180 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



The positions of the stars in the heavens change slightly from 

 year to year, owing to two causes : First, the motion of the solar sys- 

 tem as a whole in the direction toward the constellation Hercules; 

 second, the individual motions peculiar to stars themselves. The 

 first-mentioned component of proper motion increases directly as the 

 stellar distance descreases. Stellar parallax observers, being over- 

 whelmed by the enormous number of the stars, were obliged to choose 

 from them a small list for observation. It was natural to select stars 

 of large proper motion for such a purpose, since these would prob- 

 ably represent the class nearest to the earth. Hence if we take, for 

 instance, from the list of stars whose parallax has been determined 

 a group whose mean parallax is 0.1", the mean of their proper mo- 

 tions exceeds the mean proper motion, which would be found for all 

 the stars whose parallax is 0.1"'. if all those stars had been investi- 

 gated. Hence it must occur that, as the distances of more and more 

 of the stars become known, our estimated value for the mean dis- 

 tance, corresponding to stars of a given brightness or a given proper 

 motion must diminish. 



2. MOTIONS or THE STARS. 



Within the last few years gi-eat pieces of statistical investigation 

 in relation to the stars have been published which are of the highest 

 value for the progress of our knowledge of the universe. Americans 

 should be particularly proud of several of these investigations. 



The first is by the Harvard College Observatory, under the direc- 

 tion of Prof. E. C. Pickering. Volume 50 of its annals, containing 

 the Revised Harvard Photometry, gives a catalogue of the positions, 

 photometric magnitudes, and spectra of 9,110 stars, mainly of the 

 6.5 magnitude and brighter, covering both the northern and southern 

 hemisphere. A still more extensive work of a similar kind, to be 

 called the Revised Draper Catalogue, which will include data for 

 probably 200,000 stars, is now in course of preparation at the 

 Harvard College Observatory and may be expected to be finished 

 within a short time, thanks to Prof. Pickering's gi'eat care in the 

 arrangement of the work. 



The second great work to which I refer is entitled " Preliminary 

 General Catalogue of 6.188 stars for the epoch 1900. Prepared at 

 the Dudley Observatory, Albany, N. Y., by Lewis Boss and pub- 

 lished by the Department of Meridian Astrometry of the Carnegie 

 Institution of Washington, 1910." Prof. Boss says : " The general 

 catalogue of 6,188 stars herein contained is the result of an attempt 

 to deduce for these stars the most exact positions and motions that 

 are readily attainable from the means at command." In compiling 

 it he has compared about 80 star catalogues, from the catalogue of 

 Bradley, dated 1755, to modern catalogues of the period 1900. From 



