Every drop of water in the ocean, even in the deepest parts, responds
to the forces that create the tides. No other force that affects the sea is
so strong. Compared with the tides, the waves created by the wind are
surface movements felt no more than a hundred fathoms below the
surface. The currents also seldom involve more than the upper several
hundred fathoms despite their impressive sweep.
The tides are a response of the waters of the ocean to the pull of the
Moon and the more distant Sun. In theory, there is a gravitational
attraction between the water and even the outermost star of the
universe. In reality, however, the pull of remote stars is so slight as to
be obliterated by the control of the Moon and, to a lesser extent, the
Sun.
Just as the Moon rises later each day by fifty minutes, on the
average, so, in most places, the time of high tide is correspondingly
later each day. And as the Moon waxes and wanes in its monthly cycle,
so the height of the tide varies. The tidal movements are strongest
when the Moon is a sliver in the sky, and when it is full. These are the
highest flood tides and the lowest ebb tides of the lunar month and are
called the spring tides. At these times the Sun, Moon, and Earth are
nearly in line and the pull of the two heavenly bodies is added together
to bring the water high on the beaches, to send its surf upward against
the sea cliffs, and to draw a high tide into the harbors. Twice each
month, at the quarters of the Moon, when the Sun, Moon and Earth lie
at the apexes of a triangular configuration and the pull of the Sun and
Moon are opposed, the moderate tidal movements called neap tides
occur. Then the difference between high and low water is less than at
any other time during the month.