246 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1931 



sumably in the plane of the Milky Way, and proper motions for the 

 part of the Milky Way in the southern hemisphere were lacking. 

 He had to content himself with such indications of the center as 

 could be found from analysis in the plane of the Equator. It is true 

 that the direction provisionally given by Gylden for the center of 

 the system is opposite to that now generally accepted; but that is 

 because the double-period term fixes only the line to and from the 

 center, and does not decide between the two possible antipodal posi- 

 tions. He concluded: "At all events there remains an indication 

 that the motions of the stars have something in common, and that 

 they are not so at random as many astronomers have been inclined 

 to assume." 



By these researches we find the change of velocity in going toward 

 or away from the center; we do not learn the actual velocity at any 

 point. A possible way of discovering this is by observing the globu- 

 lar clusters which can be seen at very great distances up to and 

 beyond the center of the galaxy. By their great spread they will 

 have a mean motion fairly representative of the system as a whole, 

 whereas our stellar observations are limited to a comparatively small 

 region and give the local motion. The difference represents the mean 

 speed at which the stars in our neighborhood are traveling through 

 the system. The result of this determination can not at present be 

 regarded as very accurate, but is sufficient to show that our orbital 

 speed is large, probably between 200 and 300 km per sec. 



CONSEQUENCES OF THE ROTATION 



We have thus to recognize that for a broad cosmical survey the 

 standard of rest to which we have been in the habit of referring all 

 our measured velocities is an inappropriate one. We realized long 

 ago that it was too crude to take the sun as standard, and we have 

 referred velocities to the " mean of the stars " — meaning the stars 

 which come within range of ordinary measurements. Now we have 

 to recognize that this also is a very local standard affected by large 

 orbital velocity, and we must apply a further correction of two or 

 three hundred kilometers per second to reduce to the center of our 

 galaxy. I am afraid it is too much to hope that this will be our 

 final resting place; we see in outer space some hundreds of thou- 

 sands of other galaxies which will claim a share in defining a uni- 

 versal standard. Meanwhile the shift of our viewpoint to the center 

 of the galax}'^ has produced one great improvement; it has brought 

 better order into the motions of the spiral nebulae. It is the gen- 

 eral rule that spiral nebulae are receding from us at very high speed ; 

 the greater the distance, the higher the speed. But the rule was 

 marred by two notable exceptions. As these two are the largest 



