OUR REVOLVING "ISLAND UNIVERSE" — SKILLING 135 



The result actually given by the star's spectrum is the component 

 of velocity directed toward or away from the observer. (See fig. 1.) 

 The maximum value of this line-of -sight velocity is called Oort's con- 

 stant because it figures so prominently in the method devised by Oort 

 for studying galactic rotation. It is a constant quantity for any given 

 difference in distance to stars along the Milky Way : for each additional 

 1,000 light-years in any of the four directions of maximum radial mo- 

 tion, the radial motion increases about 3y 2 miles a second. Thus, for 

 a star at 8,000 light-years from us the spectroscope should show a 

 velocity of about 28 miles a second. Using the well-known laws of 

 gravitation, this value can be related to the speed of rotation of the 

 galaxy. Oort worked out an equation in which his "constant" would 

 be one of the known terms, the distance to the center another known 

 term, and the velocity of the sun the unknown term to be found. 



Based upon the above principles various observers have arrived at 

 somewhere near 170 miles a second as the velocity of the sun and its 

 planets in their revolution around the center. Most of the stars that 

 are near enough to be visible to the naked eye move at about the same 

 speed, and that is why a great telescope is needed to study the revolu- 

 tion of the galaxy. 



Knowing the velocity of the sun and its distance from the center 

 it is a mere matter of arithmetic to find its period of revolution around 

 the center. The radius of the orbit is 33,000 light-years, each light- 

 year being nearly 6 trillion miles. The period found proves to be in 

 the neighborhood of 200 million years. 



Geologists place the age of the earth at around 2,000 million years, 

 basing their estimate on the chemical analysis of rocks containing the 

 radioactive element uranium and a peculiar kind of lead into which 

 it very slowly changes at a known rate. The relative amounts of the 

 two metals show when the rock was formed. From these figures it 

 would seem that the sun and earth have had time for only about 10 

 revolutions around the distant center of the stellar system since the 

 earth was formed and solidified. 



Slow and difficult as it has been to work out and prove this theory 

 of galactic rotation, the theory simplifies astronomy by relating to one 

 another various puzzling facts about motion that are hard to under- 

 stand one by one. Such matters as "star streaming," "asymmetry of 

 motion," and "high- and low-velocity stars," are more easily explained 

 together than separately. In this respect the rotation theory has, in a 

 measure, paralleled the heliocentric theory of the solar system by 

 which Copernicus explained such riddles as "retrograde motion" of 

 planets. 



