sun's place among the stars — ADAMS 



143 



but a single star does it amount to as much as 1" of arc, and for 

 the vast majority of stars it is less than one-tenth of this amount. 

 A tenth of a second of arc with the telescopes usually employed for 

 these investigations corresponds to about 0.00015 inch on the photo- 

 graphic plate ; but such is the accuracy of the method and the skill 

 of the observers that the distances of about 3,500 stars have been 

 determined in this way. The measurements give directly a small 

 angle, and one-half of this angle is called the parallax of the star. 

 It corresponds to the angle subtended by the distance of the earth 

 from the sun as seen from the star. The full moon as seen from 

 the earth subtends an angle of about 1,900" of arc. 



This direct or trigonometric method of determining the distances 

 of the stars is an extremely valuable one, and gives us our most 



25 



20 



S15 



10 



Right Ascension 

 Figure 2. — A moving cluster of stars, the Hyades. (Lewis Boss.) 



accurate knowledge regarding the distances of the stars in the 

 neighborhood of the sun (pi. 3). Beyond distances of 300 or 400 

 light-years, however, its value in the case of individual stars falls 

 off rapidly, smce at these great distances the parallaxes are so 

 small that the inherent error of measurement becomes a large frac- 

 tion of the parallax, and finally equals or surpasses it. For the 

 most distant stars, therefore, we must find other methods that do 

 not depend upon direct measurements of position. 



Several such methods are known, some of which give us with 

 high accuracy not the distances of individual stars but the aver- 

 age distance of groups of stars of different brightness. Still others 

 are applicable ovUy to special classes such as double stars, or the 

 interesting moving clusters in which the stars move together through 

 the galaxy just as flocks of birds move together through the air 

 (fig. 2). All these methods give parallaxes and distances directly, 



