448 



COSMOS. 



7. Periods of sidereal revolution and axial rotation. We 

 shall confine ourselves here to giving the sidereal, or true 

 periods of revolution of the planets in reference to the fixed 

 stars, or a fixed point of the heavens. During such a revolu- 

 tion, a planet passes through exactly 360 degrees in its course 

 round the Sun. The sidereal revolutions of the planets must 

 be clearly distinguished from the tropical and synodic, the 

 former of which refer to the return to the spring equinox, the 

 latter to the difference of time between two consecutive 

 conjunctions or oppositions. 



In another more perspicuous form the two periods of revo- 

 ution are: 



Mercury 87 d 23 h i5 m 47 s 



Venus' 224 16 49 7 



Earth 365 6 9 10 -7496: 



ratione constabit optime pondera Planetarum omnium esse 

 inter se ut vires." "The bodies of Venus and Mercury are 

 more ripened and condensed, on account of the greater heat 

 of the Sun. The more remote planets, by want of heat, are 

 deficient in those metallic substances and weighty minerals 

 with which the Earth abounds. Bodies are denser in pro- 

 portion to their nearness to the Sun, from which reason ifc 

 will easily appear that the weight of all planets is in propor- 

 tion to their forces." 



