19U] on The Stars around the North Pole 121 



alternatives, but for an individual star it is impossible to decide 

 between them. 



7. The table given in § 6 shows how largely the proper motions 

 of stars vary in amount. They differ just as widely in direction. 

 Some signs of regularity in the directions were first detected by Sir 

 "William Herschel, who found that the movement of seven quick- 

 moving stars, situated in different parts of the sky, w^ere approxi- 

 mately directed to one point. He observed that this would result 

 if the proper motions arose not from the movement of the stars 

 themselves but from that of the point of observation in an opposite 

 direction, and concluded that the solar system was moving towards 

 a point in the constellation Hercules. This conclusion was not 

 universally admitted for some time, but researches by Argelander, 

 Airy, Bessel, and others demonstrated a regular drift among the 

 stars, such as would arise if on their otherwise irregular movements 

 were superposed this common motion. A large number of researches 

 have been made on the exact direction of the sun's motion, and it 

 is now established with some certainty that it is towards a point in 

 right ascension IS'" and declination 35' N., not far in direction from 

 the bright star Vega. The speed of 

 the sun's motion through space has 

 been determined by spectroscopic ob- 

 servations. On the average, stars near 

 Vega appear to be approaching us, 

 stars in the opposite direction to be 

 receding from us. In this way, Prof. 

 Campbell has found from the observed 

 velocities of 1500 stars that the solar 

 system is moving at the rate of 19*5 

 Mlometres a second. 



8. The fact that the sun is moving 

 with a velocity of 19*5 kilometres a ^^^- ^' 

 second in a known direction supplies 



us immediately with a means of determining the average distances of 

 groups of stars. This velocity carries the sun forward in a century 

 a distance equal to 412 times the sun's distance from the earth. If 

 at the beginning of the century the sun is at S, and at the end has 

 moved toS^, the angular distance of a star situated at P and having 

 no motion of its OAvn will have increased from ASP to AS^P. The 

 difference of these angles, which is the proper motion of the star, is 

 SPSi. Thus in the triangle SPS^ all the angles are known and the 

 small base SS^ ; therefore the side SP— the distance of the star from 

 the sun — can be deduced. The principle is the same as that used in 

 ordinary surveying, the base line being provided by the motion of 

 the sun. But we cannot say that any individual star is at rest. If, 

 however, we take a sufficiently large group of stars it is legitimate 

 to suppose that in the average the peculiar movements of the separate 



