26 



KNO\A/'LEDGE 



[Nov. 11, 1881. 



COMETS. 



Part II. 



MOST persons know tlmt the name " comet " is derived 

 from the word coma, or hair, nntl is applied to 

 relestial objects whicli appear to have a hairy appendage. 

 Modem astronomers do not, indeed, use the word coma in 

 this sense, hut draw a distinction between the rnma and 



Fig. 1. — Oianpes of a Comet whpn firet poen. 



— would not be very applicable, by the way, to any comets 

 that ha\c appeared in modem times. The Chinese applietl 

 to comets the name aui, or " broom." 



It might be supposed that the hairy, broom-like, or tail- 

 like appendage, so commonly seen in comets, is really a 

 distinctive feature cf these comets. This, however, is far 

 from being the case. A very large numl)er of comets have 

 no visible tails. We refer, of course, principally to tele- 

 scopic comets ; for very few comets which have been con- 

 spicuous to the naked eye have wanted this appendage. 



The cornn — in the modem 

 astronomical sense — is never 

 wanting. This term is applied 

 to a misty, hazy light, surround- 

 ing on every side a small bright 

 spot, which is termed the 

 nvckus of the comet. 



AVhen first seen in the 

 telescope, a comet usually 

 presents a small round disc of 

 hazy light, somewhat brighter 

 near the centre. As the comet 

 approaches the sun, the disc 

 lengthens, and, if the comet 

 is to be a tailed one, traces 

 begin to be seen of a streaki- 

 ness in the comet's light. 

 Gradually a tail is formed, 

 which is turned always frovj 



* 



the tail. There can be no doubt, however, that the part 

 now called the comet's tail was that from which these 

 objects derived their name. The word conwia or conwtes is 

 not a lately-formed one ; but was used by Cicero, Tibullus, 

 and other ancient writers, and it is worthy of notice that 

 all the names applied to comets by the Koraans had a 

 reference to fiairiness — s/e///r comanles, crinilrr, roneinnatce, 

 they are called by Ovid, Plin_y, and Cicero. The last term 

 — signifying stare which show a mrlcd or crisped hairiness 



the sun (Fig. 1). The tail grows brighter and longer, and 

 the head becomes developed into a coma surrounding a 

 distinctly-marked nucleus. Presently the comet is lost to 

 view through its near approach to the sun. L?ut after 

 awhile it is again seen, sometimes wonderfully changed in 

 aspect through the eflects of sokir heat. Some comets are 

 brighter and more striking after passing their point of 

 nearest approach to the sun (or perihelion) than before ; 

 others are quite shorn of their splendour when they re- 



