297. Halley's Comet.—Halley's comet has become one of the most celebrated of modern times. It is the first comet whose return was both predicted and observed. It made its appearance in 1682. Halley computed its orbit, and compared it with those of previous comets, whose orbits he also computed from recorded observations. He found that it coincided so exactly with that of the comet observed by Kepler in 1607, that there could be no doubt of the identity of the two orbits. So close were they together, that, were they both drawn in the heavens, the naked eye would almost see them joined into one line. There could therefore be no doubt that the comet of 1682 was the same that had appeared in 1607, and that it moved in an elliptic orbit, with a period of about seventy-five years. He found that this comet had previously appeared in 1531 and in 1456; and he predicted that it would return about 1758. Its actual return was retarded somewhat by the action of the planets on it in its passage through the solar system. It, however, appeared again in 1759, and a third time in 1835. Its next appearance will be about 1911. The orbit of this comet is shown in Fig. 326. Fig. 327 shows the comet as it appeared to the naked eye, and in a telescope of moderate power, in 1835. This comet appears to be growing less brilliant. In 1456 it appeared as a comet of great splendor; and coming as it did in a very superstitious age, soon after the fall of Constantinople, and during the threatened invasion of Europe by the Turks, it caused great alarm. Fig. 328 shows the changes undergone by the nucleus of this comet during its perihelion passage in 1835.
Fig. 328.
Fig. 329.
Fig. 330.
298. Encke's Comet.—This telescopic comet, two views of which are given in Figs. 329 and 330, appeared in 1818. Encke computed its orbit, and found it to lie wholly within the orbit of Jupiter (Fig. 324), and the period to be about three years and a third. By comparing the intervals between the successive returns of this comet, it has been ascertained that its orbit is continually growing smaller and smaller. To account for the retardation of this comet, Olbers announced his celebrated hypothesis, that the celestial spaces are filled with a subtile resisting medium. This hypothesis was adopted by Encke, and has been accepted by certain other astronomers; but it has by no means gained universal assent.
299. Biela's Comet.—This comet appeared in 1826, and was found to have a period of about six years and two thirds. On its return in 1845, it met with a singular, and as yet unexplained, accident, which has rendered the otherwise rather insignificant comet famous. In November and December of that year it was observed as usual, without any thing remarkable about it; but, in January of the following year, it was found to have been divided into two distinct parts, so as to appear as two comets instead of one. The two parts were at first of very unequal brightness; but, during the following month, the smaller of the two increased in brilliancy until it equalled its companion; it then grew fainter till it entirely disappeared, a month before its companion. The two parts were about two hundred thousand miles apart. Fig. 331 shows these two parts as they appeared on the 19th of February, and Fig. 332 as they appeared on the 21st of February. On its return in 1852, the comets were found still to be double; but the two components were now about a million and a half miles apart. They are shown in Fig. 333 as they appeared at this time. Sometimes one of the parts appeared the brighter, and sometimes the other; so that it was impossible to decide which was really the principal comet. The two portions passed out of view in September, and have not been seen since; although in 1872 the position of the comet would have been especially favorable for observation. The comet appears to have become completely broken up.
