Akademik

Employed as an astrologer by Habsburg emperors, Johannes Kepler is best remembered for his discovery of the elliptical orbits of the planets. He was relentless in pursuit of universal harmonies, and his magical conception of the cosmos led him to reject both the Ptolemaic and Copernican systems. Through observation, mathematics, and reason, he discovered those harmonies known to posterity as Kepler's laws of planetary motion.

Kepler was born in the predominantly Catholic town of Weil der Stadt in Germany to Lutheran parents. He attended local elementary schools before a religious education at the Lutheran seminary in Maulbronn and the University of Tübingen. After taking a master's degree, Kepler continued his studies, guided by the astronomer Michael Maestlin, who taught the heliocentric ideas of Nicolaus Copernicus.* Suspected of Calvinist leanings, he halted his theo­logical training in favor of astronomy.

In 1594 Kepler set out to teach mathematics at the Lutheran grammar school in Graz, in the Habsburg duchy of Styria, a land increasingly subject to pressures of the Counter-Reformation. His reputation protected him from serious threat, and in Graz he was able to compose his first great theoretical work, The Cosmographic Mystery (1596), which brought the attention of Tycho Brahe,* im­perial astronomer of Rudolf II.* By 1600 pressures forced him from Styria and into the employ of Brahe in Prague. Within a year Brahe was dead, and Kepler had inherited both his post and his compilation of astronomical observations.

This inheritance proved essential to Kepler. Working with Brahe's superior data, Kepler set about solving problems found within the Copernican system. Often distracted by calendar making and astrology, by 1609 Kepler outlined his first two laws of planetary motion in The New Astronomy. A year later his Reply to the Starry Messenger supported Galileo's* use of the telescope and explained the optical principles behind it.

By 1612, however, when Catholic reform came to Bohemia, Kepler moved to Linz in Upper Austria, spending fourteen years as provincial mathematician and remaining in Habsburg employ. Astronomical works followed: The Epitome of Copernican Astronomy (1618), a definitive textbook; The World Harmony (1619), which introduced Kepler's third planetary law; and The Rudolfine Tables (1627), a mathematical description of the cosmos based on Brahe's observations and Kepler's chief task as imperial astrologer.

Forced by the Thirty Years' War to leave Linz in 1626, Kepler continued the last years of his unsettled life attached to Albert von Wallenstein, moving to the duchy of Sagan and feeling an unwelcome outsider. While traveling to secure payment for The Rudolfine Tables, he fell ill and died in Regensburg in 1630.

Throughout his life Kepler contributed to fields of knowledge other than as­tronomy; optics was a particular love. His Dioptrics described comprehensively the behavior of light passing through lenses, and The Optical Part of Astronomy applied his findings. Additional works came on the human eye and the theory of regular solids. He completed an important early work on logarithmic analysis and infinitesimal calculus. The Six-sided Snowflake and The Dream are small masterpieces of crystallography and science fiction.

Brought into the world among religious and financial uncertainties, Kepler never entirely escaped. His entire life was marked by suspicion from religious partisans, precarious postings, a near-constant pursuit of funds owed to him, and diverse insecurities, including his mother's prosecution for witchcraft around 1620. Despite these realities, Kepler built a reputation as a brilliant astronomer who reshaped generations' views of the universe.

Kepler's chief significance lies in his astronomical theories, a curious blend of the mystical and the rational. This style of thought can best be seen in his Cosmographic Mystery, rejected by modern scientists in every detail, but clearly inspiring brilliant later discoveries. Convinced that God had not created a chaotic universe, Kepler suggested that distances among the then-known planets dis­played ratios corresponding to the five perfect solids known to the ancients. The idea is difficult to describe, but elegant in concept: the orbit of each planet is the circumference of a circle formed by each of the five polyhedrons and a sphere extending, in a certain order, outward from the sun. Kepler depicted Mercury within an octahedron (eight equilateral triangles), Venus within an ico-sahedron (twenty equilateral triangles), earth within a dodecahedron (twelve pen­tagons), Mars within a tetrahedron (four equilateral triangles), Jupiter within a cube (six squares), and Saturn within a sphere encompassing the entire system.

Although ultimately rejected by Kepler himself (despite its remarkably small margin of error), the theory represents the painstaking pursuit of universal har­monies that led him to less fanciful mathematical modeling. The results were his three laws of planetary motion outlined in The New Astronomy and The World Harmony. The first law states simply that a planetary orbit is an ellipse with the sun as one of the foci. The second stipulates that the radius of a planetary orbit, drawn from the sun, sweeps equal areas in equal periods of time.

Thus, as a planet speeds up approaching the sun, the area swept by its orbital radius, in a given interval of time, is equal to the area swept by the radius as the planet slows down departing the sun, for the same length of time. The third law outlines the relation between a planet's mean distance from the sun and the time required for it to complete one orbit. Sometimes referred to as the 3/2 ratio, the law stipulates that the cube of the distance is proportional to the square of the time.

Kepler's science was a natural theology, the relentless discovery of the mind of God in the cosmos. Shunned by some, most notably Galileo, for its mystical qualities, his work nonetheless clearly directed the course of astronomy and physics for years to come. Without Kepler, Newton's explication of the laws of gravity and motion would be hardly imaginable. Dubbed an astronomer's as­tronomer by one biographer because of his mathematical precision, Kepler was surely the first modern scientist to illustrate the "laws of nature."

J. B. Brackenridge, The Key to Newton's Dynamics, 1995.

M. Caspar, Kepler, 1993.

O. Gingerich, The Eye of Heaven, 1993.

J. R. Voelkel, Johannes Kepler and the New Astronomy, 1999.