PROBLEMS OF MODERN SCIENCE 



the astronomers of Alexandria sixteen hundred 

 years earlier. Even with modern instruments and 

 methods the detection of motion is difficult, and 

 the number of stars which have been found to be 

 in motion relative to the sidereal system as a whole 

 forms a very small percentage of the total. Still, 

 there is no doubt that all are moving, and it is only 

 distance and faintness which prevent observation of 

 the resultant changes. Dynamics shows that it is 

 impossible for bodies attracting one another in 

 accordance with the law of gravitation to be in a 

 state of relative rest, unless they are in contact with 

 one another. Either their distances apart or the 

 directions of the lines joining them must be con- 

 tinually altering, and changes in distance and 

 direction may take place simultaneously. Space 

 contains no fixed landmarks. Indeed, if there 

 were we have no means of recognising them as 

 such ; consequently absolute position and abso- 

 lute motion are meaningless terms. The question 

 whether dynamics enables us to fix absolute direc- 

 tions in space is an interesting one, but we have no 

 time to consider it now. 



The practical problem we have to deal with is 

 the determination of the motions of the stars relative 

 to ourselves, or, more strictly speaking, to the sun, 

 for the change of standpoint is easily allowed for. 



The motion of a star can be resolved into two 

 components, one, in the line of sight, producing 

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