266 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



or 4" per century. Second, the motion in the line of sight, or radial 

 velocity, measured by the spectroscope, which again varies between 



and about 250 kilometers per second, the velocity of a faint star 

 in the Southern Hemisphere determined last year. It is evident that 

 in order to get the true direction and velocity of a star we must 

 know, in addition to its radial motion, its velocity in kilometers per 

 second at right angles to the line of sight. If its proper motion is 

 known this can be readily computed when we know its distance, 

 and hence we can obtain the direction and magnitude of its motion. 



In determining these motions we have to remember that we are 

 on a moving body, the earth, which has a velocity of revolution 

 around the sun of about 20 miles per second, and we must also 

 remember that the sun, which is one of the stars, is also in motion. 

 That this is the case has long been recognized, and the direction of 

 tliis motion was determined from the proper motions of the stars by 

 Sir William Herschel over 100 years ago. The method of doing this 

 can be readily understood, for if we imagine the stars to be moving 

 in all directions at random, it is, nevertheless, evident that in the 

 portion of the sky which we are approaching, the general tendency 

 will be for them to open out, while they will tend to close in in the 

 opposite direction, and to drift backward at the sides. Hence, if the 

 motion of the stars is at random, it is only a question of mathematics 

 to determine the direction and magnitude of the sun's motion in 

 space. 



Over 20 different determinations, based upon the proper mo- 

 tions of different numbers of stars, have been worked out, which 

 all agree reasonably well in showing the sun to be moving toward the 

 dividing line between Lyra and Hercules just a little south and east 

 of the bright star Vega. This point has shifted around considerably 

 between Hercules and Lyra, but the last determination, from Boss's 

 Prehminary General Catalogue, issued only last year, places it where 



1 have just stated (R. A. 270.5°, Dec. + 34.3°). 



If we consider, on the other hand, a determination of the apex of 

 the sun's way, as this point is called, derived from the radial velocities 

 of stars, we find it to be in a somewhat different position. We have 

 had three or four determinations of the solar apex from radial velocity 

 measures; but none of these need be considered here except that 

 obtained last year by Prof. Campbell, director of the Lick Observa- 

 tory, the pioneer and foremost exponent of accurate radial velocity 

 determinations, whose methods have been universally followed and 

 their accuracy never excelled. I place the results of his 14 years' 

 work as the most important astronomical result announced during 

 the year. In determining the velocity and direction of the sun's 

 motion, the radial velocities of 1,073 stars brighter than the fifth 

 magnitude, well distributed over the sky, were used, 1,020 of which 



