TIME AND MOTION 73 



first discovery which might have given rise to 

 doubt was due to the Danish astronomer, Romer 

 (1675), who noted that the frequency of revo- 

 lution of the satelHtes of Jupiter appeared to 

 be slightly greater when the earth moved toward 

 Jupiter than w^hen it moved in the other direc- 

 tion; just as the whistle of a locomotive seems 

 to have a higher pitch when it is coming toward 

 us than when it is receding. Homer rightly ex- 

 plained this by assuming that light moves with 

 a finite velocity. This discovery at once reopens 

 the question of simultaneity. We can no longer 

 say that two events which we see at the same 

 time are simultaneous. A bright nova suddenly 

 appears in the skies, but we say that it really 

 flashed out thousands of years ago and that the 

 light has been traveling to us ever since. If we 

 knew the distance of the star and the velocity of 

 light we might make due allowance for this time 

 of travel, but is the velocity of light dependent 

 perhaps upon the motion of the earth, or upon 

 the motion of the star that we are observing? 



This was a question which later was to give 

 much concern to physicists. But in the mean- 

 time the velocity of light came to be regarded as 

 one of the fundamental constants of nature, — 

 more fundamental, indeed, than light itself ; for 



