160 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1916. 



orbit, 93,000,000 miles, subtends an angle of only 0.75 seconds. This 

 is called the star's parallax. 



Up until very recently the parallax determinations of Elkin and 

 Chase at Yale University Observatory, by direct eye observations 

 with the heliometer, were regarded as of the highest accuracy. But 

 now the photographic method as worked out at Yerkes Observatory^ 

 by Prof. Schlesinger, now director at Allegheny Observatory, -has 

 come to be preferred. This work is being pushed by Director 

 Mitchell, of Leander McCormick Observatory, and is also occupying 

 a prominent place on the program at several other observatories 

 where large telescopes are available. 



Altogether less than 1,000 star distances have been measured. It 

 is a slow, tedious work, often disappointing, a Centauri, the nearest 

 star, except the sun, is at 25,000,000,000,000 miles, while the sun is at 

 only 93,000,000 miles. 



When a measurement indicates that a. star is at 2,000,000,000,- 

 000,000 miles or more (parallax 0.01 seconds) it is the same as saying 

 that the sbar is too far away for its distance to be determined. It 

 may be ten or a hundred times as far as the measurements indicate. 

 This is about the average distance of the faintest stars visible to the 

 naked eye. The great majority of telescopic stars lie beyond this 

 distance. If observers did not choose stars expected to be relatively 

 near, most of their i*esults would come out thus indeterminately. 

 Even as it is, a great number of measurements do come out in this 

 disappointing fashion. Unless some better method of investigation is 

 discovered, measurements of individual star distances must ever be 

 in this unsatisfactory state. In treating of star motions we shall see 

 how our knowledge of the average distances of certain groups of 

 stars has been extended.^ 



MOTIONS. 



About the year 1750 the English astronomer royal, Bradley, ob- 

 served the positions in the heavens of 3,222 stars. Bradley's stars 

 and many others have been observed often in more recent years. All 

 the best work relating to about 6,000 of the brighter stars was com- 

 pared and reduced to a homogeneous system about the year 1910 by 

 the late Prof. Lewis Boss, of the Dudley Observatory and Carnegie 

 Institution. 



From Boss's work the proper motions (so called) of these stars 

 were accurately determined. All stars, including our sun, move 



1 Since this paragrapti was written Dr. Adams, of Mount Wilson Solar Observatory, has 

 discovered a spectroscopic method of determining parallaxes, which is applicable to stars 

 of classes P, G, K, and M, and is independent in accuracy of the distance of the star, if 

 it is sufficiently bright to permit a good spectrum photograph to be made. 



