COMET

a heavenly body of a luminous and nebulous appearance which approaches to and recedes from the sun. The name is derived from the Greek, a name bestowed on these bodies because they generally po e a ‘tail’ or ‘tails;’ this tail in ancient times also ' being called a ‘beard’ when the train preceded the nucleus, as is the case when the ©. is receding from the sun. Most O’s. are divided into three parts, the nucleus and the coma, which aT hae TE _ together form the head of the ©., _ the tazl. It is impo ible to exactly _ define the limits of each of these parts, as they shade gradually into the other; quite often, too, a C. is without a tail, others again (as eg. that of Cheseaux, (1744) may have half a dozen. The, nucleus is the brightest portion, the coma which surrounds it is a hazy area of light, while the tail becomes more and more faint and attenuated until it fades out. How attenuated is the Matter that composes a C. may be _ judged by the fact that stars have Tepeatediy been seen through the! RSE r + | thickest parts, and that the earth has | pa ed through the tail of a C. with- | out any observable effect. Thus Sir (2)|J. Herschel records that in 1832 he |saw a group of stars of the sixteenth magnitude through almost the center | of Biela’s ©. The composition of a C. ‘ as revealed by the spectroscope: (which was first succe fully applied to determine the constitution of O’s. by Sir William Huggins in 1868) is of tanutey. A ot Mat is oiere those of sodium and iron, have been | Observed in the spectrum of the nucleus. The spectrum of a OC. also shows that the light is partly reflected sunlight and party es act Cah 800 C’s. ee m recorded, the larger portio alf this number have had their orbit: calculated, and are found to move rs in one or other of ie cece of the HS tos conic sections, 7z.e. an ellipse, a para ola, | O° penal tert eae ncaa of O's. farce|MOVINS on elliptical orbits is comparatively few, being about eighty. rine epee pe C’s. may be a aa: turn to the sun and _ therefore | Bece arily belong to the solar system (qg.v.).. Chief among these are Biela’s, Encke’s, and Halley’s ©’s. The rest of the C’s. move in parabolic paths, except about half a dozen, which, as the result of perturbations, have a hyperbolic movement. Regarded merely spectacularly and historically, C’s. have ever been the object of man’s curiosity and sometimes his fear. Thus the dream of Julius Cesar and ‘the battle of Hastings were believed to have been heralded by C’s., a representation of the latter C. appearing in the Bayeux tapestry. The Bayeux C. has been shown by calculation to have indubitably been Halley’s C., and it is conjectured that Halley’s C. was the one recorded by the Chinese annals as having appeared in 240 and 87 B. C. The periodic return of certain C’s. has been useful in fixing or confirming historical dates, that of Halley being the first to return as predicted, viz., in 1759. The most spec and|tacular of the C’s. of the 19th century was that found by Donati on June 2, 1858. It stretched over a space of 40°, or nearly a quarter of the sky, ‘and its maximum width was about 10°. In 1921 there were four O’s., the Reid, Dubiago, Encke and Pons Winnecke. The last named returned much. behind iis scheduled time and departed widely fromm its calculated orbit. Prior to its rediscovery by Barnard, on April 10, there seemed to be a chance of a close approach of this C. to the earth. However, even the expected meteor shower failed to materialize. See Brnwa’s Comet. In 1923 the following O’s., by calculation, should return to the northern skies: May, Jiacobini; June, Coggia; December, Denning and Swift. In 1924: Barnard, June; Swift, May; Encke, October.