SCIENCE OF COMMON THINGS. 24:3 



Velocity of light determined from observations on Jupiter's satellites. 



The earth revolves around the sun in an orbit of 

 which the sun is the centre. We are able to calculate 

 the exact time when an observer standing in the centre 

 of the earth's orbit, that is, in the sun, would see an 

 eclipse of Jupiter's satellite ; but as the earth moves 

 round the sun in its orbit, it is brought at one time 

 ninety-five million of miles nearer Jupiter than the sun 

 is, and at another time it is carried ninety-five millions 

 of miles further off. Now, when the earth is nearest 

 to Jupiter, the eclipse takes place eight minutes in 

 advance of the calculated time, and when it is ninety- 

 five millions of miles farther off, the eclipse occurs eight 

 minutes later than the calculated time. This delay is 

 occasioned by the fact, that in the one case the light 

 coming from the satellite to the earth has to traverse a 

 much greater distance than in the other ; and if the 

 light requires eight minutes, or 480 seconds, to move 

 over 95,000,000 of miles, it will require one second to 

 move over 197,000 miles, or, with more exact data, 

 192.000 miles in one second. 



Fig. 50. 



The explanation above given will be made clear by reference to the 

 following diagram, Fig. 50. S represents the sun, a b the orbit of the 

 earth, and T T' the position of the earth at different and opposite points 

 of its orbit. J represents Jupiter, and E, its satellite, about to be eclipsed 

 by passing within the shadow of the planet. Now the time of the com- 

 mencement or termination of an eclipse of the satellite, as stated from 

 calculation in tables, is the instant at which the satellite would appear to 

 enter or emerge from the shadow, if it could be seen by an observer from 

 the sun, S. If the transmission of light were instantaneous, it is obvious 

 that the light coming from Jupiter's satellite, E, would be seen at the 

 same moment at the points T^ Sand T'. But repeated observation shows 



