9, io] The Daily Motion of the Celestial Sphere 9 



same time of night. Rather more careful observation, 

 carried out for a considerable time, is necessary in order 

 to see that the aspect of the sky changes in a regular way 

 from night to night, and that after the lapse of a year the 

 same stars become again visible at the same time. The 

 explanation of these changes as due to the motion of 

 the sun on the celestial sphere is more difficult, and the 



FIG. 3. The circles orthe celestial sphere. 



unknown discoverer of this fact certainly made one of 

 ihe most important steps in early astronomy. 



If an observer notices soon after sunset a star somewhere 

 in the west, and looks for it again a few evenings later at 

 about the same time, he finds it lower down and nearer to 

 the sun ; a few evenings later still it is invisible, while its 

 place has now been taken by some other star which was at 

 first farther east in the sky. This star can in turn be 

 observed to approach the sun evening by evening. Or if 

 the stars visible after sunset low down in the east are 



