294 THE POPULAR SCIENCE MONTHLY. 



tangled by cycles, epicycles, and eccentric positions, and proclaimed the 

 heliocentric system, his opponents objected that the earth's axis, pro- 

 duced to the celestial sphere, with its successive positions day after day 

 parallel to each other, should be seen to describe a circle in the heavens 

 as the earth sweeps round through its orbit of nearly 600,000,000 miles 

 in length. Copernicus replied that it does describe such a circle ; but 

 the stars, by reference to which it can alone be mapped out, are so dis- 

 tant that the circle of almost 100,000,000 miles' radius there dwindles 

 to a point and vanishes by perspective. To reverse the line of sight, 

 let us suppose ourselves transported to the pole-star and looking back 

 upon the orbit of the earth. So vast is the distance that this elliptical 

 orbit contracts almost to the infinitesimal dimensions of a point ; for, 

 at that distant station, the earth's orbital diameter subtends an angle of 

 only 0.182", or twice the angle called the parallax of the star. Such is 

 the distance that the astronomer has successfully attempted to measure, 

 starting with a primary triangle based on a determined line of only a 

 few miles in length. 



The nearest fixed star is Alpha Ce^itauri, with a parallax of 0.928", 

 corresponding to a distance of 20,518,000 millions of miles. Light, 

 traveling at the rate of 186,500 miles a second, requires 3.5 years to 

 reach us from this nearest star. So the solar system, with its immense 

 distances, is yet alone in the universe of stars ; and our central lumi- 

 nary is separated so far from other suns that the distance to its outmost 

 planet is almost a vanishing quantity in comparison with the distance 

 to its nearest starry neighbors. We gaze upon the glittering heavens 

 at night, and wrap in thought a canopy of shining stars about our 

 earth as if it were an ornamented mantle ; but could we take our sta- 

 tion on a silent planet circling round some other starry sun, our sun 

 would take its place as only one among the mazes of the stars. 



How brilliantly Sirius shines with pure white light in the evening 

 sky ! Yet the earth has circled seventeen times round the sun since the 

 light that the eye gathers to an image of the Dog-star left that glorious 

 orb. Patiently the astronomer centres that little circle around which 

 the pole-star sheds its guiding light, that he may adjust his instrument 

 to parallelism with the axis of a revolving world. But since that light 

 left its source at the pole-star, a child has grown through youth to man- 

 hood, and in his hair the gray of silver lines has begun to develop 

 under the cares of six-and-thirty years. And these are only our nearest 

 neighbors among the stars ! 



For every star visible to the naked eye under the most favorable 

 circumstances the great Washington telescope shows from 5,000 to 

 8,000 more. According to the best authorities, the first six magnitudes 

 contain 5,904 stars. Only half of these can be seen above the horizon 

 at once ; and the sixth magnitude comjorises 4,424. These can be seen 

 distinctly only on very favorable nights ; so that for ordinary observation 

 only 740 stars are visible at any moment above the horizon. There are 



