ROTATION OF THE GALAXY — EDDINGTON 243 



This distortion comes from the shearing effect when the inner part 

 of a ring travels faster than the outer part. 



Mathematically we can describe this distribution by saying that, 

 when the stars are arranged according to galactic longitude I, their 

 observed radial velocities contain a term C sin 2(Z — Zo), where U is 

 the longitude of the center of the system. Moreover, it is clear that 

 the effect is greater for greater distances being approximately pro- 

 portional to the distance of the stars considered from the observer. 

 We therefore express the term as 



Ar sin 2(? — Zq), 



where r is the distance of the stars examined, and A is a constant. 



The stars have their own individual motions superposed on the 

 general rotation of the system, and we can only expect to discover 

 this effect if we average out the individual motions by taking means 

 for a considerable number of stars. Owing to the increase of effect 

 with distance it is best to search for it in the more distant classes of 

 objects. It may be said at once that the search is successful. The 

 expected distribution of velocity is found in all classes of objects 

 that could be expected to show it, and they agree among themselves 

 both as to the magnitude of the effect and as to the direction in which 

 the center of the galaxy is situated. 



OBSERVATIONAL EVIDENCE 



Through the researches of Harlow Shapley, the center of our 

 galaxy had already been located in the direction of the great star 

 clouds of Sagittarius — the richest part of the Milky Way. He 

 deduced this from the distribution of the most distant galactic ob- 

 jects observable, particularly the globular clusters, which may be 

 supposed to outline the shape of the system. The exact center can 

 not be found with any high accuracy, but the position generally 

 adopted is in 325° galactic longitude. Oort's method of deducing 

 it from the rotation effect is entirely independent ; it generally gives 

 a rather higher longitude 330°-335°, but the difference is within the 

 probable uncertainty of the determinations. 



As already stated, the magnitude of the effect increases with the 

 distance. For stars distant 1,000 parsecs ^ it amounts to 17 km per 

 sec, that is to say the stars seen at this distance in one part of the 

 sky are in the mean moving toward us at 17 km per sec, whereas 

 those 90° away are moving from us at the same rate. For other dis- 

 tances the effect is in proportion — 81/^ km per sec. for 500 parsecs 



^1 parsec = 3.26 light-years. 

 102992—32 17 



