122 



COSMOS. 



ing long periods of time. The planets revolving in such va- 

 rious elliptical orbits are also all situated in diflerent planes. 

 In order to render a numerical comparison possible, they are 

 reduced to a fundamental plane, either fixed or movable, ac- 

 cording to certain laws. As such, the most convenient is the 

 ecliptic — the course which the Earth actually traverses — or 

 the equator of the terrestrial spheroid. "We add to the same 

 table the inclinations of the axes of rotation of the planets 

 toward their own orbits, so far as they are determined with 

 any certainty. 



The small planets are omitted here, because they will be 

 treated of further on as a separate distinct group. If thd 

 planet Mercury, situated near the Sun, and the inclination 

 of whose axis toward the ecliptic (7° 0' 5"* 9) approaches 

 very near to that of the solar equator (7° 30'), the inclinations 

 of the other seven planets will be seen to oscillate between 

 0|° and 3|-°. Jupiter exhibits, in the position of the axis of 

 rotation with reference to its own orbit, the closest approxi- 

 mation to the extreme of perpendicularity. On the contrary, 

 the axis of rotation of Uranus, to conclude from the inclina- 

 tion of the orbits of its satellites, very nearly coincides with 

 the plane of the planet's orbit. 



Since the division and duration of the seasons, the solar al- 

 titudes under various latitudes, and the length of the days, 

 depend upon the amount of the inclination of the Earth's axis 

 toward the plane of its orbit, as well as upon the obliquity of 

 the ecliptic {i.e., upon the angle which the apparent course 

 of the Sun makes with the equator at their point of intersec- 

 tion), this element is of the most extreme importance as re- 

 gards the astronomical climate, i.e., the temperature of the 

 Earth, in as far as this is a function of the meridian altitude 

 attained by the Sun and the duration of its continuance above 

 the horizon. If the obliquity of the ecliptic were great, or 



