Fig. 52.—Aspects of Remarkable Comets. or one that will carry it off into infinite space, never to return. One rate of speed on the curve indicates an elliptical orbit that returns; a greater rate of speed indicates that it will take a parabolic orbit, which never returns. The exact rate of speed is exceedingly difficult to determine; hence it cannot be confidently asserted that any comet ever visible will not return. They may all belong to the solar system; but some will certainly be gone thousands of years before their fiery forms will greet the watchful eyes of dwellers on the earth. A comet that has an elliptic orbit may have it changed to parabolic by the accelerations of its speed, by attracting planets; or a parabolic comet may become elliptic, and so permanently attracted to the system by the retardations of attracting bodies. A comet of long period may be changed to one of short period by such attraction, or vice versa.
The number of comets, like that of meteor streams, is exceedingly large. Five hundred have been visible to the naked eye since the Christian era. Two hundred have been seen by telescopes invented since their invention. Some authorities estimate the number belonging to our solar system by millions; Professor Peirce says more than five thousand millions.
Famous Comets.
The comet of 1680 is perhaps the one that appeared in A.D. 44, soon after the death of Julius Cæsar, also in the reign of Justinian, A.D. 531, and in 1106. This is not determined by any recognizable resemblance. It had a tail 70° long; it was not all arisen when its head reached the meridian. It is possible, from the shape of its orbit, that it has a periodic time of nine thousand years, or that it may have a parabolic orbit, and never return. Observations taken two hundred years ago have not the exactness necessary to determine so delicate a point.
On August 19th, 1682, Halley discovered a comet which he soon declared to be one seen by Kepler in 1607. Looking back still farther, he found that a comet was seen in 1531 having the same orbit. Still farther, by the same exact period of seventy-five years, he found that it was the same comet that had disturbed the equanimity of Pope Calixtus in 1456. Calculations were undertaken as to the result of all the accelerations and retardations by the attractions of all the planets for the next seventy-five years. There was not time to finish all the work; but a retardation of six hundred and eighteen days was determined, with a possible error of thirty days. The comet actually came to time within thirty-three days, on March 12th, 1759. Again its return was calculated with more laborious care. It came to time and passed the sun within three days of the predicted time, on the 16th of November, 1835. It passed from sight of the most powerful telescopes the following May, and has never since been seen by human eye. But the eye of science sees it as having passed its aphelion beyond the orbit of Neptune in 1873, and is already hastening back to the warmth and light of the sun. It will be looked for in 1911; and there is good hope of predicting, long before it is seen, the time of its perihelion within a day.
Biela's lost Comet.—This was a comet with a periodic time of six years and eight months. It was observed in January, 1846, to have separated into two parts of unequal brightness. The lesser part grew for a month until it equalled the other, then became smaller and disappeared, while the other was visible a month longer. At disappearance the parts were 200,000 miles asunder. On its next return, in 1852, the parts were 1,500,000 miles apart; sometimes one was brighter and sometimes the other; which was the fragment and which was the main body could not be recognized. They vanished in September, 1852, and have never been seen since. Three revolutions have been made since that time, but no trace of it could be discovered. Probably the same influence that separated it into parts, separated the particles till too thin and tenuous to be seen. There is ground for believing that the earth passed through a part of it, as before stated under the head of meteors.
The Great Comet of 1843 passed nearer the sun than any known body. It almost grazed the sun. If it ever returns, it will be in A.D. 2373.
Donati's Comet of 1858.—This was one of the most magnificent of modern times. During the first three months it showed no tail, but from August to October it had developed one forty degrees in length. Its period is about two thousand years. Every reader remembers the comet of the summer of 1875.
Encke's Comet.—This comet has become famous for its supposed confirmation of the theory that space was filled with a substance infinitely tenuous, which resisted the passage of this gaseous body in an appreciable degree, and in long ages would so retard the motion of all the planets that gravitation would draw them all one by one into the sun. We must not be misled by the term retardation to suppose it means behind time, for a retarded body is before time. If its velocity is diminished, the attraction of the sun causes it to take a smaller orbit, and smaller orbits mean increased speed—hence the supposed retardation would shorten its periodic time. This comet was thought to be retarded two and a half hours at each revolution. If it was, it would not prove the existence of the resisting medium. Other causes, unknown to us, might account for it. Subsequent and more exact calculations fail to find any retardations in at least two revolutions between 1865 and 1871. Indications point to a retardation of one and a half hours both before and since. But such discrepancy of result proves nothing concerning a resisting medium, but rather is an argument against its existence. Besides, Faye's comet, in four revolutions of seven years each, shows no sign of retardation.
The truth may be this, that a kind of atmosphere exists around the sun, perhaps revealed by the zodiacal light, that reaches beyond where Encke's comet dips inside the orbit of Mercury, and thus retards this body, but does not reach beyond the orbit of Mars, where Faye's comet wheels and withdraws.