October 3, 1913] 



SCIENCE 



469 



him, in order to set their clocks in synchron- earth moves on, the earth clock of A^ will 

 ism. reach a position opposite B (Fig. 7), who finds 



We are now in a position to measure the 

 velocity of light in both systems, and find that 

 in each case it is 60/8. We can readily believe 

 that all of nature's laws in general, and the 

 velocity of light in particular, should be the 

 same on sun, earth or planet in the Milky 

 Way; but the fact that the earth-man finds 

 the sun clocks slow, and the sun-man finds 

 the earth clocks slow, in the same ratio is the 

 startling contribution of the theory of rela- 

 tivity : that two actions simultaneous upon one 

 system shoiild not be simultaneous when 

 viewed from another system is surprising. 



Let us see what the two observers on the 

 sun have to tell us about one of the clocks on 

 the earth. 



A and B are sixty miles apart and can not 

 both see the earth clock at the same time; 

 but the earth is a moving system, hence A 

 can compare his clock with the earth clock, 

 and later B can make a similar comparison. 



that the hand of his clock has again reached 

 12, while the hand of the earth clock has 

 reached only 9; hence A and B establish the 

 fact that twelve hours on the sun are equal to 

 nine hours on the earth; that is, the earth 

 clock runs slow in the ratio of 3 to 4. 



Fig. 7 



When the earth-men agree to make observa- 

 tions on one of the sun clocks, they reach a 

 similar conclusion. A^ and 5' are sixty miles 



When the earth clock is opposite A (Fig. 6), 

 the latter finds the hand of his clock at 12, 

 and the hand of the earth clock at 12; as the 



apart and can not both see the sun clock at 

 the same time; when 5' comes to a position 

 opposite the sun clock (Fig. 8), the hand of 



