XVII

ARISTOTLE

IT was convenient to give Aristotle’s modified system of concentric spheres in close connexion with the systems of Eudoxus and Callippus, and to reserve the rest of his astronomy for separate treatment. While his modification of the beautiful theory of Eudoxus and Callippus was far from being an improvement, Aristotle rendered real services to astronomy in other respects. Those services consisted largely of thoughtful criticisms, generally destructive, of opinions held by earlier astronomers, but Aristotle also made positive contributions to the science which are of sufficient value to make it impossible to omit him from a history of Greek astronomy.

We have seen that he modified the purely geometrical hypotheses of Eudoxus and Callippus in a mechanical sense. A purely geometrical theory did not satisfy him; he must needs seek to assign causes for the motions of the several concentric spheres. We may therefore conveniently begin this chapter with an account of his views on Motion. Motion, according to Aristotle, is, like Form1 and Matter,² eternal and indestructible, without beginning or end.³ Motion presupposes a primum movens which is itself unmoved;⁴ for that which is moved, being itself subject to change, cannot impart an unbroken and uniform movement;⁵ the primum movens, then, must be one,⁶ unchangeable, absolutely necessary;⁷ there is nothing merely potential about it, no unrealized possibility;⁸ it must therefore be incorporeal,⁹ indivisible,¹⁰ and unconditioned by space,¹¹ as well as motionless and passionless;12 it is absolute Reality and pure Energy,¹³ that is, God.¹⁴ In another aspect the primum movens is the Final Cause, pure Being, absolute Form, the object of thought and desire;¹⁵ God is Thought,¹⁶ self-sufficient,¹⁷ contemplating unceasingly nothing but itself,¹⁸ the absolute activity of Thought, constituting absolute reality and vitality and the source of all life.¹⁹ The primum movens causes all the movements in the universe, not by any activity of its own²⁰—for that would be a movement and, as immaterial, it can have no share in movement—but by reason of the fact that all things strive after it and try to realize, so far as possible, its Form;²¹ it operates like a beloved object, and that which is moved by it communicates its motion to the rest.²²

Motion takes place only by means of continuous contact between the motive principle and the thing moved. Aristotle insists upon this even in a case where the contact might seem to be only momentary, e.g. where a thing is thrown. The motion in that case seems to continue after contact with the thrower has ceased, but Aristotle will not admit this; he assumes that the thrower moves not only the thing thrown but also the medium through which the thing is thrown, and makes the medium able to act as moved and movent at the same time (i.e. to communicate the movement); and further that the medium can continue to be movent even after it has ceased to be moved.²³ God then, as the first cause of motion, must be in contact with the world,²⁴ though Aristotle endeavours to exclude contiguity in space from the idea of ‘contact’, which he often uses in the sense of immediate connexion, as of thought with its object.²⁵ The primum movens operates on the universe from the circumference, because the quickest motion is that of the (outermost limit of the) universe, and things move the quickest which are nearest to that which moves them.²⁶ Hence in a sense it could be said that God is to Aristotle ‘the extremity of the heaven‘;27 but Aristotle is careful to deny that there can be any body or space or void outside the universe; what is outside is not in space at all; the ‘end of the whole heaven’ is life (), immortal and divine’.²⁸

Motion in space is of three kinds, motion in a circle, motion in a straight line, and motion compounded of the two (‘mixed’).²⁹ Which of these can be endless and continuous ? The ‘mixed’ would only be so if both the two components could; but movement in a straight line cannot have this character, since every finite rectilinear movement has terminal points at which it must turn back,³⁰ and an infinite rectilinear movement is impossible, both in itself,³¹ and because the universe is finite; hence circular motion is the only motion which can be without beginning or end.³² Simple bodies have simple motions; thus the four elements tend to move in straight lines; earth tends downwards, fire upwards; between the two are water, the relatively heavy, and air, the relatively light. Thus the order, beginning from the centre, in the sublunary sphere is earth, water, air, fire.³³ Now, says Aristotle, simple circular motion is more perfect than motion in a straight line. As, then, there are four elements to which rectilinear motion is natural and circular motion not natural, so there must be another element, different from the four, to which circular motion is natural.³⁴ This element is superior to the others in proportion to the greater perfection of circular motion and to its greater distance from us;³⁵ circular motion admits no such contraries as ‘up’ and ‘down’; the superior element therefore can neither be heavy nor light;³⁶ the same absence of contrariety suggests that it is without beginning or end, imperishable, incapable of increase or change (because all becoming involves opposites and opposite motions).³⁷ This superior element which fills the uppermost space is called ‘aether’,³⁸ the ‘first element’,³⁹ or ‘a body other and more divine than the four so-called elements’;40 its changelessness is confirmed by long tradition, which contains no record of any alteration in the outer heaven itself or in any of its proper parts.⁴¹ Of this element are formed the stars,⁴² which are spherical,⁴³ eternal,⁴⁴ intelligent, divine.⁴⁵ It occupies the whole region from the outside limit of the universe dawn to the orbit of the moon, though it is not everywhere of uniform purity, showing the greatest difference where it touches the sublunary sphere.⁴⁶ Below the moon is the terrestrial region, the home of the four elements, which is subject to continual change through the strife of those elements and their incessant mutual transformations.⁴⁷

There is, Aristotle maintains, only one universe or heaven, and that universe is complete, containing within it all the matter there is. For, he argues, all the simple bodies move to their proper places, earth to the centre, aether to the outermost region of the universe, and the other elements to the intervening spaces. There can be no simple body outside the universe, for that body has its own natural place inside, and, if it were kept outside by force, the place occupied by it would be the natural place for some other body; which is impossible, since all the simple bodies have their proper places inside. The same argument holds for mixed bodies; for, where mixed bodies are, there also are the simple bodies of which they are composed. Nor can there be any space or void outside the universe, for space or void is only that in which a body is or can be.⁴⁸ Another argument is that the prirmum movens is single and complete in itself; hence the world, which derives its eternal motion from the primum movens, must be so too.⁴⁹ If it be suggested that there may be many particular worlds as manifestations of one concept ‘world’, Aristotle replies that this cannot be; for the heaven is perceptible to our senses; hence it and other heavens (if any) must contain matter; but our heaven contains all the matter there is, and therefore there cannot be any other.50

Next, the universe is finite. In the Physics Aristotle argues that an infinite body is inconceivable, thus. An infinite body must either be simple or composite. If composite, it is composed of elements; these are limited in number; hence an infinite body could only be made up of them if one or more were infinite in magnitude; but this is impossible, because there would then be no room for the rest. Neither can it be simple; for no perceptible simple body exists except the elements, and it has been shown that none of them can be infinite.⁵¹ In the De caelo he approaches the subject from the point of view of motion. A body which has a circular motion, as the universe has, must be finite. For, if it is infinite, the straight line from the centre to a point on its circumference must be infinite; now if, as being infinite, this distance can never be traversed, it cannot revolve in a circle, whereas we see that in fact the universe does so revolve.⁵² Further, in an infinite body there can be no centre; hence the universe which rotates about its centre cannot be infinite.⁵³

Aristotle’s arguments for the spherical shape of the universe are of the usual kind. As the circle, enclosed by one line, is the first of plane figures, so the sphere, bounded by one surface, is the first of solid figures; hence the spherical shape is appropriate to the ‘first body’, the subject of the outermost revolution’.⁵⁴ Next, as there is no space or void outside the universe, it must, as it revolves, continually occupy the same space; therefore it must be a sphere; for, if it had any other form, this condition would not be satisfied.⁵⁵ [Aristotle is not strictly correct here, since any solid of revolution revolving round its axis always occupies the same space, but it is true that only a sphere can remain in exactly the same position when revolving about any diameter whatever.] Further, we may infer the spherical form of the universe from the bodies in the centre. We have first the earth, then the water round the earth, air round the water, fire round the air, and similarly the bodies above the fire; now the surface of the water is spherical; hence the surfaces of the layers following it, and finally the outermost surface, correspond.56

The fabric of the heavens is made up of spherical shells, as it were, one packed inside the other so closely that there is no void or empty space between them;⁵⁷ this applies not only to the astral spheres,⁵⁸ but right down to the earth in the middle;⁵⁹ it is necessary so far as the moving spheres are concerned because there must always be contact between the moving and the moved.⁶⁰

We have above described the working of Aristotle’s mechanical system of concentric spheres carrying the fixed stars and producing the motions of the planets respectively, and it only remains to add a word with reference to the motive power acting on the spheres other than that of the ‘outermost revolution’. The outermost sphere, that of the fixed stars, is directly moved by the one single and eternal primum movens, Divine Thought or Spirit. Only one kind of motion is produced when one movens acts on one object;⁶¹ how then do we get so many different movements in the spheres other than the outermost? Aristotle asks himself this question: Must we suppose that there is only one unmoved movens of the kind, or several, and, if several, how many are there ? He is obliged to reply that, as eternal motion must be due to an eternal movens, and one such motion to one such movens, while we see that, in addition to the simple revolution of the whole universe caused by the unmoved primum movens, there exist other eternal movements, those of the planets, we must assume that each of the latter movements is due to a substance or essence unmoved in itself and eternal, without extension in space.⁶² The number of them must be that of the separate spheres causing the motion of the separate planets.⁶³ The number of these spheres he had, as we have seen (p. 217), fixed provisionally, while recognizing that the progress of astronomy might make it necessary to alter the figures.⁶⁴ Of the several spheres which act on any one planet, the first or outermost alone is moved by its own motion exclusively; each of the inner spheres, besides having its own independent movement, is also carried round in the motion of the next sphere enveloping it. so that all the inner spheres, while themselves movent, are also moved by the eternal unmoved movent.65

In a chapter of the De caelo⁶⁶ Aristotle discusses the question which is the right side of the heaven and which the left. The disquisition is not important, but it is not unamusing.⁶⁷ He begins with a reference to the view of the Pythagoreans that there is a right and a left in the universe, and proceeds to investigate whether the particular distinction which they draw is correct or not, ‘assuming that it is necessary to apply such principles as “right” and “left” to the body of the universe’.⁶⁸ There being three pairs of such opposites, up and down (or upper and lower), right and left, before and behind (or forward and backward), he begins with the distinctions (1) that ‘up’ is the principle of length, ‘right’ of breadth, and ‘before’ of depth, and (2) that ‘up’ is the source of motion ( ),’ right’ the place from which it starts (), and ‘to the front’ () is the place to which it is directed (). Now the fact that the shape of the universe is spherical, alike in all its parts, and continually in motion, is no obstacle to calling one part of it ‘right’ and the other ‘left’. What we have to do is to think of something which has a right and left of its own (say a man) and then place a sphere round it.⁶⁹ Now, says Aristotle, I call the diameter through the two poles the length of the universe (because only the poles remain fixed), so that I must call one of the poles the upper, and the other the lower. He then proceeds to show that the proper relativities can only be preserved by calling the south (the invisible) pole the upper and the north (the visible) pole the lower, from which it follows that we live in the lower and left hemisphere, and the inhabitants of the regions towards the south pole live in the upper and right hemisphere; and this is precisely the opposite of what the Pythagoreans hold, namely that we live in the upper and right hemisphere, and the antipodes represent the lower and left. The argument amounts to this. ‘Right’ is the place from which motion in space starts; and the motion of the heaven starts from the side where the stars rise, i. e. the east; therefore the east is ‘right’ and the west is ‘left’. If now (1) you suppose yourself to be lying along the world’s axis with your head towards the north pole, your feet towards the south pole, and your right hand towards the east, then clearly the apparent motion of the stars from east to west is over your back from your right side towards your left; this motion, Aristotle maintains, cannot be called motion ‘to the right’, and therefore our hypothesis does not fit the assumption from which we start, namely that the daily rotation ‘begins from the right and is carried round towards the right ()’.70 We must therefore alter the hypothesis and suppose (2) that you are lying with your head towards the south pole and your feet towards the north pole. If then your right hand is to the east, the daily motion begins at your right hand and proceeds over the front of your body from your right hand to your left. We should nowadays regard this as giving precisely the wrong result, since motion round us in front from right to left can hardly be described as ,’ to the right’;so that hypothesis (1) would, to us, seem preferable to hypothesis (2). But Aristotle’s point of view is fairly clear. We are to suppose a man (say) standing upright and giving a horizontal turn with his right hand to a circle about a vertical diameter coincident with the longitudinal axis of his body. Aristotle regards him as turning the circle towards the right when he brings his right hand towards the front of his body, although we should regard it as more natural to apply ‘towards the right’ to a movement of his right hand still more to the right, i.e. round by the right to the back. The (to us) unnatural use of the terms by Aristotle is attested by Simplicius who says that motion is in any case towards the front (),⁷¹ and it is doubtless due to what Aristotle would regard as the necessity of making front (in the dichotomy front and back, or before and behind) correspond to right (in the dichotomy right and left), just as up (in the dichotomy up and down) must also correspond to right; this is indeed clear from his own statement quoted above that, as ‘the right’ is the place from which motion starts, so ‘to the front’ is the place towards which it is directed.

We come next to Aristotle’s view as to the shape of the heavenly bodies and the arguments by which he satisfied himself that they do not move of themselves but are carried by material spheres.

He held that the stars are spherical in form. One argument in support of this contention is curious. Nature, he says, does nothing without a purpose; Nature therefore gave the stars the shape most unfavourable for any movement on their own part; she denied to them all organs of locomotion, nay, made them as different as possible from the things which possess such organs. With this end in view, Nature properly made the stars spherical; for, while the spherical shape is the best adapted for motion in the same place (rotation), it is the most useless for progressive motion.72 This is in curious contrast to the view of Plato who, with more reason, regarded the cube as being the shape least adapted for motion ().⁷³ The second argument is from analogy. Since the moon is shown by the phases to be spherical, while we see similar curvature in the lines separating the bright part of the sun from the dark in non-total solar eclipses, we may conclude from this that the sun and, by analogy, the stars also are spherical in form.⁷⁴

With regard to the spheres carrying the stars round with them, we note first that the ‘heaven’, in the sense of the ‘outermost heaven’ or ‘the outermost revolution of the All’ ( ), which is the sphere of the fixed stars, was with Aristotle a material thing, a ‘physical body’ ()⁷⁵ Now, says Aristotle,⁷⁶ seeing that both the stars and the whole heaven appear to change their positions, there are various a priori possibilities to be considered; (1) both the stars and the heaven may be at rest, (2) both the stars and the heaven may be in motion, or (3) the stars may move and the heaven be at rest, or vice versa. Hypothesis (1) is at once ruled out because, under it, the observed phenomena could not take place consistently with the earth being at rest also; and Aristotle assumes that the earth is at rest (). Coming to hypothesis (2), we have to remember that the effect of a uniform rotation of the heaven about an axis passing through the poles is to make particular points on this spherical shell describe parallel circles about the axis; suppose then that the heaven rotates in this way, and that the stars also move. Now, says Aristotle, the stars and the circles cannot move independently; if they did, it is inconceivable that the speeds of the stars would always be exactly the same as the speeds of the circles; for, while the speeds of the circles must necessarily be in proportion to their sizes, i.e. to their radii, it is not reasonable to suppose that the stars, if they moved freely, would revolve at speeds proportional to the radii of the circles; yet they would have to do so if the stars and the circles are always to return to the same places at the same times, as they appear to do. Nor can we, as in hypothesis (3), suppose the stars to move and the heaven to be at rest; for, if the heaven were at rest, the stars would have to move of themselves at speeds proportional to the radii of the circles they describe, which has already been stated to be an unreasonable supposition. Consequently only one possibility remains, namely that the circles alone move, and the stars are fixed on them and carried round with them;77 that is, they are fixed on, and carried round with, the sphere of which the circles are parallel sections.

Again, says Aristotle, there are other considerations which suggest the same conclusion. If the stars have a motion of their own, they can, being spherical in shape, have only one of two movements, namely either (1) whirling () or (2) rolling (). Now (1), if the stars merely whirled or rotated, they would always remain in the same place, and would not move from one position to another, as everybody admits that they do. Besides, if one heavenly body rotated, it would be reasonable to suppose that they all would. But, in fact, the only body which seems to rotate is the sun and that only at the times of its rising and setting; this, however, is only an optical illusion due to the distance, ‘for our sight, when at long range, wavers’ (literally ‘turns’ or ‘spins’, ). This, Aristotle incidentally observes, may perhaps be the reason why the fixed stars, which are so distant, twinkle, while the planets, being nearer, do not. It is thus the tremor or wavering of our sight which makes the heavenly bodies seem to rotate.⁷⁸ In thus asserting that the stars do not rotate, Aristotle is of course opposed to Plato, who held that they do.⁷⁹

Again (2), if the stars rolled (along, like a wheel), they would necessarily turn round; but that they do not turn round in this way is proved by the case of the moon, which always shows us one side, the so-called face.80

It is for a particular reason that I have reproduced so fully Aristotle’s remarks about rotation and rolling as conceivable movements for stars as spherical bodies. It has been commonly remarked that Aristotle draws a curious inference from the fact that the moon has one side always turned to us, namely that the moon does not rotate about its own axis, whereas the inference should be the very opposite.⁸¹ But this is, I think, a somewhat misleading statement of the case and less than just to Aristotle. What he says is that the moon does not turn round in the sense of rolling along; and this is clear enough because, if it rolled along a certain path, it would roll once round while describing a length equal to 3·1416 times its diameter, but it manifestly does not do this. But Aristotle does not say that the moon does not rotate; he does not, it is true, say that it does rotate either, but his hypothesis that it is fixed in a sphere concentric with the earth has the effect of keeping one side of the moon always turned towards us, and therefore incidentally giving it a rotation in the proper period, namely that of its revolution round the earth. I cannot but think that the fact of the moon always showing us one side was one of the considerations, if not the main consideration, which suggested to Aristotle that the stars were really fixed in material spheres concentric with the earth.

We pass to matters which are astronomically more important. And first as to the spherical shape of the earth. Aristotle begins by answering an objection raised by the partisans of a flat earth, namely that the line in which the horizon appears to cut the sun as it is rising or setting is straight and not curved.⁸² His answer is confused; he says that the objectors do not take account of the distance of the sun from the earth and of the size of its circumference,⁸³ the fact being that you can have an apparently straight line as a section when you see it from afar in a circle which on account of its distance appears small. He should no doubt have said, first, that the sun, as we see it, looks like a flat disc of small size on account of its distance, and then that the section of an object apparently so small by the horizon is indistinguishable from a section by a plane through our eye, so that the section of the disc appears to be a straight line. He has, however, some positive proofs based on observation, (1) In partial eclipses of the moon the line separating the bright from the dark portion is always convex (circular)—unlike the line of demarcation in the phases of the moon, which may be straight or curved in either direction—this proves that the earth, to the interposition of which lunar eclipses are due, must be spherical.84 He should no doubt have said that a sphere is the only figure which can casf a shadow such that a right section of it is always a circle; but his explanation shows that he had sufficiently grasped this truth, (2) Certain stars seen above the horizon in Egypt and in Cyprus are not visible further north, and, on the other hand, certain stars set there which in more northern latitudes remain always above the horizon. As there is so perceptible a change of horizon between places so near to each other, it follows not only that the earth is spherical, but also that it is not a very large sphere. He adds that this makes it not improbable that people are right when they say that the region about the Pillars of Heracles is joined on to India, one sea connecting them. It is here, too, that he quotes the result arrived at by mathematicians of his time, that the circumference of the earth is 400,000 stades.⁸⁵ He is clear that the earth is much smaller than some of the stars.⁸⁶ On the other hand, the moon is smaller than the earth.⁸⁷ Naturally, Aristotle has a priori reasons for the sphericity of the earth. Thus, using once more his theory of heavy bodies tending to the centre, he assumes that, whether the heavy particles forming the earth are supposed to come together from all directions alike and collect in the centre or not, they will arrange themselves uniformly all round, i.e. in the shape of a sphere, since, if there is any greater mass at one part than at another, the greater mass will push the smaller until the even collection of matter all round the centre produces equilibrium.88

Aristotle’s attempted proof that the earth is in the centre of the universe is of course a petitio principii, He begins by attempting to refute the Pythagorean theory that the earth, like the planets and the sun and moon, moves round the central fire. The Pythagoreans, he says, conceived the central fire to be the abode of sovereignty in the universe, the Watchtower of Zeus, while others might say that the centre, being the worthiest place, is appropriate for the worthiest occupant, and that fire is worthier than earth. To this he replies that the centre of a thing is not so worthy as the extremity, for it is the extremity which limits or defines a thing, while the centre is that which is limited and defined, and is more like a termination than a beginning or principle.⁸⁹ When Aristotle comes to state his own view, he rightly says that heavy bodies, e. g. parts of the earth itself, tend towards the centre of the earth; for bodies which fall towards the earth from different places do not fall in parallel lines but ‘at equal angles’, i. e. at right angles, to the (spherical) surface of the earth, and this proves that they fall in the direction of its centre. Similarly, if a weight is thrown upwards, however great the force exerted, it falls back again towards the centre of the earth.⁹⁰ But, he asks, do bodies tend towards the centre because it is the centre of the universe or because it is the centre of the earth, ‘since both have the same centre’ ?⁹¹ He replies that they must tend towards the centre of the universe because, in the reverse case of the light elements, e.g. fire, it is the extremities of the space which envelops the centre (i.e. the extremities of the universe) to which they naturally tend.⁹² Even the show of argument in the last sentence does not prevent the whole of the reasoning from being a petitio principii For it is exclusively based on the original assumption that, of the four elements, earth and, next to that, water tend to move in a straight line ‘downwards’, i.e., on Aristotle’s view, towards the centre of the universe,⁹³ the effect of which is that not only do the particles of the earth tend to the centre of the universe, but a fortiori the earth itself, which must therefore occupy the centre of the universe.94

Another argument is that, according to the astronomical views of the mathematicians, the phenomena which are observed as the heavenly bodies change their positions relatively to one another are just what they should be on the assumption that the earth is in the centre.⁹⁵ The answer to this is, as Martin says, ‘How do you know ? And how can you use the argument when you have quoted, without stating any objection to it, the argument of the Pythagoreans that their theory of the motion of the earth need cause no sensible difference of parallax in comparison with the theory that the earth is at the centre ?’

The earth being in the centre of the universe, what keeps it there? Dealing with this question, Aristotle again begins by a consideration of the views of earlier philosophers. He rejects Thales’ view that the earth floats on water as contrary to experience, since earth is heavier than water, and we see water resting or riding on earth, but not the reverse. He rejects, too, the view of Anaximenes, Anaxagoras, and Democritus that it rides on the air because it is flat and, acting like a lid to the air below it, is supported by it. Aristotle points out first that, if it should turn out that the earth is round and not flat, it cannot be the flatness which is the reason of the air supporting it; according to the argument it must rather be its size than its flatness, and, if it were large enough, it might even be a sphere. With this he passes on.96 Nor does Empedocles’ theory meet with more favour; if the earth is kept in its place in the same way as water in a cup which is whirled round, this means that the earth is kept in its place by force, and to this view Aristotle opposes his own theory that the earth must have some natural tendency, and a proper place, of its own. Even assuming that it came together by the whirling of a vortex, why do all heavy bodies now tend towards it ? The whirling is at all events too far away from us to cause this. And why does fire move upwards ? This cannot be through the whirling either; and, if fire naturally tends to move to a certain region, surely the earth should too. But, indeed, the heavy and the light were prior to the whirling, and what determines their place is, not whirling, but the difference between ‘up’ and ‘down’. Finally he deals with the view of Anaximander (followed by Plato) that the earth is in equilibrium through being equidistant from all points of the circumference, and therefore having no reason to move in one direction rather than another. Incidentally comparing the arguments (1) that, if you pull a hair with force and tension exactly equal throughout, it will not break, and (2) that a man would have to starve if he had victuals and drink equally disposed all round him, Aristotle again complains that the theory does not take account of the natural tendency of one thing to move to the centre, and of another to move to the circumference. It happens incidentally to be true that a body must remain at the centre if it is not more proper for it to move this way or that, but whether this is so or not depends on the body; it is not the equidistance from the extremities which keeps it there, for the argument would require that, if fire were placed in the centre, it would remain there, whereas in fact it would not, since its tendency to fly upwards would carry it uniformly in all directions towards the extremities of the universe; hence it is not the equidistance, but the natural tendency of the body, which determines the place where it will rest.⁹⁷

In setting himself to prove that the earth has no motion whatever, Aristotle distinguishes clearly between the two views (1) of those who give it a motion of translation or ‘make it one of the stars’, and (2) of those who regard it as packed and moving about an axis through its centre.98 Though he arbitrarily adds the words ‘and moves’ to the phraseology of the Timaeus, thereby making it appear that Plato attributed to the earth a rotation about its axis, which, as we have seen, he could not have done, the second of the two views was actually held by Heraclides Ponticus, who was Aristotle’s contemporary. It seems likely, as Dreyer suggests,⁹⁹ that, in speaking of a motion of the earth ‘at the centre itself’,¹⁰⁰ Aristotle is not thinking of a rotation of the earth in twenty-four hours, i.e. a rotation replacing the apparent revolution of the fixed stars, as Heraclides assumed that it did; for he does not mention the latter feature or give any arguments against it; on the contrary, he only deals with the general notion of a rotation of the earth, and moreover mixes up his arguments against this with his arguments against a translation of the earth in space. He uses against both hypotheses his fixed principle that parts of the earth, and therefore the earth itself, move naturally towards the centre.¹⁰¹ Whether, he says, the earth moves away from the centre or at the centre, such movement could only be given to it by force; it could not be a natural movement on the part of the earth because, if it were, the same movement would also be natural to all its parts, whereas we see them all tend to move in straight lines towards the centre; the assumed movement, therefore, being due to force and against nature, could not be everlasting, as the structure of the universe requires.¹⁰²

The second argument, too, though directed against both hypotheses, really only fits the first, that of motion in an orbit.

‘Further, all things which move in a circle, except the first (outermost) sphere, appear to be left behind and to have more than one movement; hence the earth, too, whether it moves about the centre or in its position at the centre, must have two movements. Now, if this occurred, it would follow that the fixed stars would exhibit passings and turnings . This, however, does not appear to be the case, but the same stars always rise and set at the same places on the earth.’103

The bodies which appear to be ‘left behind and to have more movements than one’ are of course the planets. The argument that, if the earth has one movement, it must have two, is based upon nothing more than analogy with the planets. Aristotle clearly inferred as a corollary that, if the earth has two motions, one must be oblique to the other, for it would be obliquity to the equator in at least one of the motions which would produce what he regards as the necessary consequence of his assumption, namely that the fixed stars would not always rise and set at the same places. As already stated, Aristotle can hardly have had clearly in his mind the possibility of one single rotation about the axis in twenty-four hours replacing exactly the apparent daily rotation; for he would have seen that this would satisfy his necessary condition that the fixed stars shall always rise and set at the same places, and therefore that he would have to get some further support from elsewhere to his assumption that the earth must have two motions. Still less could he have dreamt of the possibility of Aristarchus’s later hypothesis that the earth has an annual revolution as well as a daily rotation about its axis, which hypothesis satisfies, as a matter of fact, both the condition as to two motions and the condition as regards the fixed stars.

The Meteorologica deals with the sublunary portion of the heavenly sphere, the home of the four elements and their combinations. Only a small portion of the work can be said to be astronomical, but some details bearing on our subject may be given. We have seen the four elements distinguished according to their relative heaviness or lightness, and the places which are proper to them respectively; in the Meteorologica they are further distinguished according to the tangible qualities which are called their causes . These tangible qualities are the two pairs of opposites, hot—cold, and dry—moist; and when we take the four combinations of these in pairs which are possible we get the four elements; hot and dry = fire, hot and moist = air (air being a sort of vapour), cold and moist = water, cold and dry = earth.¹⁰⁴ Of the four qualities two, hot and cold, are regarded as active, and the other two, dry and moist, as passive.105 Since each element thus contains an active as well as a passive quality, it follows that all act upon and are acted upon by one another, and that they mingle and are transformed into one another.¹⁰⁶ Every composite body contains all of them;¹⁰⁷ they are never, in our experience, found in perfect purity.¹⁰⁸ Elemental fire is warm and dry evaporation,¹⁰⁹ not flame; elemental fire is a sort of ‘inflammable material’ which ‘can often be kindled by even a little motion, like smoke’;¹¹⁰ but flame, or fire in the sense of flame, is ‘an excess of heat or a sort of ebullition’,¹¹¹ or an ebullition of dry wind¹¹² or of dry heat¹¹³; again, flame is said to be a fleeting, non-continuous product of the transformation of moist and dry in close contact.¹¹⁴ The reason for this distinction between ‘fire’ and flame is obvious, as Zeller says; for Aristotle could not have made the outer portion of the terrestrial sphere, contiguous to the aether, to consist of actual burning flame. According to Aristotle, the stars are not made of fire (still less all the spaces between them); in themselves they are not even hot; their light and heat come from friction with the air through which they move [notwithstanding that they are in the aethereal sphere]; the air in fact becomes fire through their impact on it; the stratum of air which lies nearest to them underneath the aethereal sphere is thus warmed. Especially is this the case with the sun; the sun is able to produce heat in the place where we live because it is not so far off as the fixed stars and it moves swiftly (the stars, though they move swiftly, are far off, and the moon, though near to us, moves slowly); further, the motion often causes the fire surrounding the atmosphere to scatter and rush downwards.¹¹⁵

Such phenomena as shooting stars ( or ) and meteors (of the two kinds called and ) are next dealt with. These are due to two kinds of exhalation, one more vaporous (rising from the water on and in the earth), the other dry and smoke-like (rising from the earth); these go upwards, the latter uppermost, the former below it, until, caught in the rotation at the circumference of the sublunary sphere, they take fire. The particular varieties of appearance which they present depend on the shape of the rising exhalations and the inclination at which they rise. Sometimes, however, they are the result, not of motion kindling them, but of heat being squeezed out of air which comes together and is condensed through cold; in this case their motion is like a throw rather than a burning, being comparable to kernels or pips of fruits pressed between our fingers and so made to fly to a distance; this is what happens when the star falls downwards, since but for such compelling force that which is hot would naturally always fly upwards. All these phenomena belong to the sublunary sphere.116 The aurora is regarded as due to the same cause combined with reflection lighting up the air.¹¹⁷

Aristotle has two long chapters on comets.¹¹⁸ He begins, as usual, by reviewing the opinions of earlier philosophers and so clearing the ground. Anaxagoras and Democritus had explained comets as a ‘conjunction of the planets when, by reason of coming near, they seem to touch one another’. Some of ‘the so-called Pythagoreans’ thought that they were one planet, which we only see at long intervals because it does not rise far above the horizon, the case being similar to that of Mercury, which, since it only rises a little above the horizon, makes many appearances which are invisible to us and is actually seen at long intervals only. Hippocrates of Chios and his pupil Aeschylus gave a similar explanation but added a theory about the tail. The tail, they said, does not come from the comet itself, but the comet, as it wanders through space, sometimes takes on a tail ‘through our sight being reflected, at the sun, from the moisture attracted by the comet’. Explanations by Hippocrates and Aeschylus follow, of the reasons (1) of the long intervals between the appearances of a comet: the reason in this case being that it is only left behind by the sun very slowly indeed, so that for a long time it remains so close to the sun as not to be visible; (2) of the impossibility of a tail appearing when the comet is between the tropical circles or still further south : in the former position the comet does not attract the moisture to itself because the region is burnt up by the motion of the sun in it, and, when it is still further south, although there is plenty of moisture for the comet to attract, a question of angles (only a small part of the comet’s circle being above the horizon) precludes the sight being reflected at the sun in this case, whether the sun be near its southern limit or at the summer solstice; (3) of the comet’s taking a tail when in a northerly position : the reason here being that a large portion of the comet’s circle is above the horizon, and so the reflection of the sight is physically possible. Aristotle states objections, some of which apply to all, and others to some only, of the above views. Thus (1) the comet is not a planet, because all the planets are in the zodiac circle, while comets are often outside it; (2) there have often been more than one comet at one time; (3) if the tail is due to ‘reflection’, and a comet has not a tail in all positions, it ought sometimes to appear without one; but the five planets are all that we ever see, and they are often all of them visible above the horizon; and, whether they are all visible, or some only are visible (the others being too near the sun), comets are often seen in addition. (4) It is not true that comets are only seen in the region towards the north and when the sun is near the summer solstice; for the great comet which appeared at the time of the earthquake and tidal wave in Achaea [373/2 B.C.] appeared in the region where the sun sets at the equinox, and many comets have been seen in the south. Again (5) in the archonship of Eucles, the son of Molon, at Athens [427/6 B.C.], a comet appeared in the north in the month of Gamelion, when the sun was at the winter solstice, although, according to the theory, reflection of the sight would then be impossible. Aristotle proceeds :

‘It is common ground with the thinkers just criticized and the supporters of the theory of coalescence that some of the fixed stars, too, take a tail; on this we must accept the authority of the Egyptians (for they, too, assert it), and moreover we have ourselves seen it. For one of the stars in the haunch of the Dog got a tail, though only a faint one; that is to say, when one looked intently at it, its light was faint, but when one glanced easily at it, it appeared brighter.

‘Moreover, all the comets seen in our time disappeared, without setting, in the expanse above the horizon, fading from sight by slow degrees and in such a way that no astral substance, either one star or more, remained. For instance, the great comet before mentioned appeared in the winter of the archonship of Astaeus [373/2 B.C.], in clear and frosty weather, from the beginning of the evening; the first day it was not seen because it had set-before the sun, but on the following day it was visible, being the least distance behind the sun that allowed of its being seen at all, and setting directly; the light of this comet stretched over a third part of the heaven with a great leap as it were , so that people called it a street. And it went back as far as the belt of Orion and there dispersed.

‘Nevertheless Democritus for one stoutly defended his own theory, asserting that stars had actually been seen to remain on the dissolution of comets. But in that case it should not have sometimes happened and sometimes failed to happen; it should have happened always. The Egyptians, too, say that conjunctions take place of planets with one another and of planets with the fixed stars; we have, however, ourselves seen the star of Zeus twice meet one of the stars in the Twins and hide it, without any comet resulting.’

Aristotle adds that this explanation of comets is untenable on general grounds, since, although stars may seem large or small, they appear to be indivisible in themselves. Now, if they were really indivisible, they would not produce anything bigger by coming in contact with one another; therefore similarly, if they only seem indivisible, they cannot seem by meeting to produce anything bigger.

Aristotle’s own theory of comets explains them as due, much like meteors, to exhalations rising from below and catching fire when they meet that other hot and dry substance (also here called exhalation) which, being the first (i.e. outermost) portion of the sublunary sphere and in direct contact with the revolution of the upper (aethereal) part of the heavenly sphere, is carried round with that revolution and even takes with it part of the contiguous air. The necessary conditions for the formation of a comet, as distinct from a shooting star or meteor, are that the fiery principle which the motion of the upper heaven sets up in the exhalation must neither be so very strong as to produce swift and extensive combustion, nor yet so weak as to be speedily extinguished, but of moderate strength and moderate extent, and the exhalation itself must be ‘well-tempered’ ; according to the shape of the kindled exhalation it is a comet proper or the ‘bearded’ variety . But two kinds of comets are distinguished. One is produced when the origin of the exhalation is in the sublunary sphere; this is the independent comet . The other is produced when it is one of the stars, a planet or a fixed star, which causes the exhalation, in which case the star becomes a comet and is followed round in its course by the exhalation, just as haloes are seen to follow the sun and the moon. Comets are thus bodies of vapour in a state of slow combustion, moving either freely or in the wake of a star. Aristotle maintains that his view that comets are formed by fire produced from exhalations in the manner described is confirmed by the fact that in general they are a sign of winds and droughts. When they are dense and there are more of them, the years in which they appear are noticeably dry and windy; when they are fewer and fainter, these characteristics are less pronounced, though there is generally some excess of wind either in respect of duration or of strength. He adds the following remarks on particular cases :

‘On the occasion when the (meteoric) stone fell from the air at Aegospotami, it was caught up by a wind and was hurled down in the course of a day;119 and at that time too a comet appeared from the beginning of the evening. Again, at the time of the great comet [373/2 B.C., see pp. 244, 245 above] the winter was dry and arctic, and the tidal wave was caused by the clashing of contrary winds; for in the bay the north wind prevailed, while outside it a strong south wind blew. Further, during the archonship of Nicomachus at Athens [341/0 B.C.] a comet was seen for a few days in the neighbourhood of the equinoctial circle; it was at the time of this comet, which did not rise with the beginning of the evening, that the great gale at Corinth occurred.’

It has been pointed out that Aristotle’s account of comets held its ground among the most distinguished astronomers till the time of Newton.120

Passing to the subject of the Milky Way,¹²¹ Aristotle again begins with criticisms of earlier views. The first opinion mentioned is that of the Pythagoreans, some of whom said that it was the path of one of the stars which were cast out of their places in the destruction said to have occurred in Phaethon’s time; while others said that it was the path formerly described by the sun, so that this region was, so to speak, set on fire by the sun’s motion. But, Aristotle replies, if this were so, the zodiac circle should be burnt up too, nay more so, since it is the path not only of the sun but of the planets also. But we see the whole of the zodiac circle at one time or another, half of it being seen in a night; and there is no sign of such a condition except at points where it touches the Milky Way. The remarkable hypothesis of Anaxagoras and Democritus is next controverted; we have already (pp. 83–5) quoted Aristotle’s criticisms. Next, a third view is mentioned according to which the Milky Way is ‘a reflection of our sight at the sun’, just as comets had been declared to be. Aristotle refutes this rather elaborately. (1) If, he says, the eye, the mirror (the sun) and the thing seen (the Milky Way) were all at rest, one and the same part of the reflection would belong to one and the same point of the mirror; but if the mirror and the thing seen move at invariable distances from our eye (which is at rest), but at different speeds and distances relatively to one another, it is impossible that the same part of the reflection should always be at the same point of the mirror. Now the latter of the two hypotheses is that which corresponds to the facts, because the stars in the Milky Way and the sun respectively move at invariable distances from us, but at different distances and speeds in relation to one another; for the Dolphin rises sometimes at midnight, and sometimes at sunrise, but the parts of the Milky Way remain the same in either case; this could not be so if the Milky Way were a reflection instead of a condition of the actual localities over which it extends. (2) Besides, how can the visual rays be reflected at the sun during the night? Aristotle’s own explanation puts the Milky Way on the same footing as the second kind of comets, those in which the separation of the vapour which takes fire on coming into contact with the outer revolution is caused by one of the stars; the difference is that what in the case of the comet happens with one star takes place in the case of the Milky Way throughout a whole circle of the heaven and the outer revolution. The zodiac circle, owing to the motion in it of the sun and planets, prevents the formation of the exhalations in that neighbourhood; hence most comets are seen outside the tropic circles. The sun and moon do not become comets because they separate out the exhalation too quickly to allow it to accumulate to the necessary extent. The Milky Way, on the other hand, represents the greatest extent of the operation of the process of exhalation; it forms a great circle and is so placed as to extend far beyond the tropic circles. The space which it occupies is filled with very great and very bright stars, as well as with those which are called ‘scattered’ ; this is the reason why the collected exhalations here form a concretion so continuous and so permanent. The cause is indeed indicated by the fact that the brightness is greater in that half of the circle where it is double, for it is there that the stars are more numerous and closer together than elsewhere.

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1 Metaph. Z. 8, 1033 b 16, Z. 9, 1034 b 7, Λ. 3, 1069 b 35, &c.

² Phys. i. 9, 192 a 22–32.

³ Metaph. Λ. 6, 1071 b 7.

⁴ Ibid. 1071 b 4.

⁵ Phys. viii. 6, 259 b 22; c. 10, 267 a 24.

⁶ Metaph. Λ. 8, 1073 a 25, 1074 a 36, &c.

⁷ Ibid. Λ. 7, 1072 b 7–11.

⁸ Ibid. Λ. 6, 1071 b 12.

⁹ Ibid. Λ. 6, 1071 b 20.

¹⁰ Ibid. Λ. 9, 1075 a 7.

¹¹ De caelo i. 9, 279 a 18 sq.; Phys, viii. 10, 267 b 18.

¹² De anima iii. 2, 426 a 10.

¹³ Metaph, Λ. 7, 1072 a 25.

¹⁴ De caelo, loc. cit.

¹⁵ Metaph. Λ. 7, 1072 a 26; De anima iii. 10, 433 a 18.

¹⁶ Eth. N. x. 8, 1178 b 21; Metaph. Λ. 9, 1074 b 25.

¹⁷ De caelo ii. 12, 292 b 5; Politics, H. I, 1323 b 23.

¹⁸ Metaph. Λ. 9, 1075 a 10.

¹⁹ Metaph. Λ. 7, 1072 b 28.

²⁰ De caelo ii. 12, 292 a 22; Eth. N. x. 8, 1178 b 20.

²¹ Metaph. Λ. 7, 1072 a 26.

²² Ibid. 1072 b 3.

²³ Phys. viii. 10, 266 b 27–267 a 18.

²⁴ De gen. et corr. i. 6, 323 a 31.

²⁵ Metaph. e. 10, 1051 b 24; Λ. 7, 1072 b 21.

²⁶ Phys. viii. 10, 267 b 7–9.

²⁷ Sextus Emp. Adv. Math. x. 33; Hypotyp. iii. 218.

²⁸ De caelo i. 9, 279 a 16–28.

²⁹ Phys. viii. 8, 261 b 29.

³⁰ Phys. viii. 8, 261 b 31–4.

³¹ Ibid. iii. 5, 206 a 7; c. 6, 206 a 16.

³² Ibid. viii. 8, 261 a 27–263 a 3, 264 a 7 sqq.; c. 9, 265 a 13 sq.

³³ Aristotle is careful, however, to explain that the division between air and fire is not a strict one, as between two definite layers; there is some intermixture (cf. Meteor. i. 3, 341 a 1–9). Further, the ‘fire’ is what from force of habit we call fire; it is not really fire, for fire is an excess of heat, a sort of ebullition (ibid. 340 b 22, 23).

³⁴ De caelo i. 2, 268 b 26–269 b 17.

³⁵ Ibid.

³⁶ De caelo i. 3, 269 b 18–33.

³⁷ Ibid. 270 a 12–35.

³⁸ Ibid. 270 b 1–24.

³⁹ De caelo iii. 1, 298 b 6; Meteor, i. 1, 338 b 21, &c.

⁴⁰ De gen. an. ii. 3, 736 b 29–31.

⁴¹ De caelo i. 3, 270 b 11–16.

⁴² De gen. an. ii. 3, 737 a i.

⁴³ De caelo ii. 8, 290 a 7–b 11.

⁴⁴ Metaph. Λ. 8, 1073 a 34.

⁴⁵ Ibid. 1074 a 38–b 3; Eth. N. vi. 1143 b 1.

⁴⁶ Meteor, i. 3, 340 b 6–10.

⁴⁷ Meteor, ii. 3, 357 b 30.

⁴⁸ De caelo i. 9, 278 b 8–279 a 14.

⁴⁹ Metaph. Λ. 8, 1074 a 36–8.

⁵⁰ De caelo i. 9, 277 b 27–278 a 28.

⁵¹ Phys. iii. 5, 204 b 3–35.

⁵² De caelo i. 5, 271 b 28–272 a 7.

⁵³ De caelo i. 7, 275 b 12–15.

⁵⁴ Ibid. ii. 4, 286 b 10–287 a 5.

⁵⁵ Ibid. ii. 4, 287 a 11–22.

⁵⁶ De caelo ii. 4, 287 a 30–b 4.

⁵⁷ Cf. Phys. vii. 2, 243 a 5.

⁵⁸ De caelo i. 9, 278 b 16–18.

⁵⁹ De caelo ii. 4, 287 a 5–11.

⁶⁰ Phys. vii. 1, 242 b 24–6, vii. 2, 243 a 3–5.

⁶¹ Phys, viii. 6, 259 a 18.

⁶² Metaph. Λ. 8, 1073 a 14–b 10.

⁶³ Metaph. Λ. 8. 1074 a 13–16.

⁶⁴ Ibid. 1073 b 10–17.

⁶⁵ Phys. viii. 6, 259 b 29–31.

⁶⁶ De caelo ii. 2, 284 b 6–286 a 2.

⁶⁷ This matter also is fully discussed by Boeckh, Das kosmische System des Platon, pp. 112–19.

⁶⁸ De caelo ii. 2, 284 b 9–10.

⁶⁹ Ibid. 285 b 1–3.

⁷⁰ De caelo ii. 2, 285 b 20.

⁷¹ Sirhplicius on De caelo, p. 392, 1, Heib.

⁷² De caelon. 8, 290a31–b5; c. 11, 291b 11–17.

⁷³ Plato, Timaeus 55 D, E.

⁷⁴ De caelo ii. 11, 291 b 17–23.

⁷⁵ De caelo i. 9, 278 b 11–14.

⁷⁶ Ibid. ii. 8, 289 b 1 sqq.

⁷⁷ De caelo ii. 8, 289 b 32.

⁷⁸ Ibid. ii. 8, 290 a 9–23.

⁷⁹ Plato, Timaeus 40 A.

⁸⁰ De caelo ii. 8, 290 a 26.

⁸¹ Cf. Martin, ‘Hypothèses astronomiques grecques’ in Mémoires de l’Acad.des Inscriptions et Belles-Lettres, xxx. 1881, p. 287; Dreyer, Planetary Systems, p. III, note.

⁸² De caelo ii. 13, 294 a 1.

⁸³ seems clearly to be the circumference of the sun (not that of the horizon which cuts the solar disc).

⁸⁴ De caelo ii. 14, 297 b 23–30.

⁸⁵ Ibid. 297 b 30 – 298 a 20.

⁸⁶ Ibid. 298 a 19; Meteor, i. 3, 339 b 7–9.

⁸⁷ Aëtius, ii. 26. 3 (D. G. p. 357 b 11).

⁸⁸ De caelo ii. 14, 297 a 8–b 18.

⁸⁹ Ibid. ii. 13, 293 a 17–b 15.

⁹⁰ Ibid. ii. 14, 296 b 18–25.

⁹¹ Ibid. 296 b 9–12.

⁹² Ibid. 296 b 12–15.

⁹³ Phys. iv. 4, 212 a 26; c. 8, 214 b 14; and especially De caelo iv. 1, 308 a 15–31. In the last-cited passage Aristotle, without mentioning Plato by name, attacks Plato’s doctrine that, in a perfect sphere such as the universe is, you cannot properly describe one part rather than another as ‘up’ or ‘down’; on the contrary, says Aristotle, I call the centre, where heavy bodies collect, ‘down’, and the extremities of the sphere, whither light bodies tend to rise, ‘up’, But, as usual, there is less difference between the two views than Aristotle would have us believe. Plato (Timaeus 62 D) said, it is true, that, as all points of the circumference are equidistant from the centre, it is incorrect to apply the terms ‘up’ and ‘down’ to different specific portions of that circumference, or to call any portion of the sphere ‘up’ or ‘down’ relatively to the centre, which is neither ‘up’ nor ‘down’, but simply the centre. But he goes on to say (63 B–E)that you can use the terms in a purely relative sense; any two localities may be ‘up’ and ‘down’ relatively to one another, and Plato proposes a criterion. If a body tends to move to a certain place by virtue of seeking for its like, this tendency is what constitutes its heaviness, and the place to which it tends is ‘down’; and the opposite terms have the opposite meanings. The only difference made by Aristotle is in definitely allocating the centre of the universe as the place of the heaviest element, earth, and arranging the other elements in order of lightness in spherical layers round it, so that on his system the centre of the universe becomes ‘down’, and any direction outwards along a radius is ‘up’.

⁹⁴ De caelo ii. 14, 296 b 6–9.

⁹⁵ Ibid. 297 a 2–6.

⁹⁶ De caelo ii. 13, 294 a 28–b 30.

^96a Ibid. 295 a 16–b 9.

⁹⁷ Ibid. 295 b 10 – 296 a 21.

⁹⁸ De caelo ii. 13, 293 a 20–3, b 18–20; c. 14, 296 a 25–7.

⁹⁹ Dreyer, Planetary Systems, pp. 116, 117.

¹⁰⁰ De caelo ii. 14, 296 a 29, b 2.

¹⁰¹ Ibid. 296 b 6–8.

¹⁰² Ibid. 296 a 27–34.

¹⁰³ De caelo ii. 14, 296 a 34–b 6.

¹⁰⁴ De gen. et corr. ii. 3, 330a 30–b 7.

¹⁰⁵ Meteor, iv. i, 378 b 10–13, 21–5.

¹⁰⁶ De gen. et corr. ii. 2, 329 b 22 sq.

¹⁰⁷ Ibid. c. 8, 334 b 31 sq.

¹⁰⁸ Ibid. c. 3, 330 b 21; Meteor. ii. 4, 359 b 32, &c.

¹⁰⁹ Meteor. i. 3, 340 b 29; c. 4, 341 b 14.

¹¹⁰ Ibid. 341 b 19–21.

¹¹¹ Ibid. c. 3, 340 b 23.

¹¹² Ibid. c. 4, 341 b 21–2,

¹¹³ De gen. et corr. ii. 3, 330 b 29.

¹¹⁴ Meteor, ii. 2, 355 a 9.

¹¹⁵ De caelo ii. 7, 289 a 13–35; Meteor, i. 3, 340 a 1, 341 a 12–36.

¹¹⁶ Meteor, i. 4, 341 b–342 a.

¹¹⁷ Ibid. c. 5, 342 a 34–b 24.

¹¹⁸ Ibid. cc. 6, 7, 342 b 25 – 345 a 10.

¹¹⁹ This appears to be the earliest mention of the meteoric stone of Aegospotami by any writer whose works have survived. The date of the occurrence was apparently in the archonship of Theagenides [468/7 B.C.]. The story that Anaxagoras prophesied that this stone would fall from the sun (Diog. L. ii. 10) was probably invented by way of a picturesque inference from his well-known theory that the fiery aether whirling round the earth snatched stones from the earth and, carrying them round with it, kindled them into stars (Aët. ii. 13. 3; D.G. p. 341; Vorsokratiker, i², p. 307. 16), and that one of the bodies fixed in the heaven might break away and fall (Diog. L. ii. 12; Plutarch, Lysander 12; Vorsokratiker, i², pp. 294. 29, 296. 34). Diogenes of Apollonia, too, a contemporary of Anaxagoras, said that along with the visible stars there are also stones carried round, which are invisible, and are accordingly unnamed; ‘and these often fall upon the earth and are extinguished like the stone star which made a fiery fall at Aegospotami’ (Aët. ii. 13. 9; D. G. p. 342; Vorsokratiker, i², p. 330. 5–8).

¹²⁰ Ideler, Aristotelis Meteorologica, vol. i, p. 396. Yet Seneca (Nat. Quaest. vii) had much sounder views on comets. He would not admit that they could be due to such fleeting causes as exhalations and rapid motions, as of whirlwinds, igniting them; if this were their cause, how could they be visible for six months at a time (vii. 10. 1)? They are not the effects of sudden combustion at all, but eternal products of nature (22. 1). Nor are they confined to the sublunary sphere, for we see them in the upper heaven among the stars (8. 4). If it is said that they cannot be ‘wandering stars’ because they do not move in the zodiac circle, the answer is that there is no reason why, in a universe so marvellously constructed, there should not be orbits in other regions than the zodiac which stars or comets may follow (24. 2–3). It is true, he says, that, owing to the infrequency of the appearances of comets, their orbits have not as yet been determined, nay, it has not been possible even to decide whether they keep up a definite succession and duly appear on appointed days. In order to settle these questions, we require a continuous record of the appearances of comets from ancient times onwards (3. 1). When generation after generation of observers have accumulated such records, there will come a time when the mystery will be cleared up; men will some day be found to show ‘in what regions comets run their courses, why each of them roams so far away from the others, how large they are and what their nature; let us, for our part, be content with what we have already discovered, and let our posterity in their turn contribut to the sum-of truth (25. 7).’

¹²¹ Meteor. i. 8, 345 a 11 – 346 b 10.