Osianders Preface To Revolutions

It is asserted in the preface of Revolutions that the aim of the book is to advance a mathematical scheme to aid in the prediction of planetary motions and not to develop a theory of the planetary system as it actually exists in nature. Hypotheses such as that of the Earth's motion "need not be true nor even probable," but provide only "a reliable basis for computation." Although the purpose of the preface was apparently to forestall criticism of the philosophical or irreligious implications of the heliocentric system, it created some uncertainty concerning Copernicus's intentions. The preface was unsigned, and the reader might naturally assume that these were the words of Copernicus himself. In addition, some modern scholars, in particular those who incline to an instrumentalist view of the history of astronomy, have regarded the preface as a reasonable characterization of the contents of Revolutions. Thus Neugebauer (1968, 100) writes, "it is hard for me to imagine how a careful reader could reach a different conclusion."

It was later found that the preface had been written by Andreas Osiander (1498-1552), a Lutheran charged by Rheticus with the final stages of the printing of Revolutions. In fact, the preface is not at all in line with the organization, contents, and logic of the work. In the first section or book Copernicus presented several arguments to make the motion of Earth seem physically reasonable and thus to counter the traditional common-sense Aristotelian rationale for a stationary Earth. If it were simply a matter of developing an effective mathematical predictive scheme, such an excursion into the domain of natural philosophy would have been unnecessary. As we pointed out above, the technical modifications Copernicus made to traditional models were motivated by a desire to make them physically plausible and more than mere abstract calculational devices. Copernicus's introduction of the third motion of the Earth was motivated by his belief in a physical, material system of spheres, a belief that was grounded in a realistic conception of astronomy with ample medieval precedents.

The point at issue is connected to the origins of Copernicus's system and the place of this system in the history of cosmology. There are several explanations for how Copernicus was originally led to the idea of a moving Earth and a Sun-centered universe, most of them connected to the development of his system as a geometrical and mathematical theory. The first explanation was advanced by Copernicus himself and concerned the role of the equant in Ptolemaic astronomy. Copernicus stated that his purpose was to develop a system that used only motions that were circular and uniform about the center of the circle. Hence a primary concern of Revolutions is the elimination of the equant from planetary models. The difficulty with this explanation is that attempts to rid astronomy of the equant were already well established among Islamic astronomers, work with which Copernicus was familiar either directly or indirectly, and this astronomy was geocentric. Indeed, the decision to reject the equant was based primarily on considerations about how a rigid sphere would rotate, considerations that were independent of the choice of cosmology. Despite the prominence Copernicus gave to the question of the equant, it was of secondary importance in the actual decision to adopt a heliocentric system.

The most compelling motivation for heliocentric astronomy has already been discussed above. The Copernican universe is a natural one requiring no coincidences or special assumptions concerning the directions of the various radii of the system. To configure its dimensions as a cosmological system, there is no need to invoke a nesting principle and stipulate a particular planetary order. The distances of all planets are given in terms of the astronomical unit (the distance from the Earth to the Sun), and the order of planets is determined by the fairly natural condition that the angular motion of a planet decreases with its distance from the Sun. The special role of the Sun and the phenomenon of retrograde motion are simple consequences of the assumption that the Earth is a planet and, like the other planets, revolves about the Sun.

Whatever weight we assign to these different factors, there are two things about which we can be certain: the idea for the heliocentric system originated in considerations that were primarily astronomical and mathematical in character, and Copernicus believed that his system described the universe as it actually is, that is, his new system was a cosmology as well as a predictive scheme for saving the phenomena. The characterization of Osiander and such moderns as Neugebauer can only be regarded as incorrect. Given these facts, it was necessary to deal with the problem of developing and justifying an appropriate physics for a moving Earth. The first book of Revolutions is an attempt to ground a fact (the movement of the Earth) deduced from theoretical astronomy in the traditional domain of natural philosophy, that is, in Aristotelian physics as it had been elaborated by scholastic thinkers. Although many commentators have found this part of the book to be the least successful, Copernicus's goal at least indicated his primary commitment to the physical truth of the heliocentric system.

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