468 GENERAL SCIENCE 



tides, but when the sun and moon are pulling at right angles, the 

 tides are called neap tides. The moon pulls the water with greater 

 force than the sun because the moon is 400 times nearer the earth 

 than the sun. 



The Earth. The earth is one of the small planets, and, because 

 of its rapid motion when it was in a molten mass, it became flattened 

 at the north and south poles. Such a body is called an oblate 

 spheroid. The diameter of the earth from pole to pole is 26 miles 

 shorter than through the earth at the equator. The earth's diameter 

 is about 8000 miles and its circumference 25,000 miles. The cur- 

 vature of the earth amounts to about 8 inches per mile. The amount 



FIG. 357. The broadening horizon seen from different elevations. 



of curvature increases as the square of the distance multiplied by 

 eight. For example, for 2 miles the curvature of the earth would 

 be 2 2 X8 in. equals 32 in.; for 3 miles, 3 2 X8 in. equals 72 in.; for 

 4 miles, 4 2 X8 in. equals 128 in., etc. 



The earth turns on its axis once in 24 hours; however, we all 

 know that day and night are not of the same length at all times of 

 the year. The difference in the length of the day is caused by the 

 inclination of the axis of the earth. 



The north pole has a night lasting about six months, and a day 

 equally long, as will be seen by one of the diagrams. The earth 

 travels around the sun in an orbit once every year of 365J days. 

 Because of this extra fourth of a day it is necessary to add one day 

 to every fourth year. We call this year a leap year. The true 

 length of the year is 365 days, 5 hours, 48 minutes and 46 seconds. 



