194 PROGRESS OF SCIENCE IN THE CENTURY. 



millions of yearly journeys to reach the star in ques- 

 tion." * 



It seems on the whole most convenient to use, as 

 Bessel suggested, as a unit " the light journey of one 

 year." The velocity of light is 186,300 miles a 

 second, about six billion miles a year. " Light takes 

 four years and four months to reach the earth from 

 o Centauri, yet a Centauri lies some ten billions of 

 miles nearer to us (so far as is yet known) than any 

 other member of the sidereal system 1 " f In other 

 words, we see a Centauri, not as it is now, but as it 

 was more than four years ago. Similarly, light takes 

 more than six years to reach us from 61 Cygni. 



Given a determination of the parallax and distance 

 of two stars in a system, and a knowledge of their 

 period of revolution, it became possible to calculate 

 their combined mass in terms of that of the sun ; and 

 the process of weighing the stars began. 



HerscheTs conclusion as to movement of the solar 

 system as a whole, often doubted, was repeatedly 

 confirmed ; the general direction was more carefully 

 stated ; and even the rate has been guessed at F. G. 

 W. Struve (1793-1864) continued Herschel's study 

 of double stars, and published in 1837 his monumen- 

 tal Mensura Micrometricce, which " will for ages 

 serve as a standard of reference by which to detect 

 change or confirm discovery." 



The distances of some of the nearer stars can be 

 calculated by the determination of annual parallax, 

 a method first successfully employed by Bessel 

 (1838), Henderson (1839), and Struve (1840); 

 this is historically important as a confirmation of 



* Freely translated from Dannemann's Grundrita einer 

 Qftrhirhtei drr tfatiirtnttmachaftcn, vol. 1, 1806, p. 825. 

 f A. M. Clerke. Uittory, 1885, p. 49. 



