COMETS. In addition to the planets, the solar system also contains certain somewhat erratic bodies called comets (q.v.). Nothing definite is known as to their origin or physical constitu tion; but it is certain that they move, under the influence of gravitation, in orbits that admit of calculation by the aid of Newton's and Kepler's laws. In appearance they vary greatly. At times the comet is but a trace of thin, filmy haze on the sky, so tenuous that it can barely be glimpsed in the telescope, and allowing the stars to be seen easily through it on the background of the sky. At other times, the comet is a magni ficent object. very conspicuous to the naked eye, and perhaps spreading its tail over a large part of the celestial vault. Such large comets in ancient times were usually looked upon with superstitious fear. In our day little of this re mains, except that the possibility of actual col lision with the earth is often the subject of in when new comets appear. It may be said
with confidence that the inherent probability of such a collision is so small as to be practically nil. And even if a collision were imminent, we might look forward to it without apprehension, so far as our present. knowledge of comets enables us to form an opinion. Sometimes comets are seen to divide into parts, and there is good rea son to suppose that they occasionally become completely disintegrated, subsequently changing into swarms of small particles, perhaps in most cases no bigger than the head of a pin. When the earth passes through such a swarm, friction with our atmosphere heats the particles until they become incandescent. They are then plainly visible as meteors, and even quite large ones have at times been found actually to fall upon the earth's surface. See METEORS.