SECT, iv.] ABERKATION. 35 



In consequence of the relations already mentioned in the 

 meanmotions and mean longitudes of the first three satellites, 

 they never can be all eclipsed at the same time : for, when 

 the second and third are in one direction, the first is in the 

 opposite direction ; consequently, when the first is eclipsed, 

 the other two must be between the sun and Jupiter. The 

 instant of the beginning or end of an eclipse of a satellite 

 marks the same instant of absolute time to all the inhabitants 

 of the earth ; therefore, the time of these eclipses observed 

 by a traveller, when compared with the time of the eclipse 

 computed for Greenwich, or any other fixed meridian (N. 95), 

 gives the difference of the meridians in time, and, conse- 

 quently, the longitude of the place of observation. The 

 eclipses of Jupiter's satellites have been the means of a dis- 

 covery 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' 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 attributed to the time employed 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 190,000 miles in one second. Such is its ve- 

 locity, 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 confirmed this 

 astonishing result. 



Objects appear to be situate in the direction of the rays 

 which proceed from them. Were light propagated instanta- 

 neously, every object, whether at rest or in motion, would 

 appear in the direction of these rays ; but, as light takes 



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