Lecture 7 — Solution of problems concerning shooting stars

For the question might be raised whether the 'shooting' of a 'star' is the same thing as when you put an exhalation below a lamp and it lights the lower lamp from the flame above. For here too the flame passes wonderfully quickly and looks like a thing thrown, and not as if one thing after another caught fire. Or is a 'star' when it 'shoots' a single body that is thrown?

Apparently both cases occur: sometimes it is like the flame from the lamp and sometimes bodies are projected by being squeezed out (like fruit stones from one's fingers) and so are seen to fall into the sea and on the dry land, both by night and by day when the sky is clear. They are thrown downwards because the condensation which propels them inclines downwards. Thunderbolts fall downwards for the same reason: their origin is never combustion but ejection under pressure, since naturally all heat tends upwards.

When the phenomenon is formed in the upper region it is due to the combustion of the exhalation. When it takes place at a lower level it is due to the ejection of the exhalation by the condensing and cooling of the moister evaporation: for this latter as it condenses and inclines downward contracts, and thrusts out the hot element and causes it to be thrown downwards.

The motion is upwards or downwards or sideways according to the way in which the evaporation lies, and its disposition in respect of breadth and depth. In most cases the direction is sideways because two motions are involved, a compulsory motion downwards and a natural motion upwards, and under these circumstances an object always moves obliquely. Hence the motion of 'shooting-stars' is generally oblique. So the material cause of all these phenomena is the exhalation, the efficient cause sometimes the upper motion, sometimes the contraction and condensation of the air.

Further, all these things happen below the moon. This is shown by their apparent speed, which is equal to that of things thrown by us; for it is because they are close to us, that these latter seem far to exceed in speed the stars, the sun, and the moon.

44. Because he assigned two causes for the generation of shooting stars, he now raises a certain problem about them. With respect to this he does two things.

First, he raises the problem which is this: whether the trajectory of shooting stars is the same as when the smoke-like exhalation of a lower candle is set afire by the flame of a higher candle or light (for in such a case the fire is seen to travel downward with marvelous speed and there appears to be the projection of one and the same fire instead of fire igniting in two distinct bodies); or whether the truth is that the trajectories of falling-stars are the projections of some same falling body.

45. second at apparently both (342a8), he solves this problem. Concerning it he does two things.

First, he says that the trajectories of falling stars seem to be due to both causes. For sometimes such a trajectory results from the continuous enkindling of matter, as was said of the smoke from lamps; but sometimes certain ignited substances are projected out as the result of being expelled by a higher coldness, much like a cherry pit squeezed out by one's fingers. Hence they are seen falling into the earth and sea, during the day as well as the night, when the sky is clear.

He says, during the day and not at night only, because unless such a falling fire approached the earth through movement, it would not be visible during the day. Likewise he says, when the sky is clear, because when is is cloudy, such fire would be snuffed out by the humidity of the clouds and air. But although those things expelled are on fire, and should, it would seem, on that account have to be light and therefore ascend, they are nevertheless cast downwards, because the condensation of cold driving them inclines them downwards.

This is the reason why thunderbolts fall downwards even though ignited: for the generation of all things falling in this way is due, not to their being set afire by something hot which ignites them, but to their being detached by something cold which expels themsince by nature everything hot is borne aloft.

46. Second, at when the phenomenon (342a16), he explains the difference between the trajectories of the stars arising from these two causes. And he says that whatever shooting stars are generated more in the highest region, these are caused by the combustion of an exhalation; but the ones generated farther down are caused by the moister exhalation mixing together, i.e., condensing and cooling. For this moist exhalation, now concentrated and inclining downwards, pushes and, as it were, thrusts downward the hot element, together with some condensed matter.

47. Then when he says, the motion is upwards or downwards or sideways according to the way in which the evaporation lies (342a21), he explains certain phenomena accompanying these events.

About this he does two things:

first, he explains the reason for the type of motion of such falling stars;

second, he determines the place where they are generated, at further, all these things happen below the moon (342a30).

He says therefore first that, depending on the different position of the exhalation with respect to the sides or depth [i.e., top or bottom], the falling star will be moved differently—either above or below, or to the side, of its point of ejection by the cold. For if the condensed cold matter which does the ejecting has come together above, the falling star is moved downward by the expulsion; but if the matter has collected below, then it is moved upward; if it comes together in neither place, then its [the star's] motion is sideways, as though obliquely or along the diameter. And this often happens: for the ejected hot mass is moved with two motions: by nature, as hot, it is moved upwards; but through the violence of the ejection, it is moved downwards. But all such things, whose motions are so combined, are moved according to the diameter, i.e., obliquely, since such a motion is as though a mean between ascent and descent. As a result the motion of falling stars is most often oblique.

Then he summarizes what has been set forth and says that the material cause of all the foregoing is an exhalation; but the movent cause is twofold: for sometimes it is the movement of a higher body, sometimes it is the condensing of air thickened by coldness, and subsequently forcing out the hot.

48. Then when he says, further, all these things happen below the moon (342a30), he determines the place where the aforesaid are produced and says that they are all produced below the moon. A sign of this is that they appear to us to be moving very rapidly, as do things we project, such as arrows and the like, which, for being close to us, seem to be travelling faster than the stars and sun and moon—although it is plain that, in truth, the higher bodies are moved much faster than anything here.