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Get the answer of: What causes the tide to rise and fall ?
A watch along the coast will show that the sea water not always remain at the same level. The water level swells and rises and then falls or subsides again in the course of a day. This phenomenon is called tide. This rise and fall of water level can be observed easily in steep banked coastal areas. The water of the oceans and seas thus never maintains a constant level.
The swelling and rise of sea water level is called high-tide and the fall or subsidence of water level is called ebb-tide. It takes some 12 hours from the peak of one high tide to reach the peak of the next high tide. In the same way, there is a gap of about 12 hours between the lowest limit to which the water level subsides at ebb-tide to the lowest limit in the next ebb-tide.
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In other words, from the lowest level to which the water subsides at ebb-tide, it takes about 6 hours for the water to swell to the highest level of the following high-tide. The limit of rise and fall of water level due to tides is less than 1 metre at mid-seas.
But, such fluctuations of water level are more marked in coastal areas where the sea is shallow. At Okha in Russia, the water level swells to a height of 2.5 metres. The highest and most spectacular swelling of a high-tide, ranging from 15-18 metres, is observed in the Bay of Fundy near Novascotia.
There are two main reasons which cause high and ebb-tides:
(1) The attraction of the sun and the moon on earth, and
(2) The centrifugal force caused by the rotation of the earth.
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Though the sun is vastly bigger than the moon, it is the moon which exerts greater attraction on the earth. This is because the moon is much nearer to the earth than the sun.
During the earth’s rotation high tides occur in the parts which come closest to the moon and in parts which lie in the due opposite direction (i.e., at the antipodal parts). Between these two, there are ebb-tides in the two intermediate parts.
Hence, in the course of 24 hours two high and two ebb-tides occur at the same place. Therefore, at any place in the sea, it takes about 6 hours for the water to swell to the highest level at high-tide and subside to the lowest at ebbtide.
Though tides occur twice during a day, yet the time interval between two high-tides is not exactly 12 hours. The exact interval is 12 hours 25½ minutes. The moon moves from west to east and revolves round the earth, once in every 29½ days. The moon, therefore, never crosses over a meridian precisely at 24 hours interval. This is because in 24 hours the moon itself advances a little to the east on its orbit.
As a result of this forward movement of the moon, the time interval between two consecutive crossings by the moon over a particular meridian is 24 hours 51 minutes. Hence, it takes 12 hours 25½ minutes from the occurrence of one high-tide to the next high-tide.
As shown in Fig. 5.9 suppose, the moon is over the meridian at place ‘A’. When after a complete rotation, Place ‘A’ comes back to its previous position, the moon will in the mean time move from place ‘P’ to place ‘Q’. The moon, therefore, will be over the meridian at place ‘B’. Place ‘A’ will come directly below the moon only after 51 minutes.
On the new moon day, the sun and the moon are on the same side of the earth. Hence, on that day, the same portion of the earth is attracted by both the sun and the moon.
As a result, the primary tide, which is caused in that portion of the earth due to the combined attraction of the sun and the moon, shows the highest rise of water level. The secondary tide, caused on the opposite side by combined influence of the sun and the moon, shows a relatively higher rise of water level than on other days. This tide is known as the spring-tide of the new moon.
On a full moon day, the sun and the moon are positioned on opposite sides of the earth. Hence, on that day, the portion of the earth which faces the moon shows a greater rise of water-level than on other days, under the influence of the primary tide of the moon and of the secondary tide of the sun.
Again on the opposite side of the earth, the water-level rises higher than on ordinary days under the influence of the primary tide of the sun and of the secondary tide of the moon. This high-tide is called Spring-Tide of the full moon.
On the seventh and eighth days of the moon, the sun and the moon are positioned at right angles to the earth. As a result, there ought to be ebb-tide at ‘B’ which is located at the angle of 90° from A’ and where there is primary tide caused by the moon.
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But, there is really near- primary tide at ‘B’ due to the influence of the sun’s attraction on this portion of the earth. In effect, due to the tendency for high-tides at both ‘A’ and ‘B’ the water-level does not rise conspicuously at any of these places. This tide on the seventh and eight days of the moon is known as neap-tide.
The earth rotates from the west to the east. As a result, the tides on the sea surface occur successively from east to west. This makes it appear as if the sea water is flowing from east to west during a tide. At high tide, the tidal water enters a river through the estuary with great force. This is known as tidal bore.
In rivers like Hooghly, Salween, Siene, Garonne, Severn and Amazon, the tidal water enter and rush swiftly up the rivers with great force. Such tidal bores are at times dangerous to life and property, because sea water rushes up the rivers with great speed.
But, high-tides also help in navigating ocean going liners to a considerable distance up rivers due to the rise of water level in the estuaries. Again, ebb-tides help in keeping the river mouths free of debris which are carried away to the sea.
The river can never be blocked by silt deposition. The sea water entering a river makes its water a little saline and this prevents quick freezing of the water in extreme cold weather. Thus, tides are of help to mankind in a number of ways.