NEWS FROM THE STAES — ABBOT. 161 



each with his own rate and direction, so that at the end of a century 

 the configuration of the heavens is not quite the same as at its begin- 

 ning. These " proper motions " range from 870 seconds of arc per 

 century down. (A second of arc is about the angular width of a 

 telegraph wire as seen at a distance of a half mile.) The vast ma- 

 jority of stars have a less proper motion than 20 seconds per century. 



Proper motions are observed as angles and can not be expressed 

 in miles per second without other information. We see only the 

 component of motion at right angles to the line from the earth to 

 the star. If a star is coming directly toward us, it has no proper mo- 

 tion, though its real speed may be very great. Near stars have 

 greater average proper motions than distant ones, just as men walk- 

 ing on the other side of the street apparently outdistance those a 

 block away. Two things besides proper motion are therefore needed 

 to determine the real motion of a star, namely, its distance and the 

 angle its real motion makes to the line of sight. 



Fortunately, the spectroscope can help in this matter. Although, 

 as stated above, the chemical elements are discovered in stars by the 

 reversal of their spectrum lines, careful measurement shows that 

 the positions of the stellar lines are slightly shifted, either toward 

 the red or toward the violet, with respect to the bright lines of the 

 comparison spectrum of a metal. Doppler predicted this effect nearly 

 a half century before it was observed in stai-light. It depends on the 

 motion of the star in the line of sight. 



Light travels by waves. Violet light has more waves per second 

 tlian red. If a star is approaching, its light seems to have more waves 

 per second because the star's motion is added to that of light, and 

 hence all the spectrum lines are shifted toward the violet. The lines 

 are shifted toward the red for stars that are receding. From the 

 amount of the shift the actual rate of approach or recession of the 

 star may be found. Naturally, a small correction must be made for 

 the motion of the earth on its axis and its motion round the sun. 

 We then have the actual rate of motion of the star to or from the 

 sun. It is a very valuable thing about this kind of measurement that 

 if only a star is bright enough it makes no difference at all in the 

 accuracy of the determination hovx- distant the star may be. This 

 unfortunately is not so with proper motions. 



As the sun has a motion of its own, which Sir William Herschel 

 rightly concluded in the year 1783 is toward the northern constella- 

 tion Hercules and not far from the bright star Vega, all the stellar 

 motions, of course, appear to be affected by an equal motion to that 

 of the sun but in the opposite direction. 



Director Campbell, of Lick Observatory, has rexiently published 

 a collection of the so-called " radial motions " of nearly 2,000 stars, 



