
He became a professor of mathematics at Graz, from 159398, and court mathematician to Holy Roman Emperor Rudolf II. In 1596 he wrote "Mysterium Cosmographicum", which led to exchanges with Galileo and Tycho Brahe. He became assistant to Brahe in 1599. Shortly afterwards, in 1601, Brahe suddenly died, and Kepler inherited both his master's position and his vast and very accurate collection of astronomical data on the motion of the planets. "Astronomia Nova" (1609) contained the first two of what we now call Kepler's laws; the third law appeared in 1619 in his "Harmonice Mundi". All three laws were the result of calculations based on Brahe's observations, which Kepler published in the Tabulae Rudolphinae (1627).
Halfway through the thirteenth century, after Greek manuscripts of Arabic science had been translated into Latin for study in European universities, knowledge of astronomy had spread throughout Europe. The Renaissance blossomed in the next two centuries, ushering in a new era in picturing the physical world, ending the dominance of ecclesiastical concerns. The reformation had challenged the authority of church hierarchy with "Sola Scriptura" (ie Only the Scriptures). In the atmosphere of this new intellectual freedom of thought, Copernicus came up with a simplified geometrical system of looking at the universe.
When the Renaissance and the Reformation were coming to an end in the years prior to 1600, Copernicus's work was read by a few astronomers who recognized the computational advantages of the Copernican system. However, they were not willing to take seriously its philosophical and physical implications.
Enter Johannes Kepler! Johannes Kepler (15711630), the German assistant and successor to Tycho Brahe, was a Copernican from his twenties on, and was destined to bring about acceptance of the heliocentric concept. That is, he believed the sun rather than the earth was the center of the planetary system.
The lifelong question that concerned Kepler was the nature of the timing and motion of the celestial machinery, for he was convinced that simple mathematical relations existed that could make sense of the planetary system. He saw the planetary system operating according to its own set of mathematical laws which was quite a radical idea for those times.
Kepler was a mathematician rather than an observer. Yet, Kepler was supplied with years of impeccable data by the elder Tache Brahe who had carefully marked the position of Mars in relationship to the rest of the celestial map. Kepler rejected many ideas, such as circular orbits, because they did not fit Brahe's observations. In 1609, Johannes Kepler finally published his first two laws of planetary motion in a book entitled New Astronomy. A decade later (1619), his third law was published in The Harmonies of the World.
Through these works, Kepler can be seen in many respects to mark the beginnings of what we call modern science. Kepler developed his empirical laws from Brahe's data on Mars: "By the study of the orbit of Mars," he said, "we must either arrive at the secrets of astronomy or forever remain in ignorance of them." However, in what proved to be a revolutionary step, Kepler then generalized saying that his laws applied to all the planets, including the Earth, without ever actually verifying that this was indeed true. Now we now know, they even apply to comets. Though Kepler may not have dreamed of such things, the generalization of his laws predict and explain the motion of satellites orbiting the earth. The expectation that the mathematical laws of science are universal is so readily accepted in our time that it is difficult to imagine just how important to science Kepler's actions were.
Kepler's work put to rest any notion that planets move in perfectly circular orbits because nature has decreed that the heavenly bodies must show perfection in their movements. He also put to rest in the scientific community an ancient idea that there exists a mystical complex motion of planets that somehow governs our ways. Although Kepler never knew why planets move by the empirical relationships articulated in his three laws, he diligently sought a cause of which these three laws were the effect. As he stated, "I am much occupied with the investigation of physical causes. My aim in this is to show that the celestial machine is ...... rather a clockwork..."; Kepler vaguely sensed that bodies have a natural "magnetic" affinity for each other and guessed that the Sun has an attractive force. However, it remained for Newton, half a century later, to formulate a unified theory of motion invoking gravity as the cause of planetary motion.
William L. Drennon 

