| Name. | Period, in years. | Peri- helion Passage. | Long. of Peri- helion. | Long. of Ascend- ing Node. | Inclin- ation. | Motion. | Next Return. |
|---|---|---|---|---|---|---|---|
| ° ′ | ° ′ | ° ′ | |||||
| Encke | 3·29 | 1888, June 28 | 158 36 | 334 39 | 12 53 | D | 1891 |
| Tempel(1873) | 5·20 | 1878, Sept. 7 | 306 8 | 121 1 | 12 46 | D | 1894 |
| *Barnard | 5·40 | 1884, Aug. 16 | 301 2 | 5 9 | 5 28 | D | 1895 |
| Brorsen | 5·46 | 1879, Mar. 30 | 116 15 | 101 19 | 29 23 | D | 1895 |
| Pons-Winnecke | 5·73 | 1886, Sept. 16 | 276 4 | 101 56 | 14 27 | D | 1892 |
| Tempel (1867) | 5·98 | 1879, May 7 | 238 11 | 78 46 | 9 47 | D | 1891 |
| Tempel-Swift | 5·99 | 1880, Nov. 8 | 43 0 | 296 42 | 5 31 | D | 1892 |
| *Brooks (1886) | 6·30 | 1886, June 7 | 229 46 | 53 3 | 12 56 | D | 1892 |
| *Spitaler | 6·40 | 1890, Oct. 26 | 58 24 | 45 8 | 12 52 | D | 1897 |
| Biela | 6·62 | 1852, Sept. 23 | 109 8 | 245 52 | 12 33 | D | ? |
| D’Arrest | 6·64 | 1890, September | 319 9 | 146 9 | 15 43 | D | 1897 |
| *Finlay | 6·67 | 1886, Nov. 22 | 7 34 | 52 30 | 3 2 | D | 1893 |
| *Wolf | 6·78 | 1884, Sept. 27 | 352 31 | 206 18 | 25 16 | D | 1891 |
| *Swift | 6·91 | 1889, Nov. 29 | 69 29 | 331 27 | 10 3 | D | 1896 |
| *Brooks (1889) | 7·07 | 1889, Sept. 30 | 1 26 | 17 59 | 6 4 | D | 1896 |
| Faye | 7·57 | 1888, Aug. 19 | 50 56 | 209 42 | 11 20 | D | 1896 |
| *Denning | 8·69 | 1881, Sept. 13 | 312 31 | 65 57 | 6 51 | D | 1899 |
| *Peters | 12·80 | 1846, June 1 | 240 7 | 260 28 | 30 24 | R | ? |
| Tuttle | 13·66 | 1885, Sept. 11 | 116 28 | 269 42 | 54 19 | D | 1899 |
| *Tempel (1866) | 33·18 | 1866, Jan. 11 | 60 28 | 231 26 | 17 18 | R | 1899 |
| *Stephan | 33·62 | 1867, Jan. 20 | 75 52 | 78 36 | 18 13 | D | 1900 |
| *Westphal | 67·77 | 1852, Oct. 13 | 43 12 | 346 13 | 40 59 | D | 1920 |
| Pons | 71·48 | 1884, Jan. 25 | 93 21 | 254 6 | 74 3 | D | 1955 |
| Olbers | 72·45 | 1887, Oct. 8 | 149 45 | 84 30 | 44 35 | D | 1960 |
| *Di Vico | 73·25 | 1846, Mar. 6 | 90 35 | 77 36 | 84 57 | D | 1919 |
| *Brorsen | 74·97 | 1847, Sept. 10 | 79 13 | 309 49 | 19 8 | D | 1922 |
| Halley | 76·37 | 1835, Nov. 15 | 304 32 | 55 10 | 17 45 | R | 1912 |
Halley’s Comet.—A fine comet with a tail about 15° long appeared in the summer of 1682, and Halley computed the orbit according to the method explained by Newton. He then consulted observations of previous comets, and discovered a great similarity in the paths of large comets seen in 1531 and 1607 to that of the one he himself had observed in 1682. He thereupon suspected the three bodies to be one and the same, and advised posterity to maintain a strict watch for the comet’s return in about 1758 or 1759. On pursuing his investigations still further, he alighted upon records of comets in 1305, 1380, and 1456, which greatly strengthened his opinion that the comet of 1682 moved in an elliptical path with a period of about 75½ years. He termed this body “the Mercury[48] of comets, revolving round the Sun in the smallest orbit,” and said that, should it reappear according to his prediction in about the year 1758, “impartial posterity must needs allow this to be the discovery of an Englishman.”
As the time drew near for the return of the comet, interest became intensified, and computations were made by Clairaut with a view to determine the precise epoch when it would arrive at perihelion. He found that the comet would be retarded by the action of Jupiter and Saturn, but that perihelion would be reached at the middle of April 1759, subject to an uncertainty of 30 days. The comet was rediscovered on Dec. 25, 1758, by Palitzch, an amateur astronomer at Politz, near Dresden, who employed a telescope of 8 feet focal length, and appears to have anticipated Messier and others who were on the alert for it. It arrived at perihelion on March 12, 1759, and within a month of the date announced by Clairaut. Early in May it had a tail nearly 50° long, and presented a fine aspect in the heavens. Thus the sagacity of Halley had revealed a periodical comet—the first known. It duly returned again in 1835, and received all the attention which a body so replete with historical associations deserved.
Comet 1862 III. (Aug. 19, 1862).
Sawerthal’s Comet, 1888 I. (March 25, Brooks).
Encke’s Comet.—Until the year 1819 Halley’s Comet was the only one certainly known to be periodical. Then the able deductions of Encke presented us with a veritable “Mercury[49] of comets.” He showed that a small comet, discovered by the unwearying Pons of Marseilles on Nov. 26, 1818, was really identical with three previous comets—viz. 1786 I. (Mechain), 1795 (C. Herschel), and 1805 (Thulis),—and that its period was a very short one of about 3-1/3 years. Its return to perihelion was predicted to occur on May 24, 1822, and this was observed in the southern hemisphere. It returned again on Sept. 16, 1825, and on this occasion the circumstances were more favourable. Since 1825 this object has effected nineteen returns to perihelion, and one of the most singular facts noticed in connection with it is that its period is gradually shortening. In 1795 it was 1212 days, while in 1858 it was 1210. In order to explain this contraction of orbit, it has been necessary to assume the existence of a thin medium in space capable of affording a slight resistance to the tenuous materials of a comet, though not dense enough to appreciably affect the motions of planets. If this closing-up of the orbit and shortening of period continue to operate through a vast interval of time, Encke’s Comet must be ultimately precipitated upon the Sun![50]