Light. 131 



at C (plate XI. fig. 45), an eclipse is observed of 

 one of the satellites of Jupiter, which thus re- 

 gularly suffers eclipses, at intervals of about forty- 

 two hours and a half. If the earth never left C, 

 but continued there immbveable, we should re- 

 gularly see the satellite eclipsed at the expected 

 interval of forty-two hours and a half; and also 

 in thirty times that number the spectator would 

 see thirty eclipses. But the earth is not fixed ; 

 let us, then, farther suppose that the earth in 

 moving through half its orbit from C, the place 

 of conjunction, has just placed itself in opposition, 

 near D, that is, where it would be situated be- 

 hind the sun relatively to Jupiter. If light had 

 no progressive motion, a spectator on our globe 

 would see the first satellite of Jupiter emerge 

 from the shadow after a period equal to as many 

 times 4>2f hours, as there would be eclipses 

 after the moment of conjunction. But this does 

 not happen : for the spectator at D sees the ter- 

 mination of the eclipse about sixteen minutes 

 later than the calculation predicts ; so that, in 

 all the intermediate positions between C and D, 

 the difference as far as this limit has been con- 

 tinually increasing. Now C D, the rectilinear 

 distance between these two positions, is equal 

 to the diameter of the earth's orbit, that is, 

 to about 190 millions of English miles. This 

 space, therefore, is passed over by light in 16 

 minutes; so that, assuming it to move uniformly, 

 we find, by an easy proportion, the space passed 



