[96] _I.e._ the tail of a comet. XXI Our Stoic friends, therefore, are satisfied that, like trumpet meteors 1 and beams, and other portents of the sky, comets are formed by dense air. They appear in greatest number toward the north, because there is most of the sluggish air there. Why, then, you naturally ask, does the comet not remain stationary, but advance in the sky from day to day? Let me explain. The comet, according to this account, pursues its fuel just as fires do. Although its tendency is to rise to the upper regions, still, if material fail it, it retrogrades and sinks. In the air, too, it does not pursue a direct path to right or left. It has no particular route assigned to it; wherever the supply 2 of its fuel leads it, thither it crawls; it does not advance in its orbit as a star, but feeds as a fire. Why, then, does it appear for a long period, and why is it not quickly extinguished? For the recent one which we saw during this joyous reign of Nero displayed itself to view for six months, revolving in the opposite direction to the former one in Claudius’ time. That one rising from the north up toward the zenith made for the east, always growing dimmer. This one began in the same quarter, but making toward the west, turned finally toward the south, where it withdrew from view. No doubt the former found moister elements, more suitable for its fire, and pursued them; the latter 3 in turn chose a richer and more substantial district. So they descend toward the direction in which they are invited by their material, and not by a definite path, which in the two we have seen was different, since the one moved off toward the right and the other toward the left. Now all stars[97] have their orbit in one direction, namely, contrary to the motion of the universe. The latter moves from east to west, the stars go from west to east. For this reason they have a double motion,--one, their own proper motion; the other, which carries them round along with the heavens. [97] Planets may be specially referred to; the Latin word is the generic one, _stella_. XXII