The Primary Planets. 309 



tremely protracted and eccentrical, the aphelion 

 of a comet is consequently at an immense distance 

 from the sun. In that case the light which they 

 receive from him is too feeble to be reflected to 

 us, and they are only visible when they approach 

 their perihelion. The time of their appearance 

 is, therefore, very short, compared with the time 

 of their disappearance. In order to describe the 

 course of a comet, let ABPC (PI. XXIX. fig. 

 120.) be the very long orbit of a comet, in one 

 of whose foci S is placed as the sun v ; the aphelion 

 in A ; the perihelion in P. The comet is not 

 visible to us but when it approaches towards B, 

 and during the time which it passes the arc BPC 

 of its orbit. But the time is considerably shorter 

 than that which it employs to pass the other por- 

 tion of its orbit CAB, for these two reasons : first, 

 because the arc BPC is much shorter than the 

 arc C AB ; and in the second place, because the 

 comets, like the planets, are slower in their 

 course while they depart further from the sun ; 

 and, on the, contrary, they are swifter as they ap- 

 proach the sun. It requires much less time to 

 pass over the portion BPC of their orbit which 

 is visible to us, than the other portion CAB. 



The most luminous part of the comet is com- 

 monly surrounded with a kind of atmosphere, 

 which again seems to emit from it a fainter 

 light, somewhat resembling the Aurora Borealis. 

 The interior part is called the nucleus, and the 



