ITS VELOCITY. 



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 occurred. Thus when the earth was at E, the 

 eclipse was observed 16 minutes, or about 960 seconds later than 

 when the earth was at A. The diameter of the orbit of the earth, 

 A E, measuring about 190 millions of miles, it appeared that that 

 distance produced a delay of 960 seconds, which was at the rate 

 of 198,000 miles per second. It appeared, then, that for every 

 198,000 miles that the earth's distance from Jupiter was in- 

 creased, the observation of the eclipse was delayed one second. 



Such were the facts which presented themselves to Koemer 

 How were they to be explained ? It would be absurd to suppose 

 that the actual occurrence 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 

 dependence on the position or motion of the earth, yet unquestion- 

 ably the time they appear to occur to an observer upon the earth, 

 has a dependence on the distance of the earth from Jupiter. 



To solve this difficulty, the happy idea occurred to Koemer 

 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 sometime after- 

 ward, 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 com- 

 mencement of the eclipse, and the moment of the arrival of the 

 light at the earth is the moment the commencement of the 

 eclipse is observed. 



Thus Roemer with the greatest felicity and success explained 

 the discrepancy 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 



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