376 



COMET 



of 1557. Newton succeeded in demonstrating that 

 they are guided in their movements by the same 

 principle which controls the planets in their orbits ; 

 and Halley was the first, by determining the para- 

 bolic elements of a number of comets from the 

 recorded observations, to identify the comet of 

 1682 with one which had been observed in 1607 

 and the observations recorded by Kepler and 

 Longomontanus, and also with a comet observed in 

 1531 by Apian, at Ingoldstadt, and thus confidently 

 to predict the return at the end of 1758 or beginning 

 of 1759, of a comet 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 are : 

 ( 1 ) 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 time 

 of perihelion passage. (6) The direction of motion, 

 whether direct or retrograde. 



To determine these parabolic elements, three 

 observations of the comet 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 comet is identical with 

 any that have been previously observed. To pre- 

 dict, however, with accuracy the time of the return 

 of a comet, a much more elaborate 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 comet, by the joint work of 

 Laland, Madame Lepante, and Clairaut, who 

 announced, in November 1758, just as astronomers 

 began to look out for the return of the comet, that 

 it would take 618 days more to return to the peri- 

 helion 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 

 comet passed the perihelion on the 12th March 

 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 comet of the former periods by their 

 similarity. For the next perihelion passage, the 

 different calculations executed by MM. Damoiseau, 

 Rosenberger, Lehmann, and De Pontecoulant, fixed 

 respectively the 4th, the 26th, the llth, and the 

 13th November 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 twelve 

 days from the most remote. We have adverted 

 to the perihelion passages of this comet in 1531, 

 1607, 1682, 1759, and 1835. It is also now identi- 

 fied with a comet observed in 1456, and one in 1378, 

 recorded by Chinese observations. There are no 

 sufficiently reliable European observations previous 

 to 1456, but it is conjectured by Arago tnat this 

 comet is the same with the comet of 1305 ; that of 

 1230 ; a comet mentioned in 1006 by Hali Ben 

 Rodoan ; that of 885 ; finally, a comet seen in the 

 year 52 before our era. 



This account of Halley's comet has been given at 

 length to illustrate the principles on which the 

 calculations are made. A simple method, due to 

 OJbers, enables the elements of cometary orbits 

 to be calculated in a comparatively short time, and 

 almost all comets discovered have now their paths 

 roughly determined soon after they become visible. 



There are two other periodical comets of peculiar 

 interest : ( 1 ) That of Encke, with a short period 

 of 1204 days. Its orbit does not extend so far as 



the orbit of Jupiter, and a slight acceleration in its 

 periodic times of return was regarded until lately 

 as suggesting the possibility of the space, within 

 our solar system at least, being occupied by a 

 resisting medium, though of extreme rarity. (2) 

 That of Biela or Gambart, having a period of six 

 years and three-quarters. During the visit of this- 

 comet in 1846, it was seen first at New Haven, 

 U.S., by Messrs Herrickand Bradley, on December 

 29, 1845, in the act of separating into two distinct 

 comets, which kept moving side by side till they 

 disappeared. On the return of the comet in the 

 autumn of 1852, the distance between the two 

 nuclei had much increased. Since then, although 

 repeatedly due, it has not again been seen. 



The celebrated comet of 1680, which furnished 

 Newton with the occasion for proving that comets 

 revolve around the sun in conic sections, and that, 

 consequently, they are retained in their orbits by 

 the same force as that which regulates the move- 

 ments of the planets, appears to have been about 

 the most remarkable for brilliancy of any of which 

 we have authentic accounts. This comet is sup- 

 posed to be identical with the one that appeared 

 about the time 

 of Caesar's death 

 (44 B.C.), with 

 that which was 

 seen in the reign 

 of Justinian in 

 the year 531, and 

 with another in 

 the year 1106, in 

 the reign of 

 Henry II., the 

 period of revolu- 

 tion, according 

 to the orbit cal- 

 culated for it by 

 Whiston, being 

 about 575 years. 

 There is, how- 

 ever, some doubt 

 among astrono- 

 mers as to the 

 real form of its 

 orbit, the one 

 assigned to it 

 by Encke giving 

 it a period 

 of 8813 years. 

 This comet ap- 

 proached nearer 

 to the sun than 

 any known, ex- 

 cept that of 1843. 

 The comet of 1680 

 approached the 

 sun within the 163d of the semi-diameter of the 

 earth's orbit. The annexed diagram shows a part 

 of its path ; the same diagram shows the direction 

 of the tail. 



In the first half of the 19th century, the comets 

 chiefly notable are those of 1811 and 1843. The 

 former was a most brilliant object for many weeks 

 in the northern heavens. It is periodic, though 

 travelling at aphelion to the enormous distance 

 from the sun of 40,121,000,000 miles. The latter 

 was also remarkable for its brilliance, but chiefly 

 for its very near approach to the sun, passing so 

 near that a trifling change in its direction of 

 approach would have caused a collision. Not 

 more than 32,000 miles separated their surfaces 

 when nearest. Its tail also was enormous, reach- 

 ing in length 200,000,000 miles. Since 1850 many 

 brilliant comets are recorded : Donati's in 1858 ; 

 and the great comet of 1861, through the tail of 

 which our earth passed on June 30th of that year. 



