130 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1945 



quick swing around its "perihelion," as it is moving now, during the 

 1940 and 1950 decades, when it is out beyond the orbit of Neptune. 

 As the comet falls back toward the sun it will regain its speed. Even 

 the earth, which has an orbit nearly but not quite circular, goes faster 

 in December than in June because it is then 3 percent nearer the 

 central sun. It is partly due to this greater velocity of the earth that 

 winters in the Northern Hemisphere are a week shorter than sum- 

 mers. If their orbits were exactly circular around the sun, all bodies 

 at equal distances from it would go at the same velocity, whether they 

 were planets, comets, baseballs, or grains of sand. Stars would behave 

 in the same way in going around their attracting center. 



In the great rotation of our galaxy most star orbits around the 

 common center are nearly circular, and therefore any two stars fairly 

 close together, say within a hundred light-years of each other, move 

 at nearly the same velocity. The sun is a typical star and has a nearly 

 circular orbit. The difference between the velocity the sun actually 

 has and what it would have if its orbit were exactly circular is found 

 by comparing its motion with the average for all stars in its vicinity. 

 This variation from the average velocity is called solar motion. It is 

 12 miles a second, as was found by several observers. The variation of 

 the rate of any star from the velocity it would have if its orbit were 

 circular can be similarly found. These variations in the velocities of 

 stars caused by differences in the shapes of their orbits are often spoken 

 of as their "peculiar motions," to distinguish them from the variations 

 in velocities caused by differences in distance from the center of the 

 galaxy. 



Although an occasional star is found to have a velocity with respect 

 to the sun greater than 50 miles a second, it is never going in the 

 direction of the sun's revolution around the galactic center. It is 

 always a slow star that we are overtaking. No stars have been found 

 that run as much faster than the sun as some of them run slower. 

 The reason is that they would be thrown out of the galaxy by centrif- 

 ugal effect if they did. Neighboring stars can go a little faster than 

 the sun, but they cannot go much faster. If a star should pass us 

 with a velocity as much as 30 or 40 miles a second greater than that 

 of the sun, we could bid it farewell, for such a motion would carry 

 it out of the stellar system beyond the gravitational control of the 

 stars. 



In this way the puzzling stream of "high-speed stars" was ex- 

 plained by Lindblad in simple terms of gravitation. They are really 

 low-velocity stars at the slow end of eccentric orbits, and we, and 

 other stars of fairly uniform speed, are overtaking and passing them. 

 These eccentric stars will in time fall in toward the center, regaining 

 speed, but now they are lagging badly behind. 



