DIURNAL REVOLUTION OF THE EARTH. 31 



the letters A B, after an interval of 6 hours, the spectator would 



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, 









* 



,. 



******** 



arrive under B, and perceive the star B directly overhead while the 

 star A would be just ready to sink below the horizon. After an 

 interval of 18 hours more he would again arrive under A, having 

 performed a complete revolution. Now as all the stars are 

 observed to have a perfectly uniform motion, moving once around 

 the earth in 24 hours, never changing their apparent positions 

 with regard to each other, doubtless this supposition appeared to 

 Copernicus the most rational, and its truth is now incontestably 

 proved, and universally admitted. The great motion of the 

 heavens being thus shown not to be real, but only apparent, 

 Copernicus naturally endeavored to ascertain how far certain other 

 motions, which the followers of Ptolemy explained by innumera- 

 ble cycles, and crystalline spheres, as if all their observed motions 

 were real, might be explained by a movement of our earth 

 instead of these bodies. The actual size of the sun and planets, 

 as also their actual distance from the earth, not being known at 

 that time, rendered this problem more difficult, and beside this, 

 he was wholly unacquainted with the laws of gravitation. Hence 

 it was no ordinary effort of mind to reduce the various compli- 

 cated motions of the planets and the sun to one harmonious 

 system. Pythagoras, the celebrated Greek philosopher who lived 

 500 years before Copernicus, had already suggested the idea that 

 the sun was the central body, and that the earth and planets were 

 revolving about the sun at various distances. He did not attempt 



