these two bulges is a trough girdling the earth and connect- 

 ing the poles. The two high tides are most easily under- 

 stood by considering the forces acting on the earth. At any 

 point there is an attractive force to the center of the 

 earth that depends on the mass of the moon and inversely as 

 the square of the distance. If that was the only force, 

 the earth and moon would be drawn together. There is also 

 a centrifugal force acting in the opposite direction along 

 the line of centers of the moon and the earth. The amount 

 of this centrifugal force is equal and opposite to the 

 gravitational attraction but, unlike the latter, is equal 

 and in the same direction for all points on the earth. 

 Therefore, although the vectorial sum of the two forces at 

 the center of the earth is zero, along the line of centers 

 there is a net force toward the moon on the earth's surface 

 directly under the moon, and a net force away from the moon 

 on the opposite side. Calculations show that this difference 

 (tide-producing) force varies inversely as the cube of the 

 distance to the center of the moon. 



Although the above simplified explanation implies a 

 vertical lifting of the sea surface, actually the high tide 

 is formed by a very small horizontal component of the vector 

 for all points not on the line of centers which is unopposed 

 whereas the lifting force is opposed by the gravitational 

 pull toward the center of the earth. 



26-4 



