MIRACLES OF SCIENCE 



over, there are exceptional stars the apparent motion 

 of which is notable. Thus there is an eighth 

 magnitude star, detected by Professor Kapteyn in 

 the photographic charts taken at the Cape of Good 

 Hope, which seems to move by an amount represent- 

 ing 8.7 seconds of arc per year; a shift that would 

 carry it across an apparent distance equal to the 

 diameter of the full moon in two centuries. 



With changes such as these the astronomer deals 

 readily enough; yet we must reflect that the annual 

 change in position of this, most rapidly moving star 

 is equivalent to a shift of only one foot in an object 

 viewed at a distance of four and a half miles. 

 Furthermore there is no other star known to move 

 anything like as fast as this; and there are only six- 

 teen stars all told that change position one-third as 

 fast. The only one of these that is visible to the 

 naked eye is the famous star Alpha Centauri, our 

 nearest neighbor, which appears at second magni- 

 tude, and which would shift position across the face 

 of the moon in about five hundred years. 



DIRECTION OF THE SUN*S FLIGHT 



A fractional part of such a movement as this suf- 

 fices, ho.wever, for the tests of the astronomer; and 

 as comparison of the present-day positions of the 

 stars may be made with certain accurate star charts 

 of earlier generations (notably that of Bradley made 

 about the middle of the eighteenth century), the 

 astronomer of our day can show the exact direction 

 of backward drift or proper motion of a very large 

 number of stars. Photographic plates taken at in- 



42 



