The universal theory of gravitation

The Cartesian vortex theory

When it comes to our Solar System, pretty much the whole of modern-day predictive astronomy is based on the law of universal gravitation, introduced by Isaac Newton. Newton's work was the crowning achievement of a century of investigations into the subject of mechanics, which began with the work of Galileo. A detailed history of the development of dynamical ideas in the seventeenth century is beyond the scope of this book, but we will discuss briefly the main ideas and attitudes that shaped astronomical thought.

By the mid seventeenth century, it was clear that the Ptolemaic universe, with the Moon, Sun and planets orbiting a stationary Earth, did not correspond to reality. Evidence suggested that the Earth was of a similar nature to the Moon and the other planets, and Aristotelian explanations of planetary phenomena in terms of natural motions were no longer acceptable. What was now required was an explanation in terms of terrestrial physics, a subject that, with Aristotle gone, was wide open. One such attempt was Kepler's, and his ultimate conclusions in the form of his laws of planetary motion were of immense significance, but little attention was paid to the precise nature of his physics. Another bold attempt was that of Rene Descartes, whose vortex theory was published in 1644.

Whereas Kepler had been concerned with the technical details of planetary astronomy, Descartes was interested in cosmology in a much broader sense. Kepler was led to his discoveries by a study of the small irregularities in planetary motions, but Descartes was led to his vortex theory by a consideration of the structure of the Universe as a whole. He accepted the Copernican system, since it explained the phenomena with the fewest assumptions, though he realized

1 For a thorough discussion of this topic, see Westfall (1971).

that the same phenomena could be explained by a geoheliocentric universe. As a predictive tool, the vortex theory was way behind Keplerian astronomy, but it was immensely influential, particularly in The Netherlands and France, and played a key role in the subsequent development of ideas about the mechanics of the heavens.

The nature of the scientific method was a keen subject of debate in the early seventeenth century. Galileo had set the ball rolling (so to speak) with his insistence that mathematical theory was backed up by experiment, and in 1620, Francis Bacon, one of the most distinguished philosophers of science during the Renaissance, published the New Organon in which he espoused a

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