Excerpt from Science Reports, 1923, Vol. 1: A Collection of Non-Technical Papers on Recent Progress in Science
Until a few years ago our only method of finding the dis tance of any individual star was by triangulating to it from opposite Sides of the earth's orbit. By Observing the Slightly different direction of the star from the ends of this enormous baseline, it was possible to calculate the long Sides of this resulting triangle, and SO to determine the distance of the star from the solar system. But Since this difference of direction is in no case more than and for all but a few of the nearer stars is too small to measure at all, we were confronted with the probability that we Should never know the whereabouts of more than a few thousand among the hundreds of millions of the starry host; a discouraging pros pect, Since we can not find out much about an Object unless we know its distance from us; we can observe the angular motion of an individual star across the line of sight, but the actual speed with which it is traveling remains unknown; we can measure the apparent brightness Of a star, but this gives no clue to its actual or intrinsic brightness if its dis tance is unknown.
If, however, we have determined the distance of a star, we can convert its angular motion into velocity, and also from its apparent brightness we can calculate its intrinsic bright ness. Now it is by a reversal of one or the other Of these two processes that methods have recently been developed for finding, from their apparent brightness, or from their appar ent motion, the distance, not of all stars but of certain classes of stars, some of them exceedingly remote.
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