revolution in about 1,210 days. Its period is shortened at each revolution by about its eleven-thousandth part, which, if continued at the same rate, will cause it to fall into the sun after about twenty or thirty thousand years. very The mass of Mercury is very difficult to determine. Lagrange determined it by assuming the density of the planets to vary inversely as their mean distances from the sun. Encke's Comet, when near its perihelion, passes at times near to Mercury, and the disturbance which that planet produces in the motion of the comet enables the astronomer to compute the value of its mass. In 1838 the comet was in the required position, and M. Encke found the mass of the planet considerably less than the value assigned by Lagrange. The major axis of the orbit of Encke's Comet lies almost exactly in the plane of Jupiter's motion; and since the aphelion of the comet's orbit is near the orbit of Jupiter, it is evident that the planet may sometimes exert a powerful influence on the motion of the comet. This happened at the comet's revolution of 1819-'22. M. Encke computed the perturbations of the comet, allowing for the effects of a resisting medium, and assuming an indeterminate correction to Jupiter's mass. Equating the effects thus produced to the observed errors of the comet's motion, he obtained a number of equations of conditions, the subsequent combination and solution of which gave a value of Jupiter's mass agreeing very closely with that found from the perturbations of the smaller planets.* One of the most curious and interesting facts recorded in the history of comets, is their occasional separation into two or more distinct bodies. The cause which produces this is at present unknown. Grant refers it to the influence of the sun; but since our observations can only extend but little beyond the orbit of Mars, it seems pretty difficult to decide whether the breaking-up always takes place in that part of the solar system where the sun's influence is powerful, or in * Grant's Hist. Phys. Ast., p. 135. more remote regions of space. It is not impossible that a comet may encounter some of the meteoric rings which seem to exist in the planetary system. Seneca relates that Ephorus, an ancient Greek author, makes mention of a comet which separated into two distinct bodies before it disappeared.* Seneca doubted the fact, but Kepler thought it quite probable; † and he further thought it quite probable that two comets which appeared in the same region of the heavens in the year 1618, were fragments of a separated comet. We are informed by Hevelins that Cysatus perceived in the head of the great comet of 1618 unequivocal symptoms of a breaking-up into two distinct bodies. When the comet first appeared, in the month of November, it resembled a round mass of concentrated light. On the 8th of December it seemed to be divided into several parts. On the 20th of the same month its appearance was that of a multitude of small stars. Hevelins himself informs us that he witnessed a similar appearance in the head of the comet of 1661. § The most remarkable example, however, which we have of the separation of a comet into two distinct bodies, is that of Biela's Comet in the year 1846, or perhaps the last of 1845. This comet is a faint nebulous object, invisible to the naked eye, with a diameter of about 50,000 miles. At all previous returns to that of 1846 it appeared as a single body; and when first observed in 1845 it presented no unusual appearance. About the middle of December, however, it showed signs of violent disturbance, with indications of a separation. On the 19th it was pear-shaped. On the 29th it was seen attended by a faint nebulous spot. On the 13th of January, 1846, it was observed at the Naval Observatory, Washington, by Lieut. Maury to consist of two distinct bodies, each moving in an independent orbit. On the 15th the division was generally noticed in Europe, and carefully observed. Each body exhibited the usual charac *Nat. Quæst., lib. vii. cap. xxi. § Ibid., p. 417. teristics of a comet, a condensation about the middle parts, and an elongation in a direction opposite the sun. The central condensation of each seems to prove that the comet is of the nature of a fluid, or of fine, disconnected particles, which are drawn together by their mutual gravitation. Professor Hubbard, of the Naval Observatory, Washington, from an elaborate investigation of the motion of the two comets,* found that the maximum distance between them in 1846 was less than the distance of the moon from the earth; and that the observations are satisfied by supposing each part to describe an independent ellipse around the sun. The next return of the comet to the perihelion would take place in 1852. As the time drew near and the attention of astronomers began to be directed towards the subject, much interest was manifested relative to the phenomena that might be presented. On the 28th of Angust of that year, Professor Secchi, at Rome, discovered one of the nuclei, but the other was not found till September 15th, when the "new one" was too faint for Secchi to determine its exact position. But what seems curious, it was found on comparison that the new one was really the one that was discovered in August, and the bright one was the new one. On the 16th the southern nucleus (first discovered) was observed, the other not being visible, and on the 17th and 18th only the northern one could be seen; but on the 19th both were observed by Secchi, and the double observation was repeated at Rome and at Pulkowa on the 20th, 23d, and 25th, while on the 21st only the southern and on the 22d only the northern one was visible. "We have thus," says Professor Hubbard, "a most interesting repetition of the alterations exhibited in 1845–46, which now appear more remarkable only in consequence of the extreme faintness of the comets, which was such that the slightest change of light sufficed to carry them either within or beyond the reach of vision." The apparent distance of the two nuclei when discovered was little more than half a degree, but the real distance was about seven times as * Gould's Ast. Jour., vols. iii., iv., vi. Ast. Jour., vol. vi., p. 160. great as in 1846. It seems to be nearly impossible to tell which of the two nuclei corresponds to the original one of 1846. The hypothesis generally adopted, is that the one discovered in August is the original nucleus. The return of the comets to the perihelion in 1849 was not observed, owing to the fact that they were in the immediate vicinity of the sun. It was expected that their return in 1866 would be generally observed; but owing to some circumstances, but few observations, of a doubtful character, were made upon one of the nuclei. * Owing to the small mass of comets, and their occasional near approach to the larger planets, they are sometimes thrown entirely out of their original orbits and caused to describe new ones differing very much from the former ones. Especially is this the case when they pass near to the planet Jupiter, which, as Sir John Herschel says, "seems, by some strange fatality, to be constantly in the way of comets, and to seem as a perpetual stumbling-block to them." On the 14th of June, 1770, a comet was discovered by Messier, at Paris, and it was observed till the next October the 2d. It passed its perihelion on the 14th of August, and on the 1st of July it was distant from the earth only about six times the distance of the moon, or 1,400,000 miles. It was found that parabolic elements would not represent the motion of the comet. Six years after it had disappeared. Lexell found that it revolved in an elliptical orbit with a period of five and a half years. Its aphelion distance was about 250,000,000 miles, and its perihelion distance 65,000,000 miles. Since the comet was a brilliant object in 1770 it was a curious circumstance that it had not been seen before, and that it was not seen in 1776. Lexell found from the elements of its orbit that in 1776 it was fifty-seven times as near to Jupiter as to the sun, or the fifty-eighth part of Jupiter's mean distance, and at that time the disturbing influence of Jupiter was so great that it threw the comet into an entirely new orbit. It 1776 it could not be seen on account of the relative situations of the comet and the earth at the time, * Monthly Notices of the Royal Ast. Soc., vol. xxvi. pp. 241 271. the former being enveloped in the sun's rays. In 1779 it again passed so near Jupiter as to have its orbit entirely changed a second time, so as to render the comet afterwards invisible. Laplace afterwards undertook the analytical investigation of this interesting question, and his results confirm the deductions of Lexell. * He found that the greater axis of the orbit before 1767 was, 2,526,000,000 miles, and its period forty-eight and a half years; and after 1779 the greater axis of the orbit was 1,400,000,000, and its period twenty years. The perihelion distance was double the mean distance of the planet Mars, so that the comet would always remain invisible. The question rested here for nearly half a century, when Leverrier undertook a thorough revision of Laplace's investigations. He found the observations of 1770 not sufficiently accurate to warrant the definite conclusions at which Laplace had arrived. He found that the action of Jupiter might have thrown the comet into a parabolic or hyperbolic orbit, in which it would depart from our system, probably never again to return; or its orbit might have been elliptic with a very long period (Laplace's conclusion); or, finally, its orbit might have been simply modified somewhat without destroying its character of short periodicity—a result which renders it somewhat probable that the comet may yet be recognized. On the 22d of November, 1843, M. Faye, of the Royal Observatory of Paris, discovered a comet whose period was found to be about seven and a half years. Since its period and path are about the same as those of Lexell's comet, M. Valz advanced the hypothesis that the two are identical. M. Le Verrier undertook a thorough investigation of the subject, and his conclusion was that the two comets cannot possibly be the same body. It is possible that Faye's comet was introduced into, or rather fixed in, our system by the attraction of Jupiter. This is a question that cannot be definitely settled, but Le Verrier has shown that the comet must have been introduced into the system at least as early as 1747, so that it had made not less than thirteen revolutions previous to its discovery. Mec. Celeste, liv. ix. chap. ii. |