XXIII
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In none of the ordinary fires in the sky is the route curved; it is distinctive of a star (planet) that it describes a curve in its orbit. Whether other comets had this circular orbit I cannot say. The two in our own age at any rate had. Again, everything kindled by a temporary cause quickly gives out. Thus torches gleam only while they flit across the sky; thus lightning has strength for just one stroke; thus so-called shooting and falling stars fly past, cutting through the air. No fires have any considerable duration unless their strength is inherent. I mean the divine fires which the universe maintains eternally, because they are its parts and works. These, I say, are always active; they have an orbit the even tenor of which they preserve, and they are uniform. They would on alternate days be larger or smaller if the fire was merely casual, the sudden outcome of some accidental cause. Such a fire would be greater or less according as it was fed more abundantly or more scantily. I said a moment ago that no fire could be lasting which arose from some defect in the atmosphere. I have now to add further, that it can by no means be fixed and steady. Both torch and lightning and shooting star, and any other kind of fire forced out of the air by pressure, are in flight; none of them is visible save in the course of its fall. But a comet has its own settled position. For that reason it is not expelled in haste, but steadily traverses its course; it is not snuffed out, but takes its departure. If it were a wandering star (i.e. planet), says some one, it would be in the zodiac. Who, say I, ever thinks of placing a single bound to the stars? or of cooping up the divine into narrow space? These very stars, which you suppose to be the only ones that move, have, as every one knows, orbits differing one from another. Why, then, should there not be some stars that have a separate distinctive orbit far removed from them? What reason is there why there should not be passages into the heavens at some part of them?98 But if you are convinced that every star (planet) cannot but touch the zodiac, then I say the comet might have such a wide orbit that at some point it may coincide with the zodiac. This is not necessary, but it is possible.