TO KNOW THE STARRY HEAVENS 



135 



on the sky. Just as when one is walk- 

 ing through a wood or riding on a rail- 

 way he sees all objects apparently mov- 

 ing past him in a direction exactly op- 

 site to that in which he is going. The 

 nearer objects, because they are nearer, 

 appear to move the most rapidly, while 

 those which are very distant may seem 

 to be moving: verv little indeed. 



observations were begun with the most 

 accurate instruments that had then ever 

 been constructed in order to see if any 

 such dis])lacement could be found, but 

 apparent!}' no trace of it existed. The 

 reason is that the stars are so very far 

 away that the greatest displacement of 

 even the nearest of them is quite too 

 small to have been detected with these 



Fi^. 3. The Mi-ridian Circle in the United States Naval Observatory. Washington. An 

 instrument for determining^ the positions of stars on the sky with higrh accuraey. 



In exactly the same way, if the earth 

 is really passing around a circular path, 

 any star should seem to be moving in 

 direction exactly opposite to that in 

 which the earth is moving at the instant 

 of observation. A little reflection will 

 show the reader that the total effect of 

 this apparent motion of the star will 

 be to cause it to apjiarently describe 

 a small closed jiath in the heavens, 

 completing its circuit alxjut this p;ilh 

 in exactly one year. 



Three centuries aeo most careful 



earl}- instruments. In fact, very nearly 

 all of the stars are so distant that even 

 in our most accurate instruments they 

 seem not to Idc displaced in the slightest 

 degree while we on the earth change 

 our position 186,000,000 of miles in 

 going from one side of our sun to the 

 other. If in the course of all this jour- 

 ney a star is displaced in its position 

 on the sk}- b}- the thickness of the finest 

 spider thread, we can measure the dis- 

 placement and so lind the distance 

 awav of the star. This work illustrates 



