Lecture 15 — Swiftness and slowness in the motion of the planets proportionate to their distance from the first sphere and the earth

314 With their order—I mean the position of each, as involving the priority of some and the posteriority of others, and their respective distances from the extremity—with this astronomy may be left to deal, since the astronomical discussion is adequate.

315 This discussion shows that the movements of the several stars depend, as regards the varieties of speed which they exhibit, on the distance of each from the extremity. It is established that the outermost revolution of the heavens is a simple movement and the swiftest of all, and that the movement of all other bodies is composite and relatively slow, for the reason that each is moving on its own circle with the reverse motion to that of the heavens. This at once leads us to expect that the body which is nearest to that first simple revolution should take the longest time to complete its circle, and that which is farthest from it the shortest, the others taking a longer time the nearer they are and a shorter time the farther away they are. For it is the nearest body which is most strongly influenced, and the most remote, by reason of its distance, which is least affected, the influence on the intermediate bodies varying, as the mathematicians show, with their distance.

431. After deciding about the nature and motion of the stars, the Philosopher here determines their order and position, and especially with respect to the planets—for with respect to the fixed stars it is plain that all are situated in the outermost sphere. Concerning this he does two things:

first he shows that in this matter the natural philosopher should take the mathematician's suppositions;

secondly, he shows what pertains strictly to the natural philosopher to consider in this matter, at this discussion shows (291a32; [432]).

He says therefore (291a29) that in regard to the coordination of the stars, namely, as to how each is arranged, in such a way that some are prior and some posterior, i.e., higher and lower, and how they are related as to elongations, i.e., as to how far one is distant from another, must be considered from what is stated in astronomy where these particular aspects are sufficiently determined. For these matters cannot be derived from the principles of natural philosophy, but from mathematical principles, i.e., from the proportions existing between magnitudes. Now Anaximander is said to have been the first to find out the notions of the magnitudes of the stars, and of their distances one from the other and from the earth; while it is to the first Pythagoreans that credit is given for grasping the order of position of the planets, although these matters were given more careful and more perfect consideration by Hipparchus and Ptolemy.

432. Then, at this discussion shows (291a32), he shows what belongs to the natural philosopher to consider on these points, namely, the swiftness and slowness of their motions. He says, therefore, that it is reasonable for the motions of any stars to be slower and faster depending on their proportionate distance from the first sphere and from the earth. For we suppose as sensibly evident that the outermost revolution of the heaven is simple, i.e., not composed of several motions, because no irregularity appears in it; moreover, it is the swiftest of all, inasmuch as, in the briefest time, namely, in the course of one day it makes the circuit of the largest circle that encompasses the whole. But the circlings of the planets are both slower and plural, not only because the motions of the different planets are each different, but also because the motion of each planet is a composite of various motions. For each of the planets, as regards its peculiar motion in its own circle, has a movement contrary to the movement of the first heaven—here contrariety is taken in a wide sense, for, strictly speaking, contrariety does not exist among circular motions, as has been said in book I—in the sense that the motion of the first heaven is from east to west, whereas the movements of the planets in their own circles are from west to east. Hence, it is reasonable that the planet nearest to the simple and first revolution, against which it is moving in its own circle, should consume the most time in traversing its own circle—thus Saturn takes 30 [actually 29] years to traverse its orbit. On the other hand, the planet most distant from the outermost sphere, namely, the moon, traverses its orbit in the least time, namely, in the space of one month or even less. Among the other planets, that nearer to the outermost sphere always traverses its circle in a greater time—Jupiter in 12 years, Mars in two, Venus, Mercury and the Sun in a year, more or less. Thus the farther they are from the outermost sphere, the less time is required for traversing their orbit, the reason being that the first sphere most greatly dominates the planet closest to it, thus making its contrary motion slower, but dominates least the planet farthest away, because of its distance—hence the contrary motion is swifter in it, i.e., in the moon. The intermediate planets behave according to the proportion of the distance, as the mathematicians show, in such a way, namely, that the higher planets move more slowly in their proper motions. But with respect to the motion whereby they are moved with the motion of the first mobile body, the higher they are, the swifter they are, as has been shown above.

433. However, from what Aristotle says here, it seems that violence exists among the heavenly bodies, namely, if the motion of the planets nearer to the outermost sphere is slowed up by the greater influence exerted upon it by the motion of the first sphere on account of being near it. Now if there is any violence present, it follows that these motions would not last forever as Aristotle wishes—for nothing violent can be eternal, as was mentioned above.

To this Alexander responds that although the domination of the outermost sphere does indeed produce a necessary slowing down in the nearer planet, yet not in a violent manner. For those heavenly motions are controlled by intellect and will. But in motions subject to will that is not violent which is in conformity with the will, even though there be an element of necessity involved. Now the aim of the will of the one moving the highest planet is to move its mobile body in accord with the motion of the higher mobile body, to which it desires to be likened. Consequently, it does not follow that the slowness in the motion of the first planet is violent.

434. But this does not wholly solve the difficulty in such a way as to preserve the principles supposed by Aristotle to the effect that a larger body is moved more swiftly by a proper and natural motion. Hence if that motion whereby a planet is moved in its own circle, is proper and natural to it, the consequence is that the sphere of a higher planet, since it is larger, will be moved more swiftly according to its own motion. Then, too, it seems that a strange order is being preserved if a body which is farther from the immobile earth, but nearer to the most rapid motion of the first mobile, should move slower in its own motion.

435. Hence, others said that in the heaven there is but one motion, namely, the one whereby the entire heaven is revolved from east to west by the motion of the first mobile: and with respect to this motion the higher body has the swifter motion not only according to the size of the circle but also according to the brevity of time, the result being that a higher sphere will traverse a greater circle in less time, Hence a lower star fails to return to the same point according to time, but not because it moves with a motion contrary to the first motion. This explanation saves the teaching that since the higher planet fails only by a little to match the first motion, whereas a lower planet fails more, the higher planet is swifter and the lower planet slower.

436. Indeed that is the situation if, as Ptolemy says, the motion of the planets takes place on circles equidistant from the equinoxial circle [equator] and upon the same poles. Yet the opposite is apparent, since the planets at one time decline to the north and at another time to the south. Hence, it seems that the failure to keep up with the first motion should be explained in terms of another motion of the planets, by which they are moved from west to east, rather than in terms of a sole failure to keep pace with the first motion, a failure that makes a higher planet seem to be moved more slowly.

437. But Alexander assigns another cause to this besides the one which Aristotle assigns from the dominance of the first motion. For he says that a higher planet requires more time to traverse its circle, not because its motion is slower, but because the distance is greater. For a motion that requires more time can be swifter or as swift as another, if the excess in distance traversed is greater or equal to the excess of time. But this does not appear in the planets. For since Saturn traverses its circle in 30 years, while the moon traverses its circle in approximately one month, the ratio of the size of Saturn's sphere to that of the moon would have to be according to the ratio of the aforesaid times. But that is not observed in this case, nor in the other planets.

438. Hence, it seems that a different explanation should be given, namely, that we must consider two natures in the universe: first, the nature of eternal permanence which is above all in the separated substances, and secondly, generable and corruptible nature which is present in lower bodies. Now heavenly bodies, being intermediate, share somewhat in both, according to two motions. For the first motion, which is the diurnal, is the cause of eternal duration in things; but the second motion, which is in the oblique [i.e., zodiacal] circle from west to east, is the cause of generation and corruption and of other changes, is is plain from the words of the Philosopher in On Generation II. According to this, the first mobile, as being at once the most noble and the nearest to the separated substances in the order of nature, has only the first motion, which pertains to the nature of uniformity. But the other heavenly bodies, to the extent that they depart from the unchangeable substances and approach generable and corruptible substances, share somewhat in the other motion, which pertains to the nature of difformity, and to a lesser extent, according as the body is higher and more noble. According to this, then, the higher planet, namely, Saturn has the least portion of the second motion on account of the nobility of its nature; consequently this motion in Saturn is slower. But the moon, on account of the closeness of its nature to generable and corruptible bodies, shares the most in the second motion, which in it is most swift. The intermediate planets behave in an intermediate way: for Jupiter, which is immediately under Saturn traverses its circle with its own motion in about 12 years; Mars in about two years; the Sun, Venus and Mercury take nearly uniformly one year.

439. Nor does the proportion of speed need to be according to the proportion of the distances, because the heavenly motions are not only natural, but also voluntary and for a desired end. And therefore, insofar as these motions are natural, the general rule is that the higher planets are slower of motion; but insofar as their motions are voluntary, the proportion of their speed varies in a particular case, not according to distance, but according to what is better. Hence, because the motions of Venus and Mercury are, as it were, bound to the motion of the Sun, as serving the Sun in producing its effects, they are moved as though uniformly with the Sun.

440. Thus Aristotle's statement that the supreme sphere exercises more influence on the highest planet and less on a remote one, is not to be understood in the sense of a compulsory force, but in the sense of a natural impression insofar, namely, as the nature of a higher thing is participated in more by something closer to it than by something farther away from it.

Thus are the principles of Aristotle preserved. For although both motions are natural to a planet, namely, the diurnal motion and that in its own circle, nevertheless the former is natural to it according to what is more noble in its nature. Therefore, it is only with respect to that motion that Aristotle's principle is saved, to the effect that a larger body is moved more swiftly. Likewise in man, who has a sensitive and an intellective nature, we say that the more a man is noble, the more he has of the motion of the nobler nature, namely, the intellective, and the less of the motion of the less noble, namely, the sensitive.