COMET, a luminous heavenly body which, in general, consists of a nucleus or "head" with, or frequently without, a tail, the whole moving in the heavens, first toward, then around, and finally away again from the sun, like a planet at one part of its elliptic orbit. Comets have in every age excited attention, and, till recently, have inspired terror in the general public, or at least in ignorant minds. Tycho Brahe, about 1577, showed that a certain comet was at a greater distance from the earth than the moon. Hevelius, in 1668, ascertained that the orbit of a comet was concave and not a straight line, the latter erro neous view having been held by Tycho Brahe, already mentioned, and Kepler. Doerfel (1861) believed comets to move in parabolas. In 1682 Halley proved the comet, subsequently called after him, to be periodic in its returns. In 1704 Sir Isaac Newton proved comets to obey the law of gravitation, and held that in all probability they moved in elliptic orbits.
Present state of knowledge and opin ion.—More than 600 comets, according to J. R. Hind, have been taken note of, but about 17,500,000 are believed to exist in connection with the solar system. The head or nucleus is much less solid than it seems. Thus, in 1832, Sir John Herschel saw a group of stars only of the 16th magnitude, almost through the center of Biela's comet. When such a body might be expected to exercise gravitation its influence is too small to be perceptible. Regarding orbit, the comets may be divided into two very distinct classes: First, those whose orbits are so long that they are usually regarded as parabolas, and second, those whose orbit and period are both short. Of the latter class, about 12 or 15 are known. The first seem to have come to us from outside space; the sec ond set, originally belonging to the former, to have had their direction changed so as to produce their present short elliptic orbits by the action on them of some planet. In 1866 Professor Schiaparelli, of Milan, discovered that the orbit of Tuttle's comet, the third which had appeared in 1862, was nearly identical with that of the August me teors, and Tempel's comet, the first of 1866, with that of the November me teor stream. In consequence of this dis covery, Prof. P. G. Tait published the view that the sudden development of tails many millions of miles in length, the occurrence of comets with many tails, and the observed fact that there is no definite relation of direction be tween a comet's tail and its solar radius vector, may be accounted for on the sup position that a comet is a cloud of small masses, such as stones and fragments of meteoric iron, shining by reflected light alone, except where these masses impinge on each other, or on other matter circulating around the sun, and thus produce luminous gases along with considerable modifications of their rela tive motion. The differences of motion
of the meteoric fragments relatively to the earth present appearances analo gous to those of a flock of sea-birds fly ing in one plane, and only becoming as a long streak when the plane of the flock passes approximately through the spectator's eye. The so-called envelopes surrounding them are compared with the curling wreaths of tobacco smoke emitted from a pipe. On June 24, 1881, Wm. Huggins examined the bright comet then in the sky with the spectroscope. Assuming this to be similarly composed to other comets, he came to the conclu sion that part of their light is reflected sunlight and part original light, and further that carbon is present in the cometary matter.
Among the best known periodic comets are: (1) Biela's Comet: named after M. Biela, an Austrian officer, who discovered it at Prague on Feb. 27 or 28, 1826, a comet which has a periodic time of about 6% years or 338 weeks. It returned in September, 1832, again in 1839, then in 1845; when, between Dec. 19, 1845, and Jan. 13, 1846, it separated into two comets, which went off in company, com ing back together in 1852, since which time they have returned no more; but it has been discovered that when, toward the end of November, the earth inter sects the lost double comet's path, there is a display of meteors. This was notably seen on Nov. 30, 1867, and on Nov. 27, 1872. Biela's is called also Gambart's Comet.
(2) Donati's Comet: named after Dr. Donati of Florence, a comet discovered by Donati on June 2, 1858. Periodic time about 2,000 years.
In 1910 two brilliant comets appeared; 1910A and Halley's. The latter had long been expected, but the other was new and came as a surprise to the astronomers. Consult "Story of the Comets," Chambers (1910).
(3) Encke's Comet: named after Johann Franz Encke, Director of the Ob servatory at Berlin, a comet the perio d icity of which was detected by Encke in 1819. He proved it identical with Mechain and Messier's comet of 1786, with Herschel's of 1795, and Pons' of 1805. It appeared again in 1822, 1828, and at such intervals as to show that its periodic time is 3.29 years, or 1,210 days. Its orbit is everywhere nearer the sun than that of Jupiter.
(4) Halley's Comet: named after the celebrated Edmund Halley, the friend of Newton, and, from 1720 to 1741-1742, English Astronomer-royal, a comet the first whose periodic time was ascer tained. It is about 75 years. It was identical with the comets of 1456, 1531, and 1607, and appeared again in 1759 and 1835. This comet was due in 1910 when it appeared in January. Also in the same year a new comet-1910A. Two new comets appeared in 1918, one dis covered by Reid, June 12, the other by Bergdorf, Nov. 23. Five comets ap peared in 1919, four of which were previously known.