350 GENERAL SCIENCE 



direction he advanced, while others at the same time would 

 be passing out of sight below the opposite horizon (the direc- 

 tion from which the observer is moving). If, on the other 

 hand, an observer remains at the same location, and is 

 carried round and round by reason of rotation of the earth, 

 the same phenomena of the rising and setting of celestial 

 bodies is experienced. 



Why the rate of the earth's rotation is what it is must 

 remain unknown. But this rate is uniform, and its period 

 for one rotation is the natural time unit known as a day. 

 The rate of rotation so far as known varies for the different 

 planets, and their day periods differ somewhat from ours. 



The whole of the earth's surface moves on together in its 

 motion around the axis. Those portions at and near the 

 equator, which have a much longer circumference to be 

 carried through in the same time of rotation, must have a 

 correspondingly greater eastward velocity. Near the poles 

 the number of miles through which any position on the 

 surface must be carried to make a completed round of motion 

 is less, and the eastward velocity there of the earth's surface 

 is relatively small. 



Because of the earth's spherical form, as an observer travels 

 northward his northern horizon dips lower and lower. The 

 North Star, which at the equator is approximately upon the 

 horizon, seems to rise higher and higher as he advances 

 northward. Its altitude (distance above horizon) is for him 

 all the time the same as his distance from the equator (his 

 latitude). Both the altitude of the star, and the latitude 

 of the observer, are expressed in degrees. 



SUMMARY 



The rising and setting of the sun, moon, and stars is a direct result 

 of the rotation of the earth. As the earth in its rotation carries us as 

 observers around on its surface, our eastern horizon passes star after 



