Complete Dictionary of Scientific Biography | 2008 COPYRIGHT 2008 Charles Scribner's Sons.


Kepler, Johannes

(b. Weil der Stadt, Germany, 27 December 1571; d. Regensburg, Germany, 15 November 1630)

astronomy, physics.

Although Kepler is remembered today chiefly for his three laws of planetary motion, these were but three elements in his much broader search for cosmic harmonies and a celestial physics. With the exception of Rheticus, Kepler became the first enthusiastic Copernican after Copernicus himself; he found an astronomy whose clumsy geocentric or heliostatic planetary mechanisms typically erred by several degrees and he left it with a unified and physically motivated heliocentric system nearly 100 times more accurate.

When Kepler was twenty-five and much occupied with astrology, he compared the members of his family with their horoscopes.1 His grandfather Sebald, mayor of Weil in 1571, when Kepler was born, was “quick-tempered and obstinate.” His grandmother was “clever, deceitful, blazing with hatred, the queen of busybodies.” His father, Heinrich, was described as “criminally inclined, quarrelsome, liable to a bad end” and destined for a “marriage fraught with strife.” When Kepler was three years old, his father joined a group of mercenary soldiers to fight the Protestant uprising in Holland, thereby disgracing his family. Soon after his return in 1576, he again joined the Belgian military service for a few years; and in 1588 he abandoned his family forever.

Although Kepler describes his mother, the former Katharina Guldenmann, as “thin, garrulous, and bad-tempered,” he adds that “treated shabbily, she could not overcome the inhumanity of her husband.” Katharina showed her impressionable son the great comet of 1577. Later, Kepler spent many months between 1617 and 1620 preparing a legal defense when his aged but meddlesome mother was accused of and tried for witchcraft.

Kepler first attended the German Schreibschule in Leonberg, where his family had moved in 1576; shortly after, he transferred to the Latin school, there laying the foundation for the complex Latin style displayed in his later writings. In 1584 he entered the Adelberg monastery school; and two years later enrolled at Maulbronn, one of the preparatory schools for the University of Tübingen. In October 1587 Kepler formally matriculated at Tübingen; but because no room was available at the Stift, the seminary where, as a scholarship student supported by the duke of Württemberg, he was expected to lodge, he continued at Maulbronn for another two years. On 25 September 1588 he passed the baccalaureate examination at Tübingen, although he did not actually take up residence there until the following year.

At Tübingen, Kepler’s thought was profoundly influenced by Michael Maestlin, the astronomy professor. Maestlin knew Copernican astronomy well; the 1543 De revolutionibus he owned is probably the most throughly annotated copy extant; he edited the 1571 edition of the Prutenicae tabulae, and he used them to compute his own Ephemerides. Although Maestlin was at best a very cautions Copernican, he planted the seed that with Kepler later blossomed into a full Copernicanism. The ground was fertile. Kepler’s quarterly grades at the university, still preserved, show him as a “straight A” student; and when he applied for a scholarship renewal at Tübingen, the senate noted that he had “such a superior and magnificent mind that something special may be expected of him.” Nevertheless, Kepler himself wrote concerning the science and mathematics of his university curriculum that “these were the prescribed studies, and nothing indicated to me a particular bent for astronomy.”2

On 11 August 1591 Kepler received his master’s degree from Tübingen and thereupon entered the theological course. Halfway through his third and last year, however, an event occurred that completely altered the direction of his life. Georgius Stadius, teacher of mathematics at the Lutheran school in Graz, died; and the local authorities asked Tübingen for a replacement. Kepler was chosen; and although he protested abandoning his intention to became a clergyman, he set out on the career destined to immortalize his name.

Graz and the Mysterium Cosmographicum . On 11 April 1594, the twenty-two-year-old Kepler arrived in southern Austria to take up his duties as teacher and as provincial mathematician. In the first year he had few pupils in mathematical astronomy and in the second year none, so he was asked to teach Vergil and rhetoric as well as arithmetic. But the young Kepler made his mark in another way; soon after coming to Graz, he issued a calendar and prognostication for 1595, which contained predictions of bitter cold, peasant uprisings, and invasions by the Turks. All were fulfilled, to the great enhancement of his local reputation. Five more calendars followed in annual succession, and later in Prague he issued prognostications for 1602 to 1606. These ephemeral items are now extremely rare, some surviving in unique copies; and all the copies of nearly half the editions are totally lost.

Kepler’s personal reaction to astrology was mixed. He rejected most of the commonly accepted rules, and he repeatedly referred to astrology as the foolish little daughter of respectable astronomy. In De fundamentis astrologiae certioribus (1601) he wrote: “If astrologers sometimes do tell the truth, it ought to be attributed to luck.”3 Nevertheless, his profound feeling for the harmony of the universe included a belief in a powerful concord between the cosmos and the individual. These views found their fullest development in the Harmonice mundi. Furthermore, his astrological opinions continually provided welcome supplementary justification for his office as imperial mathematician. At least 800 horoscopes are still preserved in his manuscript legacy. Included are many for himself; if we are to believe the deduced time of his conception (16 May 1571, at 4:37 A.M. on his parents’ wedding night), then he was a seven-month baby.

Concerning the calendars, Kepler later wrote: “Because astrology has no language other than that used by common man, so the common man will not understand otherwise, knowing nothing of the generalities of abstractions and seeing only the concrete, will often praise a calendar in an accidental case that the author never intended or blame it when the weather doesn’t come as he expects: so much trouble have I brought upon myself, that I finally have given up writing calendars.”4 Nevertheless, Kepler later produced a series from 1618 to 1624, excusing himself with the remark that when his salary was in arrears, writing calendars was better than begging.

Meanwhile, just over a year after his arrival in Graz, Kepler’s fertile imagination hit upon what he believed to be the secret key to the universe. His own account, here greatly abridged, appears in the introduction to the resulting work, the Mysterium cosmographicum of 1596.

When I was studying under the distinguished Michael Maestlin at Tübingen six years ago, seeing the many inconveniences of the commonly accepted theory of the universe, I became so delighted with Copernicus, whom Maestlin often mentioned in his lectures, that I often defended his opinions in the students’ debates about physics. I even wrote a painstaking disputation about the first motion, maintaining that it happens because of the rotation of the earth. I have by degrees—partly our of hearing Maestlin, partly by myself—collected all the advantages that Copernicus has over Ptolemy. At last in the year 1595 in Graz when I had an intermission in my lectures, I pondered on this subject with the whole energy of my mind. And there were three things above all for which I sought the causes as to why it was this way and not another—the number, the dimensions, and the motions of the orbs.5

After describing several false attempts, Kepler continues:

Almost the whole summer was lost with this agonizing labor. At last on a quite trifling occasion I came near the truth. I believe Divine Providence intervened so that by chance I found what I could never obtain by my own efforts. I believe this all the more because I have constantly prayed to God that I might succeed if what Copernicus had said was true. Thus it happened 19 July 1595, as I was showing in my class how the great conjunctions [of Saturn and Jupiter] occur successively eight zodiacal signs later, and how they gradually pass from one trine to another, that I inscribed within a circle many triangles, or quasi-triangles such that the end of one was the beginning of the next. In this manner a smaller circle was outlined by the points where the line of the triangles crossed each other [see Fig. 1].

The proportion between the circles struck Kepler’s eye as almost identical with that between Saturn and Jupiter, and he immediately initiated a vain search for similar geometrical relations.

And then again it struck me: why have plane figures among three-dimensional orbits? Behold, reader, the invention and whole substance of this little book! In memory of the event, I am writing down for you the sentence in the words from that moment of conception: The earth’s orbit is the measure of all things; circumscribe around it a dodecahedron, and the circle containing this will be Mars; circumscribe around Mars a tetrahedron, and the circle containing this will be Jupiter; circumscribe around Jupiter a cube, and the circle containing this will be Saturn. Now inscribe within the earth an icosahedron, and the circle contained in it will be Venus; inscribe within Venus an octahedron, and the circle contained in it will be Mercury. You now have the reason for the number of planets.

Kepler of course based his argument on the fact that there are five and only five regular polyhedrons.

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