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Tides

A rocky islet at low tide, Bay of FundyA rocky islet at low tide, Bay of Fundy Sea levels rise and fall twice a day. This pattern is known as the tides. The tidal range is the difference between high and low tide. Seas that are almost surrounded by land, such as the Mediterranean, have a very small tidal range. Tides are caused by the the combined gravitational pull of the Moon and the Sun and the rotation of the Earth. Most coastlines have two low tides and two high tides each day, while others have only one of each or as many as four.


The gravitational pull by the Sun and MoonThe gravitational pull by the Sun and Moon

How are tides caused?

Tides are caused by the pull of gravity by the Sun and Moon on the Earth. As the Earth spins, the ocean waters on the side of the Earth closest to the Moon (and the opposite side) bulge outwards, causing a high tide. At the same time, the rest of the Earth has a low tide.




Spring and neap tidesSpring and neap tides When the Sun, Moon and Earth are in line, the Sun’s gravity combines with that of the Moon to increase the pull on the water. This makes high tides higher and low tides lower. These are called spring tides (1). When the Sun and Moon are at right angles, the difference between high and low tides is at its lowest. These are neap tides (2).



Surfing the Turnagain Arm tidal boreSurfing the Turnagain Arm tidal boreClick to play video

Tidal bore

The highest tidal range is found in the Bay of Fundy, between Nova Scotia and New Brunswick, Canada. The greatest average spring tide range is 14.5 m (48 ft).

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