48 THE WONDERFUL CENTURY. CHAP. vi. 



The accuracy of this method under favorable condi- 

 tions is very great, as has been proved by those cases in 

 which we have independent means of calculating the 

 real motion. The motion of Venus toward or away 

 from us can be calculated with great accuracy for any 

 period, being a resultant of the combined motions of the 

 planet and of our earth in their respective orbits. The 

 radial motions of Venus were determined at the Lick 

 Observatory in August and September, 1890, by spectro- 

 scopic observations, and also by calculation, to be as 

 follows : 



BY OBSERVATION. BY CALCULATION. 



Aug. 16th. 7.3 miles per second. 8.1 miles per second. 



11 22d. 8.9 " 8.2 



" 30th. 7.3 " 8.3 



Sept. 3d. 8.3 8.3 



" 4th. 8.2 " 8.3 



showing that the maximum error was only one mile per 

 second, while the mean error was about a quarter of a 

 mile. Owing to the greater difficulty in observing the 

 spectra of stars, the accuracy in their case is probably 

 not quite so great. This has been tested by observations 

 of the same star at times when the earth's motion in its 

 orbit is toward or away from the star, whose apparent 

 radial velocity is, therefore, increased or diminished by 

 a known amount. Observations of this kind were made 

 by Dr. Vogel, Director of the Astrophysical Observa- 

 tory at Potsdam, showing, in the case of three stars, of 

 which ten observations were taken, a mean error of about 

 two miles per second. 



The same observer, from his study of the spectra of 

 the variable star Algol, has been able to determine that 



