198 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



MOTION AND DISTANCE ORDINARILY MEASURED BY " TYING UP " 



TO DEFINITE OBJECTS. 



So we come to one of the main conceptions of the theory of rela- 

 tivity, the moving frame of reference. 



We ordinarily refer our measurements and indeed our notions 

 of distance and of motion to some frame, what the mathematician 

 would call some system of co-ordinates, which, so to speak, is " tied " 

 to some definite objects — ordinarily to that portion of the earth's 

 surface on which we may have set ourselves or over which we may 

 be traveling at the moment. 



Though we and all our well-informed ancestors for two centuries 

 have known very well that this frame of reference is not at rest but 

 is in rapid and intricate motion, we are, nevertheless, still accus- 

 tomed to referring our motions to this moving frame and saying 

 that a thing has not budged when its position with respect to the 

 ground has not altered. 



And in doing this we not only follow the promptings of common 

 sense, but find a practical and working basis for the scientific de- 

 scription of almost all terrestrial affairs. 



But the moment we begin to look off the earth into space things 

 are different. It then becomes obvious that the earth is not at rest 

 but moving, both on its own axis and about the sun. 



I say "obvious;" but it is worth remembering that these facts — 

 at present so familiar even to the man in the street — aroused, when 

 their truth was first advocated, the most violent disbelief and agi- 

 tation, and that it took a century or more of controversy to displace 

 the old innate belief in the fixity of the earth, that is, of our frame 

 of reference, and substitute the belief that it was in motion. 



NECESSITY OF FINDING OTHER MEANS OF MEASURING MOTION AND 



DISTANCE. 



So far as our solar system goes we may comfortably treat the sun 

 as being at rest and attach our frame of reference to it. But when 

 we come to look still farther afield at the stars we find them in 

 motion and later detect a drifting tendency among them which indi- 

 cates beyond question that our sun itself is moving. 



So next we hitch our frame of reference on to a sort of average 

 position of all the stars visible to the naked eye, and find that with 

 respect to this new frame of reference the sun and planets are 

 moving at the rate of about 12 miles per second in a definitely known 

 direction. 



We were content with this until within the last decade, when 

 observations upon the nebulae, which we know now to be enormously 



