LIGHT. 290 



immense is their distance from our earth. If, therefore, one of the remote fixed 

 Btars were to-dav blotted from the heavens, several generations on the earth 

 •would have passed away before the obliteration could be known to man. 



The following comparison between the velocity of light and the speed of a 

 locomotive engine has been instituted : — Light passes from the sun to the 

 eartli in about eight minutes ; a locomotive engine, traveling at the rate of 

 a mile in a minute, ■would require upward of one hundred and eighty years to 

 accomplish the same journey. 



Who first as- 650. The velocity of light was first deter- 

 rei^ityofu<^ht? mined by Yon Roemer, an eminent Danish 

 astronomeij from observations on the satellites 

 of Jupiter. 



Ex lain th ^^® method by •which Ton Roemer arrived at this result 



method by may be explained as follows : — The planet Jupiter is sur- 

 l^citT of li^ht rounded by several satellites, or moons, which revolve about 

 was deterniined it in certain definite times. As they pass behind the planet, 

 of'°Jupiter''sKiU ^^^Y disappear from the sight of an observer on the earth, or 

 ellites. in other words, they undergo an eclipse. 



The earth also revolves in an orbit about the sun, and in the course of its 

 revolution is brought at one time 192 millions of miles nearer to Jupiter than 

 it is at another time, when it is in the most remote part of its orbit. Suppose, 

 now, a table to be calculated by an astronomer, at the time of year when the 

 earth is nearest to Jupiter, showing, for twelve successive months, the exact 

 moment when a particular satellite would be observed to be eclipsed at that 

 point. Six months afterward, when the earth, in the course of its revolution, 

 has attained a point 192 millions of miles more remote from Jupiter than it 

 formerly occupied, it would be found that the echpse of the satellite would 

 occur sixteen minutes, or 960 second?, later than the calculated time. This 

 delay is occasioned by the fact that the light has had to pass over a greater 

 distance before reaching the earth than it did when the earth was in the op- 

 posite part of its orbit, and if it requires sixteen minutes to pass over 192 mil- 

 lions of miles, it will require one second to move over 200,000 miles. "When, 

 on the contrary, the earth at the end of the succeeding six months has as- 

 sumed its former position, and is 192 millions of miles nearer Jupiter, the 

 eclipse will occur sixteen minutes earlier, or at the exact calculated time given 

 in the tables. The velocity of light, therefore, in round nimibers, maybe con- 

 sidered as 200,000 miles per second.* A more exact calculation, founded on 

 perfectly accurate data, gives as the true velocity of hght 192,500 miles per 

 Becond. 



• The explanation ahove given ■will be made clear by reference to the following dia- 

 gram. Fig. 23'2. 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 bv passing within the shadow of the planet. Now the lima 

 of the commencement or termination of an eclipse of the satellite, is the instant at which 

 the satellite would appear, to an observer on the earth, to enter, or emerge from tho 



