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Proudman Oceanographic Laboratory on Tides
The definition of a tide is not as obvious as it might sound – it is not just the rise and fall of water in our seas and oceans.

A tide is the regular and predictable movement of water caused by astronomical phenomena and the topography of the water basins (i.e. the depths and shape of the coastline).

The influence of winds and variations in atmospheric pressure lead to what are called surges.

The principle cause of tides is the gravitational effects of the Moon and Sun on the Earth-Moon-Sun system, and the movements of those three bodies within the system.

If you consider just the effect of the Moon (see Fig A), the gravitational pull on the Earth’s water masses combined with the rotation of the Earth and Moon around their common centre creates two bulges of water on opposite sides of the Earth. The rotation of the Earth on its axis every 24 hours causes points on the Earth to pass under these 2 bulges each day leading to the 2 high waters experienced in most places on the Earth.

The Sun also creates two bulges of water (see Fig B), and the relative position of the Moon and the Sun over the Earth governs whether these two bulges line up to give the larger spring tides (during full and new Moon) or pull against each other to give neap tides (first and third quarters).

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