PORTION OE THE COSMOS. THE PLANETS* 



327 



The small planets have been omitted, because they will be 

 treated subsequently as a detached group. With the excep- 

 tion of Mercury, which is situated so near the sun, and the 

 inclination of whose orbit to the ecliptic (7 0' 5"*9) is very 

 nearly the same as that of the sun's equator 7 30'), we see 

 that the inclinations of the other seven planetary orbits 

 oscillate between OJ and 3J. In respect to the position 

 of each planet's axis of rotation relatively to its own orbit, 

 it is Jupiter which approaches most nearly to the extreme 

 case of perpendicularity. In Uranus, on the other hand, 

 to judge by the inclination of the paths of the satellites, the 

 axis of rotation of the planet almost coincides with the plane 

 of its orbit. 



As it is on the inclination of the Earth's axis to the plane 

 of its orbit, therefore on the obliquity of the ecliptic (i. e. on 

 the angle which the apparent path of the sun makes at its 

 intersection with the terrestrial equator,) that the distribu- 

 tion and duration of the seasons, the altitudes of the sun in 

 different latitudes, and the length of the day depend ; so 



