STRTJCTUEE OF THE GALAXY PLASKETT 207 



column, than is the case in the stellar velocities. No one who bears 

 this agreement in mind can have any reasonable doubt that not only 

 tlie stars but the intervening exceedingly diffuse matter, 4 ounces in 

 the volume of the earth, are rotating around a very distant center 

 in the direction of the constellation Sagittarius. This direction of 

 328° is determined from the least-squares solutions at the same time 

 as the rotational effect rA. Our conviction of the reality of the 

 galactic rotation will be considerably strengthened when we learn 

 that the geometrical center of the galaxy as determined by Shapley 

 from the distribution of the globular clusters is at galactic longitude 

 327°, agreeing exactly, within the errors of observation, with that 

 determined dynamically on the assumption of a rotation of the 

 galaxy from the distribution of the velocities. 



It will be of interest to derive some more general results of the 

 rotation. I am sure it is not necessary to tell you that the velocities 

 we have been discussing are not the rotational velocities around the 

 galactic center but only the differences in velocity produced by the 

 change in rotational velocity at different distances from the center. 

 The orbital or rotational velocity is much greater, though not yet 

 very rigorously determined. One way of obtaining it is from 

 measures of the radial velocity of the globular clusters, which, as we 

 have seen, have a low rotational motion and whose mean observed 

 velocity should hence give the orbital velocity of the sun. The 

 result shows that the rotational speed is of the order of 300 kilo- 

 meters, or 200 miles, per second. This becomes greater as we go 

 toward and smaller as we go away from the center. Computing 

 the change, it appears that 5,000 light-years nearer the center the 

 speed will be 335 kilometers per second, and 5,000 light-years farther 

 away, 2G5 kilometers per second. As you have seen, it is these 

 changes in rotational speed that cause the velocity differences dis- 

 cussed. 



As compared with terrestrial velocities, the rotational speed of 

 300 kilometers per second is tremendous indeed, nearly 2,000 times 

 faster than man has ever traveled on the earth, than the speed 

 attained in the recent Schneider Cup trials of 400 miles per hour, 

 but yet so vast are the dimensions of the galaxy that it will take the 

 sun about 250,000,000 years to complete one revolution. Hence, dur- 

 ing the whole span of geologic time on the earth our sun has only 

 made some five or six revolutions in its orbit. This velocity of 300 

 kilometers per second only corresponds to a change of angle or longi- 

 tude of "0.006 per year, hence the hopelessness of detecting the rota- 

 tion by observing the proper motion of external objects is obvious. 



The magnitude of the rotational effect — the value of A, which we 

 have found to be 1 kilometer per second for stars 200 light-years 

 away — and the orbital speed of the sun, 300 kilometers per second, 



