KEPLER (KEPLERO), JOHANNES. – One of the four great founders of modern astronomy, together
with Copernico, Galilei, and Newton. Born in Weil (Württemberg, Germany) on 16 March 1571, into a noble but by then greatly impoverished family, K. was a herder as a child; but at the age of thirteen, with the help of charitable persons, he was able to enter the Protestant Seminary of Adelberg, from which he then went to that of Maulbronn and subsequently to the University of Tübingen. At Tübingen he met the astronomer Michele Mästlin, who inspired in him a great love for the science of the heavens and
from whom he learned the new ideas of Copernicus, then fiercely opposed by the Lutherans (it is well known that Luther considered Copernicus insane and that Melanchthon had declared that his opinions would not be tolerated), of which he became an ardent supporter; this earned him expulsion from the Seminary of Tübingen, where he lived with other students.
In 1594 K. went to teach mathematics at a school in Graz (Styria), where he married a widow who some time later fell gravely ill; consequently K. was forced to compile almanacs and horoscopes, since his meagre salary was insufficient to meet so many expenses. After various vicissitudes, in 1599 K. went to Prague as an assistant to Tycho Brahe, then astronomer at the court of the Emperor Rudolf II, in the compilation of his Tabulae Rudolphinae. When Tycho died (24 October 1601), K. obtained appointment as his successor, but his salary was reduced once again; consequently the poor astronomer, already burdened with a family, was forced to live a very miserable life. Nevertheless, he managed to publish in 1609 his Astronomia nova, seu physica coelestis tradita commentariis de motibus stellae Martis, that is, his major work.
But K.’s circumstances worsened still further when his wife died (3 July 1611) and, shortly afterward (20 January 1612), the Emperor Rudolf, his protector, also died. Having left Prague, the astronomer went to Linz (Austria), where he gave lessons in mathematics and philosophy and where he married again, so as to have someone to look after his numerous children; but the new union was even less fortunate than the former, since Susanna Reutlinger, who was twenty years younger than her husband, was also very different in character. Despite his poverty, and with incredible perseverance in his work, K. published in 1622 his Harmonices mundi, in which the third law of planetary motion is set forth, and later, in 1627, the Tabulae Rudolphinae, which served for
In the final years of his life, owing to the wars and revolts that were ravaging the country, K. was forced to move to Ratisbona and Ulma, even descending to the miserable condition of a farm laborer in order to support his wife, his eleven children, and himself. Finally, through the good offices of an illustrious Jesuit mathematician, Fr. Paolo Guldin (discoverer of the well-known geometrical theorems that today bear his name), K. was admitted to the court of Wallenstein and in 1630 appointed professor at Rostock. But his life was already nearing its end; stricken with pneumonia, he died on 15 November 1630, abandoned in a country inn near Ratisbona. His grave, in the paupers’ cemetery, still remains unknown.
K. was the first astronomer to carry out a true celestial triangulation, making use chiefly of the observations of Mars made by Tycho Brahe and deriving from them the three fundamental laws governing the motion of the planets around the sun, which, in his honor, astronomers still call Keplerian laws. They are as follows:
I) The planets describe plane orbits around the sun with constant areal velocity. II) These orbits are ellipses, with the sun occupying one focus. III) 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 make it possible to calculate the positions of the planets precisely, but also because they serve as the foundation for the theory of universal gravitation, established later (1687) by Newton. Indeed, with certain formulas of mechanics and infinitesimal calculus, from the first Keplerian law one deduces that the planets are acted upon by an attractive force directed toward the sun; from the second law one deduces that this force is inversely proportional to the square of their distance from the sun; and from the third law one deduces that the masses of the planets are extremely small in comparison with the mass of the sun, which thus constitutes the center of gravity of the entire planetary system.
K. also deserves credit for perfecting the telescope and for constructing a sufficiently accurate table of astronomical refraction in his Optica ad Vitellionem (dedicated to Fr. Witel, a Polish Dominican who had devoted considerable attention to questions of optics), and subsequently in the Dioptrica.