Ocean Tides

a diurnal tidal pattern.

and 2 high tides and 2 low tides in one day is semi-diurnal.
Neap tide
a high tide is where the water is really high and a low tide is where the water is low and you have to be carfull
The highest of the high tide is called a spring tide and the lowest of the low tide is called a neap tide.

I.INTRODUCTION

Tide, periodic rise and fall of all ocean waters, including those of open sea, gulfs, and bays, resulting from the gravitational attraction of the moon and the sun upon the water and upon the earth itself. See Gravitation.

II.LUNAR TIDES

The moon, being much nearer to the earth than the sun, is the principal cause of tides. Because the force of gravity decreases with distance, the moon exerts a stronger gravitational pull on the side of the earth that is closer to it and a weaker pull on the side farther from it. The earth does not respond to this variation in strength because the planet is rigid instead, it moves in response to the average of the moon's gravitational attraction. The world's oceans, however, are liquid and can flow in response to the variation in the moon's pull. On the side of the earth facing the moon, the moon's stronger pull makes water flow toward it, causing a dome of water to rise on the earth's surface directly below the moon. On the side of the earth facing away from the moon, the moon's pull on the oceans is weakest. The water's inertia, or its tendency to keep traveling in the same direction, makes it want to fly off the earth instead of rotate with the planet. The moon's weaker pull does not compensate as much for the water's inertia on the far side, so another dome of water rises on this side of the earth. The dome of water directly beneath the moon is called direct tide, and the dome of water on the opposite side of the earth is called opposite tide.

As the earth rotates throughout the day, the domes of water remain aligned with the moon and travel around the globe. When a dome of water passes a place on the earth, that place experiences a rise in the level of the ocean water, known as high tide or high water. Between successive high tides the water level drops. The lowest water level reached between successive high tides is known as low tide or low water. Low and high tides alternate in a continuous cycle. The variations that naturally occur in the level between successive high tide and low tide are referred to as the range of tide. At most shores throughout the world, two high tides and two low tides occur every lunar day, the average length of a lunar day being 24 hours, 50 minutes, and 28 seconds. One of these high tides is caused by the direct-tide dome and the other by the opposite-tide dome. Two successive high tides or low tides are generally of about the same height. At various places outside the Atlantic Ocean, however, these heights vary considerably; this phenomenon, which is known as diurnal inequality, is not completely understood at the present time.

III.SOLAR TIDES

Tides

The sun likewise gives rise to two oppositely situated domes, but because the sun is far from the earth, its tide-raising force is only about 46 percent that of the moon. The sum of the forces exerted by the moon and sun result in two sets of domes, the positions of which depend on the relative positions of the sun and moon at the time. During the periods of new and full moon, when the sun, moon, and earth are directly in line, the solar and lunar domes coincide. This results in the condition known as spring tides, in which the high tide is higher and the low tide is lower than usual. When the moon is in first or in third quarter, however, it is at right angles to the sun relative to the earth, and the height of the tides is subject to the opposing forces of the sun and moon. This condition produces neap tides, in which the high tide is lower, and the low tide is higher, than normal. Spring and neap tides occur about 60 hours after the corresponding phases of the moon, the intervening period of time being known as the age of the tide or age of the phase inequality. The interval of time between the crossing of a meridian by the moon at one point and the next high tide at that point is called the lunitidal interval, or the high-water interval for that point. The low-water interval is the period between the time the moon crosses the meridian and the next low tide. Average values for the high-water lunitidal intervals during periods of new and full moon are known as the establishment of the port. Values for the intervals during other periods of the month are often referred to as the corrected establishment.

IV.TIDAL CURRENTS AND WAVES

Tidal Pool

Tidal PoolThe fluctuation of the tide allows for a unique environment along shorelines. The current continually circulates and replenishes a rich supply of nutrients along beaches, but organisms living there must be adapted to both buffeting waves and frequent shifts from open air to complete submersion. Marine organisms adapt to the constantly changing surroundings in a variety of ways. Starfish use suction-cup feet, barnacles fix permanently to large objects like rocks and boats, and seaweed anchors firmly to the ocean floor. When the tide goes out, pockets of water remain trapped in rocks, depressions in the sand, and natural basins called tidal pools, like the one shown here during low tide.Encarta EncyclopediaPat O'Hara Photography

Full Size

Accompanying the vertical rise and fall of water are various horizontal or lateral movements commonly known as tidal currents or tidal streams, which are very different from the common ocean currents (see Ocean and Oceanography). In confined areas, a tidal current flows for about 6 hours, 12 minutes in an upstream or shoreward direction, corresponding to high tide; it then reverses and flows for approximately the same time in the opposite direction, corresponding to low tide. During the period of reversal, the water is characterized by a state of rest, or calm, known as slack water or slack tide. A current flowing toward the shore or upstream is called flood current; that flowing in a direction away from land or downstream is known as ebb current.

At various times gigantic waves strike the surrounding shore with tremendous force and cause considerable damage to life and property. Although sometimes called tidal waves, these waves are not caused by tidal phenomena. Earthquakes, undersea volcanic eruptions, and underwater landslides can cause large waves to wash ashore that are known as tsunamis, while hurricanes can cause a dome of water to wash ashore that is known as a storm surge.

Another related phenomenon is the seiche, which usually occurs in landlocked seas, coves, bays, or lakes, such as San Francisco Bay in California and Lake Geneva in Switzerland. The water surface is observed to oscillate between a few centimeters and several meters mainly because of tidal forces or local variations in atmospheric pressure aided by high winds, but sometimes as a result of a distant seismic shock. The movement of water occurs in long waves and may last from a few minutes to several hours.

V.TIDAL ENERGY

The energy of tides has been harnessed to produce electricity. In the summer of 1966, a tidal power plant with a capacity of 240,000 kw went into operation on the Rance River, an estuary of the English Channel in northwestern France. The incoming tide of the river flows through a dam, driving turbines, and then is trapped behind the dam. When the tide ebbs, the trapped water is released and flows back through the dam, again driving the turbines. Such tidal power plants are most efficient if the difference between high and low tides is great, as in the Rance estuary, where the difference is 8.5 m (28 ft). The highest tides in the world occur in the Bay of Fundy in Canada, where the difference between high and low tide is about 18 m (about 60 ft). The erection of a tidal power plant across Passamaquoddy Bay, an arm of the Bay of Fundy, has long been contemplated; however, the project has not yet been begun.

Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

Another name for incoming tide is the flood tide.

because of the gravitational pull of the moon

it pulls the tide up or push it down

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Not just the moon. The moon along with the sun and the earth's gravitational pull itself.

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no its only the moon!

the moons gravitational pull causes the tides. high tide and low tide. Also each tide occurs twice each day

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the moon pulls the water part of the earth

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if you saw dispicable me then they took the moon away and the surfer fell, so it's just the moon. (even though its a movie)

Tides are caused by the Gravity of the Sun, Earth, and Moon in certain positions.

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I'm going to clear this all up: The tides are caused by the gravitational pull of the moon AND Sun. It says so in my textbook.

= Why Two High Tides a Day? = ---- Long before Newton, it was obvious that the moon exerted a force on the oceans. After all, day in and day out, year after year, ever since the oceans formed, there have been two high tides a day, once with the moon at its zenith and once again nearly 12 hours later. When the concept of universal gravitation was introduced, it quantified the forces involved and neatly accounted for the ocean tides. Today, anyone can tell you that the tides are caused by the "gravity" of the moon. Or are they? The oceans form a thin, incompressible, but easily deformable layer on most of the earth's surface. One might expect that the moon's gravitational force would pull the entire ocean layer towards it, distending the layer (producing a high tide) in the direction of the moon on the near side of the earth, while simultaneously compressing it (producing a low tide) on the opposite side of the earth (see Figure 1, not to scale). This model is half right and half wrong: half of all high tides are generated in exactly this way, but instead of there being a low tide on the far side of the earth, there is also a high tide. There must be some other explanation.

Indeed there is. The motion of the earth-moon system, taken in isolation, is a classical two-body problem where each body exerts an attractive force (gravity) on the other. Solving the equations of motion, one finds that the two bodies rotate about their common centre of mass, like a rigid asymmetric dumbbell spinning around an axis perpendicular to the bar. As shown in Figure 2, the centre of mass of the earth-moon system turns out to be inside the earth, about three-quarters of an earth radius from its center, along the line joining the earth and moon.

The earth, therefore, orbits the center of mass in a tight circle (almost) while the moon orbits in a large one. This orbital motion causes the earth to experience a centrifugal (pseudo-) force, which distends the ocean layer in the direction away from the moon, not unlike what happens to the water in a pail when you swing it in a circle. Combining this orbital effect with the direct gravitational pull of the moon explains the simultaneous high tides on opposite sides of the earth: on the near side the direct pull dominates and causes the oceans to bulge in the direction of the moon; on the far side the centrifugal effect dominates and causes the oceans to bulge in the direction away from the moon. As the earth spins on its axis, a given seaside location will experience a high tide when the moon is at its closest, and then another one about 12 hours later when it is at its furthest. Reality, of course, is never quite so simple. Although the sun is much farther away from the earth than the moon, it is also much more massive, so its gravitational pull on the earth is relatively large (almost half of what the moon exerts). It therefore plays a significant role in determining both the timing and strengths of the tides. So, are the tides caused by the "gravity" of the moon? In large part, yes. But the gravity of the earth is as much responsible for the two-body rotation as that of the moon. The tides are caused by the combined gravitational forces of the moon, earth, and sun. Here is a simpler explanation: Looking down from the North Star, the Earth rotates anticlockwise by about 180 degrees in 12 hours. During that same period of time, the moon travels on its orbit around the earth clockwise by about 180 degrees. The result is that the moon crosses the same earth longitude (but not necessarily the same latitude) every 12 hours, or twice every day. This results in 2 local high tides. The 2 low tides happen between times.