18 



EFFECT OF ROTATION OF THE EARTH AND THE MOVEMENT OF THE MOON 

 AND THE EARTH IN THEIR ORBITS 



33. The effect of the rotation of the earth. — As shown in paragraph 25, 

 the lunar and solar tide-producing forces each create two areas of high 

 potential on the surface of the earth, one facing the moon, or sun, 

 and the other opposite. As the earth spins around its axis, these 

 areas make the circuit of the earth and set up the slight oscillations of 

 the oceans which make the tides. The rise and fall of the actual tide 

 at any locality, and the times of high water and low water, depend on 

 the conformation of the ocean shores and beds and on the momentum 

 of the water masses as well as on the tide-producing potential. The 

 equilibrium tide affords a measure of the effect of tide-producing 

 potentials alone. The variations of equilibrium tides resulting from 

 the movements of the moon and earth in their orbits indicate the 

 variations to be expected in the actual tides because of these move- 

 ments. 



34. Efied of the declination of the moon and sun. — The surface of 

 equiUbrium of the oceans due to the lunar tide-producing potential 



Figure 9. 



Figure 10. 



Effect of moon's declination on tides. 



has been shown to be a prolate spheroid, with its axis pointing to the 

 moon (par. 26). When the moon is in the plane of the earth's equator, 

 as shown in figure 9, the equilibrium tide at any point P on the earth's 

 surface quite evidently goes through two equal fluctuations during 

 one rotation of the earth around its axis NS, as measured from the 

 position of the moon; i. e., two equal lunar equilibrium tides then 

 occur each lunar day. At the earth's equator the range of these two 

 tides is 1.75 feet when the moon is at its mean distance from the earth 

 (par. 30), this range decreasing with the latitude of the tidal station. 

 When, on the other hand, the moon is above or below the plane of the 

 earth's equator (fig. 10) the two daily fluctuations of the lunar equi- 

 librium tide quite obviously are unequal, except on the earth's equator, 

 the inequality depending on the latitude of the tidal station, and the 



