366 A Short History of Astronomy [CH. xin. 



upon the ratio of the velocity of light to that of the earth 

 in its orbit round the sun ; and observations of Jupiter's 

 satellites after the manner of Roemer (chapter vin., 162) 

 give the light-equation, or time occupied by light in 

 travelling from the sun to the earth. Either of these 

 astronomical quantities of which aberration is the more 

 accurately known can be used to determine the velocity 

 of light when the dimensions of the solar system are known, 

 or vice versa. No independent method of determining the 

 velocity of light was known until 1849, when Hippolyte 

 Fizeau (1819-1896) invented and successfully carried out 

 a laboratory method. 



New methods have been devised since, and three com- 

 paratively recent series of experiments, by M. Cornu in 

 France (1874 and 1876) and by Dr. Michelson (1879) 

 and Professor Newcomb (1880-82) in the United States, 

 agreeing closely with one another, combine to fix the velocity 

 of light at very nearly 186,300 miles (299,800 kilometres) 

 per second ; the solar parallax resulting from this by means 

 of aberration is very nearly 8" '8.* 



284. Encke's value of the sun's parallax, 8"'57i, deduced 

 from the transits of Venus (chapter x., 227) in 1761 and 

 1769, and published in 1835, corresponding to a distance 

 of about 95,000,000 miles, was generally accepted till past 

 the middle of the century. Then the gravitational methods 

 of Hansen and Leverrier, the earlier determinations of the 

 velocity of light, and the observations made at the opposition 

 of Mars in 1862, all pointed to a considerably larger value 

 of the parallax; a fresh examination of the i8th century 

 observations shewed that larger values than Encke's could 

 easily be deduced from them; and for some time from 

 about 1860 onwards a parallax of nearly 8"'95, correspond- 

 ing to a distance of rather more than 91,000,000 miles, was 

 in common use. Various small errors in the new methods 

 were, however, detected, and the most probable value of the 

 parallax has again increased. Three of the most reliable 

 methods, the diurnal method as applied to Mars in 1877, 

 the same applied to the minor planets in 1888-89, and 



* Newcomb's velocity of light and Nyren's constant of aberration 

 (2o"'492i) give 8"794; Struve's constant of aberration (2O"'445), 

 Loewy's (2o"'447), and Hall's (2o"'454) each give 8"'8i, 



