508 POPULAR SCIENCE MONTHLY. 



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 (t) Lyras are 3' apart; 

 yet, to ordinary vision they appear simply as a single star. To appre- 

 ciate 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 dis- 

 tinguished and accurately measured by the aid of a telescope of ordinary 

 power. 



On the other hand, if we measure the motions by terrestrial stand- 

 ards 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 magni- 

 tude. It happens thus because the number of stars of smaller magni- 

 tudes 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, cooperating 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 pho- 

 tographs it soon became evident that the star had been previously seen 

 or photographed, but always in slightly varying positions. An examina- 

 tion of the observed positions at various times showed that the star had 

 a more rapid proper motion than any other yet known. Yet, great 



