SKCT. VII. SOLAR PARALLAX. 53 



the sun's horizontal parallax was found to be 8"-72. 

 But by other considerations it has been reduced by 

 Professor Encke to 8"-5776 ; from which the mean 

 distance of the sun appears to be about ninety-five mil- 

 lions of miles. This is confirmed by an inequality in the 

 motion of the moon, which depends upon the parallax of 

 the sun, and which, when compared with observation, 

 gives 8"- 6 for the sun's parallax. 



The parallax of Venus is determined by her transits ; 

 that of Mars by direct observation, and it is found to be 

 nearly double that of the sun, when the planet is in 

 opposition. The distance of these two planets from 

 the earth is therefore known in terrestrial radii, conse- 

 quently their mean distances from the sun may be 

 computed ; and as the ratios of the distances of the 

 planets from the sun are known by Kepler's law, of the 

 squares of the periodic times of any two planets being 

 as the cubes of their mean distances from the sun, their 

 absolute distances in miles are easily found (N. 132). 

 This law is very remarkable, in thus uniting all the 

 bodies of the system, and extending to the satellites as 

 well as the planets. 



Far as the earth seems to be from the sun, Uranus is 

 no less than nineteen times farther. Situate on the 

 verge of the system, the sun must appear to it not 

 much larger than Venus does to us. The earth cannot 

 even be visible as a telescopic object to a body so re- 

 mote. Yet man, the inhabitant of the earth, soars 

 beyond the vast dimensions of the system to which his 

 planet belongs, and assumes the diameter of its orbit 

 as the base of a triangle whose apex extends to the 

 stars. 



Sublime as the idea is, this assumption proves in- 

 effectual, except in a very few cases ; for the apparent 

 places of the fixed stars are not sensibly changed by the 

 earth's annual revolution. With the aid derived from 

 the refinements of modern astronomy, and of the most 

 perfect instruments, a sensible parallax has been de- 

 tected only in a veiy few of these remote suns, a Cen- 

 tauri has a parallax of one second of space, therefore it 

 is the nearest known star, and yet it is more than two 

 hundred thousand times farther from us f han the sun 



K2 



