comet

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A celestial body, observed only in that part of its orbit that is relatively close to the sun, having a head consisting of a solid nucleus surrounded by a nebulous coma up to 2.4 million kilometers (1.5 million miles) in diameter and an elongated curved vapor tail arising from the coma when sufficiently close to the sun. Comets are thought to consist chiefly of ammonia, methane, carbon dioxide, and water.

Century Dictionary and Cyclopedia

1. n. One of a class of celestial bodies which move about the sun in greatly elongated orbits, usually elliptical or parabolic. The typical comet, as it approaches the sun, has the appearance of a bright star-like point (the nucleus) surrounded by a mass of misty light (the coma), which is extended away from the sun into a stream of light (the tail) reaching a length of from 2° to 90°. Comets which follow a parabolic orbit appear but once, their orbit being infinite, and are called parabolic comets; those moving in ellipses return periodically, and are called periodic comets. The fact of the periodicity of some comets was first established by Halley with reference to the comet of 1682. The paths in which they move are not, like those of the planets, all nearly in the same plane as the orbit of the earth, but are inclined to that orbit at all angles; and their motion along their paths, though generally direct, that is, in the same direction as that of the earth and the other planets, is sometimes retrograde. Some comets have no nucleus; and this is the case with every one while it is still very remote, when it appears as a mere nebulous patch. In this state it is called a telescopic comet. As it approaches the sun, the nucleus is gradually formed as a central but not sharply defined point of light; later, the tail, consisting of vaporous matter driven back by some repellent influence of the sun, often with enormous velocity, is formed; and lastly, if the comet is a bright one, a series of bright envelops rise successively from the nucleus, each extending back into the tail, and gradually disappearing. The matter of which comets are composed is so transparent that the faintest stars are seen through them without the slightest diminution of their luster. Of their physical constitution little is definitely known. The most remarkable discovery of recent times regarding them is the identity of the course of some of them with the orbit of certain showers of shooting stars. This was first demonstrated by the Italian astronomer Schiaparelli, who proved the agreement between the orbit of the great comet of 1862 and that of the star-shower seen annually about August 1st–10th. Very remarkable comets appeared in 1456, 1680, 1811, 1841, 1858 (Donati's), 1861, and 1874. They have always been objects of superstitious fear. See cut under envelop.
2. n. In heraldry, same as blazing-star.
3. n. One of a group of humming-birds with long forked tails: as, the Sappho comet, Cometes sappho; the Phaon comet, Cometes phaon.
4. n. A game of cards, somewhat like speculation, invented and popular in the reign of Louis XV. of France.
5. n. In photography, a comet-shaped defect appearing on gelatin dry plates.

Wiktionary

1. n. astronomy A celestial body consisting mainly of ice, dust and gas in a (usually very eccentric) orbit around the Sun and having a "tail" of matter blown back from it by the solar wind as it approaches the Sun.
2. n. A celestial phenomenon with the appearance given by the orbiting celestial body.

GNU Webster's 1913

1. n. (Astron.) A member of the solar system which usually moves in an elongated orbit, approaching very near to the sun in its perihelion, and receding to a very great distance from it at its aphelion. A comet commonly consists of three parts: the nucleus, the envelope, or coma, and the tail; but one or more of these parts is frequently wanting. See Illustration in Appendix.

WordNet 3.0

1. n. (astronomy) a relatively small extraterrestrial body consisting of a frozen mass that travels around the sun in a highly elliptical orbit