[Illustration: Fig. 315.] It will be seen that the comet, as it approaches the sun, travels head foremost; but as it leaves the sun it goes tail foremost. The apparent length of the tail of a comet depends partly upon its real length, partly upon the distance of the comet, and partly upon the direction of the axis of the tail with reference to the line of vision. The longer the tail, the nearer the comet; and the more nearly at right angles to the line of vision is the axis of the tail, the greater is the apparent length of the tail. In the majority of cases the tails of comets measure only a few degrees; but, in the case of many comets recorded in history, the tail has extended half way across the heavens. The tail of a comet, when seen at all, is usually several million miles in length; and in some instances the tail is long enough to reach across the orbit of the earth, or twice as far as from the earth to the sun. The tails of comets are apparently hollow, and are sometimes a million of miles in diameter. So great, however, is the tenuity of the matter in them, that the faintest stars are seen through it without any apparent obscuration. See Fig. 316, which is a view of the great comet of 1264. [Illustration: Fig. 316.] [Illustration: Fig. 317.] [Illustration: Fig. 318.] [Illustration: Fig. 319.] [Illustration: Fig. 320.] The tails of comets are sometimes straight, as in Fig. 316, but usually more or less curved, as in Fig. 317, which is a view of _Donati's_ comet as it appeared at one time. The tail of a comet is occasionally divided into a number of streamers, as in Figs. 318 and 319. Fig. 318 is a view of the great comet of 1744, and Fig. 319 of the great comet of 1861. No. 1, in Fig. 320, is a view of the comet of 1577; No. 2, of the comet of 1680; and No. 3, of the comet of 1769. [Illustration: Fig. 321.] Fig. 321 shows some of the forms which the imagination of a superstitious age saw depicted in comets, when these heavenly visitants were thought to be the forerunners of wars, pestilence, famine, and other dire calamities. 289. _Visibility of Comets._--Even the brightest comets are visible only a short time near their perihelion passage. When near the sun, they sometimes become very brilliant, and on rare occasions have been visible even at mid-day. It is seldom that a comet can be seen, even with a powerful telescope, during its perihelion passage, unless its perihelion is either inside of the earth's orbit, or but little outside of it. Motion and Origin of Comets.