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LIGHT. 227 



earth returned to the same relative position with the planet, the observed time 

 corresponded precisely with the predicted time.* 



From this course of observation and inquiry it became apparent that the 

 lateness of the eclipse depended altogether on the increased distance of the 

 earth from Jupiter. The greater that distance, the later was the occurrence 

 of the eclipse as apparent to the observers, and on calculating the change of 

 distance, it was found that the delay of the eclipse was exactly proportional 

 to the increase of the earth's distance from the place where the eclipse occur- 

 red. Thus when the earth was at E, the eclipse was observed 16 min- 

 utes, or about 1,000 seconds later than when the earth was at A. The diame- 

 ter of the orbit of the earth, A E, measuring about two hundred millions of 

 miles, it appeared that that distance produced a delay of a thousand seconds, 

 which was at the rate of two hundred thousand miles per second. It appear- 

 ed, then, that for every two hundred thousand miles that the earth's distance 

 from Jupiter was increased, the observation of the eclipse was delayed oiie 

 second. 



Such were the facts which presented themselves to Roemer. How were 

 they to be explained ? It would be absurd to suppose that the actual occur- 

 rence of the eclipses was delayed by the increased distance of the earth from 

 Jupiter. These phenomena depend only on the motion of the satellite and the 

 position of Jupiter's shadow, and have nothing to do with, and can have no de- 

 pendance on the position or motion of the earth, yet unquestionably the time 

 they appear to occur to an observer upon the earth, has a dependance on the 

 distance of the earth from Jupiter. 



To solve this difficulty, the happy idea occurred to Roemer that the moment 

 at which we see the extinction of the satellite by its entrance into the shadow 

 is not, in any case, the very moment at which that event takes place, but some- 

 time afterward, viz.: such an interval as is sufficient for the light which left 

 the satellite just before its extinction to reach the eye. Viewing the matter 

 thus, it will be apparent that the more distant the earth is from the satellite, 

 the longer will be the interval between the extinction of the satellite and the 

 arrival of the last portion of light which left it, at the earth ; but the moment 

 of the extinction of the satellite is that of the commencement of the eclipse, 

 and the moment of the arrival of the light at the earth is the moment the com- 

 mencement of the eclipsed is observed. 



Thus Roemer with the greatest facility and success explained the discrep- 

 ancy between the calculated and the observed times of the eclipses ; but he 

 saw that these circumstances placed a great discovery at his hand. In short, it 

 was apparent that light is propagated through space with a certain definite 

 speed, and that the circumstances we have just explained supply the means of 

 measuring that velocity. 



We have shown that the eclipse of the satellite is delayed one second more 

 for every two hundred thousand miles that the earth's distance from Jupiter is 

 increased, the reason of which, obviously is, that light takes ane second to 

 move over that space ; hence it is apparent that the velocity of light is at the 

 rate, in round numbers, of two hundred thousand miles per second. 



Such was the discovery which has conferred immortality upon the name of 

 Roemer ; a discovery to which, as we have shown, he was accidentally led 

 when seeking to determine the velocity of one of the moons of Jupiter. The 

 velocity thus determined would, in the corpuscular theory, be regarded as that 

 with which the particles of light issuing from the surface of a visible object move 



* Strictly speaking ie interval is longer than twelve months, bat the circumstance is not iinpor- 

 ant here. 



