414 ASTRONOMY. 



see the heavenly bodies move. The sky is not a 

 real substance ; the blue colour is only owing to 

 the refraction of the rays of light which pass 

 through the atmosphere. 



On considering with attention, for one or more 

 nights, the motions of the stars, you find each star 

 describing a circle in about twenty-four hours. 

 Those stars that appear northward describe smaller 

 circles than those that are more to the south. If 

 you look towards the south, you observe some 

 stars just appearing above the horizon, grazing 

 this circle, but not rising above it, and then 

 vanishing ; others, a little farther from the south, 

 rise above the horizon, making a small arc, and 

 then go down ; while some again describe a larger 

 arc, and take a longer time in setting. If you now 

 turn to the north, you will find that some just skim 

 the horizon, mount to the top of the heavens, and 

 then descend, and again touch the horizon and 

 mount, without ever disappearing. Others, that 

 are higher, describe complete circles in the sky, 

 without coming to the horizon ; and these circles 

 diminish, till at last we arrive at a star that scarce 

 seems to move from the point where it is stationed, 

 the rest wheeling round it. 



It may be easily conceived, that as there is a 

 hemisphere above, there is also another beneath, 

 though invisible ; and that, of course, the horizon 

 is a great circle of the sphere, dividing the concave 

 heavens into two parts, the visible above, and the 

 invisible below. The general appearance, there- 

 fore, of the starry heavens, is that of a vast 

 concave sphere turning round two fixed points 

 diametrically opposite to each other; the one in 

 the northern hemisphere visible to us, and the 

 other in the southern hemisphere. 



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