Competitive Exams: Oceanography Tides geography notes on ocean tides and types of tides

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What Are Tides? What Causes Tides?

Ocean Tides

The rise and fall of sea water due to gravitational forces, centripetal of the Sun and Moon are called tides. The sea waves generated by the tides are called Tidal Waves.

The earth rotates from west to east and revolves around the sun following an elliptical orbit. Similarly, the moon rotates from west to east and revolves around the earth following an elliptical orbit. The period of the farthest distance between the moon and the earth (4,07,000 km) is called 'Apogee'. While the period of the nearest distance (3,56,000 km) is called 'Perigee'.

The surface of the earth with its diameter of 12,800 km (8000 miles) is 6400 km nearer to the moon then its centre

The centre of the moon is 3,84,800 km (2,40,000 miles) away from the centre of the Earth. The earth's outer surface is 3,77,600 km away from the outer surface of the moon.

Therefore the gravitational force of the moon will be maximum at the earth's surface facing the moon, while it will be minimum at the opposite side of the earth.

Consequently the water of the earth's surface facing the moon is attracted and pulled and high tides occur. At the same time low tide is formed at the opposite side of the Earth.

Therefore, two tides and Ebbs are experienced twice at every place on earth's water surface within 24 hours.

The moon. Thus the tide centre takes 24 hours 52 min. to come under the moon.

The average difference in water level between high and low tides at any place is called 'AMPLITUDE' of the tide.

The tidal range is generally 13 m. In the Mediterranean and Baltic, the range is very small but highest tide is experienced in the Bay of Fundy (East Canada), the tide may rise 1520m

Types of Tides

  1. Spring Tides: The spring tides are the highest when the moon, earth and the sun are in the straight line. They occur at new and full moon; especially on 1 Jan it is highest. The straight line is called 'Syzygy', and this position is called 'Conjuction'. When the position of earth is in between sun and moon it is called 'Opposition'.

  2. Neap Tides: The position is quadrature i.e. sun, earth and moon are in a position of right angle. They occur at the moon's first and third quarter i.e. on the 7th and 8th day of every fortnight and the direct force is produced by the sun and the Working in opposite direction and thus low tide is formed. The height neap tide is 20% lower than general tides.

  3. Tropical and Equatorial Tides: like the sun, there is also northward and southward position of the moon in relation to the equator of the earth. If the sun completes its northward and southward position in one year (nearly 365 days), the moon completes it in 29.5 days (1 synodic month). When there is maximum declination of the moon to the north of the equator, the moon's rays fall vertically on the tide centres (near the Tropic of Cancer) hence spring tides are caused. Such tropical tides move westward along the Tropic of Cancer and also occur along the Tropic of Capricorn which is opposite to the Tropic of Cancer. The tides occurring along the Tropics of Cancer and Capricorn are called the 'Tropical Tides'.

  4. Apogean and Perigean tides.

  5. Daily and Semidiurnal Tides.

  6. Equinocital tide.

Theories Related to Origin of Tides

  1. Equilibrium Theory by Newton.

  2. Progressive Way Theory by William Whewell.

  3. Canal Theory by G.B.Airy.

  4. Stationary Wave Theory by R.A.Herish.

  5. latitudes.