TidesNearly all marine coastlines experience the rhythmic rise and fall of sea level called tides. The daily oscillation in ocean level is a product of the gravitational attraction of the Moon and Sun on Earth's oceans and varies in degree worldwide. Tidal action is an important force behind coastal erosion and deposition as the shoreline migrates landward and seaward. Causes of TidesThe gravitational attraction of the Sun is about half that of the of the Moon on the Earth. Gravitational attraction is a function of both the mass of the objects and the distance between them. Even though the Moon is much smaller in mass than the Sun it is closer and thus has a greater influence on the Earth than does the Sun. The gravitational pull of the Moon and the Sun stretches both solid and fluid surfaces of the Earth. This creates a tidal bulge in the atmosphere, the oceans and to a very slight extent the Earth's crust.
Figure OC. 7 The two forces that result in tides Gravity is not the only force responsible for a tidal bulge. Inertia, the tendency of moving objects to continue moving in a straight line or stay motionless, also affects the tidal bulge. As the gravitational force draws water closer to the Moon the inertial force tries to keep it in place. The tidal bulge forms as the gravitation force exceeds the inertial force on the near side. The gravitational force of the far (opposite) side is less because it is farther away from the Moon. On this side the inertial force exceeds the gravitational force. Here the water attempts to keep going in a straight line, moving away from the Earth, creating another, smaller bulge. Thus tidal bulge, is greatest on the side of the Earth facing the Moon or Sun ("near side") simply because it's closer than the "far side" of the Earth. Tidal CurrentsWatching the tide "come in" one gets the impression that ocean water is moving in and out along the shoreline. The landward and seaward movement are a result the Earth rotating into and out of a semi-fixed tidal bulge as it changes its position relative to the Sun and Moon. Any point on Earth rotates through two bulges every 24 hours and 50 minutes producing two high tides and two low tides called each day. The difference in height between consecutive high and low tides is the tidal range. During a high tide water moves landward as a flood current. During low tide water recedes seaward as an ebb current. The two high tides and the two low tides do not have to be of equal height because the angle between the Moon and Earth changes each day. The tidal range is the difference in height between high and low tide Figure OC.8 Flood and Ebb Currents
Spring and Neap Tides
Figure OC.9 Spring and neap tides.
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The
Sun and Moon are said to be in conjunction with they are aligned
with the Earth. The highest tides, and greatest gravitational
attraction, occur when the Sun and Moon are on the same side of the
Earth creating a spring tide. This occurs during the new moon
phase. A second lower spring tide occurs when the Sun and Moon are on
opposite sides of the Earth during the full Moon phase. A neap tide
results when the Sun and moon are at right angles to the Earth.
There are two neap tides, at the first-quarter and second quarter phases
of the Moon.