SECT. IV. VELOCITY OF LIGHT. 31 



(N. 96), are later by 16' 26"'6 than those which take place when 

 the planet is in opposition. As Jupiter is nearer to us when in 

 opposition by the whole breadth of the earth's orbit than when in 

 conjunction, this circumstance is to be attributed to the time em- 

 ployed by the rays of light in crossing the earth's orbit, a distance 

 of about 190,000,000 of miles ; whence it is estimated that light 

 travels at the rate of 192,000 miles in one second. Such is its 

 velocity, that the earth, moving at the rate of nineteen miles in 

 a second, would take two months to pass through a distance 

 which a ray of light would dart over in eight minutes. The 

 subsequent discovery of the aberration of light has fully con- 

 firmed this astonishing result. 



Objects appear to be situate in the direction of the rays which 

 proceed from them. Were light propagated instantaneously, 

 every object, whether at rest or in motion, would appear in the 

 direction of these rays ; but, as light takes some time to travel, 

 we see Jupiter in conjunction, by means of rays that left him 

 16 m 26 8< 6 before ; but, during that time, we have changed our 

 position, in consequence of the motion of the earth in its orbit : 

 we therefore refer Jupiter to a place in which he is not. His true 

 position is in the diagonal (N. 97) of the parallelogram, whose 

 sides are in the ratio of the velocity of light to the velocity of the 

 earth in its orbit, which is as 192,000 to 19, or nearly as 10,000 

 to 1. In consequence of the aberration of light, the heavenly 

 bodies seem to be in places in which they are not. In fact, if 

 the earth were at rest, rays from a star would pass along the axis 

 of a telescope directed to it ; but, if the earth were to begin to 

 move in its orbit with its usual velocity, these rays would strike 

 against the side of the tube ; it would, therefore, be necessary to 

 incline the telescope a little, in order to see the star. The angle 

 contained between the axis of the telescope and a line drawn to 

 the true place of the star is its aberration, which varies in 

 quantity and direction in different parts of the earth's orbit ; but, 

 as it is only 20"-481, it is insensible in ordinary cases (N. 98). 



The velocity of light deduced from the observed aberration of 

 the fixed stars perfectly corresponds with that given by the 

 eclipses of the first satellite. The same result, obtained from 

 sources so different, leaves not a doubt of its truth. Many such 

 beautiful coincidences, derived from circumstances apparently 

 the most unpromising and dissimilar, occur in physical astronomy, 



