Johannes Kepler was the born into a poor German family. His father worked as a taverner and later became a mercenary soldier, apparently dying in battle when Johannes was a boy. Kepler himself was a sickly child and fell victim to small pox. His sight was severely affected as a result. His family were Lutherans and Kepler originally went to school hoping to become a minister. He studied both theology and mathematics at University of Tubingen. He was encouraged to gravitate toward the sciences and became particularly skilled in math and astronomy. He was tutored under the foremost authority of astronomy at that time. Although he seemed disappointed by the roadblocks to his entering the ministry, he came to see that his work in astronomy glorified God as he discovered the planetary order ordained in the universe. Kepler was a deeply religious man and believed that the God who created the universe made it work in beauty and harmony, with mathematical precision.
Kepler found himself in tension with the religious and scientific worldviews of his time. He disagreed theologically with the Catholics and scientifically with the Lutherans. Lutheranism was not popular among the predominantly Catholic areas. This forced him at times to choose between conversion to Catholicism during the Counter-Revolution or to flee to territory more favourable to Protestantism. However, because Kepler was committed to a Copernican system in which the Earth and planets traveled around the sun (heliocentric view), he was shunned by the Lutheran establishments which mocked such a view for they held to the ancient Ptolemaic geocentric view: that is, the sun and planets travel around the Earth. The Greeks had believed that the Earth was the fixed center of the universe and later Christians concurred. The latter found it reasonable to believe that if God had made mankind as His special creation and in His image that the Earth would occupy the central place in the universal scheme. Since Tubingen was a Lutheran center, it was not surprising that Kepler would have to seek work in territory that was more sympathetic or indifferent to his unorthodox Copernican views.
Kepler moved to the University of Graz (Austria) where he taught mathematics. While there, he wrote the Mysterium Cosmographicum (1596) which gives voice to his support of the Copernican system. However, as anti-Lutheran pressure increased from Catholic rulers, Kepler was forced to move on. The university itself was shut down so he went to Prague to work with Tycho Brahe, the Imperial Mathematician. Although this was an unwelcome move, it provided Kepler with his big break. Brahe had kept meticulous records on planetary motion for years. He was very secretive about these records. He assigned Kepler to work on the orbit of Mars, which actually proved to be a good assignment for Mars’ orbit is quite amenable to proving its elliptical path. In less than a year, Tycho died and Kepler took his records to continue the astronomical puzzle. Kepler also inherited Tycho’s position as Imperial Mathematician. Tycho had hoped that Kepler would vindicate his own belief that the sun moved around the Earth, and the other planets circled the sun. This was a hybrid of the Ptolemaic and Copernican systems, but was not a terribly satisfying solution to the problem.
While studying the orbits, Kepler tried out various theories of his own to explain the data. One perennial question was why planets do not travel in uniform speed in their orbit. Another centered on why the absolute distances did not correlate well with the time it took them to complete one orbit. In other words, there wasn’t a clean formula that said “Double the orbital distance and double the time required to traverse it.” Kepler worked with variations on the circular and epicyclical theories that were in vogue at the time. They were particularly complicated but not especially accurate. After several years of work on the problem, Kepler discovered the elliptical nature of Mars’ orbit. He published his findings in 1609 in a work entitled Astronomia Nova (The New Astronomy). In it, Kepler’s outlines the first two laws of planetary motion. A third law was discovered and added later to the earlier findings. Kepler’s laws state that:
- Planets follow orbits that are elliptical, not circular. The sun is at one focus of the ellipse.
- A line drawn between a planet and the sun, will sweep out equal areas in equal amount of time. This means that planets move faster as they get closer to the sun. (The time:vector area is a constant.) Kepler’s initial understanding was couched in terms of an animated force within the system; later he came to reconceptualize it in terms of a mechanical force which drove the planets with greater propulsion when they were closer to the sun and with a “quasi-exhausted” force when distant.
- There is a fixed ratio between time required to complete one orbit and the average distance from the sun. This is time squared: average distance cubed. This law was published in The Harmony of the World (1619). Knowing orbital time thereby allowed for calculation of stellar location.
Although Kepler is best known for his work in establishing the veracity of the Copernican system of astronomy, he was also an important figure in discovering the telescope, in understanding the imaging process of the eye, in developing logarithms, and discovering new mathematical geometric forms.
Johannes Kepler died in Regensburg in 1630. He was going to collect a debt owed him by the imperial treasury for his work. Nothing remains of his burial site which was destroyed shortly thereafter during the Thirty Years War.