THE NON-LIVING WORLD ifg 



enormous base, the amount of their remoteness remains 

 unascertainable save in some thirty instances, as (with 

 these exceptions) they present no appreciable difference 

 of position, or, as it is called, no parallax, when viewed 

 from opposite points of the earth's orbit. 



The sun is 852,900 miles in diameter, and 1,252,700 

 times the volume of the earth. 



Our satellite, the moon which is only 238,813 miles 

 distant from us, and has but a diameter of 2,160 miles- 

 circles round us in about four weeks and turns once on 

 her own axis during each such revolution. Therefore 

 the same side of the moon is ever turned towards the 

 earth. The so-called changes in the moon, of course, 

 simply result from the different amounts of her surface 

 which are, at different times, illuminated by the sun's 

 rays. Similar changes are also shown by the planet 

 Venus. 



The moon appears to be devoid of both air and water, 

 or if such substances exist there, they seem to have 

 retreated into the moon's interior, and give no signs of 

 their presence on its much scarred surface. 



As to the shape of the paths followed by the earth and 

 the other planets in their revolution round the sun, they 

 follow precisely the same laws as regulate any body so 

 moving round another that it cannot fall to the surface 

 of the latter. We have already seen * that any body pro- 

 jected from a point external to the earth's surface, and 

 with a certain velocity, would be constrained to revolve 

 round it in an ellipse and that its radius vector must 

 always pass through equal areas in equal times and always 

 in the same plane. This is the precise law of the earth's, 

 and of all the other planets', revolutions round the sun. 



* See ante, p. 65. 



