Kepler's Physical Astronomy, and: The Music of the Heavens: Kepler's Harmonic Astronomy (review)

Reviews 243 Stephenson, Bruce, Kepler's Physical Astronomy, Princeton, Princeton University Press, rpt. 1994; paper; viii, 216, 50 figures; R.R.P. US$16.95, £12.95. Stephenson, Bruce, The Music of the Heavens: Kepler's Harmonic Astronomy, Princeton, Princeton University Press, 1994; cloth; pp. xii, 260; R.R.P. US$42.50, £30.00. A m o n g the major figures of the Scientific Revolution, no one was as prolix or complex as Johannes Kepler. Much of his voluminous writing has remained obscure because of the great difficulty modern scholars encounter in respect to both his language and his thought. It is the good fortune of historians of science that Bruce Stephenson now can serve as an expert guide through some of the densest parts of Kepler's cosmology and astronomy. Stephenson wrote Kepler's Physical Astronomy as his doctoral dissertation andfirstpublished it in 1987. Its successful reception led to this paperback edition. There seem to be no changes made, certainly none of any substance, in the new printing. The starting point of the book is the view that Kepler was the 'last great practitioner of the science of ancient astronomy' (p. 1). As such he served both as its culmination and its .termination. Kepler's work was the crowning achievement of ancient astronomy, among whose practitioners the author includes several of Kepler's recent predecessors such as Regimontanus and Copernicus himself. That discipline used geometrical models to predict the motion of the heavenly bodies w e now call the solar system, and Kepler was very much within that tradition when he formulated the laws of planetary motion, the achievement for which he gained permanent fame. Yet he also acted to undermine ancient astronomy by being thefirstto see astronomy as being part of physics, which required a physical explanation for the motion of the planets. Stephensonfirstexamines Kepler's Mysterium Cosmographicum (1596), which he finds to be still in the geometrical traditional of ancient astronomy, despite its advocacy of the heliocentric theory. Stephenson sees Copernicus, for all of his importance in the Scientific Revolution, as belonging to that tradition, because his support of a sun-centred universe was based on geometry, not physics. The author devotes most of his book, in an overlylong chapter of 116 pages, to Kepler's Astronomia Nova (1609). Stephenson argues that the astronomer broke new ground in it, not so much for its presentation of hisfirsttwo laws of planetary motion, the work's usual cause for mention in history of science textbooks, but for the attempt to fashion a 244 Reviews physical theory for the planets. Stephenson persuasively argues that Kepler was far more systematic and organised in Astronomia Nova than previous historians of science have seen him, such as Arthur Koestler, who used the term 'sleepwalking' to describe Kepler's mode of thought. Stephenson painstakingly follows the order of the book to lay out Kepler's goals: 'to demonstrate that the models used by all the astronomers were wrong, and that they could not be salvaged in any reasonable manner'; second, 'to develop his new astronomy, and show it to be accurate to the limits of the Tychonic observations';finally,'to argue that... his theories were much more likely to represent what was actually taking place in the heavens' (p. 22). The 'conquest of Mars', successfully fitting into his theory the motions of Mars, which had caused so much trouble for ancient astronomy, allowed Kepler to claim success. Stephenson turns more briefly to Kepler's Epitome Astronomiae Copernicanae (1621), in which Kepler argued more explicitly for a 'dynamic physical astronomy centred on the sun' (p. 139). The physical force that produced the motion of the earth and the other planets around the sun was a type of planetaryfilament,similar to but not identical with magnetism. In this way Kepler anticipated Newton's law of universal gravitation in sensing that both mass and distance were factors in the strength of that force. In his first book Stephenson largely avoided any discussion of the more esoteric aspects of Kepler's work, which make him so difficult for m o d e m scholars to understand. In The Music of the Heavens, Stephenson tackles Kepler at...