XV. THE EARTH AS A PLANET 



DAY AND NIGHT, AND THE EARTH'S ROTATION 



It is not strange that the ancients believed the earth was 

 the center around which the sun, moon, and stars revolved, 

 or that from the time of Ptolemy (A. D. 100-170) to Coper- 

 nicus (1473-1543) it was taught that the earth was the 

 centre of the universe. In these days we believe that the 

 earth's rotation accounts for the daily round of the heavenly 

 bodies as they rise, pass meridian, and set. But it is neces- 

 sary to make a more or less conscious effort really to think 

 in terms of the Copernican rather than of the Ptolemaic 

 teaching. The very language we use tends to preserve the 

 older views. We say that the sun sets, the moon rises, etc., 

 and we fail to devise any better expression of our meaning. 

 It is not wholly a misfortune, however, that so much of 

 mental effort has to be made to distinguish between what is 

 real and what is apparent in the phenomena occasioned by 

 the earth's rotation. In the study of Astronomy there are 

 ever recurring demands actually to "see" (to visualize) mo- 

 tions of celestial bodies in space that are similar to those of the 

 earth. The development of an imagination based upon 

 known realities, and of a power to grasp abstract mathe- 

 matical notions, is of greatest educational importance. 



If the earth as a spherical body were to become fixed in 

 space (to stand still), then for an observer at any one place 

 all objects above his horizon would remain in sight all the 

 time, and objects below that horizon would always be hidden. 

 In case the observer were to travel about, his horizon would 

 change with the place of observation. New bodies would 

 come into view as his horizon dipped below them in whatever 



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