Fragmentary Notes on Aristotle s On the Heavens Aristotle had considered and rejected both the proposition that the Earth revolves about some center of the Universe (he mentions the Pythagorean central fire ) and that the Earth rotates about its axis, which he interprets Timaeus as holding. i At the beginning of On the Heavens II, 14, he offers three arguments against these propositions. The first argument, which seems to be against both, is that, since the natural motion of the element earth is towards the center of the universe, circular motion would be unnatural ( constrained ), and therefore could not be eternal. ii The second argument amounts to the assertion that the Earth s revolution around a center would result in stellar parallax, which is not observed. iii (This is an interesting case in which Aristotle, while reasoning soundly, gets the empirical facts wrong because of the imprecision of his available observational methods.) The third argument, which has only to do with the Earth s rotation, is that projectiles thrown straight up come straight down again, whereas if the Earth rotated, they would undergo a lateral displacement. iv Galileo s Dialogue Concerning the Two Chief World Systems (1632) argues in favor of the Copernican and against the Aristotelian-Ptolemaic system. It was this work that prompted his trial and conviction. v Dialogue has four parts, which are called Days. The Second Day is devoted largely to rebutting Aristotelian arguments, both those of Aristotle himself and those of his followers, against the revolution and rotation of the Earth. Among the arguments of Aristotle himself, it is the third to which Salviati, the interlocutor who represents Galileo s viewpoint, devotes the most attention. vi Various forms of this argument are considered, including assertions not only that a projectile thrown straight upwards, but also that a heavy object dropped from the mast of a
stationary ship or from a tower, would experience a lateral displacement if the Earth rotated. To all these cases Salviati s basic line of argumentation is the same: the Aristotelian argument is question-begging, because it assumes that a piece of earth has a natural motion towards the enter of the universe, which is also the center of the Earth. On the contrary, says Salviati, all bodies near the surface of the Earth are carried around with the Earth s rotary motion, and therefore, in appearing to fall straight down towards the base of the mast or the tower, they are merely being carried around together with the mast or tower in the Earth s rotation. vii One might think that Salviati should have replied to Aristotle by appealing to the law of inertia, or Newton s First Law, arguing that locally the surface of the Earth is a Galilean frame (i. e., a reference frame with respect to which the law of inertia is fulfilled), and that, throughout the event of throwing a projectile straight up or dropping it, its lateral motion will remain unchanged due to inertia. Yet it is hard to find such a bald statement in the text of the Dialogue, and in his Forward to Drake s translation Albert Einstein says that the Dialogue does not evince a full-blown theory of inertia. viii... A third concept of mechanism is based on the distinction between material and psychic powers. Aristotle had posited certain powers material ones that act of necessity. In Physics II, 9, Aristotle inquires about the role of necessity in natural things. He concludes that there is absolute necessity only in matter that to the extent that things act for an end, there is only the conditional necessity of requisite preconditions. ix
In On the Heavens Aristotle tentatively solves a certain abstruse puzzle that results from his geocentric cosmology. All the stars undergo the same motion, that of the Heaven, says he, and each of the planets undergoes a motion different from the stars and also from each others. The longer period planetary motions, we ought to conclude, are undergone by planets closest to the stars. And since the stars undergo but a single motion (the diurnal rotation of the Heaven), the simplicities of the planetary motions ought to be proportional to their proximity to the Heaven. We see, however, something else namely that the motions of the Sun and Moon are simpler than those of some of the planets that are closer to the Heaven. Aristotle s suggested solution is that we ought to regard the stars and planets, not merely as bodies, but as living beings, possessed of goods and of action for the sake of those goods. And just as among terrestrial animals there is not a strict proportion of the simplicity of action to the nobility of good, so among the stars and planets. x Kepler understands Aristotle as holding that the planets are affixed to spheres or orbs, and that for each orb there is not only an incorporeal (and thus immovable) intelligence that is capable, by means of its infinite power, of moving the sphere for an infinite time, but also a soul that both is moved by the intelligence and moves the sphere directly. xi In Astronomia Nova, and again in Epitome of Copernican Astronomy, Kepler asserts that Tycho Brahe has disproved the physical reality of the orbs. xii In Astronomia Nova he then goes on:...if there are no orbs, the conditions under which the intelligences and moving souls must operate are made very difficult, since they have to attend to so many
things to introduce to the planet two intermingled motions. They would at least have to attend at one and the same time to the principles, centers, and periods of the two motions. But if the Earth is moved [i. e., if Copernicanism is true], I show that most of this can be done with physical rather than animate faculties, namely magnetic ones. xiii He makes similar points in the Epitome. xiv i On the Heavens, Book II, Chapters 13 and 14. In surveying the opinions of his predecessors, Aristotle discusses the central fire at 293a17-293b16 and Timaeus at 293b32. The passage of Timaeus in question is Stephanus 40b. ii 296a24-34. iii 296a34-b6. Stellar parallax is the change in the apparent relative positions of the stars that follows change in the Earth s orbital position. iv 296b7-24. v Galileo, Dialogue, Translator s Preface, pp. xxiii-xxv. vi Salviati credits the parallax argument with great ingenuity and force, but argues later in the Dialogue that because of the great distance of the stars, their parallax is very difficult to detect (pp. 364ff.). vii Dialogue, Second Day, pp. 138ff. viii Page xi. ix 199b33-200b7. x On the Heavens, Book II, Chapter 12; 291b24-292b25.
xi Epitome of Copernican Astronomy, Book IV, Part II, Chapter 2; pp. 50-51. xii Astronomia Nova, pp. 8-9; Epitome of Copernican Astronomy, pp. 16-17. xiii Astronomia Nova, p. 9. xiv Epitome of Copernican Astronomy, pp. 52-3.