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.
| Designation. | 1st Perih. Passage. | Last Perih. Passage obs. | Period Years. | Least Dist. Ast. Units. | Gr. Dist. Ast. Units. |
| 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.