COMET (AS., Lat. cometa, comet, from Gk. Kopajrns, kometFs, having long hair, from Koimip, koman, to wear long hair, from Kban, komr:, hair). The word 'comet' had its origin in the hairy appearance often exhibited by the haze or luminous vapors, the presence of which is at first sight the most striking characteristic of the celestial bodies called by this name. The gen eral features of a comet are: a definite point or nucleus, a nebulous light surrounding the nu cleus, and a luminous train preceding or follow ing it. Anciently, when a train preceded the nucleus—as is the case when a comet has passed its perihelion and recedes from the sun—it was called 'the beard.' being termed 'the tail' when seen following the nucleus as the sun is ap proached. This distinction has disappeared from all modern astronomical works, and the latter name is given to the appendage, whatever its apparent position. Neither this luminous at tendant, the tail, nor the nucleus, is now con sidered an essential cometary element, but all bodies in the solar system are classed as comets if they have a motion of their own, and describe orbits of an extremely elongated form. There are several plain points of difference between comets and planets. The planetS move in the same direction, from west to east, which is astronomieally called 'direct motion;' hut the movements of comets are often from east to west, or retrograde. The orbits of all the planets are confined to a zone of no great breadth on either side of the ecliptic; but the paths of comets cut the ecliptic in every direction, some being almost perpendicular to it. The orbits of all the planets are nearly circular: or, more properly speaking. are ellipses of very small eccentricity. The orbits of comets, on the other hand. present great variety of eccentricity, many of them being ellipses or elongated closed orbits of various degrees of elongation; others, though very rarely, may he hyperbolas: while the majority have a form of orbit not differing sensibly from the parabola. which is the limiting form of curve to which both the ellipse and hyperbola usually approximate. Any attraction. however, of an extraneous body, like a planet, interfering with the attraction of the sun. might change the orbit from the ellipse to the hyper bola, and vice versa, or from the parabola to either. As. however, there is only one parabola corresponding to infinite sets of ellipses and hy perbolas, an interfering cause is not likely to change the orbit from an ellipse or hyperbola to the parabolic form. Of about 350 comets whose orbits have been obtained with more or less accuracy, GO appear to have described el lipses, 275 orbits cannot be distinguished from parabolas, and in two cases the hyperbolic form of orbit is extremely probable. The discovery that comets are celestial bodies extraneous to our atmosphere is due to Tycho Brahe, who ascertained the fact by observations of the comet of 1577. Newton succeeded in demonstrating that their movements are subject to the same law which controls the planets in their orbits. Halley was the first, by determining the para bolic elements of a number of comets from re corded observations, to identify the comets of 1682 with one which had been observed in 1607 and with one observed by Apiau at lngolstadt in 1531. and thus confidently to predict the return. at the end of 1758 or the beginning of 1739, of a comet which would have the same parabolic elements. This prediction of the first 'periodic' comet moving in a closed oval orbit simply meant that the portion of the closed orbit lying nearest the sun, and therefore the only observ able portion of the orbit, would very closely resemble the parabolas or open curves in which this comet had been supposed to be moving at its earlier appearances.
Parabolic cometary elements are the following: (1) The inclination: (2) the longitude of the node; (3) the longitude of the perihelion or point of nearest approach of the sun: (4) the perihelion distance, or nearness of approach to the sun: (5) the direction of motion, whether direct or retrograde. The first two of these
elements determine the plane of the orbit. To determine these parabolic elements, three ob servations of the comet are sufficient; and by a table of such elements, calculated from re corded observations, it is possible at once to ascertain, as Halley did, whether any newly observed comet is identical with any that has been previously observed. However, to predict with accuracy the time of the return of a comet, a much more accurate calculation must be made of the orbit, taking into account the perturba tions of the planets to whose influence it is subject. This difficult problem was .01x-ed. in the case of Halley's comet, by the joint work of Lalande, Mine. Lepante. and Clairaut, who an nounced in November, 175S, just as astronomers began to look out for the return of the comet, that it would take GIS days more to return to the perihelion than on the preceding revolution. The perihelion passage was fixed about the mid dle of April. 1759; but Clairaut distinctly stated that. pressed for time, he had neglected small values which collectively might. amount to about a month in the seventy-six years. The comet passed the perihelion on March 12, 1759, exactly a month before the time announced, but within the assigned limits of divergence from that date. The elements of its orbit proclaimed it to lie the comet of the former appearances by their similarity. For the next perihelion pas sage, the different calculations executed by Damoiseau and He Pontecoulant fixed the 4th, the 7th. and the 13th of November, 1835. Sub sequently, observations indicated the lfith—that is to say, a deviation of only three days from what turned out the most accurate calculation, and a deviation of twelve days from the most remote. We have adverted to the perihelion passages of this comet in 1531, 1607, 1682, 1759, and 1835. It is also now identified with a comet observed in 1456, and one in 1378, reeo•ded by Chinese observations. There are no sufficiently reliable European observations previous to 1456, but it is conjectured that this comet is the same with the comet of 1501, with that of 1145, with a comet mentioned in 1066 by Hall ben Rodoan, with that of 989, and finally, with a comet seen in the year 12 before our era. This account of Halley's comet has been given at length, to illustrate the principles on which the calculations are made. There are, in all, eighteen comets whose periodi city is established by the fact that their return has been actually observed: (1) That of Dicke, with a short period of 3.303 years; its orbit does not extend so far as the orbit of .Jupiter, and a slight acceleration in its periodic times of return has suggested the possibility of space. within our solar system at least, being occupied by a resisting medium, though of extreme rarity. (2) That of Hiatt or Gambart, having a period of 6.692 years. During the visit of this comet in 1846, it was seen to separate into two distinct parts. which kept moving side by side till they disappeared. On the return of the comet in the autumn of 1852. the distance between the two nuclei had much increased. Since then. although many times due, it has not again been seen. (3) That of Faye. with a period of 7.566 years. The orbits of Biela's and Faye's comets extend beyond the orbit of Jupiter, but not so far as that of Saturn. (4) That of Winneeke, with a period of 5.831 years. (5) That of Bro•sen, with a period of 5.456 years. (6) That of Temple (No. 1). with a period of 6.538 years. (7) That of Temple (No. 2), with a period of 5.281 years. (8) That of D'Arrest, with a period of 6.675 years. (9) That of Tuttle. with a period of 13.667 years. (10) That of Swift, with a period of 5.547 years. (11) That of E. Swift. with a period of 5.803 years. (12) That of Finlay. with a period of 6.556 years. (13) That of Wolf, with a period of 6.845 years. (14) That of Holmes. with a period of 6.874 years. (15) That of Brooks, with a period of 7.097 years.