178 OUTLINES OF NATURAL PHILOSOPHY. 



Rotation of the Planets. 



176. Four of the planets, Venus, Mars, Ju- 

 piter and Saturn, when examined with the te- 

 lescope, appear to revolve on axes, in the same 

 direction in which they revolve in their orbits ; 

 the axis of each remaining always parallel*, or 

 nearly parallel to itself. 



a. This conclusion is derived from the motion of cer- 

 tain spots, which are distinguished, by the colour 

 or intensity of their light, from the other parts of 

 the planetary disk. In this way the time of rotation 

 is also determined, as in the case of the Sun, 



( 



b. It is thus found, that Venus revolves in 23** 21m 9s, 

 on an axis which makes a very small angle with 

 the plane of the ecliptic. This was first observed 

 by the elder CASSINI. LA LANDE, J 3341. 



c. Mars revolves in 1 day 39 minutes, on an axis in- 

 clined at an angle of 59 42' to the ecliptic. 



d. Jupiter revolves in 9^ 56m, on an axis nearly per- 

 pendicular to the ecliptic. 



<?. Saturn revolves on his axis in 10 h 16 m . 



/ Though 



