KEPLER (KEPLERO), JOHANNES. — One of the four great founders of modern astronomy, along with Copernicus, Galileo, and Newton. Born in Weil (Württemberg, Germany) on 16 March 1571, into a noble but then greatly impoverished family, Kepler as a boy tended livestock; but at thirteen, with the help of charitable persons, he was able to enter the Protestant Seminary at Adelberg, from which he later moved to Maulbronn and then to the University of Tübingen. At Tübingen he met the astronomer Michael Mästlin, who inspired in him a great love for the science of the heavens.
In 1594 Kepler went to teach mathematics at a school in Graz (Styria), where he married a widow who soon fell gravely ill; thus Kepler was forced to compile almanacs and horoscopes, since his meager salary was insufficient to meet so many expenses. After various vicissitudes, in 1599 Kepler came to Prague as an assistant to Tycho Brahe, then astronomer at the court of Emperor Rudolf II, in the compilation of the latter’s *Tabulae Rudolphinae*. After Tycho’s death (24 Oct. 1601), Kepler was appointed his successor, though his salary was further reduced; thus the poor astronomer, already burdened with a family, was forced into a very wretched life. Nevertheless, in 1609 he managed to publish his *Astronomia nova, seu physica coelestis tradita commentariis de motibus stellae Martis*, that is, his major work.
Kepler’s circumstances worsened still further when his wife died (3 July 1611) and shortly afterward (20 Jan. 1612) Emperor Rudolf, his patron, also died. Having left Prague, the astronomer went to Linz (Austria), where he gave lessons in mathematics and philosophy and remarried, so as to have someone to care for his many children; but this new union was no more fortunate than the first, since Susanna Reutlinger, who was twenty years younger than her husband, was also of a very different temperament. Despite his poverty and with incredible perseverance in his work, in 1622 Kepler published his *Harmonices mundi*, in which he expounded the third law of planetary motion, and later, in 1627, the *Tabulae Rudolphinae*, which for a long time served astronomers in calculating the positions of the planets.
In the final years of his life, because of the wars and revolts devastating the country, Kepler was forced to move to Regensburg and Ulm, reducing himself even to the wretched condition of a farm laborer so as to support his wife, his eleven children, and himself. Finally, through the good offices of a distinguished Jesuit mathematician, Fr. Paul Guldin (discoverer of the well-known geometric theorems that today bear his name), Kepler was admitted to the court of Wallenstein and in 1630 appointed professor at Rostock. But his life was now at an end; stricken with pneumonia, he died on 15 Nov. 1630, abandoned in a country inn near Regensburg. His grave, in the cemetery for the poor, remains unknown to this day.
Kepler was the first astronomer to perform a true celestial triangulation, chiefly by using the observations of Mars made by Tycho Brahe and deducing from them the three fundamental laws that govern the motion of the planets around the sun and that, in his honor, astronomers still call Kepler’s laws. They are as follows:
1) The planets describe around the sun plane orbits with constant areal velocity.
2) These orbits are ellipses of which the sun occupies one focus.
3) The squares of the times taken by the planets to complete one revolution around the sun are proportional to the cubes of their mean distances from the sun.
These laws are extremely important, not only because they allow the exact calculation of the positions of the planets, but also because they serve as the basis for the theory of universal gravitation, later established (1687) by Newton. Indeed, with some formulas of mechanics and infinitesimal calculus, from Kepler’s first law it is deduced that the planets are acted upon by an attractive force directed toward the sun; from the second law it is deduced that this force is inversely proportional to the square of their distance from the sun; and from the third law it is deduced that the masses of the planets are extremely small in comparison with the mass of the sun, which thus constitutes the barycenter of the entire planetary system.
Kepler is also credited with perfecting the telescope and constructing a fairly accurate table of astronomical refraction in his *Optica ad Vitellionem* (dedicated to Fr. Witelo, a Polish Dominican who had devoted much attention to optical questions) and later in his *Dioptrica*.