Competitive Exams: Hydrosphere
The uppermost surface of our earth is constituted partly of land and partly of water. The water mass or the envelope of water which covers, a great part to the surface of the earth is called the Hydrosphere. It is made up of oceans and seas, which occupy nearly 71 per cent of the whole surface of the world. The bigger ones are called oceans and the smaller ones seas.
Tides: Sea level rises and falls twice a day. This rising and falling of the surface of the sea water is called the Tide. At high tide the water becomes much deeper and even very large vessels can go far up, they are partly carried up by the strong tidal current. Even big ships often wait for the tide to carry them up or down the harbour. On account of the force called gravitation, everything in the universe attracts the earth, and, in its own tum, the earth attracts them. This mutual attraction is gravitation. Since the moon is nearest to the earth, the earth is more affected by its pull, but since the earth is very heavy in comparison to the moon; its effect is not felt so much on land. The pull of the earth, however, exerts a greater effect on earth. It is the moon trying to draw the water of the ocean towards itself which causes the tide. This pull is the greatest when the moon is directly over the place, i.e.. when it is on the meridian.
Spring Tides and Neap Tides: When the sun, moon and the earth are nearly in the same line at the full and the new moon, “Spring Tides” occur. Here the gravitational effects of the sun and the moon combine to produce tides which 1m higher than usual. Contrary to this, when the pull of the moon is at right angle to the pull of the solar and lunar effects. They neutralize each other. Hence high tides and low tides occur lower than usual. Such tides are called “Neap Tides” Spring tides and neap tides occurs every alternate week.