The Life and Times of Johannes Kepler

Johannes Kepler, was a German astronomer and natural philosopher, noted for formulating and verifying the three laws of planetary motion. These laws are now known as Kepler’s laws. Johannes Kepler was born in Weil der Stadt in Swabia, in southwest Germany. From 1574 to 1576 Johannes lived with his grandparents; in 1576 his parents moved to nearby Leonberg, where Johannes entered the Latin school. In 1584 he entered the Protestant seminary at Adelberg, and in 1589 he began his university education at the Protestant university of Tbingen. Here he studied theology and read widely.

He passed the M. A. examination in 1591 and continued his studies as a graduate student. There he was influenced by a mathematics professor, Michael Maestlin, an adherent of the heliocentric theory of planetary motion first developed by the Polish astronomer Nicolaus Copernicus. Kepler accepted Copernican theory immediately, believing that the simplicity of Copernican planetary ordering must have been God’s plan. In 1594 Kepler accepted an appointment as professor of mathematics at the Protestant seminary in Graz in the Austrian province of Styria.

He was also appointed district mathematician and calendar maker. For six years, Kepler taught, geometry, Virgil, arithmetic, and rhetoric. There he worked out a complex geometric hypothesis to account for distances between the planetary orbits-orbits that he mistakenly assumed were circular. Kepler then proposed that the sun emits a force that diminishes inversely with distance and pushes the planets around in their orbits. Kepler published his account in a thesis entitled Mysterium Cosmographicum (“Cosmographic Mystery”) in 1596.

This work is significant because it presented the first comprehensive and logical account of the geometrical advantages of Copernican theory. Kepler held the chair of astronomy and mathematics at Graz University from 1594 until 1600. Because of his talent as a mathematician, displayed in his thesis, Kepler was invited by Tycho Brahe to Prague to become his assistant and calculate new orbits for the planets from Tycho’s observations. Kepler moved to Prague in 1600. Kepler served as Tycho Brahe’s assistant until the Brahe’s death.

On the death of Brahe in 1601, Kepler assumed his position as imperial mathematician and court astronomer to Rudolf II, Holy Roman emperor. In 1609 his Astronomia Nova (“New Astronomy”) appeared, which contained his first two laws: planets move in elliptical orbits with the sun as one of the laws, and a planet sweeps out equal areas in equal times. This means that the closer a planet comes to the sun, the more rapidly it moves. Whereas other astronomers still followed the ancient precept that the study of the planets is a problem only in kinematics, Kepler took an openly active approach, introducing physics into the heavens.

In 1612 Kepler became mathematician to the states of Upper Austria. Between 1617 and 1621 Kepler published Epitome Astronomiae Copernicanae (“Epitome of Copernican Astronomy”), which became the most influential introduction to heliocentric astronomy; in 1619 he published Harmonice Mundi (“Harmony of the World”), in which he derived the heliocentric distances of the planets and their periods from considerations of musical harmony. In this work we find his third law, relating the periods of the planets to their mean orbital radii.

Equally important, it became the first textbook of astronomy to be based on Copernican principles, and for the next three decades it was a major influence in converting many astronomers to Keplerian-Copernicanism. The last major work to appear in Kepler’s lifetime was the Tabulae Rudolfinae (“Rudolfine Tables”). The new tables, based on Tycho Brahe’s accurate observations, were calculated according to Kepler’s elliptical astronomy. The English mathematician Sir Isaac Newton relied heavily on Kepler’s theories and observations in formulating his theory of gravitational force.

Kepler was a great author by many standards. Besides his astronomical books, he is credited with having written the first science fiction novel, his Somnium, published in 1634 and describing a voyage to the Moon. He wrote extensively on geometrical optics, and was the first to correctly sort out once and for all the production of real versus virtual images by mirrors and lenses. Kepler also made contributions in the field of optics and developed a system of infinitesimals in mathematics, which was a forerunner of calculus. Kepler died on November 15, 1630, in Regensburg.


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