and Galileo. In the capacity of Imperial mathematician, he 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 he 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 orbits, of equal areas, and of the relations between periods and distances. (See ; .) The first two of these laws appeared in his greatest work, Astronomia Nova, etc. (1609). Other important features of this work were discoveries in regard to gravitation, and the explanation of the tides by lunar attraction. In 1616, in Linz, Kepler calculated the first ephemerides based on his laws. In 1619, in his treatise Harmonice Mundi, he published his third law. In September, 1627, he finished the Rudolphinian Tables, the appendix of which contained a catalogue of 1005 stars. In 1629 he called the attention of astronomers to the approaching transits. That of Mercury, which occurred 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 important 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 lenses, and the principle of the inverting telescope. His theory of infinitesimals prepared the way for Cavalieri's theory of indivisibles and the invention of the calculus by Newton and Leibnitz. He was also very active in introducing logarithms into Germany. His principal writings, besides those already mentioned, include: Prodromus Dissertationum Cosmographicarum Seu Mysterium Cosmographicum (1596); De Stella Nova in Pede Serpentarii (1606); Astronomiæ Pars Optica (1604); Nova Stereometria Doliorum (1613); De Cometis (1619); Ephemcrides Novæ Motuum Cœlestium (1616); Epitomes Astronomiæ Copernicanæ (1618–21); Chilias Logarithmorum (1624). His extant manuscripts were purchased by Empress Catharine II. 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 Kepleri Opera Omnia (1858–71).

Consult: Breitschwert, Johann Keplers Leben und Wirken (Stuttgart, 1831); Brewster, Lives of Galileo, Tycho Brahe, and Kepler (London, 1874); Reitlinger, Johann Kepler (Stuttgart, 1868); Apelt, Johann Keplers astronomische Weltansicht (Leipzig, 1849); Müller, Die Keplerschen Gesetze (Brunswick, 1871); Reuschle, Kepler und die Astronomie (Frankfort, 1871); Göbel, Ueber Keplers astronomische Anschauungen (Halle, 1872); Masner, Tycho Brahe und Kepler in Prag (Prague, 1872); Günther, “Kepler und der tellurisch-kosmische Magnetismus,” in Penck's Geographische Abhandlungen (Vienna, 1888); Forster, Johnnn Kepler und die Harmonie der Sphären (Berlin, 1862); Wolt, Geschichte der Astronomie (Munich, 1877).