COMETS. In addition to the planets and their satellites, the sun's retinue contains a very large number of comets. These are distinguished from the planets in three ways: (f ) their orbits, instead of being appreciably circular, like those of the planets, are elongated ellipses, which, in the majority of cases, are almost parabolic; (2) their masses are exceedingly small compared with those of the major planets, and are probably much less even than those of the minor planets; (3) they are generally surrounded by hazy or nebulous envelopes; these are styled the "coma" when their outline is nearly circular, or the "tail" when it is greatly ex tended. The name "comet" is derived from the Greek and Latin words for "hair," from a fancied resemblance between the tails of comets and long hair streaming in the v,incl. The terms for them used in China and Japan, translated "besom star," may come either from a comparison between a comet's tail and the bundle of twigs in a broom, or from a fancied resemblance between the motion of a comet's tail across the constellations and that of a broom over a floor.
The distances of comets from the sun vary within enormously wide limits; the comets 188o I. and 1887 I. approached within 9o,000 miles of the sun's surface, and several other comets approached almost as closely. The greatest distance to which comets recede from the sun can only be given vaguely; but it seems to be a large fraction of the distance (25 millions of millions of miles) that separates us from the nearest fixed star. Indeed, in a few cases comets are f ound to be moving in hyperbolic orbits, which implies that they may pass beyond the sun's control into that of some other star ; but such cases are exceptional. The periods of comets, that is, the times required to accomplish a single revolution in their orbits, likewise vary enormously; the shortest known period is that of Encke's comet, three and one-third years, whilst the longest is in the neighbourhood of a million years. The average periods of comets are very long, certainly many thousands of years, and the number of comets of long period that approach the sun in a century is about 3oo (making some allowance f or undetected ones). The majority of these show no appreciable deviation from a parabolic orbit, and we are probably not overestimating their average period if we put it at 4o,000 years. We thus obtain 120,000 as the total number of comets in the solar system, which makes them by far the most numerous class of objects in the system; the minor planets are probably to be reckoned by thousands, but the highest estimate of their number is about 5o,000.
Comets appear at irregular inter vals, and their rapid, capricious movements, combined with the occasional size and brilliance of their tails, have caused them to be regarded with mingled interest and apprehension. Since the belief was long prevalent that the movements of the heavenly bodies influenced the affairs of men, it is not unnatural that comets were regarded as particularly potent in such influences; later they were associated with plague, famine and war. These erroneous ideas had, however, the useful result of causing the movements of comets to be noted with great care. Many such notices have come down to us, especially in the Chinese annals; they have been useful in enabling the history of Halley's comet to be carried back 2,000 years, and in giving probable identifica tions of a few other comets.
There were two schools of opinion concerning the nature of comets in early times; one regarded them as true celestial bodies, the other as vaporous exhalations within the earth's atmosphere. The former view was held by the Roman philosopher, Seneca, who made the sagacious forecast : "Some day there will arise a man who will demonstrate in what regions of the heavens the comets take their way; why they journey so far apart from the other planets; what their size, their nature." Unfortunately Euro pean astronomers for many centuries exhibited less sagacity than Seneca, and adopted the view that comets were atmospheric exhalations, though this was capable of easy refutation by the fact that comets shared in the daily revolution of the heavens, rising and setting like the stars. The erroneous view had the effect of causing very little attention to be paid to the positions of comets among the stars, so that up to the 6th century the Euro pean cometary records are far inferior in accuracy to the Chinese ones. It was one of the useful achievements of Tycho Brahe to establish, by comparison of observations made at two distant ob servatories, that comets are more remote than the moon, so that they must be regarded as heavenly bodies.
