Kepler, Johannes
1571–1630
German astronomer

German astronomer Johannes Kepler was a key figure in Renaissance science. He made groundbreaking discoveries in the fields of astronomy, mathematics, and optics (the study of the properties of light). One of his most important achievements was explaining the physical laws that govern the motions of the planets. His discovery provided support for Nicolaus Copernicus's theory that Earth and the other planets revolve around the Sun.

Early Career. Kepler was born to a Lutheran family in Weil der Stadt, a Catholic city in what is now Germany. In the mid-1570s his family moved to the Protestant duchy* of Württemberg, where Kepler began his education. A scholarship enabled Kepler to study at a Latin school, and from there he moved on to the University of Tübingen to prepare for the Lutheran ministry. He also studied mathematics and astronomy. One of his teachers introduced him to the theories of Copernicus, and Kepler became convinced that the Sun stood at the center of the universe.

After Kepler had spent three years at the university, school officials gave him a position teaching mathematics in Graz, Austria. In addition to teaching, Kepler served as district mathematician, a job that required him to produce astrological* calendars of coming events. In Graz, Kepler wrote his first book, Secret of the Universe (1596). It discussed the Copernican system and set forth many major ideas that Kepler pursued in his later work, such as the belief that simple mathematics could describe the nature of the universe.

Kepler's book established his reputation as an astronomer and prompted the Danish astronomer Tycho Brahe to invite Kepler to work with him. Kepler was eager to accept, as he hoped to use Tycho's excellent collection of astronomical data to support his theories about the structure of the solar system. In 1600 Kepler joined Tycho and other intellectuals at the court of Rudolf II in Prague, where Tycho served as court mathematician.

Kepler's years in Prague were the most productive of his life. During this period, he discovered the first two laws of planetary motion. The first describes the shapes of planetary orbits as ellipses, or ovals. The second describes the area traced out by a line drawn between the Sun and an orbiting planet. Kepler showed that this area is always equal for a given period of time. He also began to study optics. After learning about Galileo Galilei's pioneering work with telescopes, Kepler wrote two books about the use of lenses in telescopes. In addition, he wrote about astrology, trying to reform the subject in ways that would make it fit in with his new discoveries about the physics of space.

Later Career. Kepler left Prague in 1612 and found work as district mathematician in the city of Linz in Austria. Trouble arose when his mother was accused of being a witch, but Kepler used his contacts to protect her from torture during her arrest, and she was eventually released.

Despite his problems, Kepler continued to make useful discoveries. In 1615 he conducted a study of the quantity of wine that a barrel could hold. This work later proved useful in the development of calculus, a form of advanced mathematics. In 1619 Kepler published Harmony of the World, which he saw as the climax of his studies. It contained the third law of planetary motion, which laid out the relationship between planets' average distance from the Sun and the time it takes them to orbit the Sun. Around this time Kepler also wrote a textbook on Copernican astronomy.

After a brief stay in the duchy of Sagan, Bohemia, Kepler died while on his way to collect some money owed to him for his astronomical work. Four years after his death, his final work was published. Called Dream, it was a forerunner of modern science fiction, describing how the solar system would look to a traveler to the Moon.

(See alsoScience. )

* duchy

territory ruled by a duke or duchess

* astrological

relating to astrology, the study of the supposed influences of the stars and planets on earthly events