Halley concluded that all the three orbits belonged to the same comet, of which the periodic time was about 76 years. After a rough estimate of the perturbations it must sustain from the attraction of the planets, he predicted its return for 1757,—a bold prediction at that time, but justified by the event, for the comet again made its appearance as was expected, though it did not pass through its perihelion till the month of March 1759, the attraction of Jupiter and Saturn having caused, as was computed by Clairault previously to its return, a retardation of 618 days. This comet had been observed in 1066, and the accounts which have been preserved represent it as having then appeared to be four times the size of Venus, and to have shone with a light equal to a fourth of that of the moon. History is silent respecting it from that time till the year 1456, when it passed very near to the earth: its tail then extended over 60° of the heavens, and had the form of a sabre. It returned to its perihelion in 1835, and was well observed in almost every observatory. But its brightness was far from comparing with the glorious accounts of its former apparitions. That this should have been due to the process of dissipation does not seem possible in so short a period; we must therefore consider either that the earlier accounts are greatly exaggerated, or that the brightness of the comet is subject to changes from some unknown cause. Previous appearances of Halley’s comet have been calculated by J. R. Hind, and more recently by P. H. Cowell and A. C. D. Crommelin of Greenwich, the latter having carried the comet back to 87 B.C. with certainty, and to 240 B.C. with fair probability. It was detected by Max Wolf at Heidelberg on plates exposed on Sept. 11, 1909, and subsequently on a Greenwich plate of Sept. 9.
The known comet of shortest period bears the name of J. F. Encke, the astronomer who first investigated its orbit and showed its periodicity. It was originally discovered in 1789, but its periodicity was not recognized until 1818, after it had been observed at several returns. This comet has given rise to a longer series of investigations than any other, owing to Encke’s result that the orbit was becoming smaller, and the revolutions therefore accelerated, by some unknown cause, of which the most plausible was a resisting medium surrounding the sun. As this comet is almost the only one that passes within the orbit of Mercury, it is quite possible that it alone would show the effect of such a medium. Recent investigations of this subject have been made at the Pulkova Observatory, first by F. E. von Asten and later by J. O. Backlund who, in 1909, was awarded the Gold Medal of the Royal Astronomical Society for his researches in this field. During some revolutions there was evidence of a slight acceleration of the return, and during others there was not.
The following is a list (compiled in 1909) of comets which are well established as periodic, through having been observed at one or more returns. In addition to what has already been said of several comets in this list the following remarks may be made. Tuttle’s comet was first seen by P. F. A. Méchain in 1790, but was not recognized as periodic until found by Tuttle in 1858, when the resemblance of the two orbits led to the conclusion of the identity of the bodies, the period of which was soon made evident by continued observations. The comets of Pons and Olbers are remarkable for having an almost equal period. But their orbits are otherwise totally different, so that there does not seem to be any connexion between them. Brorsen’s comet seems also to be completely dissipated, not having been seen since 1879.
List of Periodic Comets observed at more than one Return.
|Last Perih. |
|Least Dist. |
|Halley||1456 June 8.2||1835 Nov. 15.9||75.9||0.58||35.42|
|Biela||1772 Feb. 16.7||1852 Sept. 23.4||6.67||0.98||6.18|
|Encke||1786 Jan. 30.9||1905 Jan. 11.4||3.29||0.34||4.08|
|Tuttle||1790 Jan. 30.9||1899 May 4.5||13.78||1.03||10.53|
|Poris||1812 Sept. 15.3||1884 Jan. 25.7||72.28||0.78||33.70|
|Olbers||1815 April 26.0||1887 Oct. 8.5||73.32||1.21||33.99|
|Winnecke||1819 July 18.9||1898 Mar. 20.4||5.67||0.77||5.55|
|Faye||1843 Oct. 17.1||1896 Mar. 19.3||7.50||1.69||5.93|
|De Vico||1844 Sept. 2.5||1894 Oct. 12.2||5.66||1.19||5.01|
|Brorsen||1846 Feb. 11.1||1879 Mar. 30.5||5.52||0.65||5.63|
|D’Arrest||1851 July 8.7||1897 May 21.7||6.56||1.17||5.71|
|Tempel I.||1867 May 23.9||1879 May 7.0||5.84||1.56||4.82|
|Tempel-Swift||1869 Nov. 18.8||1891 Nov. 15.0||5.51||1.06||5.16|
|Tempel II.||1873 June 25.2||1904 Nov. 10.5||5.28||1.34||4.66|
|Wolf||1884 Nov. 17.8||1898 July 4.6||6.80||1.59||5.57|
|Finlay||1886 Nov. 22.4||1893 July 12.2||6.64||0.99||6.17|
|Brooks||1889 Sept. 30.3||1903 Dec. 6.5||7.10||1.95||5.44|
|Holmes||1892 June 13.2||1899 April 28.1||6.89||2.14||4.50|
There are also a number of cases in which a comet has been observed through one apparition, and found to be apparently periodic, but which was not seen to return at the end of its supposed period. In some of these cases it seems likely that the comet passed near the planet Jupiter and thus had its orbit entirely changed. It is possible that in other cases the apparent periodicity is due to the unavoidable errors of observation to which, owing to their diffused outline, the nuclei of comets are liable. (S. N.)
COMET-SEEKER, a small telescope (q.v.) adapted especially to searching for comets: commonly of short focal length and large aperture, in order to secure the greatest brilliancy of light.
COMILLA, or Kumilla, a town of British India, headquarters of Tippera district in Eastern Bengal and Assam, situated on the river Gumti, with a station on the Assam-Bengal railway, 96 m.from the coast terminus at Chittagong. Pop. (1901) 19,169. The town has many large tanks and an English church, built in 1875.
COMINES, or Commines (Flem. Komen), a town of western Flanders, 13 m. N.N.W. of Lille by rail. It is divided by the river Lys, leaving one part on French (department of Nord), the other on Belgian territory (province of West Flanders). Pop. of the French town 6359 (1906); of the Belgian town, 6453 (1904). The former has a belfry of the 14th century, restored in the 17th and 19th centuries, and remains of a château. Comines carries on the spinning of flax, wool and cotton.
COMITIA, the name applied, always in technical and generally in popular phraseology, to the most formal types of gathering of the sovereign people in ancient Rome. It is the plural of comitium, the old “meeting-place” (Lat. cum, together, ire, to go) on the north-west of the Forum. The Romans had three words for describing gatherings of the people. These were concilium, comitia and contio. Of these concilium had the most general significance. It could be applied to any kind of meeting and is often used to describe assemblies in foreign states. It was, therefore, a word that might be employed to denote an organized gathering of a portion of the Roman people such as the plebs, and in this sense is contrasted with comitia, which when used strictly should signify an assembly of the whole people. Thus the Roman draughtsman who wishes to express the idea “magistrates of any kind as president of assemblies” writes “Magistratus queiquomque comitia conciliumve habebit” (Lex Latina tabulae Bantinae, l. 5), and formalism required that a magistrate who summoned only a portion of the people to meet him should, in his summons, use the word concilium. This view is expressed by Laelius Felix, a lawyer probably of the age of Hadrian, when he writes “Is qui non universum populum, sed partem aliquam adesse jubet, non comitia, sed concilium edicere debet” (Gellius, Noctes Atticae, xv. 27). But popular phraseology did not conform to this canon, and comitia, which gained in current Latin the sense of “elections” was sometimes used of the assemblies of the plebs (see the instances in Botsford, distinction between Comitia and Concilium, p. 23). The distinction between comitia and contio was more clearly marked. Both were formal assemblies convened by a magistrate; but while, in the case of the comitia, the magistrate’s purpose was to ask a question of the people and to elicit their binding response, his object in summoning a contio was merely to bring the people together either