COMETS. . 545 



eastwards from the Sun is directed towards the east, calcu- 

 lating from the nucleus ; but if the comet appears to the 

 west of the Sun, the tail turns towards the west." 13 Fracas- 

 toro and Apianus say, still more correctly, " that a line 

 produced through the head of a comet in the direction of the 

 axis of the tail meets the Sun." The words of Seneca are 

 also characteristic : " The tails of comets fly from the Sun's 

 rays." (Nat. Qittfst. vii. 20.) While among the yet known 

 planets and comets the periods of rotation dependent upon 

 the half-major axis, the shortest and the longest of the planets 

 are in the proportion of 1 : 683, the proportion in the case 

 of the comets is as 1 : 2670. Mercury (87d'97.) is here 

 compared with Neptune (60,126d'7.), and the Comet of 

 Encke (3-3 years) with the Comet of 1680 (8814 years) 

 observed by Gottfried Kirch at Coburg, Newton, and Halley. 

 The distance of the fixed star nearest to our solar system (a 

 Centauri) from the aphelion (point of greatest distance from 

 the Sun) of the last-named comet is determined by Encke in 

 an excellent treatise. The small velocity of its motion (ten 

 feet in a second) in this outermost part of its orbit, the 

 greatest proximity which the Comet of Lexell and Burckhardt 

 (1770) has attained to the earth (six times the distance of the 

 Moon), and the Comet of 1680 (and still more that of 1843) to 

 the Sun, I have already treated of in Cosmos, vol. i. p. 95, and 

 vol. iv. pp. 351 353. The second comet of the year 1819, 

 which appeared, in Europe, suddenly to break forth from out 

 of the Sun's rays in considerable magnitude, passed, according 



clioloa smoked again ;" this is the name of the planet Venus, 

 also called Tlazoteotl in the Aztec language (see my Vues 

 des Cordilleres, torn. ii. p. 303): this is probably, in the 

 Grecian as well as the Mexican sky, a phenomenon of atmo- 

 spheric refraction the appearance of small haloes. 



13 Edward Biot, in the Comptes rendus, torn. xvi. 1843, 

 p. 751. 



