PROGRESS IN ASTROPHYSICS — ABBOT. 



183 



established by numerous nearly simultaneous measurements at Mount 

 Wilson, Cal., and Bassour, Algeria. 



4. Indications of two wholly independent kinds incline us to think 

 that these variations of solar radiation are caused within the sun, 

 and not by interposing meteoric or other matter. 



STELLAR PROBLEMS. 



1. THE DISTANCES OF THE STARS. 



The actual distances of several hundred of the stars can be said 

 to be Imown within moderate limits of accuracy. Various methods 

 are used for determining the distances of the stars, but they gen- 

 erally depend upon the fact that the earth, by reason of its revolu- 

 tion about the sun, occupies places separated by 186,000,000 miles 

 at intervals six months apart. This corresponds to the surveyor's 

 base line, and allows us to triangulate for the distances of the stars. 

 Another method of estimating the stellar distances may be based 

 upon the fact that the solar system is approaching the constellation 

 Hercules at the rate of about 20 kilometers (12 miles) per second, so 

 that the position occupied by the earth in space to-day is different 

 from that which will be occupied to-morrow by reason of the motion 

 of the solar system^ but this method involves assumptions in regard 

 to the motions peculiar to the stars observed. 



It is customary to express the distances of the stars in light-years, 

 for the distances of the stars, if given in kilometers or miles, or even 

 in terms of the radius of the earth's orbit, are so enormous as to 

 require many figures. Light, however, traveling at the rate of 186,000 

 miles per second, in the course of a year travels about 6,000,000,- 

 000,000 miles. In terms of this unit the nearest star is at a distance 

 of four and a half light-years. The stellar distances are considered 

 up to such enormous quantities as a thousand or more light-years. 

 It is also customary to speak of the parallaxes of stars. By this is 

 meant the angle which the radius of the earth's orbit would subtend 

 if viewed most favorably from the star in question. The parallaxes 

 of the stars range from about one second of arc (1") downward. 

 The following table shows the relation between miles, light-years, 

 and parallaxes of stars : 



000,000 omitted. 



The first successful measurements on stellar parallaxes were made 

 by Struve at Dorpat on the star Vega in the years 1835 to 1838, and 



