GENERAL CHARACTERISTICS 



13 



earth, though that of the moon is considerably greater than that 

 of the sun ; hence, if the sun, earth, and moon are in a straight 

 line, as they are when the moon is full, at which tune she and the 

 sun are on opposite sides of the earth, and also when new, at which 

 time she is between the earth and sun, the sun's tide is added to 

 the moon's tide, thus producing the well-marked spring tides ; 

 while, when the moon is in her quarters, occupying a position at 

 right angles from the sun as viewed from the earth, the two bodies 

 tend to produce high tides on different parts of the earth at the 

 same time, and thus we have the moon's greater tides reduced by 



FIG. 11. NEAP TIDES 



the amount of the lesser tides of the sun, with the result that the 

 difference between high and low tides is much lessened. 



Again, the difference between high and low water marks is not 

 always exactly the same for the same kind of tide the spring tide 

 for a certain period, for example, not having the same limits as the 

 same tide of another time. This is due to the fact that the moon 

 revolves round the sun in an elliptical orbit, while the earth, at the 

 same time, revolves round the sun in a similar path, so that the 

 distances of both moon and sun from the earth vary at different 

 times, And, since the tide-producing influences of both these 



