Lecture 12 — That the stars do not move themselves concluded from the motions proper to the spherical shape

302 Again, since the stars are spherical, as our opponents assert and we may consistently admit, inasmuch as we construct them out of the spherical body, and since the spherical body has two movements proper to itself, namely rolling and spinning, it follows that if the stars have a movement of their own, it will be one of these. But neither is observed. Suppose them to spin. They would then stay where they were, and not change their place, as, by observation and general consent, they do. Further, one would expect them all to exhibit the same movement: but the only star which appears to possess this movement is the sun, at sunrise or sunset, and this appearance is due not to the sun itself but to the distance from which we observe it. The visual ray being excessively prolonged becomes weak and wavering. The same reason probably accounts for the apparent twinkling of the fixed stars and the absence of twinkling in the planets. The planets are near, so that the visual ray reaches them in its full vigour, but when it comes to the fixed stars it is quivering because of the distance and its excessive extension; and its tremor produces an appearance of movement in the star: for it makes no difference whether movement is set up in the ray or in the object of vision. On the other hand, it is also clear that the stars do not roll. For rolling involves rotation: but the 'face', as it is called, of the moon is always seen. Therefore, since any movement of their own which the stars possessed would presumably be one proper to themselves, and no such movement is observed in them, clearly they have no movement of their own.

402. Having presented the first argument to show that the stars are moved by the motion of their circles, which argument was based on a comparison of the stars with their circles or orbs, the Philosopher here presents a second argument based on the shape of the stars (290a7). The argument is this: the stars are spherical in shape; hence, if they were to be moved, they would have to be moved with a motion that is proper to a spherical body, which is twofold, namely, "volutation" and "circumgyration." But the stars are moved with neither of these motions. Therefore, they are not moved according to themselves but what appears of their motion is due to their being moved according to the motion of the circles.

403. First, therefore, he proposes that the stars are spherical in shape and this he manifests in two ways. In one way, because everyone else speaks thus, namely, to the effect that the stars are spherical: for which reason it should be accepted as probable. In another way, from an argument taken from what has already been determined. For it has been said that the stars are made out of the nature of the heavenly bodies. Hence we must profess that they have the same shape as the heaven. But it has been shown above that the heaven is spherical in shape. Hence the stars must be spherical in shape.

404. Then he shows the difference between the circular motions proper to spherical bodies. And he says that there are two motions that belong per se to a spherical body, i.e., in virtue of its spherical shape, namely, "volutation" and "circumgyration." Now these two motions differ according as the axis and poles upon which the spherical body is understood to be moving are diverse—and these differences are reckoned in relation to us. For if the stellar body is reckoned to be in motion upon two poles, one of which is in the surface facing us and the other in the surface opposite, in such a way that we take the axis as a line passing through the star's depth, then the star is being moved with a motion of "circumgyration." In this motion the stars keep the same face toward us after the manner in which a millstone is moved. But if the stellar body is understood to be in motion upon two poles, both of which are taken at some point where it joins the body of its sphere, then the star in its motion will not always keep the same face toward us. In this case the motion will be "volutation." Therefore, because these are the two motions proper to a spherical body, if the stars are moved with motions of their own, they should be moved with one or the other of these.

405. Then he shows that the motion seen in the stars is due to neither of these two motions.

First he shows that the motion seen in the stars is not one of circumgyration; and he proves this in two ways.

First, because if the stellar bodies were being moved with the motion of circumgyration, then, even though the parts of the star exchanged places as to subject, the star as a whole would have to remain in the same place as to subject, the place being varied only according to notion, as is clear from what was proved in Physics Vi. For that is the way things turn out for a spherical motion due to its relation to a center and to poles that are stationary. But we cannot admit such a situation in the stars, since the contrary is evident to sense—for we see stars sometimes in the east and sometimes in the west. Likewise, everyone says that the stars do not remain always in the same place but are transferred from one place to another. Therefore, the motion that appears to be in the stars is not one of circumgyration.

He shows the same thing another way, from the fact that if the motion of circumgyration should befit the stars, it would be reasonable for all to be moved with that kind of motion, since they are all of one nature, namely, the nature of heavenly body as was shown above. But such a motion is not seen in all stars but in the sun alone, and not when it is in just any part of the heaven, but only when it rises and sets. And these appearances are due not to the fact that the sun is circumgyrated but to the increased distance between the sun and our vision—for our vision, since it is so far from the sun, wavers, i.e., quivers, on account of its feebleness, insofar as it is overwhelmed by the sun's exceeding brightness.

And this may also be why the stars seem to twinkle, namely, because of their being at the greatest distance from us, since they are in the eighth sphere. On the other hand, the planets are not seen to twinkle, because being nearer to us, our vision is powerful enough to reach them in full strength. But when looking at the stationary, i.e., fixed, stars, our vision quivers as though from being extended to something very far off, due to the distance separating those stars from us. Now it is the quivering that occurs in our vision that makes the stars seem to be in motion, either according to twinkling, as in the case of a fixed star, or according to circumgyration in the case of the sun. For in order that something appear to be in motion, it makes no difference whether it is our vision, or the thing observed, that is in motion. This is plain in the case of those sailing along the coast, where, because they are in motion, it seems to them that the mountains and the land are in motion.

406. With respect to what is said here, one should consider that the Philosopher says here that our vision quivers as greatly extended, when looking at the fixed stars, not because seeing takes place by means of something sent out from the sight—for he disproves this theory in De sensu et sensato—but because in such a situation it makes no difference whether vision takes place because sight sends something out or because it receives something within it. For our vision strains to see things afar, not only if it should have to emit a visual ray to the distant body, but also if it has to receive a species travelling from the distant body, for the impression from a distant body is weaker and is, accordingly, more difficult to sense. However, Aristotle uses a manner of speaking in this instance as though sight took place by sending something out, because mathematicians so use it in their proofs, and many people speak in such terms. Now, as he himself says in Topics II, words are to be used as most people use them.

407. It should also be noted that he calls certain stars "fixed" or "stationary," not because they are not moved at all when their sphere is moved, as are the planets, which are called "wandering," but because, unlike the planets, they always maintain the same relative position to one another and present the same configurations.

Likewise his assertion that the planets do not twinkle, must, as Simplicius says, be understood as applying to the majority of planets—for Mercury twinkles, and hence in Greek it is called Stilbon, from "twinkling." The sun, too, seems to twinkle and even circumgyrate. But the twinkling appears due to the fact that our sight cannot properly apprehend the things seen: in the case of the fixed stars this occurs on account of their distance; in the case of the sun on account of its exceeding brilliance. The circumgyration on the other hand is seen due to the fact that the seen object is able to transmute the sight in such a way that, the visual spirit having been turned about, the sun itself seems to turn about. Hence it is that the sun especially seems to rotate more when it rises and sets, for then our vision can fix itself more on it because its brilliance is not as powerful, on account of earthly vapors. But when it has risen, because of the excess of its brightness the eye cannot fix itself on it long enough to make it appear to rotate, but sees it scintillate.

But Alexander says that the reason why the sun appears to rotate at sunrise and sunset is because one senses its twofold motion, namely, the diurnal and its own proper motion, in comparison to the motionlessness of the earth. But this is not believable, namely, that the motion of the sun, especially that by which it is moved with its own proper motion, should be perceived in so short a space—when it is hardly felt even over many days. Aristotle says also, in the letter of the text, that this rotation appears, not from the sun, but from the distance of our vision.

408. One should know that Plato supposed that the stars are subject to a motion of circumgyration in addition to the motion they undergo in virtue of their orbs. Simplicius tries in various ways to show that this is so.

First, because, since the stars are natural bodies, they must have some natural motion; and since they share in the nature of the heaven, they must of themselves be moved with a circular motion, which is circumgyration.

Secondly, because the stars, in the opinion of many, are animated bodies and therefore must have self-movement: although they are in some sense parts of their orbs, since they have in themselves their own integrity and their own circumgyration.

Thirdly, because, since a spherical figure is most suitable for circular motions and most unsuitable for other motions, it seems that stars are of themselves moved circularly with their own motion of circumgyration. In line with this, Plato proposed that the fixed stars are moved with two motions, namely, with a motion of circumgyration according to themselves, and with the motion of the orb (because they are seen to be moved from east to west). But the wandering stars are, according to him, moved with three motions: namely, with a motion of circumgyration, and with the motion of their own orb, and with the motion of the outermost orb, which is diurnal motion.

Simplicius also says that Aristotle is not here concerned with disproving this position. For he does not show that the stars are not circumgyrated at all, but rather that the motion which is manifest to sense in the stars is not one of circumgyration, since circumgyrated things remain in their entirety in the same place, whereas the stars, so far as the motion observed in them is concerned, do not remain in the same place. And because circumgyration is more clearly apparent in the sun at sunrise and sunset, he therefore shows that what is seen in it of such a motion, is not because of it, but because of a transmutation of our vision.

But since it was Aristotle's intent not to get away from sensible appearances, consequently, since such a circumgyration does not sensibly appear in the stars, he therefore did not assert this motion to exist in the stars, even though he did not directly disprove it. Likewise, he also did not do so because the movements of the heavenly bodies cause the motions of the lower bodies according as the former approach or recede from us—yet there is no effect noticed in lower bodies according to such a circumgyration of the stars, nor do the stars approach or recede from us according to this motion. For this reason, Aristotle is not concerned with attributing this motion to the stars.

409. Then he shows that the stars are not moved with a motion of "volutation." For whatever is revolved must be so turned that the same face is not always turn to the observer. But we see that in one of the stars, namely, in the moon, the same surface always appears to us, namely, that surface which is called the "face," because there appear in it certain distinguishing marks, just as there is in the face of a man a certain distinguishing according to lineament. From this it is clear that the moon is not moved with a motion of volutation. For the same reason no other stars are, because, since the same nature is in all the stars, the same reason is seen to hold for one and for the others.

Thus he concludes: since it would be reasonable for the stars, if they were moved on their own, to be moved with proper motions, namely, "regyration" and "volutation," and yet, as has been shown, they are not moved with these motions, therefore they are not moved of themselves.

410. It should be noted that the diversity which appears on the surface of the moon is variously explained by different ones. For some say that the cause of this diversity is the interposition of some body between us and the moon, thus preventing us from seeing its full brightness. Hence, in that portion where such bodies are interposed between us and the moon, it appears to be dark because we do not see the brightness of the moon in that part. However, this cannot be, for such an interposed body would not be interposed in the same way between the moon and the sight of a man in any part of the world whatever. Consequently a like disposition would not be observed in the moon from everywhere in the world, just as there is not seen a similar view of a solar eclipse from everywhere in the world, when the moon gets between the sun and the earth. But this does not occur in connection with the aforesaid diversity in the moon—for it is seen in a similar way from all parts of the earth, whether east, west, south, or north.

Others hold that this darkness appearing in the moon is a certain likeness of some body, such as the earth, or the sea, or mountains, which results in the moon after the manner in which a form results in a mirror. But this too is voided for the same reason. Because if such forms of this sort were mirrored by a reflection of visual rays or visual forms, there would not appear a similar variation from everywhere on earth, any more than a form in a mirror appears according to a same disposition no matter from what angle one looks at it. The reason for this is that an image is reflected back at determined places according to the position of the bodies reflected. Moreover, the proposed explanation would nullify Aristotle's argument, because it could be said that the reason why the same diversity always appears to us in the moon is not because the same surface is always turned toward us, but because whatever the surface, it receives from the aforesaid causes such an appearance when it is turned toward us.

And therefore other say—and this is a better explanation—that such a diversity [on the surface of the moon] is seen in the moon because of the disposition of its substance, and not because of the interposition of some body or because of some reflection. There is a twofold opinion among them. For some say that the forms of effects are in a certain sense in their causes, in such a way, however, that, the higher the cause, the more uniformly the various forms of its effects are in it, whereas the lower the cause, the more distinctly are the forms of its effects in it. Now the heavenly bodies are the cause of the lower bodies, and among the heavenly bodies, the moon has the lowest rank. Therefore, in the moon, according to its lower surface there is contained, so to speak, the diversity of generable bodies as in an exemplar. This was Iamblichus' opinion.

But others say that, although the heavenly bodies are of another nature than the four elements, yet the properties of the elements pre-exist in the heavenly bodies as in their causes—not in the same way as in the elements however, but in a certain more excellent manner. Now among the elements the top rank is held by fire, which has the greatest light, while the lowest in rank is held by earth which has a minimum of light. Consequently, the moon, which is the lowest of the heavenly bodies, is proportionate to earth and is in some sense similar to it in nature—for which reason it cannot be totally lit up by the sun. Hence, in that portion where it is not perfectly lit up by the sun, there is seen to be a certain darkness. And this darkness always appears according to the same disposition in the moon—something that would not be if the moon were revolving, for then the look of that obscurity would gradually change.