and the Sun's Distance. 525 



wave, allowance must be made for the time taken by the news 

 of an audible or visible event to come to us. Only the vast 

 spaces of astronomy are commensurable with the great velocity 

 of light, and furnish a sufficiently large theatre for a direct 

 experiment upon it. But in stellar astronomy the magnificence 

 of the extent of view so far transceds in magnitude even the 

 velocity of light, that the luminous ray, vast as is its velocity, 

 seems to loiter upon its long way. 



Hence in astronomy a distinction exists between the actual 

 interval of successive events and the apparent interval. For 

 example, the first satellite of Jupiter revolves around its primary 

 in about 42J hours, and therefore enters the shadow of Jupiter, 

 and is eclipsed, once every 42^- hours. As it takes light more 

 than 40 minutes to pass over the average distance of Jupiter, 

 the eclipse is not seen until so many minutes on the average 

 after it has happened. If this delay were constant, the interval 

 of successive eclipses would not be changed. But in the 

 course of six months the distance of the earth from Jupiter 

 increases by the diameter of the earth's orbit, and in the next 

 six months changes back again ; and when the earth is nearest 

 to Jupiter, the news of an eclipse reaches us in about 32 minutes, 

 whereas, if the earth is at the greatest distance, 50 minutes are 

 required. 



Consequently the intervals between successive eclipses, as 

 they exist for our eyes, are variable, being sometimes larger and 

 sometimes smaller than the real intervals. This irregularity in 

 the apparent intervals of the eclipses of the same satellite, at 

 first attributed to errors of observation, finally conducted Homer, 

 in 1675, to the discovery of the velocity of light. Delambre, 

 after discussing 1000 of these eclipses observed between 1662 

 and 1802, calculated the velocity of light to be such as to 

 require 493*2 seconds to pass over the mean distance of the sun. 

 If this time divides 95,360,000 statute miles, which is the sun's 

 distance as given by the transits of Venus in 1761 and 1769 

 according to Encke's computations, the quotient, or 193,350 

 statute miles, is the velocity of light in a second. 



The second process which astronomy has supplied for obtain- 

 ing the velocity of light may be called the indirect method. It 

 demands not a space but a velocity which is commensurable with 

 the velocity of light. If two such velocities are compounded 

 together, according to the principle of the parallelogram of 

 motions, there is a resultant motion, the direction of which devi- 

 ates sensibly from that even of the largest motion which enters 

 into the composition. In nature the velocity of the earth is 

 compounded in this way with the velocity of light, and imparts 

 to the light an apparent path differing by a small angle from the 



