Fig. 9.—Spring Tides at Full Moon
Here again we find that the changes exactly coincide with changes in the position of the moon with regard to the sun and the earth. Thus, the spring tides—those which rise very high and fall very low—always occur when the moon is full or new; while the less vigorous neap tides occur when the moon is in her quarters and presents only one-half of her illuminated disc to the earth. And, as the moon passes through a complete cycle of changes from new to first-quarter, full, last-quarter, and then to new again in about twenty-nine days, so the tides run through four changes from spring to neap, spring, neap, and then to spring again in the same period.
Fig. 10.—Spring Tides at New Moon
The reason for this is not far to seek, for we have already seen that both sun and moon exert a tide-producing influence on the 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 time 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 the amount of the lesser tides of the sun, with the result that the difference between high and low tides is much lessened.
Fig. 11.—Neap Tides
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 bodies must increase as their distance from the earth diminishes, it follows that there must be occasional appreciable variations in the vigour of the tidal movements of the ocean.