Page:Popular Science Monthly Volume 14.djvu/308

294 by cycles, epicycles, and eccentric positions, and proclaimed the heliocentric system, his opponents objected that the earth's axis, produced 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 distant 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 Centauri, 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 luminary 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 station 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 manhood, 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 comprises 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