CHAPTER V 

 PLANETARY EVOLUTION 1 



AMONG the notable phenomena presented 

 by the structure of our planetary sys- 

 ^ tern, there are some which have become 

 so familiar to us that we commonly overlook 

 them altogether, and through sheer inatten- 

 tiveness fail to realize their significance. For 

 example, all the planets revolve about the sun 

 in the same direction, which coincides with the 

 direction of the sun's own rotation upon his axis. 

 All the planets, moreover, revolve in planes 

 which are but slightly inclined to the plane 

 of the sun's equator. Satellites conduct them- 

 selves similarly with reference to their primaries. 

 Every satellite revolves about its primary in 

 the direction of the primary's axial rotation, and 

 in a plane but little inclined to the plane of the 

 primary's equator. Again, with the single inter- 

 esting exception of Uranus — and possibly also 

 of Neptune — all the planets, as well as the 

 sun, rotate upon their axes from west to east, in 

 the same direction with their orbital revolutions. 

 And lastly, all the planets, both primary and 



1 [See Introduction, § 16.] 

 249 



