THE SIZE OF THE UNIVERSE.

1895

Miles Are Too Insignificant to Be Used In Its Computation.

While it is interesting to know the distance of some of the stars in miles, when stated in that way the numbers are so large that they frequently convey very indistinct conceptions to the mind. For this reason it is customary to estimate stars' distances in "light years." A light year is the distance that light, moving at the rate of 186,300 miles per second, travels in one year. This amounts in round numbers to 5,880,000,000,000 miles. The distance of Alpha Centaur is 4.35 light years, that of Sirius, the dog star, is almost exactly twice as great, or 8.6 light years. In other words, light requires 8.6 years to come to us from Sirius. And these are among the very nearest of the stars. Some whose parallaxes have been rather estimated than measured appeared to be situated at a distance which light could not traverse in less than one or two centuries. The great still Arcturus, for instance, has, according to Dr. Elkin, a parallax of only eighteen-thousandths of a second. Its distance must, in that case, be about 181 light years, or more than a thousand million million miles. And if its distance is so great, then, since light varies inversely as the square of the distance from its source, it can be shown that Arcturus must actually give forth 5,000 or 6,000 times as much light as the sun yields.

Yet Arcturus is evidently much nearer than the vast majority of the stars are. Not one in a million is known to have a parallax large enough even to be intelligently guessed at. There may be stars whose light requires thousands instead of hundreds of years to cross the space separating them from us.

We thus see that only a few points on the nearer shores of the starry universe lie within reach of our measurements — here and there a jutting headland, while behind stretches the vast expanse over which the hundreds of millions of stars known to exist are scattered. — Garrett P. Serviss in Chautauquan.