The giant water waves which rise and fall periodically in the sea are called tides. Sometimes these tides are so high that they appear like high walls moving on the surface of the sea. What causes these tides in the sea?
The main cause behind the formation of these tides in the sea is the force of attraction between the earth and the moon. It is well known that all the bodies in the universe attract one another with a force known as gravitational force. This force also acts between the earth and the moon. The moon is pulled towards the earth and the earth, towards the moon. The force of gravitation acting between the earth and the moon is so strong that it can easily break a steel rod with a diameter of 400 kilometers. This huge force affects the solid part of the earth slightly; not so much as to bend or crack its crust, but it causes rise and fall in the seawater noticeably.
At any point in the sea, the water level keeps on rising for six hours, and subsequently, for the next six hours, it keeps on falling. Thus rise and fall of the water level in the sea is known as tide. The water level rises every 12 hours 25 minutes. This means that during one day and one night, that is 24 hours and 50 minutes, every place in the sea experiences tides two times. Tides are formed both in the seas facing the moon, as well as those on the other side. Since the moon comes to the opposite side of the earth after 12 hours and 25 minutes, tides are again formed on the same two spots.
The force of attraction between the sun and the earth also affects the formation of tides, but only to a very small extent. When the sun, earth and moon come in a straight line, the earth experiences the maximum force of attraction. And the tide formed on such occasions is very large. It is known as the spring tide. The sea experiences such tides on the full moon and the new moon days. When the sun and moon are in perpendicular direction, the earth experiences the minimum force of gravitation. The tide formed on such occasions is very small and is called the neap tide. Such tides occur between the full moon and the new moon days.
The tides can be as high as 15 meters. Tides are useful in many ways. They bring many precious items of the sea on to the shore, such as corals, pearls and mineral deposits from the deep. The tidal power of the sea is also being used to generate electricity. This method involves capturing the energy of the flowing tides during ebbing(retreating) and surging(rising). Tidal energy generators which are basically large underwater turbines are kept in highly tide-prone areas where there is a frequent flow of water during the rise and fall of tides. The constant movement of water causes these turbines to spin, which in turn generate electric power. Tidal energy, though rarerly used is considered to be one of the most potential renewable energy resources on the earth.
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