SECT, vii.] SOLAR PAKALLAX. 6 i 



depends upon the parallax of the sun, and which, when com- 

 pared with observation, gives 8"'6 for the sun's parallax; 



The parallax of Venus is determined bj 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, consequently their mean distances 

 from the sun may be computed ; "and as the ratios of the dis- 

 tances 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 remote. Yet man, the in- 

 habitant 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 ineffectual, 

 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 detected only in a very few of 

 these remote suns, a Centauri 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 

 than the sun is. At such a distance not only the terrestrial 

 orbit shrinks to a point, but the whole solar system, seen in 

 the focus of the most powferful telescope, might be eclipsed 

 by the thickness of a spider's thread. Light, flying at the 



