OUR REVOLVING "ISLAND UNIVERSE" AND ITS 

 SPIRALING COUNTERPARTS 1 



By William T. Shilling 

 Professor of Astronomy, retired, San Diego State College, San Diego, Calif. 



[With 2 plates] 



Our earth is an infinitesimal part of a great whirlpool of stars called 

 the galaxy, or Milky Way. Not only are the sun, moon, and planets 

 of the solar system moving as a very small unit that drifts with the 

 rotation of the galaxy, but all the visible constellations of the sky are 

 so close to us that they, too, are our fellow travelers in one small part 

 of this cosmic eddy. Only recently has the tremendous velocity of 

 our galactic rotation been measured and found to be 170 miles a second, 

 in the direction of the constellation Cygnus. 



On account of the smoothness of our travel and the freedom from 

 any sharp turns in our course we do not feel this motion any more than 

 we do the lesser speed of the revolution of the earth about the sun at a 

 mere 18^ miles a second. It is not the motion but abrupt changes in 

 motion that we feel when riding in an automobile or train, and the 

 earth makes no such changes in speed. The curve that the earth fol- 

 lows in its motion about the sun, 93,000,000 miles away, is so gradual 

 that in each second the earth departs only one-ninth of an inch from 

 a straight line while going forward lSy 2 miles. Only by observing 

 other moving bodies and the so-called fixed stars are we able to meas- 

 ure or even perceive our annual motion, and it is little wonder that 

 astronomers have been slow in observing the vaster sweep of the whole 

 solar system and its relatively near neighbors around a center that is 

 2 billion times as far away as the sun. 



Soon after the beginning of this century the great Dutch astronomer 

 Kapteyn, as a result of his studies in his native Holland, announced 

 the discovery of two "star streams" flowing in particular directions. 

 It has long been known that stars have some motion with respect to 

 one another. But this previously known star motion, including that 

 of the sun, was looked upon as being wholly at random. Kapteyn 



1 Reprinted by permission from the Scientific Monthly, vol. 60, No. 2, February 1945. 

 676212—46 9 125 



