STRUCTURE OF THE GALAXY — PLASKETT 203 



in rotation about the sun and be able to find the direction to the 

 sun from these motions even if the hitter were or became invisible. 

 This was done by Gylden as far back as 1871, when from the mo- 

 tions of some of the asteroids he determined the direction to the 

 sun within 6°. Exactly the same relation holds in the stellar 

 system for, even if only part of the matter is concentrated at the 

 center, the revolution will follow some intermediate course between 

 constant angular velocity and planetary moticm, and it is obvious 

 that as long as there is any condensation toward the center, there 

 will be relative motion between the neighboring stars which can be 

 measured and the rotation determined. This can be shown perhaps 

 more clearh' b}^ a diagram, figure 6. 



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FiGUUE C. — Differential velocities produced by galactic rotation. 



In the lower group of stars with the sun in the center, the differ- 

 ential rotation is shown by the stars nearer the center having a 

 higher rotational speed. If an equal and opposite velocity to that 

 of the sun is impressed upon this group it brings the sun to rest, and 

 we have the motions shown in the upper group. The components of 

 these velocities in the direction of the sun, the radial velocities, are 

 shown in tlie center group. It will at once be seen that the relative 

 motion or the rotational effect depends not only on the angle between 

 the star and the direction to the center but also on the distance of 

 the star from the sun. There is no rotational effect along, and per- 

 pendicular to, the direction to the center, while the maximum rota- 

 tional effects are midway between the zero values. 



