402 



II. 



THE PLANET3. 



GENERAL comparative considerations of \*noie ciu,-,s oi 

 eosmical bodies must here precede their individual descrip- 

 tion. These considerations refer to the 22 principal planets 

 and 21 moons [satellites, or secondary planets] which have 

 been discovered up to the present time ; not to the planetary 

 bodies in general, among which the comets whose orbits have 

 been calculated are alone tenfold more numerous. The 

 planets possess, upon the whole, a feeble scintillation, inas- 

 much as they shine by the reflected light of the Sun, and 

 their planetary light emanates from discs. (Cosmos, vol. iii. 

 p. 101.) In the ash-coloured light of the Moon, as well as 

 in the red light of its obscured disc, which is seen with 

 groat intensity between the tropics, the Sun's light under- 

 goes, in reference to the observer upon the Earth, a twice 

 repeated change in its direction. Attention has been already 

 directed elsewhere 1 to the fact, that the Earth and other 

 planets possess in themselves a feeble power of emitting 

 light, as is specially proved by some remarkable phenomena 

 upon that portion of Venus which is turned away from 

 the Sun. 



We shall consider the planets according to their number, 



1 Cosmos, vol. i. p. 196, and note p 197. 



