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Ocean Tides


It refers to general rise and fall in the water level of sea which is evident in advance and receding of water on the sea coast.
NEWTON’S EQUILIBRIUM THEORY
Postulates
  • Tides are caused by differential gravitational pull of sun and moon. Pull of moon and sun is in the ratio of 11:5.
  • A bulge is created at the surface lying directly below moon, as gravitational pull of moon is greater than sun resulting in high tide
  • Another bulge is produced on the sea surface lying just opposite side of the earth due to reactionary centrifugal force again resulting in high tide
  • Meridians lying at right angles witness lowering of sea level in order to balance overall volume of water.These are low tides.
  • Thus, at a time earth witness 2 high tides and two low tides. Also every longitude on earth witness two high tides and two low tides in a day.
Criticism
  • Gravitational of sun and moon will not be effective considering that earth is not a complete watery body but large portion is continents.
  • Bulging process in tide formation which shifts with moon position requires horizontal movement of ocean water which has been neglected by Newton. Later he amended to include westward horizontal movement of ocean water in form of wave  with crest lying aways below moon.
  • Contrary to Newton’s theory, in reality all the points on a meridian do not witness tides simultaneously but there is time lag.
  • Time for the tidal movement around the earth will not be equal to what calculated by Newton as there will be friction due to continents, ocean bottom reliefs,etc.
  • Amplitude and magnitude variations
PROGRESSIVE WAVE THEORY
Incorporates following facts
  • Earth is not purely water body but composed of continents too.
  • Two places on same longitudes can have tides at different times
  • Time lag in tides away from source
  • Variation in magnitudes and amplitudes of tide at different places.
  • Tides are in the form of tidal wave which travel from east to west.Crest forms tides and troughs form ebbs.
Postulates
  • Continents are considered to have north south alignment and  due to less continentality of southern hemisphere, tidal pull of moon is more effective there.
  • Tidal waves are generated in southern hemisphere by gravitational pull of moon which travel westward.These are primary waves.
  • Primary waves get obstructed by continents in north and they are diffracted to form secondary or derived waves moving towards north.Further minor waves generated out of these waves.
  • Amplitude and magnitude of these derived waves goes on decreasing as it move towards north also they are not controlled by pull of moon like primary waves.They also lag in time from the source.
Criticism
  • It has been observed that spring tide forms at the same time from cape of good hope to greenland.
  • There is spatial variations as in different seas nature of tides differ.
  • Local phenomenon rather a global phenomenon.
STATIONARY WAVE THEORY
Postulates

  • Not due to progressive wave but Stationary wave that originates in each ocean.So a regional phenomenon.
  • Water tray analogy: Consider a water tray containing water.If external shock is provided then water starts oscillating in the tray along a nodal line.Similarly oceans get shock from gravitation pull of moon and they start to oscillate.Though they don’t oscillate about a line but about a point and also rotate about them under influence of earth’s rotation in anti-clockwise direction.
  • Thus, amplitude and magnitude of stationary depends upon the ocean basin and gravitation pull of moon respectively.
  • These stationary wave after formation move towards coasts.Crests causes tides and troughs, ebbs

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