ASTRONOMY. 209 



SECT. XIIL 



DIMENSIONS OF THE SOLAR SYSTEM. 







HITHERTO, the distance of the Sun from the 

 Earth has served as the unit, by which we have 

 measured all other distances in the planetary sys- 

 tem. It now remains, (in order to have a precise 

 idea of those distances), to compare this unit with 

 the diameter of the Earth, and of consequence 

 with the known measures in which that diameter 

 has already been expressed. This depends on 

 the parallax of the Sun, which has been shewn 

 to be less than 10", and, on account of its small- 

 ness, difficult to be ascertained. The method 

 which first presents itself, does not lead to any 

 thing more precise than the limit just mentioned. 



Since the ratios of the distances of the 

 planets from the Sun, to the distance of the 

 Earth from the Sun, are known, if the parallax 

 of any of the planets were discovered, that of 

 the Sun would, of consequence, become known. 



This follows, readily, from the laws of the planetary 



motions. 

 VOL. II. O Mars, 



