16 



THE SKY. 



being inhabited, of course it can never be answered with 

 certainty, but it is a great deal more likely that they are 

 than that the moon is, which, having neither water nor atmo- 

 sphere, can hardly be supposed to give habitation to any 

 beings similar or analogous to those on earth, while Mars pos- 

 sesses a climate not greatly differing from that of the earth, 

 and has both air and water. Mars 

 has, at certain positions of the earth 

 with respect to it, a partially " gib- 

 bous " form, that is, a small portion 

 of the non-illuminated part comes 

 within the lines of pur vision) it is 

 then of the form seen in fig. 14), but 

 this can only occur when the earth 

 FIG. 14. and planet occupy positions some- 



what near to that represented in 



fig. 15, in which s is the sun, E E opposite positions of 

 the earth, and M Mars. 



Jupiter is by far the largest of the planets, it is 87,030 



