Page:The New International Encyclopædia 1st ed. v. 05.djvu/249

* COMET. 301 COMET. wluit turned out the most accurate calculation, and a deviation of twelve daj's from the most remote. We have adverted to the perihelion passages of tliis comet in 1531, 1607, lOS'i, 1759, and 1835. It is also now identified with a comet observed in 1456, and one in 1378, recorded by Chinese observations. There are no sulliciently reliable European observations previous to 1456, but it is conjectured that this comet is the same with the comet of 1501, with that of 1145, with a comet mentioned in 106G by Hali ben Rodoan, with that of 989, and finally, with a comet seen in the j-ear 12 before our era. This account of Halley's comet has been given at length, to illustrate the principles on which the calculations are made. There are, in all, eighteen comets whose periodi- city is established by the fact that their return has been actually observed: (l)That of Encke, with a short period of 3.303 years; its orbit does not extend so far as the orbit of Jupiter, and a siight acceleration in its periodic times of return has suggested the possibility of space', within our solar system at least, being occupied by a resisting medium, though of extreme rarity. (2) That of Ciela or Gambart, having a period of 6.692 years. During the visit of this comet in 1846, it was seen to separate into two distinct parts, 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 nianv times due, it has not again been seen. (3) That of Faye. with a period of 7.566 years. The orbits of Biela's and Faye's comets extend beyond the orbit of Jupiter, but not so far as that of Saturn. (4) That of Winnecke, with a period of 5.831 years. (5) That of Brorsen. with a period of 5.456 years. (6) That of Temple (No. 1), with a period of 6.538 years. (7) That of Temple (No. 2), with a period of 5.281 years. (8) That of D'Arrest. with a period of 6.675 years. (9) That of Tuttle, with a period of 13.667 years. (10) That of Swift, with a period of 5.547 years. (11) That of E. Swift, with a period of 5.863 vears. (12) That of Finlay, with a period of 6.556 years. (13) That of Wolf, with a period of 6.845 years. (14) That of Holmes, with a period of 6.874 years. (15) That of Brooks, with a period of 7.097 years. (16) That of Pons, with a period of 71.56 years. (17) That of Olbers, with a period qf 72.65 years. (18) That of Halley, ->vith a period of 76.08 years. The orbits of sixty comets appear to be "ellipses. These are probably periodical. The paths of about two himdred cannot be dis- tinguished from paraljnlas. These are possibly periodical, as the ellipse, when very eccentric, cannot near perihelion be distinguished from a ])arabola. Only about a dozen have orbits pos- sible hyperbolic: and. as we have said, in only two cases is the hyperbolic character of the path established upon foirly reliable evidence. In the month of June. 1770, Messier discovered a comet which remained visible a long time, and enabled Lexell to ascertain the orbit to be an ellipse whose major axis was only three times the diameter of the earth's orbit, and corre- sponded to a periodic revolution of five and one- half years. This result suggested grave difficul- ties. It had been found impossible to identify this comet with any previously observed, and yet it was difficult to coiiceive that a bright comet with so short a period of return should have previously escaped observation. What was still more remarkable, it was never seen again, though anxiously looked for in the places where Lexell's orbit would have brought it. It became popularly called 'Lexell's lost comet,' and gave occasion to many sarcasms by the wits of the day at the expense of astronomers, who had boasted of having found the key to the cometary movements. In the present day the exi)lanation is complete. The comet was never seen before 1770, because its orbit previously had been totally diiferent, its nearest point to the sun having been as distant as the path of Jupiter. Its appearance that year arose out of the fact that in 1767 it was in such close contact with Jupiter, moving in the same direction and nearly in the same plane, that the attraction of that planet entirely chnnged its orbit. But why has the comet not since been seen? Its passage to the point of perihelion in 1776 took place by day; and in 1779, before another return, it again encountered the vast body of .Jupiter, and suffered a fresh orbital derangement — the attraction of the planet deflecting it into more distiint regions, and so changing the form of the orbit that if it had again been visible, it would probablj' not have been recognized as identical with Lexell's comet. 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 i-egulates the movements of the planets, appears to have been about the most remarkable for brilliancy of any of which we have authentic accounts. This comet is supposed to be identical with the one that appeared about the time of Ca;sar's death (B.C. 44), 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 revolution, according to the orbit calculated for it by Whiston, being about 575 years. There is, however, some doubt among astronomers as to the real form of its orbit, the one assigned to it by Encke giving it a period of 8.813 years. The' tail of a comet is nearly always away from the sun, frequently assuming a curved form. It increases in length with its proximity to the sun, but does not acquire its greatest length till after passing the perihelion. These are usual characteristics of comets, which were exemplified by this one in a remarkable de- gree. The phenomena might be accounted for. if we were to regard the train as vaporization produced by the intense heat to which the body of the comet is exposed in its approach to the sun. In the nineteenth century the comets most remarkable for brill iancv were the comets of 1811. 1843, 1858 (Donati's), 1861, 1880, 1881. and the great comet (b) of 1882. Spectroscopic investig.ation, so far as yet pursued, points to the conclusion that comets are self-luminous.' and do not shine merely by reflecting solar light. It has been discovered, in determining the tracks of those streams of dark bodies that cause meteoric showers, that some of the tracks coincide with the orbits nf well-known comet.s. From this it is inferred that star-showers and comets mny be only ditTerent manifestations of the same thing. (See JIeteors.) What the matter of the comets consists of is, of course, only a subject for