L. Page — A Century's Progress in Physics. 329 



plicated results of experiments in light, and partly in 

 order to provide a vehicle for the propagation of gravi- 

 tational forces. 



Now an ether, if it is to explain anything at all, must 

 have at least some of the simpler properties of material 

 media. The most fundamental of these, perhaps, is posi- 

 tion in space. As a first approximation in explaining 

 optical phenomena on the earth's surface, the earth 

 might be supposed to be at rest relative to the ether. 

 But the establishment of the Copernican system made the 

 sun the center of the solar system and gave the earth an 

 orbital speed of eighteen miles a second. It may be 

 remarked parenthetically that the speed of a point on the 

 equator due to the earth's diurnal rotation is quite insig- 

 nificant compared to its orbital velocity. Hence as a 

 second approximation the sun might be considered at 

 rest relative to the ether and the earth as moving 

 through this unresisting medium. 



The first indication of this motion lay in the discovery 

 of aberration by the British astronomer Bradley in 1728. 

 Bradley noticed that stars near the pole of the ecliptic 

 describe small circles during the course of a year, while 

 those in the plane of the ecliptic vibrate back and forth 

 in straight lines, stars in intermediate positions describ- 

 ing ellipses. The surprising thing, however, was that 

 the time taken to complete one of these small orbits is in 

 all cases exactly a year. Bradley concluded that the 

 phenomenon is in some way dependent on the earth's 

 motion around the sun, and he was not long in reaching 

 the correct explanation. For suppose the earth to be at 

 rest. Then in observing a star at the pole of the ecliptic 

 it would be necessary to keep the axis of the telescope 

 exactly at right angles to the plane of the earth's orbit. 

 However, as the earth is in motion, the telescope must be 

 pointed a little forward, just as in walking rapidly 

 through the rain an umbrella must be inclined forward so 

 as to intercept the raindrops which would otherwise fall 

 on the spot to be occupied at the end of the next step. 

 The angle through which the telescope has to be tilted is 

 known as the angle of aberration, and the tangent of this 

 angle may easily be shown to be equal to the ratio of the 

 velocity of the earth to the velocity of light. Knowing 

 the velocity of the earth, the velocity of light can then be 



