CHARTING THE UNIVERSE 



drift, just as another railroad train may pass ours 

 even when we are going at best speed. Or the star 

 may be moving at an angle to our line of flight, and 

 hence the direction of its backward drift may be 

 angular. 



TESTING PROPER MOTION 



Ignoring for the moment complications due to the 

 actual motion of the stars, it will be obvious that, 

 generally speaking, the backward drift the astrono- 

 mer calls it "proper motion" will be greatest in 

 case of the nearest stars and relatively less as stars 

 become more and more distant. It is equally obvious 

 that, inasmuch as the earth goes forward by about 

 367,000,000 miles in a year, the backward drift or 

 proper motion of the stars should be very; conspicu- 

 ous, unless the stars are infinitely distant. But we 

 have already seen that the stars are exceedingly dis- 

 tant; and perhaps nothing brings their aloofness more 

 vividly to our comprehension than to be told that the 

 shift in position of even the very nearest stars is so 

 slight year by year that it would not change the 

 naked-eye aspect of the heavens appreciably in a 

 thousand years, although the earth has gone forward 

 uninterruptedly at a speed of 45,000 miles an hour 

 during the entire period. 



But of course shifts in position that are quite in- 

 appreciable to the naked eye are magnified by thje 

 telescope to measureable and even conspicuous di- 

 mensions. So changes that would not be noted by 

 the naked eye in a century or a millenium may be 

 recorded by the telescope even year by; year. More- 



4 41 



