Page:Popular Science Monthly Volume 5.djvu/441

Rh from us—at the horizon of the heavens, so to speak—and our baseline of 184,000,000 miles is as nothing in comparison with this remoteness. But if it is displaced, then we know that it annually describes a small ellipse, corresponding to the annual revolution of the earth. Every one has remarked, while traveling by rail, how the trees and other objects near at hand move in a direction contrary to our own, their speed being greater in proportion to their nearness; whereas distant objects on the horizon remain fixed. This same effect is produced in space, in consequence of our annual motion round the sun. But though we move incomparably swifter than an express-train, our rate being 1,632,000 miles per day, and 68,000 per hour, the stars are so distant that they scarcely budge. Our 184,000,000 miles of displacement are almost nothing as concerns even the nearest of them. The inhabitants of Jupiter, Saturn, Uranus, or Neptune, with their orbits five, nine, nineteen, and thirty times as large as ours, could determine the distance of a far greater number of stars than we.

This mode of measuring the distance of the stars by the perspective effect produced by the earth's annual displacement was anticipated by the astronomers of the eighteenth century, and in particular by Bradley, who, while attempting to measure the distances of the stars by comparing together observations made at an interval of six months, discovered—something else. Instead of finding the distance of the stars on which his observations were directed, he discovered a very important optical phenomenon, viz., the aberration of light, the effect produced by the motion of light and the motion of the earth combined. Similarly, William Herschel, while seeking the parallaxes of the stars by comparing bright stars with their nearest neighbors, discovered the systems of double stars. So, too, Fraunhofer, while seeking the limits of the colors in the solar spectrum, discovered the absorption rays, the study of which has given rise to Spectrum Analysis. The history of the sciences shows that frequently discoveries have been made in the course of investigations which had but little to do with them directly. Columbus discovered the New World while aiming to reach the eastern coast of Asia by sailing to the west. He would never have discovered it, would never have sought for it, had he known the true distance between Portugal and Kamtchatka.

It was not till 1840 that the distance of any of the stars was ascertained. This discovery is, therefore, of recent date, and we are only now beginning to form an approximate idea of the real distances which separate us from the stars. The parallax of the star 61 in the Swan, which was the first to be determined, was ascertained by Bessel, and was the result of observations made at Königsberg from 1837 to 1840. In 1812, Arago and Mathieu had made observations on this star, but without reaching any certain results. The parallax of Alpha in the Lyre was found by Struve, in the course of observations made at Dorpat between 1835 and 1838; but it was not