KEPLER, JOHANNES (1571–1630). Astronomer and mathematician best known for his discovery of three laws of planetary motion. Bom in Weil der Stadt, in the Duchy of Würtemburg, Kepler was educated at Lutheran seminaries in Adelberg and Maulbronn, enrolling at the University of Tübingen in 1589. Kepler’s exceptional intellectual abilities were apparent from an early age, and his family intended a church career for him. He indulged his strong scientific interests at the university and greatly admired the astronomy professor Michael Maestlin, who publicly taught the Ptolemaic scheme while privately endorsing Copernicanism. Kepler himself defended Copernican astronomy in a public debate. Because Luther himself had mocked the Copernican scheme and quoted scripture to refute it, Kepler’s defense of Copernicanism, together with his reservations about the Augsburg confession, precluded the church career for which he had prepared.
Instead, Kepler was offered a professorship of astronomy in the Austrian city of Graz, where he went in 1594. One of the duties of his professorship was to make astrological predictions. Notwithstanding a pronounced skepticism about astrology, Kepler successfully predicted a cold winter and an invasion by the Turks, prognostications that won him a salary increase and powerful patrons. His first important work was the 1596 Mysterium Cosmographicum (“Cosmographic Mystery”) in which he accounted for the numbers, sizes, and distances of planetary orbits by inscribing them within and circumscribing them about the corresponding spheres in the five Platonic regular solids. Using the idea of a rationally ordered but fundamentally mysterious cosmos as a starting point, Kepler conceived a general plan for reforming astronomy, optics, and music on the model of a quasi-theological vision of divine harmony and order.
In 1598 the ruling Hapsburg family closed the Protestant educational institutions in the Austrian provinces, and although Kepler was permitted to stay in Graz, he was compelled to leave in 1600. Moving to Prague, Kepler became an assistant to the great Danish astronomer Tycho Brahe, who served the emperor Rudolph II as the imperial astronomer. Brahe demanded that Kepler write a book attacking the (recently deceased) astronomer Nicholas Ramerius Ursus, a former imperial mathematician with whom Tycho had quarreled for years; the result was Apologia pro Tychone contra Ursum (“A Defense of Tycho against Ursus”), which remained unpublished in Kepler’s lifetime but contains important methodological arguments. On Tycho’s death in 1601, Kepler succeeded him as imperial mathematician and undertook an ambitious program of publication in astronomy and optics. The most important of these publications was the 1609 Astronomia nova (“New Astronomy”), which substituted physical reasoning for the geometrical models of all previous astronomical theories and propounded the first two of Kepler’s three laws of planetary orbit: that planetary orbits are elliptical, with the sun at one focus of the ellipse, and that the orbital radius of each planet sweeps out equal areas in equal times. In April 1610 Kepler received a copy of the Siderius Nuncius (“Starry Messenger”) in which Galileo Galilei described the results of his recent telescopic observations. Kepler was astounded by these discoveries and wrote a letter to Galileo that he then published under the title Dissertatio cum Nuncio Siderio (“Conversation with the Starry Messenger”). Kepler’s continuing interest in optics led to another influential publication from this period, the 1611 treatise Diotrica (“Dioptrics”).
After the forced abdication and subsequent death of Rudolph II in 1612, Kepler was appointed mathematician to the states of Upper Austria in Linz, where he remained for the next 14 years. His 1615 Nova stereometria doliorum vinariorum (“New Means of Measuring Wine Casks”) was a significant contribution to pure and applied mathematics; it presents a means of calculating volumes by the use of techniques that became part of the infinitesimal calculus. The culmination of his harmonic approach to cosmology appeared in his 1619 Harmonices Mundi (“Harmonies of the World”) and stated his third law of planetary motion: the squares of the planets’ sidereal periods are proportional to the cubes of the semimajor axes of the ellipses of their orbits. In 1621 Kepler published a vastly revised edition of the Mysterium cosmographicum. His final major project was a collection of astronomical tables, the Tabulae Rudolphinae (“Rudolphine Tables”) of 1627, which remained the standard work on the subject for decades. Kepler left Linz to enter the service of the Czech Duke Albrecht von Wallenstein in 1628, relocating to Sagan in Silesia. His death in 1630 in the city of Regensburg was the result of a fever contracted while traveling to Linz to settle unresolved business matters.
Descartes was familiar with Kepler’s contributions to astronomy, geometry, and optics but made little explicit reference to him. One amusing exception to this rule is Descartes’s reference to the 1611 treatise De nive sexangula (“On the Six-Cornered Snowflake”), which he mentioned to Marin Mersenne in a letter from March 1630 in the course of commenting on the mild winter in Holland, which saw neither snow nor frost (AT, vol. I, p. 127). When Jean de Beaugrand accused Descartes of taking fundamental ideas in geometric optics from Kepler, he denied that he had seen anything of substance in his Dioptrica, but declared Kepler his “first master in optics” and his predecessor who had known the most about the subject (AT, vol. II, p. 86).
KNOWLEDGE. See CERTAINTY; INTUITION.