Page:Popular Science Monthly Volume 5.djvu/440

424 themselves the system of the universe as being bounded by the frontier 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, spherically 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, beyond the space which was vaguely supposed to be occupied by the stars. The discovery of Neptune, in 1846, again removed these limits to a distance that would have appalled our fathers; the orbit described by this planet being 2,862,000,000 miles from the sun.

But the attractive force of the sun extends farther still. Beyond the orbit of Uranus, beyond the dark route slowly traversed by Neptune, 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 distances 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 represent 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 affords 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 astronomical problems.

In revolving round the sun, at the distance of 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 perspective, owing to the annual displacement of the earth around the sun. If it remains fixed, this is because it is at an infinite distance