Lecture 6 — Shooting stars and meteors—their cause and difference

Having determined these principles let us explain the cause of the appearance in the sky of burning flames and of shooting stars, and of 'torches', and 'goats', as some people call them. All these phenomena are one and the same thing, and are due to the same cause, the difference between them being one of degree.

The explanation of these and many other phenomena is this. When the sun warms the earth the evaporation which takes place is necessarily of two kinds, not of one only as some think. One kind is rather of the nature of vapor, the other of the nature of a windy exhalation. That which rises from the moisture contained in the earth and on its surface is vapor, while that rising from the earth itself, which is dry, is like smoke. Of these the windy exhalation, being warm, rises above the moister vapor, which is heavy and sinks below the other. Hence the world surrounding the earth is ordered as follows. First below the circular motion comes the warm and dry element, which we call fire, for there is no word fully adequate to every state of the fumid evaporation: but we must use this terminology since this element is the most inflammable of all bodies. Below this comes air.

We must think of what we just called fire as being spread round the terrestrial sphere on the outside like a kind of fuel, so that a little motion often makes it burst into flame just as smoke does: for flame is the ebullition of a dry exhalation.

So whenever the circular motion stirs this stuff up in any way, it catches fire at the point at which it is most inflammable. The result differs according to the disposition and quantity of the combustible material.

If this is broad and long, we often see a flame burning as in a field of stubble: if it burns lengthwise only, we see what are called 'torches' and 'goats' and shooting-stars. Now when the inflammable material is longer than it is broad sometimes it seems to throw off sparks as it burns. (This happens because matter catches fire at the sides in small portions but continuously with the main body.) Then it is called a 'goat'. When this does not happen it is a 'torch'. But if the whole length of the exhalation is scattered in small parts and in many directions and in breadth and depth alike, we get what are called shooting-stars. The cause of these shooting-stars is sometimes the motion which ignites the exhalation. At other times the air is condensed by cold and squeezes out and ejects the hot element; making their motion look more like that of a thing thrown than like a running fire.

38. Having laid down those things introduced to explain what is to follow, the Philosopher begins:

first, to determine concerning things generated on high out of dry matter;

second, things generated on high from moist matter, at **let us go on to treat of the region **(346b16).

The first is divided into three parts:

in the first he determines about falling stars and things having a like cause;

in the second about comets at **let us go on to explain the nature of comets **(342b25);

in the third about the milky circle called the "galaxy" at **let us now explain the origin, cause, and nature **(345a11).

About the first he does two things:

first, he determines about falling stars and other similar things;

second, about certain other apparitions seen in the air, at sometimes on a fine night (342a34).

About the first he does two things:

first, he states his intention and says that, having determined the foregoing, we must explain the cause of the appearance in the heaven of burning flames and of shooting stars and of so-called dali (341b1), i.e., torches, and aeges, i.e., goats. They will be discussed at one and the same time, because they are all alike in kind and produced by the same cause, and differ only in degree, as will be clear below.

39. second at the explanation of these (341b5), he determines his proposition, about which he does two things.

First, he states the causes generating the aforesaid, and says that the principle, both active and passive, of the aforesaid phenomena and of many others is what he will indicate. For when the earth has been warmed by the sun's motion, a certain exhalation is necessarily released from the earth. This is not of one sort, as some think, but is twofold: one is more vaporous and moist, the other more foam-like and dry—for from the aqueous moisture upon the earth's surface there is released and lifted on high a vaporous exhalation which is moist; from the earth itself, which is by nature dry, there is raised a fume-like or foam-like exhalation. Of these, the foam-like exhalation rises above the other on account of warmth which dominates in it and renders it more subtle: for the dry and warm is light—and fire is of this nature. But the vaporous exhalation, which is more moist, finds its place under the foam-like, being heavier, for it is not so fine: hot and moist pertain to the nature of the air, which is below fire, which is hot and dry.

The very order of the elements surrounding the earth attests to this. For under the circular motion of the heaven there is first located what is hot and dry and which is commonly called "fire," though that is not its proper name, as has been said above: for, since the item common to every smoky exhalation has no name, and such is especially apt to burn, consequently, it was necessary to use words in keeping, and so such a fume-like evaporation comes to be called "fire." Under this fume-like exhalation is air. Thus we have posited both the effective cause of the aforesaid passions, which is the sun's movement, and the material cause, namely, the fume-like exhalation.

40. Second, at we must think (341b18), he determines concerning the generation of the aforesaid passions. About this he does two things:

first, he assigns the cause of their generation;

second, the reason why certain things accompany them, at the motion is upwards or downwards or sideways according to the way in which the evaporation lies (342a21).

About the first he does three things:

first, he gives the cause of the aforesaid passions in common;

second, their mutual differences, at so whenever the circular motion stirs this stuff up (341b22);

third, he raises a question concerning what he has determined at for the question might be raised (342a3).

He says therefore first that, in the light of the foregoing, we must understand what we have just now called "fire" to be as a certain fuel (341b18), i.e., a combustible material, and that it is situated in the sphericity which is about the earth in the last place (beginning, that is, from the earth), Hence, on account of its proximity to the heavenly motion it often bursts into flame, being heated when only slightly moved, i.e., when slightly stirred by the motion of the body above it, as happens in the case of smoke, when it is ignited and becomes flame: for a flame is nothing but the burning of a dry spirit, i.e., smoke. Therefore the ignition of the above-mentioned fuel, commonly speaking, is responsible for the generation of the aforesaid passions, when matter which is prepared is placed in the proximity of the efficient cause.

41. Then when he says, so whenever the circular motion stirs this stuff up (341b22), he explains the differences among the aforesaid passions. About this he does two things.

First he shows what is the basis for the difference. And he says that from whatever source the aforesaid matter is had (regardless of how this consistency is obtained, namely, the aforesaid matter for burning), and when it is most perfectly disposed to be ignited, then it is so ignited in such a way by heating from the circular motion of the heaven: and the ignited passion varies according to the position and amount of the aforesaid matter.

42. second at if this (341b25), he determines the differences among the aforesaid passions. And he says that if the aforesaid matter has great width and length, there appears to be a certain flame enkindled in the heaven, similar to stubble burning in a field [area]. But if it does not have great width, but only length, then dali, i.e., torches, and aeges, i.e., goats, and shooting stars are generated and appear there.

For if the aforesaid matter is more in length than in width, and when it burns, the fire scintillates, i.e., seems to leap and run about like aeges, i.e., goats (which happens because not all the matter begins to be ignited at once but according to certain small sections, beginning from some starting-point in the matter), when, I say, this happens, it is called aeges, i.e., a goat.

But when the burning of the aforesaid matter takes place without the aforesaid passion, i.e., without scintillation, because the entire material is ignited at once, then it is called a dalus, i.e., a torch.

However, when the exhalation is not continuous, but frequent, and scattered in small areas, and in many ways, both according to length and according to width and even depth, then appear stars that seem to fly, because the material is rapidly consumed and ceased to be where it was previously burning, as happens with flax, if a small amount of it is laid down lengthwise and ignited: for the combustion seems to run along and seems similar to the movement of some fiery body.

In this way, it is therefore plain that the burning flame has most material; a medium amount what are called "torches" and "goats," and least for shooting stars, which accounts for their rather frequent occurrence.

43. But because shooting stars have an additional cause of their generation, he adds that sometimes an exhalation ignited by the sun's movement generates them, but sometimes, too, when cold causes air to thicken, that which is hot within, being thickened, is forced out downward and is separated from the cold; this causes the thickened mass to ignite and a falling star is seen. That is also why the motion of stars falling in that way is not assimilated to a burning, but rather to a projecting.