424 THE POPULAR SCIEJSrCE MONTHLY, 



themselves the system of the universe as being bounded by the fron- 

 tier of Saturn's orbit, at a distance from the central sun equal to 

 109,000 times the diameter of the earth, or about 860,000,000 miles. 

 The stars were fixed^ sj)herically distributed, at a distance but a little 

 greater than that of Saturn. Beyond this limit a vacant space was 

 supposed to surround the universe. The discovery of Uranus, in 

 1785, did away at once with this belt, consisting of Saturn's orbit, 

 and the frontier of solar domination was pushed out to a distance of 

 1,900,000,000 miles from the centre of the system, that is to say, be- 

 yond the space which was vaguely supposed to be occupied by the 

 stars. The discovery of Xeptune, in 1846, again removed these lim- 

 its to a distance that would have appalled our fathers ; the orbit de- 

 scribed by this planet being 2,862,000,000 miles from the sun. 



But the attractive force of the sun e?:tends farther still. Beyond 

 the orbit of Uranus, beyond the dark route slowly traversed by Nep- 

 tune, the frigid wastes of space are traveled over by the comets in their 

 erratic courses. Of these, some, being controlled by the sun, do not 

 leap from system to system, but move in closed^ curves, though at dis- 

 tances far greater than those of Uranus and Neptune. Thus Halley's 

 comet recedes to a distance of over 3,200,000,000 miles from the sun ; 

 the comet of 1811, 36,000,000,000; and that of 1680, '75,000,000,000. 

 The period of the last-named comet is 8,800 years. 



Still these figures can scarcely be compared to those which repre- 

 sent the distances of the stars. "What means have we of measuring 

 these distances ? Here the diameter of the earth will not serve as the 

 base of the triangle, as when we measure the moon's distance; nor 

 can we, as in the case of the sun, get. any assistance from another 

 planet. However, fortunately for us, the arrangement of ; our system 

 afibrds us a means of measuring these distant perspectives ; and this, 

 while demonstrating over again the earth's motion round the sun, 

 turns that motion to account for the solution of the greatest of astro- 

 nomical problems. . 



In revolving round the sun, at the distanceof 92,000,000 miles, the 

 earth annually describes an ellipse of about 500,000,000 miles. The 

 diameter of this orbit is 184,000,000 miles.- As the earth's revolution 

 round the sun is performed in a year, the earth, at any given instant, 

 will be opposite to the point where it stood six months before, as also 

 to the point where it will stand six months later. Here is a line of 

 sufficient length to serve as base of a triangle the apex of which shall 

 be a star. 



The process, then, for measuring the distance of a star from the 

 earth consists in minutely observing this star at an interval of six 

 months, or better, for a whole year, noting whether it remains fixed, 

 or whether it undergoes some little appreciable displacement of per- 

 spective, owing to the annual displacement of the earth around the 

 sun. If it remains fixed, this is because it is at an infinite distance 



