by Aristotle
Enough has been said, without further argument, to show that the causes brought forward to explain comets are false.
[5] 7 · We consider a satisfactory explanation of phenomena inaccessible to observation to have been given when our account of them is free from impossibilities. The phenomenon available suggest the following account of the matters in [10] question. We suppose that the dry and warm exhalation is the outermost part of the terrestrial world which falls below the circular motion. It, and a great part of the air that is continuous with it below, is carried round the earth by the motion of the circular revolution. In the course of this motion it often ignites wherever it may happen to be of the right consistency, and this we maintain to be the cause of the shooting of scattered stars. We may say, then, that a comet is formed when the [15] upper motion introduces into a condensation of this kind a fiery principle not of such excessive strength as to burn up much of the material quickly, nor so weak as soon to be extinguished, but stronger and capable of burning up much material, and when exhalation of the right consistency rises from below and meets it. The kind of comet [20] varies according to the shape which the exhalation happens to take. If it is diffused equally on every side the star is a comet, if it stretches out in one direction it is called bearded. And just as when a phenomenon of this kind moves we seem to have a shooting-star, so when it stands still we seem to have a star standing still. We may compare these phenomena to a heap or mass of chaff into which a torch is thrust, or [25] a spark thrown. That is what a shooting-star is like. The fuel is so inflammable that the fire runs through it quickly in a line. Now if this fire were to persist instead of running through the fuel and perishing away, its course through the fuel would stop [30] at the point where the latter was densest, and then the whole might begin to move. Such is a comet—like a shooting-star that contains its beginning and end in itself.
When the matter begins to gather in the lower region independently the comet appears by itself. But when the exhalation is constituted by one of the fixed stars or the planets, owing to their motion, one of them becomes a comet. The fringe is not [344b1] close to the stars themselves. Just as haloes appear to follow the sun and the moon as they move, when the air is dense enough for them to form along under the sun’s [5] course, so too the tail stands in the relation of a halo to the stars, except that the colour of the halo is due to reflection, whereas in the case of comets the colour is something that appears actually on them.
Now when this matter gathers in relation to a star the comet necessarily appears to follow the same course as the star. But when the comet is formed [10] independently it falls behind; for such is the motion of the terrestrial sphere. It is this fact, that a comet often forms independently, indeed oftener than round one of the regular stars, that makes it impossible to maintain that a comet is a sort of reflection, not indeed, as Hippocrates and his school say, to the sun, but to the very [15] star it is alleged to accompany—in fact, a kind of halo in the pure fuel.
As for the halo we shall explain its cause later.
The fact that comets when frequent foreshadow wind and drought must be [20] taken as an indication of their fiery constitution. For their origin is plainly due to the plentiful supply of that secretion. Hence the air is necessarily drier and the moist evaporation is so dissolved and dissipated by the quantity of the hot exhalation as not readily to condense into water.—But this phenomenon too will be explained more clearly later when the time comes to speak of the winds.—So when [25] there are many comets and they are frequent, it is as we say, and the years are clearly dry and windy. When they are fewer and fainter this effect does not appear in the same degree, though as a rule the wind is found to be excessive either in [30] duration or strength. For instance when the stone at Aegospotami fell out of the air—it had been carried up by a wind and fell down in the daytime—then too a comet happened to have appeared in the west. And at the time of the great comet the winter was dry and north winds prevailed, and the wave was due to an opposition [345a1] of winds. For in the gulf a north wind blew and outside it a violent south wind. Again in the archonship of Nicomachus10 a comet appeared for a few days about the equinoctial circle (this one had not risen in the west), and simultaneously with it there happened the storm at Corinth.
[5] That there are few comets and that they appear rarely and outside the tropic circles more than within them is due to the motion of the sun and the stars. For this motion does not only cause the hot principle to be secreted but also dissolves it when it is gathering. But the chief reason is that most of this stuff collects in the region of [10] the milky way.
8 · Let us now explain the origin, cause, and nature of the milky way. And here too let us begin by discussing the statements of others on the subject.
Of the so-called Pythagoreans some say that this is the path of one of the stars [15] that fell from heaven during the alleged destruction at the time of Phaethon. Others say that the sun used once to move in this circle. Thus this region was scorched or met with some other affection of this kind, because of the motion of these bodies.
But it is absurd not to see that if this were the reason the circle of the Zodiac [20] ought to be affected in the same way, and indeed more so than that of the milky way, since not the sun only but all the planets move in it. We can see the whole of this circle (half of it being visible at any time of the night), but it shows no signs of any such affection except where a part of it touches the circle of the milky way.
[25] Anaxagoras, Democritus, and their schools say that the milky way is the light of certain stars. For, they say, when the sun passes below the earth some of the stars are hidden from it. Now the light of those on which the sun shines is invisible, being obscured by the rays of the sun. But the milky way is the peculiar light of those stars [30] which are shaded by the earth from the sun’s rays.
This, too, is obviously impossible. The milky way is always unchanged and among the same constellations (for it is clearly a greatest circle), whereas, since the sun does not remain in the same place, what is hidden from it differs at different [35] times. Consequently with the change of the sun’s position the milky way ought to change its position too; but we find that this does not happen. Besides, if current [345b1] astronomical demonstrations are correct and the size of the sun is greater than that of the earth and the distance of the stars from the earth many times greater than [5] that of the sun (just as the sun is further from the earth than the moon), then the cone made by the rays of the sun would terminate at no great distance from the earth, and the shadow of the earth (what we call night) would not reach the stars. On the contrary, the sun shines on all the stars and the earth screens none of them.
[10] There is a third theory about the milky way. Some say that it is a reflection of our sight to the sun, just as they say that the comet is.
But this too is impossible. For if the eye and the mirror and the whole of the object were severally at rest, then the same part of the image would appear at the same point in the mirror. But if the mirror and the object move, keeping the same [15] distance from the eye which is at rest, but at different rates of speed and so not always at the same interval from one another, then it is impossible for the same image always to appear in the same part of the mirror. Now the constellations included in the circle of the milky way move; and so does the sun, the object to [20] which our sight is reflected; but we stand still. And the distance of those two from us is constant and uniform, but their distance from one another varies. For the Dolphin sometimes rises at midnight, sometimes in the morning. But in each case the same parts of the milky are found near it. But if it were a reflection and not a genuine affection of these regions, this ought not to be the case. [25]
Again, we can see the milky way reflected at night in water and similar mirrors. But under these circumstances it is impossible for our sight to be reflected to the sun.
These considerations show that the milky way is not the path of one of the planets, nor the light of s
tars unseen by the sun, nor a reflection. And those are pretty well the only views handed down by others hitherto. [30]
Let us recall our fundamental principle and then explain our views. We have already laid down that the outermost part of what is called the air has the powers of fire and that therefore when the air is dissolved by motion, there is separated off a kind of matter—and of this matter we assert that comets consist. We must suppose [35] that what happens is the same as in the case of the comets when the matter does not form independently but is formed by one of the fixed stars or the planets. Then these [346a1] stars appear as comets, because matter of this kind follows their course. In the same way, a certain kind of matter follows the sun, and we explain the halo as a reflection [5] from it when the air is of the right constitution. Now we must assume that what happens in the case of the stars severally happens in the case of the whole of the heavens and all the upper motion. For it is natural to suppose that, if the motion of a single star excites a flame, that of all the stars should have a similar result,11 and especially in that region in which the stars are biggest and most numerous and [10] nearest to one another. Now the circle of the zodiac dissolves this kind of matter because of the motion of the sun and the planets, and for this reason most comets are found outside the tropic circles. Again, no tail appears round the sun or moon; [15] for they dissolve such matter too quickly to admit of its formation. But this circle in which the milky way appears to our sight is the greatest circle, and its position is such that it extends far outside the tropic circles. Besides the region is full of the biggest and brightest constellations and also of what are called ‘scattered’ stars (you [20] have only to look to see this clearly). So for these reasons all this matter is continually and ceaselessly collecting there. A proof of the theory is this: in the circle itself the light is stronger in the half where the milky way is double, and in it the constellations are more numerous and closer to one another than in the other [25] half; which shows that the cause of the light is the motion of the constellations and nothing else. For if it is found in the circle in which there are most constellations and at that point in the circle in which they are densest and contain the biggest and the [30] most stars, it is natural to suppose that they are the most appropriate cause of the affection in question. The circle and the constellations in it may be seen in the diagram. The so-called ‘scattered’ stars it is not possible to set down in the same way on the sphere because none of them has an evident permanent position; but if you [35] look up to the sky the point is clear. For in this circle alone are the intervals full of these stars: in the other circles there are obvious gaps. Hence if we accept the cause [346b1] assigned for the appearance of comets as plausible we must assume that the same kind of thing holds good of the milky way. For the tail which in the former case is an [5] affection of a single star here forms in the same way in relation to a whole circle. So if we are to define the milky way we may call it the tail attaching to the greatest circle, and due to the matter secreted. This, as we said before, explains why there are few comets and why they appear rarely; it is because at each revolution of the heavens this matter has always been and is always being separated off and gathered into this region.
[10] We have now explained the phenomena that occur in that part of the terrestrial world which is continuous with the motions of the heavens, namely, shooting-stars and the burning flame, comets and the milky way, these being the [15] chief affections that appear in that region.
9 · Let us go on to treat of the region which follows next in order after this and which immediately surrounds the earth. It is the region common to water and air, and the processes attending the formation of water above take place in it. We must consider their principles and causes too.
[20] The efficient and chief and first of the principles is the circle in which the sun moves. For the sun as it approaches or recedes, obviously causes dissipation and condensation and so gives rise to generation and destruction. Now the earth remains [25] but the moisture surrounding it is made to evaporate by the sun’s rays and the other heat from above, and rises. But when the heat which was raising it leaves it, in part dispersing to the higher region, in part quenched through rising so far into the upper [30] air, then the vapour cools because its heat is gone and because of the place, and condenses again and turns from air into water. And after the water has formed it falls down again to the earth.
The exhalation of water is vapour: air condensing into water is cloud. Mist is what is left over when a cloud condenses into water, and is therefore rather a sign of [35] fine weather than of rain; for mist might be called a barren cloud.
So we get a circular process that follows the course of the sun. For according as [347a1] the sun moves to this side or that, the moisture in this process rises or falls. We must think of it as a river flowing up and down in a circle and made up partly of air, partly of water. When the sun is near, the stream of vapour flows upwards; when it [5] recedes, the stream of water flows down; and the order of sequence, at all events, in this process always remains the same. So if ‘Oceanus’ had some secret meaning in early writers, perhaps they may have meant this river that flows in a circle about the earth.
So the moisture is always raised by the heat and descends to the earth again when it gets cold. These processes and, in some cases, their varieties are [10] distinguished by special names. When the water falls in small drops it is called a drizzle; when the drops are larger it is rain.
10 · Some of the vapour that is formed by day does not rise high because the ratio of the fire that is raising it to the water that is being raised is small. When this cools and descends at night it is called dew and hoar-frost. When the vapour is [15] frozen before it has condensed to water again it is hoar-frost; and this appears in winter and is commoner in cold places. It is dew when the vapour has condensed into water and the heat is not so great as to dry up the moisture that has been raised, nor [20] the cold sufficient (owing to the warmth of the climate or season) for the vapour itself to freeze. For dew is more commonly found when the season or the place is warm, whereas the opposite, as has been said, is the case with hoar-frost. For obviously vapour is warmer than water, having still the fire that raised it: consequently more cold is needed to freeze it. [25]
Both dew and hoar-frost are found when the sky is clear and there is no wind. For the vapour could not be raised unless the sky were clear, and if a wind were blowing it could not condense.
The fact that hoar-frost is not found on mountains contributes to prove that these phenomena occur because the vapour does not rise high. One reason for this is [30] that it rises from hollow and water places, so that the heat that is raising it, bearing as it were too heavy a burden, cannot lift it to a great height but soon lets it fall again. A second reason is that the motion of the air is more pronounced at a height, and this dissolves a gathering of this kind. [35]
Everywhere, except in Pontus, dew is found with south winds and not with north winds. There the opposite is the case and it is found with north winds and not with south. The reason is the same as that which explains why dew is found in warm [347b1] weather and not in cold. For the south wind brings warm, and the north, wintry weather. For the north wind is cold and so, bringing wintry weather, quenches the heat of the evaporation. But in Pontus the south wind does not bring warmth enough [5] to cause evaporation, whereas the coldness of the north wind surrounds and concentrates the heat, so that there is more evaporation and not less. This is a thing which we can often observe in other places too. Wells, for instance, give off more vapour in a north than in a south wind. But the north winds quench the heat before any considerable quantity of vapour has gathered, while in a south wind the [10] evaporation is allowed to accumulate. The water itself does not freeze in the way that it does in the region of the clouds.
11 · From that region there fall three bodies condensed by cold, namely water, snow, hail. Two of these correspond to the phenomena on the lower level and are due to the sa
me causes, differing from them only in degree and quantity. [15]
Snow and hoar-frost are one and the same thing, and so are rain and dew: only there is a great deal of the former and little of the latter. For rain is due to the cooling of a great amount of vapour; for the region from which and the time during [20] which the vapour is collected are considerable. But of dew there is little; for the vapour collects for it in a single day and from a small area, as its quick formation and scanty quantity show.
The relation of hoar-frost and snow is the same: when cloud freezes there is snow, when vapour freezes there is hoar-frost. Hence snow is a sign of a cold season [25] or country. For a great deal of heat is still present and unless the cold were overpowering it the cloud would not freeze. For there still survives in it a great deal of the heat which12 caused the moisture to rise as vapour from the earth.
Hail on the other hand is found in the upper region, but the corresponding phenomenon in the vaporous region near the earth is lacking. For, as we said, to [30] snow in the upper region corresponds hoar-frost in the lower, and to rain in the upper region, dew in the lower. But there is nothing here to correspond to hail in the upper region. Why this is so will be clear when we have explained the nature of hail.
12 · But we must go on to collect the facts bearing on the origin of it, both [35] those which raise no difficulties and those which seem paradoxical.
Hail is ice, and water freezes in winter; yet hailstorms occur chiefly in spring [348a1] and autumn and less often in the late summer, but rarely in winter and then only when the cold is less intense. And in general hailstorms occur in warmer, and snow in colder places. Again, there is a difficulty about water freezing in the upper [5] region. It cannot have frozen before becoming water; and water cannot remain suspended in the air for any space of time. Nor can we say that the case is like that of particles of moisture which are carried up owing to their small size and rest on the [10] air (the water swimming on the air just as small particles of earth and gold often swim on water). In that case large drops are formed by the union of many small, and so fall down. This cannot take place in the case of hail, since frozen bodies cannot coalesce like liquid ones. Clearly then drops of that size were suspended in the air or else they could not have been so large when frozen.