The German astronomer Johannes Kepler (1571-1630) was one of the chief founders of modern astronomy because of his discovery of three basic laws underlying the motion of planets.

Johannes Kepler was born on Dec. 27, 1571, in the Swabian town of Weil. His father, Heinrich Kepler, was a mercenary; although a Protestant, he enlisted in the troops of the Duke of Alba fighting the Reformed insurgents in the Low Countries. Kepler's grandmother brought him up; for years he was a sickly child. At 13 he was accepted at a theological seminary at Adelberg.

Kepler wanted to become a theologian, and following his graduation from the University of Tübingen, as bachelor of arts in 1591, he enrolled in its theological faculty. But he was also interested in French literature and astronomy. His poor health and proclivity to morbidness singled him out no less than did his precocious advocacy of the doctrine of Copernicus.

It seems that the University of Tübingen gladly presented Kepler for the post of the "mathematician of the province" when request for a candidate came from Graz. He arrived there in April 1594 and set himself to work on one of his duties, the composition of the almanac, in which the main events of the coming year were to be duly predicted. His first almanac was a signal success. The occurrence of two not too unlikely events, an invasion by the Turks and a severe winter, which he had predicted, established his reputation.

Far more important for astronomy was the idea that seized Kepler on July 9, 1595. It appeared to him that the respective radii of the orbits of the planets corresponded to the lengths determined by a specific sequence in which the five regular solids were placed within one another, with a sphere separating each solid from the other. The sphere (orbit) of Saturn enveloped a cube which in turn enveloped another sphere, the orbit of Jupiter. This circumscribed a tetrahedron, a sphere (the orbit of Mars), a dodecahedron, a sphere (the orbit of earth), an icosahedron, a sphere (the orbit of Venus), an octahedron, and the smallest sphere (the orbit of Mercury). The idea was the main theme of his Mysterium cosmographicum (1596).

The next year Kepler married Barbara Muehleck, already twice widowed, "under an ominous sky," according to Kepler's own horoscope. Of their five children only one boy and one girl reached adulthood. It was with reluctance that Kepler, a convinced Copernican, first sought the job of assistant to Tycho Brahe, the astrologer-mathematician of Rudolph II in Prague. He took his new position in 1600. On the death of Tycho the following year, Kepler was appointed his successor.

His Three Laws

Kepler's immediate duty was to prepare for publication Tycho's collection of astronomical studies, Astronomiae instauratae progymnasmata (1601-1602). Kepler fell heir to Tycho's immensely valuable records. Their outstanding feature lay in the precision by which Tycho surpassed all astronomers before him in observing the position of stars and planets. Kepler tried to utilize Tycho's data in support of his own layout of the circular planetary orbits. The facts, that is, Tycho's observations, forced him to make one of the most revolutionary assumptions in the history of astronomy. A difference of 8 minutes of arc between his theory and Tycho's data could be explained only if the orbit of Mars was not circular but elliptical. In a generalized form this meant that the orbits of all planets were elliptical (Kepler's first law). On this basis a proper meaning could be given to another statement of his which he had already made in the same context. It is known as Kepler's second law, according to which the line joining the planet to the sun sweeps over equal areas in equal times in its elliptical orbit.

Kepler published these laws in his lengthy discussion of the orbit of the planet Mars, the Astronomia nova (1609). The two laws were clearly spelled out also in the book's detailed table of contents. Thus they must have struck the eyes of any careful reader sensitive to an astronomical novelty of such major proportion. Still, Galileo failed to take cognizance of them in his printed works, although he could have used them to great advantage to buttress his advocacy of the Copernican system.

The relations between Galileo and Kepler were rather strange. Although Galileo remained distinctly unappreciative of Kepler's achievements, the latter wrote a booklet to celebrate Galileo's Starry Messenger immediately upon its publication in 1610. On the other hand, Kepler argued rather vainly in his Conversation with the Starry Messenger (1610) that in his Astronomiae pars optica (1604), or Optics, which he presented as a commentary to Witelo's 13th-century work, one could find all the principles needed to construct a telescope.

In 1611 came Rudolph's abdication, and Kepler immediately looked for a new job. He obtained in Linz the post of provincial mathematician. By the time he moved to Linz in 1612 with his two children, his wife and his favorite son, Friedrich, were dead. Kepler's 14 years in Linz were marked, as far as his personal life was concerned, with his marriage in 1613 to Suzanna Reuttinger and by his repeated efforts to save his aged mother from being tried as a witch.

As for Kepler the scientist, he published two important works while he was in Linz. One was the Harmonice mundi (1618), in which his third law was announced. According to it the squares of the sidereal periods of any two planets are to each other as the cubes of their mean distances from the sun. The law was, however, derived not from celestial mechanics (Newton's Principia was still 6 decades away) but from Kepler's conviction that nature had to be patterned along quantitative relationships since God created it according to "weight, measure and number." Shortly after his first book appeared, he wrote in a letter: "Since God established everything in the universe along quantitative norms, he endowed man with a mind to comprehend them. For just as the eye is fitted for the perception of colors, the ear for sounds, so is man's mind created not for anything but for the grasping of quantities." In the Harmonice mundi he wrote merely a variation on the same theme as he spoke of geometry which "supplied God with a model for the creation of the world. Geometry was implanted into human nature along with God's image and not through man's visual perception and experience." The second work was the Epitome astronomiae Copernicanae, published in parts between 1618 and 1621. It was the first astronomical treatise in which the doctrine of circles really or hypothetically carrying the various planets was completely abandoned in favor of a physical explanation of planetary motions. It consisted in "magnetic arms" emanating from the sun.

Kepler was already in Ulm, the first stopover of the wanderings of the last 3 years of his life, when his Tabulae Rudolphinae (1628) was published. It not only added the carefully determined position of 223 stars to the 777 contained in Tycho's Astronomiae instauratae progymnasmata but also provided planetary tables which became the standard for the next century. Kepler died on Nov. 15, 1630. He was a unique embodiment of the transition from the old to the new spirit of science.

Further Reading

• The standard modern biography of Kepler was written by Max Caspar and was translated and edited by C. Doris Hellmann as Kepler (1959). The section on Kepler in Arthur Koestler's The Sleepwalkers (1959) is also available as a separate volume, The Watershed: A Biography of Johannes Kepler (1960). For a rigorous discussion of Kepler's astronomical theories see Alexander Koyré, The Astronomical Revolution: Copernicus, Kepler, Borelli (1961; trans. 1969).