Comets are thus distinguished by their tails, and differ very much in their orbits from the planets. The latter are direct in their wanderings, but comets are most irregular and eccentric. The name bestowed upon comets is from the Greek Kome, hair; for when the comet recedes from the sun the “tail” may be said to come out of the head, and appear as a hair in front, so to speak. But though all comets have tails, there are many luminous bodies (classed with comets) which have no tails.
Fig. 537.—Halley’s Comet.
The comet which created the most excitement was Halley’s in 1456, of which we append an illustration (fig. 537). A comet had been observed in 1607, and Halley made a calculation that it would reappear in 1757. The calculation for its actual appearance was made by Claivant, and the expected visitor passed the perihelion in March 1759. This comet, on its appearance at Constantinople, is said to have caused much consternation, and Christians regarded it as a “sign,” for the Turks had just then captured Constantinople, and were threatening Europe. Pious people included it in their supplications for deliverance from their most dreaded enemies, and “Lord, save us from the Turks, the devil, and the comet,” was a common prayer.
Fig. 538.—Great comet of 1811.
There have been several very beautiful comets. Encke’s, Coggia’s, etc., and the comet of 1858 (Donatis) must be in the recollection of middle-aged readers. Others came in 1861 and 1874. In 1881 two comets appeared. Some comets of antiquity were very remarkable, and are reputed to have equalled the sun in magnitude. One tail is usually supposed to be the distinguishing mark of a comet, but in 1774 one appeared with six tails, arranged something like a fan. Sometimes the tail is separated from the head. Of the actual consistency of comets we cannot give any lengthened details. They apparently consist of elements similar to the meteors—namely, of solid masses, and have been supposed to be aggregations of meteors. Some appear at regular intervals, and their approach can be determined with accuracy. Of course we only see those which are attracted by the sun, or those which revolve in the solar system. There must be thousands of other comets which we never see at all.
The diagram (fig. 540 in the next page) represents a portion of the path of the comet of 1680. This visitor pursued its course for two months at a velocity of 800,000 miles an hour. The tail was estimated to extend 123,000,000 of miles, and a length of 60,000,000 of miles was emitted in two days. When this great comet was approaching the sun, or its perihelion, as such approach is termed, three minutes more would have seen it rush into the orb had its enormous pace been slackened, but as it was proceeding so rapidly, and being just then 144,000 miles away, it escaped. We can scarcely estimate the results of such a collision. This comet appeared B.C. 34, and again at intervals of about 575 years. It may be expected in 2255. It is to Halley that the discovery of the elliptical orbits of comets is due.
Fig. 539.—Path of Biela’s Comet.