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.


II LUNAR TIDES
The Moon, being much nearer to the Earth than the Sun, is the principal cause of tides. When the Moon is directly over a given point on the surface of the Earth, it exerts a powerful pull on the water, which therefore rises above its normal level. Water covering the part of the Earth farthest from the Moon is also subject to this pull, so that another distinct dome of water is formed on the farther side of the Earth providing the basis for a second wave. The lunar wave crest directly beneath the Moon is called direct tide, and the crest on the side of the Earth diametrically opposite is called opposite tide. At both crests, the condition known as high water prevails, while along the circumference of the Earth perpendicular to the direct-opposite tidal axis, phases of low water occur.
Low and high waters alternate in a continuous cycle. The variations that naturally occur in the level between successive high water and low water are referred to as the range of the tide. At most shores throughout the world, two high waters and two low waters occur every lunar day, the average length of a lunar day being 24 hr, 50 min, and 28 sec. One of these high waters is caused by the direct-tide crest and the other by the opposite-tide crest. Two successive high waters or low waters 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.



III SOLAR TIDES
The Sun likewise gives rise to two oppositely situated wave crests, but because the Sun is far from the Earth, its tide-raising force is only about 46 per cent that of the Moon. The sum of the forces exerted by the Moon and Sun result in a wave consisting of two crests, 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 waves coincide. This results in the condition known as spring tides, in which the high water is higher and the low water lower than usual. When the Moon is in its first or third quarter, however, it is at right angles to the Sun relative to the Earth, and the height of the waves is subject to the opposing forces of the Sun and Moon. This condition produces neap tides, in which the high water is lower, and the low water higher, than normal. Spring and neap tides occur about 60 hr 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 water 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 water. Average values for the high-water lunitidal intervals during periods of new and full moon are known as the establishment of a port. Values for the intervals during other periods of the month are often referred to as the corrected establishment.



IV TIDAL CURRENTS AND WAVES
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. In confined areas, a tidal current flows for about 6 hr, 12 min in an upstream or shoreward direction, corresponding to high water; it then reverses and flows for approximately the same time in the opposite direction, corresponding to low water. During the period of reversal, the water is characterized by a state of rest, or calm, known as slack water. A current flowing towards 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 on the open sea gigantic waves, called tsunamis or tidal waves, strike the surrounding shore with tremendous force and cause considerable damage to life and property. These waves are not caused by natural tidal phenomena, but by earthquakes, midocean volcanic eruptions, and serious atmospheric disturbances.
Another related phenomenon is the seiche, which usually occurs in landlocked seas or lakes, such as San Francisco Bay in California and Lake Geneva in Switzerland. The water surface is observed to oscillate between a few centimetres and several metres mainly because of 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 estuary of the River Rance in north-western 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 high 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.