The Present State of our Knowledge of Stellar Motion. 145 



ON THE PRESENT STATE OF OUR KNOWLEDGE OF 

 STELLAR MOTION. 



By Truman Henry Safford, Ph. D., Field Memorial Professor of 

 Astronomy in Williams College. 



The stellar motions, the so-called proper motions of the 

 stars, which used to be thought fixed, seem to us very small, 

 because the suns we call stars are so very far away. 



Bodies which move at least as rapidly as our earth does 

 about its sun, seem to move with velocities rarely exceeding 

 one second a year; and this large stellar motion is only 

 sufficient to produce a displacement equal to the moon's 

 diameter as we see it in about eighteen centuries. 



Under these circumstances another cause besides the ap- 

 parent slowness makes the problem in question a trouble- 

 some one — the fact, namely, that accurate astronomical 

 observation is a thing of less than two centuries past — and 

 even Flamsteed, who commenced it, is now antiquated — 

 and we are obliged to take Bradley's later observations — 

 from 1750 on — as the real beginning. 



Moreover, as years go by, it is found possible and neces- 

 sary to re-reduce the older star-catalogues, in order to make 

 use of the past records to the greatest possible extent. 



Thus Bessel's Bradley's catalogue superseded in 1818 

 Bradley's own catalogue; and Professor Auwers has already 

 published the chief results of a new reduction, which, using 

 Bessel's principles and methods, supersedes Bessel's results. 



Another element of difficulty in the problem lies in the 

 comparative ease of making new observations upon the 

 stars in a routine way, and the greater difficulty of select- 

 ing the fittest objects and observing those only. The prac- 

 tical astronomer who has learned simply how to observe, is 

 far inferior to him, whose ability lies in a combination of 

 the power to select well his working list, that to observe to 

 the last degree of accuracy, and that to discuss results with 

 skill and completeness. And the German school of astron- 

 omers, established by Bessel, Gauss and Struve, is at the 

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