PORTION OF THE COSMOS. THE PLANETS. 325 



of time between two successive conjunctions or oppositions. 



In another, and perhaps still more readily intelligible 

 form, the true periods of revolution are 



From the last-named of these periods (that of the Earth) 

 the tropical period of revolution, or the length of the solar 

 year, is found to be 365-24222, or 365 d , 5 h , 48 m , 47 S '8091 : 

 by the effect of the precession of the equinoxes the length 

 of the solar year becomes St 595 shorter in the course of 

 100 years: 



