CHAPTERS ON THE STARS. 513 



apparent movement of the star. Hence our actual result in the case of 

 each separate star is a relation between the motion of the star and the 

 motion of the sun. 



I say the motion of the sun and not of the earth, because although 

 the observer is actually on the earth, yet the latter never leaves the 

 neighborhood of the sun, and, as a matter of fact, the ultimate result in 

 the long run must be a motion relative to the sun itself as if we made 

 our observations from that body. The question then arises whether 

 there is any criterion for determining how much of the apparent motion 

 of any given star should be attributed to the star itself and how much 

 to a motion of the sun in the opposite direction. 



If we should find that the stars, in consequence of their proper 

 motions, all appeared to move in the same direction, we would naturally 

 assume that they were at rest and the sun in motion. A conclusion of 

 this sort was first reached by Herschel, who observed that among the 

 stars having notable proper motions there was a general tendency to 

 move from the direction of the constellation Hercules, which is in the 

 northern hemisphere, towards the opposite constellation Argo, in the 

 southern hemisphere. 



Acting on this suggestion, subsequent astronomers have adopted the 

 practice of considering the general average of all the stars, or a position 

 which we may regard as their common center of gravity, to be at rest, 

 and then determining the motion of the sun with respect to this center. 

 Here we encounter the difficulty that we cannot make any absolute 

 determination of the position of any such center. The latter will vary 

 according to what particular stars we are able to include in our estimate. 

 What we can do is to take all the stars which appear to have a proper 

 motion, and determine the general direction of that motion. This gives 

 us a certain point in the heavens toward which the solar system is 

 traveling, and which is now called the solar apex, or the apex of the solar 

 way. 



The apparent motion of the stars due to this motion of the solar sys- 

 tem is now called their parallactic motion, to distinguish it from the 

 actual motion of the star itself. 



The interest which attaches to the determination of the solar apex 

 has led a great number of investigators to attempt it. Owing to the 

 rather indefinite character of the material of investigation, the uncer- 

 tainty of the proper motions, and the additions constantly made to the 

 number of stars which are available for the purpose in view, different 

 investigators have reached different results. Until quite recently, the 

 general conclusionn was that the solar apex was situated somewhere in 

 the constellation Hercules. But the general trend of recent research 

 has been to place it in or near the adjoining constellation Lyra. This 



VOL. LVII.— 33 



