31. What causes the tides? 



Tides are caused by gravitational forces of the moon, the sun, and 

 various other celestial bodies. The moon, being nearest, has the greatest 

 effect. The sun, despite its greater mass, exerts only a secondary effect, 

 less than half that of the moon. 



High tides are generated on the sides of the earth nearest to and 

 farthest from the moon. At times of new moon and full moon, the 

 sun's attraction reinforces that of the moon, producing higher (spring) 

 tides. Halfway between new and full moon, solar attraction does not 

 coincide with lunar attraction and therefore the difference between 

 high and low tides is less; these lesser tides are called neap tides. 



When the moon is over the Equator as the earth rotates, a point on 

 the earth passes through two high and two low areas each day. When 

 the moon is north or south of the Equator, the two highs are unequal in 

 height or there may be only one high tide. 



LOW TIDE 



LOW TIDE 



Engel, Leonard and Editors of LIFE 



The Sea, Life Nature Library, Time, Inc., 1961. 

 Gaskell, T. F. 



World Beneath the Oceans, American Museum of Natural History, 



1964. 

 Stewart, Harris B., Jr. 



Deep Challenge, Van Nostrand, 1966. 



33 



