30 ABERRATION. SECT. IV. 



meridian (N. 95), gives the difference of the meridians 

 in time, and, consequently, the longitude of the place of 

 observation. The eclipses of Jupiter's satellites have 

 been the means of a discovery which, though not so 

 immediately applicable to the wants of man, unfolds 

 one of the properties of light that medium without 

 whose cheering influence all the beauties of the creation 

 would have been to us a blank. It is observed, that 

 those eclipses of the first satellite, which happen when 

 Jupiter is near conjunction (N. 96), are later by 16 m 

 26"6 than those which take place when the planet is in 

 opposition. As Jupiter is nearer to us when in opposi- 

 tion by the whole breadth of the earth's orbit than 

 when in conjunction, this circumstance is attributed to 

 the time employed by the rays of light in crossing the 

 earth's orbit, a distance of about 191X000,000 of miles ; 

 whence it is estimated that light travels at the rate of 

 190,000 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 dis- 

 tance which a ray of light would dart over in eight 

 minutes. The subsequent discovery of the aberration 

 of light confirmed this astonishing result. 



Objects appear to be situated in the direction of the 

 rays which proceed from them. Were light propagated 

 instantaneously, every object, whether at rest or in mo- 

 tion, 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 be- 

 fore ; but, during that time, we have changed our posi- 

 tion, 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 190,000 to 19, or 10,000 to 1. In conse- 

 quence 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 



