Kepler and the Laws of Nature

Article excerpt

In 1609, the same year in which Galileo and others began to use the telescope for astronomical purposes, Johannes Kepler published his Commentary on the Motions of Mars, a book today generally cited by its short title, Astronomia nova. But that abbreviated title conceals its real challenge to the Aristotelian order of things. Kepler's work was truly the "new astronomy," but the title goes on, "based on causes, or celestial physics," and it was the introduction of physics into astronomy that was Kepler's most fundamental contribution.

Aristotle's De coelo, "On the heavens," which dealt with the geometrical motions in the heavens, was the province of astronomy professors. However, it was his Metaphysics that concerned the fundamental reasons for the motions--Aristotle implied that it was the love of God that made the spheres go round (1)-and Metaphysics was the property of the philosophy professors. Kepler unified this dichotomy, demanding physically coherent explanations as to why planets sometimes went faster than at other times. He realized that when Mars was closest to the sun, it went fastest in its orbit. It seemed to him unreasonable that the earth, on the contrary, would always travel at the same speed regardless of its distance from the sun. And when he got that straightened out, he single-handedly improved the accuracy of predicted positions by an order of magnitude. You may have thought that finding the elliptical shape of Mars' orbit made the major leap forward in accuracy. Wrong! It was getting the earth's orbit positioned correctly. His teacher Michael Maestlin criticized him for mixing up physics and astronomy, (2) but it was this insight that drove Kepler to his major break throughs. And that approach laid the essential framework for Rene Descartes and Isaac Newton.

Kepler's celestial physics pointed the way to a lawful universe that could be understood in terms of underlying physical principles. Kepler is rightly famous for his three laws of planetary motion, but he never called them laws; they were not specially singled out and ordered as a group of three until 1774 in J.-J. Lalande's Abrege d'astronomie, something probably conceived by the French astronomer himself. Nor did Kepler use the expression "laws of nature," and neither, for that matter, did Galileo. In fact, "laws of nature" in the modern sense did not come about until the philosophical inquiry starting from first principles as elaborated by Descartes. Let me first situate the origins of Kepler's laws within the larger framework of his discoveries and his cosmology, and then reflect on the construction of the modern concept of laws of nature.

Kepler's Discoveries and His Cosmology

In October of 1600 the young Kepler, who had lost his job as a high school teacher due to the Counter-Reformation, arrived in Prague from the Austrian provinces to work as an apprentice to Tycho Brahe, the greatest observational astronomer the world had yet known. Kepler's working notebook, which still survives, seems to show that he had not got off to a good start. The opening page of triangles and numbers is crossed off. (3) No doubt Christian Longomontanus, the senior staff assistant, looked over Kepler's shoulder and remarked, "Young man, we have a much easier way to do that here!" Sure enough, on the second page the problem is attacked using precepts from Tycho's manuscript handbook of trigonometric rules.

Nevertheless, Kepler had not come to his new post totally unprepared. Kepler owned a secondhand copy of Copernicus' major work, the De revolutionibus, and at the university in Tubingen he had sat with his mentor, Michael Maestlin, and together they examined a previously highlighted section of the book. (4) It was the chapter in which Copernicus inquired as to what was the center of the universe, the sun itself or the center of the earth's orbit (which were two different points because of the earth's eccentrically positioned circle). …