KEPLER, or KEPPLER, JOHANN, one of.the greatest astronomers of all ages, was b. at Magstatt, a small village in Whrtemberg. 10 m. from Stuttgart, Dec. 27, 1571. While a mere child he was left to his own resources, and his early education in consequence would have been entirely neglected had he not been admitted into the convent of Maulbronn. Kepler afterwards studied at the university of Tilbingen, and devoted nearly the whole of his time to mathematics and astronomy. In 1593 be was appointed professor of mathematics at Grittz. At this time Kepler's views of astronomy, as seen in his Prodromus, were somewhat mystical; he supposed the sun, stars, and planets were typical of the Trinity, and that God distributed the planets in space in accordance with the regular polyhedrons, etc. Yet this searching after harmony led him to the discovery of the three remarkable truths called Kepler's laws. Kepler, about 1596, -commenced a correspondence with Tycho Brahe, and in 1599 went to Prague to aid him in his researches. Tycho obtained for him a government appointment; but the salary was not paid, and Kepler lined for eleven years there in great poverty. He then •obtained a mathematical appointment at Linz, and, fifteen years afterwards, was removed to the university of Rostock; but poverty from the same cause still pursued him, and he died shortly after at Ratisbon, Nov. 15, 1630. Kepler's connection with Tycho Brahe had a. salutary effect upon his fiery enthusiasm, but, happily for science, the timid councils of the old astronomer were only partially followed. Kepler estab lished the law of the diminution of light in proportion to the inverse square of the distance, and was acquainted with the fact " that the attractive force of the sun decreases as his light;" it is strange that this latter fact did not lead him to anticipate the discovery .of Newton. In 1609 he published his nova, a commentary on the motions 'of Mars, in which, taking for his base of operations the observations of Tycho, he determined the eccentricity and aphelion of this planet, on the supposition of a circular orbit, and found the results quite irreconcilable with observation. This led him to his
first law, that the planets more in ellipses with the sun in one of the foci. The second law, that the radius-vector (q.v.) sweeps over equal areas in equal times, he at first asserted dog matically, and was for a long time puzzled to find some proof of it (the infinitesimal calculus not having at that time been invented); but at last he hit upon the expedient of dividing the ellipse into an immense number of small triangles, whose areas could be easily found. His third law (the first discovered) was an attempt to harmonize in some way the period and mean distance of the planets, and after twenty-two years of vigorous application, he discovered that the square of the periodic time is proportional to the cube of the mean distance. These discoveries, great as they undoubtedly are, are rendered still more so when we take into account the little real knowledge of the heavenly bodies existing at that time, and the scanty means in the hands of astronomers for making dis coveries. Kepler also affirmed the essential inertia of matter, the first of Galileo's laws. of motion; the dependence of the curvature of the path of planets on the attraction of the sun (Kepler unfortunately thought it was magnetic attraction) and the proportionality of the mutual attraction of bodies to their respective masses; he demonstrated the four new planets of Galileo to be satellites of Jupiter; gave a complete theory of solar eclipses; and calculated the exact epoch of the transits of Mercury and of Venus across the sun's disk. He also made numerous discoveries in optics, general physics, and geometry. A collected edition of Kepler's works was published by Frisch (1858-71).—See Brewster's Lires of Galileo, Tycho de Brahe, and Kepler (1841); and Reitlinger, Neumann, and Gruner, Johannes Kepler (1868).