Types and importance of tides

Tides by frequency:

• Semi-diurnal: the most common: two high and two low tides a day, each pair of about the same height.
• Diurnal: one high and one low tide a day, of about the same height.
• Mixed: tides of varying heights, found along the west coast of North America and on many Pacific islands.

Tides by the positions of sun, moon and earth:

• Spring tides: when the sun, moon and earth are in a straight line, tides are higher. They come twice a month, at full moon and new moon.
• Neap tides: about seven days after spring tides, the sun and moon are at right angles to the earth, and their pulls partly cancel. The moon's pull is more than twice the sun's, but is reduced by it.
• When the moon is closest to the earth (perigee), once a month, tides are unusually high and low: the tidal range is greater. About two weeks later, at apogee (moon farthest), ranges are smaller.
• When the earth is closest to the sun (perihelion, around 3 January), ranges are also greater; at aphelion (around 4 July), smaller.

• Ebb: the falling tide, from high to low. Flow or flood: the rising tide, from low to high.

Importance of tides:

• Because the positions of earth, moon and sun are known exactly, tides can be predicted well in advance, helping navigators and fishermen plan.
• Tidal height matters for harbours near rivers and in estuaries with shallow bars at the entrance; ships may enter only at high tide.
• Tides help desilt estuaries and flush out polluted water.
• Tides are used to generate electricity (in Canada, France, Russia and China). The book mentions a 3 MW tidal power project at Durgaduani in the Sundarbans, West Bengal.

Example (new): fishermen in a creek village time their trips by the tide table: they slip out on the ebb, when the falling tide helps carry boats seaward, and return on the flood, when the rising water floats them over the mud banks.