コペルニクスの太陽中心説の動機
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This paper discusses how Copernicus conceived heliocentrism. The most relevant statement by Copernicus himself to this theme is found in his dedication to Pope Paul III he made at the commencement of the De Revolutionibus. According to that, he was impelled to consider a different system of deducing the motions of the universe's spheres, because he realized that astronomers did not agree among themselves in their investigations of this subject. He continued to say that they were, in the first place, unable to establish and observe a constant length of the tropical year. This issue closely related to Church Calender reform finally induced Copernicus to conceive the earth's daily rotation on its axis, as was clearly proved in my paper entitled The Precession of the Equinoxes and the Genesis of the Copernican System last year. Next, he expressed his dissatisfaction with geometrical devices designed to construct the motions of the universe's spheres, namely homocentrics, eccentrics and epicycles. As for the first device, Copernicus pointed out that it could not give any incontrovertible results in absolute agreement with the heavenly phenomena. In the meantime, he admitted that the second one seemed in large measure to have solved the problem of the apparent motions with appropriate calculations. But he vehemently attacked this device, saying that those who employed eccentrics were found either to have omitted something essential or to have admitted something extraneous and wholly irrelevant. Clearly he meant to say that the theory of eccentrics violated the principle of uniform motion which had been observed as most fundamental in astronomy by the ancient Greek mathematicians and philosophers, and that Ptolemy in particular was wrong in introducing a wholly irrelevant device of "equant". Copernicus said nothing about the theory of epicycles in the dedication, but I suppose that he assumed a negative attitude toward it as well, since Ptolemy had proved eccentrics and epicycles to be mathematically equivalent. As a result, they were genuinely combined by the Alexandrian astronomer to work as a single unit, and this sophisticated mechanism has been called "eccentrepicycles." Therefore, the criticism of eccentrics must lead to that of epicycles. Now, what should perplex historians in search of a Copernican motive of heliocentrism might be the fact that Copernicus employed eccentrepicycles in order to reconstruct the motions of the heavenly spheres from a heliocentric point of view. Contrary to my expectations, most of the historians do not seem to be terribly worried about this Copernican inconsistency, but I have been unable to disregard it. Moreover, I have had difficulty guessing why Copernicus expressly mentioned the Eudoxan theory of homocentrics in the dedication, although the device was thought in his time to have been totally obsolete, once eccentrics or epicycles expelled it from astronomy in the second century B.c. As far as I am concerned, a keyword to fix these difficulties can be found in "Averroes". This is the Latin name of Ibn Rushd, a famous Arabian philosopher who lived in Spain in the twelfth century. He was known as the greatest commentator on Aristotle in the West into the Renaissance. As did other Spanish Muslims, he found eccentrics and epicycles to be in conflict with Aristotle's natural philosophy and searched for an alternative to the Ptolemaic system. In this case they adopted and tried to improve the obsolete system of Eudoxan homocentrics on which Aristotle himself had based an influential cosmology. However, all their attempts at rebellion against the Ptolemaic system failed, because it was absolutely impossible for anyone to explain by means of homocentrics the fact that not only the sun and moon but also five phanets approach closer to the earth and recede farther from it, provided you position the terrestrial globe at the center of the homocentric system. But no Aristotelians were allowed to remove the earth from the middle of the universe, since geocentrism was one of the indispensable suppositions of the Aristotelian natural philosophy. I think that Copernicus was well informed of the dilemma, between natural philosophy and scientific explanation of phenomena, into which stuck the Spanish-Arabian philosophers and astronomers in the twelfth century. Among them was Alpetragius the most noted astronomer in Islamic Spain whose name Copernicus mentioned in Chapter X of the De Revolutionibus. In addition, Rheticus, the only disciple of Copernicus, made the following comment upon Averroes's criticism of the Ptolemaic system in the Narratio Prima which is said to have been composed under the watchful eyes of Copernicus: Averroes, who was in other respects a philosopher of the first rank, concluded that epicycles and eccentrics could not possibly exist in the realm of nature and that Ptolemy did not know why the ancients had posited motions of rotation. His final judgement is: "The Ptolemaic astronomy is nothing, so far as existence is concerned; but it is convenient for computing the nonexistent." Now, if Copernicus refuted Ptolemy for his violation of the principle of uniform motion, then it means that he temporarily adopted homocentrics and inevitably was stuck in the same dilemma as Averroes and Alpetragius. Nevertheless, he was able to free himself of the Aristotelian binding, when he rotated the earth on its axis, induced by the purely technical problem of the precession of the equinoxes. So we can easily assume that Copernicus wriggled free from the dilemma mentioned above by positioning the sun at the center of the universe and making the earth revolve around it. In this great endeavor Copernicus might have been suggested by Philolaus and other Phthagoreans who had already proposed to move the earth, in annual revolution or in daily rotation, at the very beginning of the history of Greek cosmology. I believe I was able to deduce, in my paper The precession of the Equinoxes and the Genesis of the Copernican System Last year, the reason why Copernicus conceived the daily rotation of the earth on its axis, while I suppose I could make clear,. in this paper, how Copernicus was impelled to reconstruct astronomy from a heliocentric point of view. So at first glance, the genesis of the Copernican system appears to have two different sources, but from the viewpoint of scientific method it has the unique source, namely the principle of simplicity of nature, of which Copernicus was fully aware, as was shown in Chapter X of the De Revolutionibus as follows: 'Just as it (the wisdom of nature or the simplicity of nature) especially avoids producing anything superfluous or useless, so it frequently prefers to endow ,a single thing with many effects."
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