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Page:Popular Science Monthly Volume 57.djvu/518

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motion in angular measurement as so many seconds per year or per century. The keenest eye would not, without telescopic aid, be able to distinguish between two stars whose apparent distance is less than 2' or 120" of arc. The pair of stars known as (ε) Lyræ are 3' apart; yet, to ordinary vision they appear simply as a single star. To appreciate what 1" of arc means we must conceive that the distance between these two stars is divided by 200. Yet this minute space is easily distinguished and accurately measured by the aid of a telescope of ordinary power.

On the other hand, if we measure the motions by terrestrial standards they are swift indeed. Arcturus has been moving ever since the time of Job at the rate of probably more than 200 miles per second—possibly 300 miles. Generally, however, the motion is much smaller, ranging from an imperceptible quantity up to 5, 10 or 20 miles a second. Slow as the angular motion is, our telescopic power is such that the motion in the course of a very few years (with Arcturus the motion in a few days) can be detected. As accurate determinations of positions of the stars have been made only during a century and a half, no motions can be positively determined except those which would become evident to telescopic vision in that period. Only about 3,000 stars have been accurately observed so long as this. In the large majority of cases the interval of observation is so short or the motion so slow that nothing can be asserted respecting the law of the motion.

The great mass of stars seem to move only a few seconds per century, but there are some whose motions are exceptionally rapid. The general rule is that the brighter stars have the largest proper motions. This is what we should expect, because in the general average they are nearer to us, and therefore their motion will subtend the greatest angle to the eye. But this rule is only one of majorities. As a matter of fact, the stars of largest proper motion happen to be low in the scale of magnitude. It happens thus because the number of stars of smaller magnitudes is so much greater than that of the brighter ones that the very small proportion of large proper motions which they offer over-balances those of the brighter stars.

The discovery of the star of greatest known proper motion was made by Kapteyn, of Groningen, in 1897, coöperating with Gill and Innes, of the Cape Observatory. While examining the photographs of the stars made at this institution, Kapteyn was surprised to notice the impression of a star of the eighth magnitude which at first could not be found in any catalogue. But on comparing different star lists and different photographs it soon became evident that the star had been previously seen or photographed, but always in slightly varying positions. An examination of the observed positions at various times showed that the star had a more rapid proper motion than any other yet known. Yet, great