MIRACLES OF SCIENCE 



are shifted in one direction if you are approaching 

 the light-giving object, and in the other direction if 

 you are receding from it. With the great resolving 

 power of the modern spectrograph used in connection 

 with a big telescope, it is possible thus to measure 

 accurately the shift of lines and deduce the speed of 

 the moving body. For the purposes of this measure- 

 ment it does not matter whether the light-producing 

 object is moving toward you, or whether you are 

 moving toward the light-producing body. 



In the case under consideration, the astronomer 

 measures the motion of the earth plus the motion of 

 the star. But the earth is traveling in opposite direc- 

 tions at the two periods when the photographs are 

 taken, whereas presumably the flight of the star is un- 

 varying. So the difference between the velocities 

 recorded in the two measurements will represent 

 twice the actual speed of the earth in its flight about 

 the sun. This speed being thus accurately measured, 

 it is a very simple problem in arithmetic to determine 

 the distance which the earth traverses in a year; 

 which is obviously equivalent to 'determining the size 

 of the earth's orbit and hence the average distance 

 of the sun. 



In the number of the Publications of the Astro- 

 nomical Society of the Pacific for October, 1912, Pro- 

 fessor Arthur B. Turner of the College of the City 

 of New York, tells of recent estimates of the sun's 

 distance made by this method. But he points out 

 that the problem is by no means as simple as it seems 

 at first sight, because of certain practical complica- 

 tions that cannot be avoided. 



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