The perihelion passage of 1456 was, until recently, the earliest known. It was shown by Laugier, however, in 1843, that among the notices of comets extracted by Edward Biot from the Chinese records, were observations of a body in 1378, which was undoubtedly the comet of Halley. Further researches among these annals enabled the same astronomer to recognize two ancient returns, one in 760, the other in 451. Still more recently the distinguished English astronomer, Mr. Hind, has traced back the returns to the year 11 B.C. He remarks, however, that previous to that epoch, "the Chinese descriptions of comets are too vague to aid us in tracing any more ancient appearances," and that "European writers of these remote times render us no assistance." Let us now inquire whether the comet had probably made any former approach to the sun in an orbit nearly identical with the present. It is well known that the modern period of this body is considerably less than the ancient. Thus, the mean period since A.D. 1456 has been 75.88 years; while from 11 B.C. to 1456 A.D. it was 77.27 years. In determining the approximate dates of former returns, the ancient period should evidently be employed. Now, it is a remarkable fact that of more than 70 comets,[4] or objects supposed to be comets, whose appearance was recorded during the six centuries immediately preceding the year 11 B.C., but one—that of 166 B.C.—was observed at a date corresponding nearly to that of a former return of Halley's comet. Of this object it is merely recorded that "a torch was seen in the heavens." Whether this was a comet or some other phenomenon, it is impossible to determine. But as the comet of Halley was more brilliant in ancient than in modern times, it seems highly improbable that seven consecutive returns of so conspicuous an object should have been unrecorded, especially as twelve comets per century[5] were observed during the same period. It would appear, therefore, that the perihelion passage of 11 B.C. was in fact the first ever made by the comet, or at least the first in an orbit nearly the same as the present.
The motion of Halley's comet is retrograde. The point of its nearest approach to the sun is situated within the orbit of Venus. Its greatest distance from the centre of the system is nearly twice that of Uranus, or 36 times that of the earth. The comet is, consequently, subject to great changes of temperature. When nearest the sun its light and heat are almost four times greater than the earth's; when most remote, they are 1200 times less. In the former position, the sun would appear much larger than to us; in the latter, his apparent diameter would not greatly exceed that of Jupiter, as viewed from the earth. It would be difficult to conjecture what the consequences might be, were our planet transported to either of these extremes of the cometary path. In the perihelion, the waters of the ocean would undoubtedly be reduced to a state of vapor; in the aphelion, they would be solidified by congelation.
II. Encke's Comet.
It was formerly supposed that all comets have their aphelia far beyond the limits of the planetary system. In 1818, however, a small comet was discovered by Pons, the orbit of which was subsequently found to be wholly interior to that of Jupiter. Its elements were presented by Bouvard, in 1819, to the Board of Longitude at Paris. The form and position of the orbit were immediately found to correspond with those of a comet observed by several astronomers in 1805. The different appearances were consequently regarded as returns of the same body. Its elliptic orbit was calculated by Encke, who found its period to be only about three years and four months. Its perihelion is within the orbit of Mercury; its aphelion, between the asteroids and the orbit of Jupiter.
Encke's comet is invisible to the naked eye, except in very favorable circumstances; it has no tail; its motion, like that of the planets, is from west to east; and its orbit is inclined about 13° to the ecliptic.
A comparison of the successive periods of this interesting object has led to the discovery that its time of revolution is gradually diminishing; a fact regarded by Encke and other astronomers as indicating the existence of an ethereal medium.
III. Biela's Comet.
The discovery of Encke's comet of short period was followed, in 1826, by that of another, whose revolution is completed in about six years and eight months. It was observed on the 27th of February, by M. Biela, an Austrian officer; accordingly it has since been known as Biela's comet. On computing its elements and comparing them with those of former comets, it was found to have been observed in 1772 and 1805. Damoiseau having calculated the dimensions of the comet's elliptic path and the time of its return, announced as the result of his computations the remarkable fact that the orbits of the earth and comet intersect each other, and that the comet would cross the earth's path on the 29th of October, 1832. This produced no little alarm among the uneducated, especially in France. Even some journalists are said to have predicted the destruction of our globe by a collision with the comet. When the latter, however, passed the point of intersection at the predicted time, the earth was at a distance of 50,000,000 miles.
At the return of 1845-6, Biela's comet exhibited a most remarkable appearance. Instead of a single comet, it appeared as two distinct bodies moving together side by side, at a distance from each other somewhat less than that of the moon from the earth. Astronomers, anxious to determine whether the cometary fragments had continued separate during an entire revolution, awaited the next return with no ordinary interest. The two bodies appeared at the predicted time (September, 1852); their distance apart having increased to 1,250,000 miles. In 1859 the comet, on account of its proximity to the sun, entirely escaped detection. At the return in 1865-6 the position of the object was quite favorable for observation, yet the search of astronomers was again unsuccessful. In 1872 the body escaped detection both in Europe and America. One fragment was seen, however, at Madras, India, on the mornings of the 2d and 3d of December,—several weeks after its perihelion passage. The comet's non-appearance in 1866 and its greatly diminished magnitude in 1872 leave no room to doubt its progressive dissolution. This subject will again be referred to in discussing the phenomena of meteoric showers.
IV. Faye's Comet.