Comet (Gr. long-haired), a celestial body presenting a nebulous aspect, and travelling under the sun's attraction. Many of these bodies are distinguished by a remarkable taillike appendage. The greater number of those hitherto known have revolved round the sun on a path whose observed portion belonged to an exceedingly elongated ellipse, or was even parabolic or hyperbolic. A few, however, travel in closed orbits around the sun in known periods. It has been supposed that some among the ancients suspected the periodic motions of the planets; but the only evidence we have on the subject is vague and indefinite. Tycho Brahe was the first to prove by direct observation that comets are not mere phenomena of our own atmosphere, but certainly further away than the moon. Newton, after establishing the theory of gravitation, asserted that comets obey the laws of solar attraction, and therefore move either on elliptic, parabolic, or hyperbolic paths. From observations of the. comet of 1680 (commonly called Newton's comet) Dorfel, a clergyman of Saxony, was led to the conclusion that the course of this object was parabolic.
But the first real proof of the nature of cometary orbits was afforded by the researches of Halley into the motions of the comet of 1682 (Halley's comet). Halley computed the orbit of this comet, and having found that the figure of the orbit was either parabolic or a very extended ellipse, he examined the records of ancient cornets, and after incredible labor succeeded in discovering two whose motions agreed very closely with those of the comet of 1682. One had been observed by Appian in 1531, the other by Kepler in 1607; and Halley noticed that the intervals between the three years 1531, 1607, and 1682 are near enough to equality to suggest that one and the same comet had been observed on all three occasions. Finding that comets were observed in 1305, 1380, and 1456, he was further confirmed in the idea of the periodicity of this comet's returns; and he was thus led to predict the return of the comet about the end of 1758 or the beginning of 1759. He placed the return somewhat later than the former observed intervals would have suggested, because he found that the attraction of Jupiter would retard the comet. "When the time for its return approached, many eminent mathematicians recomputed the date of its perihelion passage, and Clairaut announced that this passage would occur between March 13 and May 13, 1759. The event actually took place on March 13, 1759; and it has been shown that a large part of the discrepancy between this date and the mean date of Clairaut's two months would have been removed had Clairaut known of the existence of Uranus, and so taken the disturbing influence of that planet into account.
On the next return of the comet in 1835, the epoch of perihelion passage was predicted much more accurately; indeed, the actual event occurred within two or three days of the dates severally announced by Pontecoulant and Rosenberger. The observations of other comets have still further confirmed Newton's theory of cometic motions. - All comets show a coma or haze of light. In nearly all cases there is a bright nucleus within this haze, and in a considerable number of instances, but not by any means in all, the comet shows a tail. When a large and complete comet, that is, a comet which possesses a coma, nucleus, and tail, is approaching the sun, the haze of light usually changes from a rounded to an elongated figure. Afterward the comet's light presents a streaky or "combed out" appearance, and then presently a tail is thrown out on the side away from the sun. The tail usually grows longer and brighter as the comet approaches the sun, and continues in existence for some time after the comet has begun to pass away from the sun's neighborhood. But there is a considerable variety in this respect among different comets. Some which have shown beautiful tails as they neared the sun, have reappeared after the perihelion passage with only a short tail or without any tail at all.
Others which have shown only insignificant tails while approaching their perihelion, have "reappeared magnified and glorified, throwing out an immense tail and exhibiting every appearance of violent excitement." Most of the comets of short period are tailless or have tails barely discernible. An examination of the drawings prepared for the third volume of the "Annals of the Observatory of Harvard College," to accompany the record of Prof. Bond's observations on Dona-ti's comet of 1858, will teach more respecting the actual processes of change which large comets undergo than any amount of verbal description. It has been justly remarked by Sir John Herschel that these "engravings, in point of exquisite finish and beauty of delineation, leave far behind everything hitherto done in that department of astronomy." - Among the comets most remarkable either for great splendor or enormous real dimensions in recent times must be mentioned those of 1780, 1807, 1811, 1815, 1819, 1825, 1843, 1847, 1858, and 1861. Among the most remarkable phenomena presented by individual comets we may mention the six tails of the great comet of 1744, and the division of Biela's comet into two distinct comets, each having coma, nucleus, and tail.
The latter phenomenon was first observed on Jan. 12, 1846, at the Washington observatory. Three days later European observers noted the same phenomenon. The two comets pursued their course side by side, with singular interchanges of lustre, now one, now the other appearing the brighter. At the return of the comet in 1852 both the comets were still visible in the same telescopic field of view. The perihelion passage of 1859 took place (if at all) under circumstances unfavorable for observation. The return of 1865 should have been readily observable; but the comet was not seen, nor has it since made its appearance. "Can it have come," says Sir John Herschel, "into contact with some asteroid as yet undiscovered, or peradventure plunged into and got bewildered among the rings of meteorites, which astronomers more than suspect? " - The recent discovery of the fact that the November and August meteor systems follow in the track of two comets (the November meteors following the telescopic comet No. I., 1866, and the August meteors following the conspicuous comet of 1862), has led to some interesting speculations respecting the nature of comets and meteors.
Schiaparelli, to whom the discovery is in part due, considers the meteors to be dispersed portions of the comet's original substance, that is, of the substance with which the comet entered the solar domain. Thus comets would come to be regarded as consisting of a multitude of relatively minute masses. Others, however, regard comets as chiefly gaseous, and the meteors as due to the solidification of portions of the gaseous coma which have been swept off by the repulsive action which forms the tail. Spectroscopic analysis has thrown some light on cometic structure, though hitherto only faint comets have been subject to careful analysis according to recent methods. Four comets examined by Dr. Hug-gins of England showed spectra indicative of gaseousness, so far as the nucleus and the brighter part of the coma are concerned. The outer part of the coma seems to shine in part by reflecting solar light. Two of the comets thus examined have shown a spectrum singularly like one of the spectra of carbon. Yet it is difficult to understand how carbon can be present in the form of luminous gas under the conditions actually existing in the case of these comets.
The spectroscopic observations by Dr. Huggins on the latest arrival, Encke's comet, have been in all respects confirmed by Prof. Young of Dartmouth college. The motions of Encke's comet, observed on many successive returns, seem to indicate the existence of a resisting medium; but Sir John Herschel has suggested another explanation; and Prof. Asaph Hall has shown in the "American Journal of Science and Arts" for December, 1871, that if resistance is actually in question, such resistance affects Encke's comet in an exceptional manner, for other well known periodic comets show no traces of its effects. All the comets having a period not exceeding seven years' travel in the same direction around the sun as the planets. Among comets with periods less than 80 years long, five sixths travel in the same direction as the planets.