WHY HIGH TIDE VARIES DAILY.
The moon travels round the earth once in twenty-eight days. In Fig. 231 the point a is nearest the moon at, say, twelve noon. At the end of twenty-four hours it will have arrived at the same position by the compass, but yet not be nearest to the moon, which has in that period moved on 1⁄28th of a revolution round the earth.43 Consequently high tide will not occur till a has reached position b and overtaken the moon, as it were, which takes about an hour on the average. This explains why high tide occurs at intervals of more than twelve hours.