304 



Astronomy. 



[Lecture 19. 



TABLE o the mean distances of the primary pla- 

 nets from the sun, in French leagues of 2283 

 fathoms each, and in English miles in round 

 numbers. 



The revolutions of the planets may be consi- 

 dered as relative to the sun, or as relative to the 

 earth. In the first case they are called periodi- 

 cal revolutions ; that is, the time which the pla- 

 nets employ in revolving about the sun in coming 

 again to a fixed point in the heavens. In the 

 second, they are called sy nodical revolutions ; 

 that is, the time which the planets seen from the 

 earth employ in returning to the sun ; or the 

 time which passes between the mean conjunction 

 and the next following. This time is very dif- 

 ferent from that of periodical revolutions, as may 

 be seen in the following table. 



