ASTRONOMY 



change in distance, and the other, at right angles 

 to the line of sight, giving rise to change in direc- 

 tion. The magnitudes of these two components 

 are found in entirely different ways. The former 

 or radial velocity is found by observing with the 

 spectroscope the amount by which the lines in the 

 spectrum of the star are displaced from their 

 normal positions. If the lines are displaced to the 

 blue end of the spectrum the star is approaching us ; 

 if the displacements are towards the red end, the 

 star is receding. These observations yield an 

 actual speed of approach or recession in miles per 

 minute. The thwartwise motion is observed as a 

 change in position upon the celestial sphere, and 

 is called proper motion. Its amount when multi- 

 plied by the distance of the star gives the speed in 

 miles per second at right angles to the line of sight. 

 Unfortunately the number of stars whose distances 

 are known with any great degree of accuracy is 

 very small, and we have for the present to be 

 content with making what deductions we can from 

 the proper motions alone. 



It is natural to assume that stars possessing 

 large proper motions are on the whole nearer to us 

 than those whose proper motions are small. More- 

 over, as we have already seen, apparent magnitude 

 depends upon distance. The determination of 

 the relations between these three quantities is one 

 of the most interesting statistical problems upon 



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