COMET. The word C. is derived from the Gr. kome, hair, a title which had its origin in the hairy appearance often exhibited by the haze or luminous vapor, the presence of which is at first sight the most striking characteristic of the celestial bodies called by this name. The general features of a C. are—a definite point or nucleus, a nebulous light surrounding the nucleus, and a luminous train preceding or following the nucleus. Anciently, when the train preceded the nucleus—as is the case when a C. has passed its perihelion, and recedes from the sun—it was called the beard, being only termed the tail when seen following the nucleus as the sun is approached. 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 attendant, the tail, nor the nucleus, is now considered an essential cometary element, but all bodies are classed as comets which 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 w. to e., which is astronomi cally called " direct motion;" but the movements of comets are often from e. to w., 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 even 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 every variety of eccentricity, some of them being ellipses or elongated closed orbits of various degrees of elongation; others, 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 approximate, under given conditions.
Any attraction, however, of an extraneous body interfering with the attraction of the sun might change the orbit from the ellipse to the hyperbola, and rice versa., or from the parabola to either. As, however, there is only one parabola corresponding to infinite sets of ellipses and hyperbolas, an interfering cause is not likely to change the orbit from an ellipse or hyperbola to the parabolic form. Of about 200 comets whose orbits , have been obtained with more or less accuracy, 40 appear to have described ellipses, 7 hyperbolas, and 150 orbits that cannot be distinguished from parabolas.
The discovery that comets are celestial bodies, extraneous to our atmosphere, is due to Tycho Bralle, who ascertained the fact by observations of the C. of 1537. Newton succeeded in demonstrating that they are guided in their movements by the same prin ciple which controls the planets in their orbits; and Halley was the first, by determin ing the parabolic elements of a number of comets from the recorded observations, to identify the C. of 1682 with one which had been observed in 1607 and the observations
recorded by Kepler and Longoinoutanus, and also with a C. observed in 1531 by Apian, at Ingoldstadt, and thus confidently to predict the return, at the end of 1758 or begin ning of 1759, of a C. which would have the same parabolic elements. These parabolic elements are elements of a parabola nearly coincident with the elongated elliptic orbit of the comet. They The inclination. 2. the longitude of the node. These two determine the plane of the orbit. 3. The longitude of the perihelion, or point of nearest approach to the sun. 4. The perihelion distance, or nearness of approach to the sun, 5. The direction of motion, whether direct or retrograde.
To determine these parabolic elements, three observations of the C. are sufficient; and by a table of such elements deduced from the recorded observations, it is possible at once to ascertain whether any newly observed C. is identical with any that have been previously observed. To predict, however, with accuracy the time of the return of a C., a much more accurate calculation must be made of the orbit, taking into account the perturbations of the planets to whose influence it is subject. This difficult problem was solved, in the case of Halley's C., by the joint work of Leland, Mine. Lepante, and Clairaut, who announced, in Nov. 1758, just as astronomers began to look out for the return of the C., that it would take 618 days more to return to theperihelion than on the preceding revolution. The perihelion passage was fixed about the middle of April, 1759; but Clairaut distinctly forewarned the world that, being pressed for time, he had neglected small values, which collectively might amount to about a month in the seventy-six years. The C. passed the perihelion on the 12th Mar., 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 be the C. of the former periods by their simi larity. For the next perihelion passage, the different calculations executed by MM. Damoiseau and De Pontecoulant, fixed the 4th, the 7th, and the 13th Nov. 1835. Subsequently, observations indicated the 16th—that is to say, a deviation of only three days from what turned out the most accurate calculation, and a deviation of 12 days from the most remote. We have adverted to the perihelion passages of this C. in 1531, 1607, 1682, 1759, and 1835. It is also now identified with a C. observed in 1456, and one in 1373, recorded by Chinese observations. There are no sufficiently reliable European observations previous to 1456, but it is conjectured by Arago, that this C. is the same with the C. of 1305; that of 1230; a C. mentioned in 1006 by Hali Ben Rodoan; that of 885; finally, a C. seen in the year 52 before our era.