IN DESCRIPTIVE ASTRONOMY. 119 



52. What planets would float in water ? 



According to Chambers' Astronomy, the density of Saturn 

 is .68 that of water ; Uranus, .99 ; Neptune, .96. According to 

 Newcomb, Saturn's density is .75. 



53. How must the moons of Jupiter appear during 

 their transit across the disk of that planet ? 



The satellites appear on the disk of their primary as round 

 luminous spots, preceded or followed by their shadows, which 

 show as round black or blackish spots. CHAMBERS. 



54. " The shadow of the satellite precedes the satellite 

 itself when Jupiter is passing from conjunction to oppo- 

 sition) but follows it between opposition and conjunc~ 

 tion." Explain. 



"When actually in conjunction, the shadow is in a right line 

 with the satellite, and the two may be superposed. 



55. What facts point to the conclusion that Mars may, 

 perhaps, have passed his planetary prime ? 



The proportion of land and water, and the appearance of 

 the seas, all point to a conclusion somewhat similar to the one 

 stated in the following quotation : 



" Mars' orbit being outside the earth's, he was probably 

 formed earlier. The mass of Mars is not much more than \ 

 the earth's, and the surface about |- ; if he possessed the same 

 degree of heat as the earth, he would have only \ the amount 

 to radiate, and the supply would not last so long. Though 

 having only \ the surface of the earth, he would still cool off 3 

 times as rapidly as the earth. Mars must, therefore, be at 

 least three times as far on the way toward planetary decrepi- 

 tude and death as our earth." Proctor's Poetry of Astronomy. 



50. Why may ^ve conceive that Saturn and Jupiter are 

 yet in their planetary youth ? 



(See Astronomy, note, p. 163.) 



Vast planets, like Saturn and Jupiter, must have required 

 for cooling a far longer time than the earth, and thus the va- 

 rious stages of development would occupy a much greater 



