1!^:^0] The Theory op Relativity 21 



clearly and simply the cause of the loops in the paths of the planets, 

 the phases of Venus and the Moon, and the motions of planets, satel- 

 lites and comets. 



It led very soon to the discovery of Kepler's laws and the epoch- 

 making work of Newton, including the development of the Newtonian 

 mechanics and the differential and integral calculus. Two hundred years 

 after Copernicus, Herschel again upset our frame of reference, for he 

 showed that the whole Solar System is moving through space with a 

 velocity of something like fifteen miles a second in the approximate 

 direction of the star Vega, and that in addition the "fixed stars" are 

 anything but fixed. All of them seem to be travelling in different 

 directions through space with enormous velocities. One of them, in- 

 deed, No. 1830 in Groombridge's catalogue, has such a large proper 

 motion that it is called the ' ' runaway star. ' ' 



So our fixed frame of reference again fails us, and in our attempt 

 to find another we adopted the theory of a stationar^^ ethei* filling all 

 space, and affording the medium in which light waves are propagated. 

 Such a medium seemed necessary to explain the experiments in optics, 

 electrostatics, magnetism and electromagnetism, and since the same 

 kind of medium with the same properties was needed for the explana- 

 tion of phenomena in each of the branches of physics just named. 

 Maxwell pointed out that this fact constituted strong evidence for the 

 existence of the ether. 



Now if it does exist, and if it is stationary in space, and // we can 

 determine the absolute motion of the earth with respect to it, then 

 we shall have found our much-desired frame of reference fixed m 

 space. 



The early experiments seemed to show that the ether is really at 

 rest in space. In 1727 Bradley discovered the aberration of light, and 

 the velocit}'- of light as calculated from his formula agreed well with 

 that found fifty years earlier by Roemer from observations of the 

 eclipses of Jupiter 's satellites. Now since the fact of aberration seems 

 inexplicable on any other basis than a fixed or stationary' ether, be- 

 cause if the ether in the telescope tube travels with it there would 

 be no reason for any aberration, it was considered that the aberration 

 of light clearly indicated that the ether is at rest in space, and that the 

 material composing the earth moves through the ether just as a tennis 

 racquet moves through air, allowing it to flow through the holes in the 

 stringing. But it seemed incredible that any medium could stream 



