CHAPTER V. 



PLANETARY EVOLUTION. 



Among? the notable phenomena presented by the structure 

 of onr planetary system, there are some which have become 

 so familiar to us that we commonly overlook them altogether, 

 and through sheer inattentiveness fail to realize their signifi- 

 cance. 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, more- 

 over, revolve in planes which are but slightly inclined to the 

 plane of the sun's equator. Satellites conduct themselves 

 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 interesting 

 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 

 secondary, move in elliptical orbits of small or moderate 

 eccentricity. 



We are so accustomed to acquiesce in these facts, as if 

 they were ultimate, that we seldom stop to consider them 

 in their true light, as unimpeachable witnesses to the past 



