THE PLANETS. 449 



whence it follows that the period of the tropical revolution, or 

 the length of the solar year, is 365 d *24222, or 365d. 5h. 48m. 

 47"'8091; the length of the solar year is shortened 0"'595 

 in 100 years on account of the precession of the equinoxes : 



Mars 1 year, 321 d 17 h 30 m 41 s 



Jupiter ... 11 years, 314 20 2 7 



Saturn 29 years, 166 23 16 32 



Uranus 81 years, 5 19 41 36 



Neptune ... 164 years, 225 17 



The rotation is most rapid in the case of the exterior 

 planets, which have at the same time a longer period of 

 revolution ; slower in the case of the smaller interior planets, 

 which are nearer to the Sun. The periods of revolution of 

 the asteroids between Mars and Jupiter are very various, and 

 will be spoken of in the enumeration of the individual planets. 

 It is, therefore, sufficient in this place to give a comparative 

 result, and to observe that among the small planets, Hygeia 

 has the longest, and Flora the shortest period of revolution. 



8. Inclination of the planetary orbits and axes of rotation. 

 Next to the masses of the planets, the inclination and eccen- 

 tricity of their orbits are among the most important elements 

 upon which the disturbances depend. The comparison of 

 these, in the order of succession of the interior, small inter- 

 mediate and exterior Dlanets (from Mercury to Mars, from 

 Flora to Hygeia, from Jupiter to Neptune), presents manifold 

 similarities and contrasts, which lead to considerations as to 

 the formation of these cosmical bodies, and their changes 

 during long periods of time. The planets revolving in such 

 various elliptical orbits, are also all situated in different 

 planes. In order to render a numerical comparison possible, 

 they are reduced to a fundamental plane, either fixed or 

 moveable, according to certain laws. As such the most con- 

 venient is the ecliptic the course which the Earth actually 

 traverses or the equator of the terrestrial spheroid. We 



