Wave Graphic

What causes tides?

Tide-generating forces (TGF) are a result of the gravitational attraction between the earth, the sun, and the moon and the centrifugal force due to the relative motions of the moon around the earth, and the earth around the sun.  While these forces exactly balance on average, the local mismatch at the earth's surface creates a horizontal force directed towards the surface points closest and farthest from the moon (the "lunar" TGF) and the sun (the "solar" TGF). 

The crust of the earth is slightly elastic, so that it is deformed by the TGFs, creating lunar and solar tidal budges (high land) at the points closest and furthest from the moon and sun respectively. To an observer fixed on the earth's surface, these tidal budges move from east to west around the earth as it rotates each day, thus causing two luner and two solar high earth tides about each day. The period of the solar tide is exactly 12.00 hours, while the period of the lunar tide is slightly longer, 12.42 hours, due to the moon's revolution around the earth every 27 days. These tides are called the "semidiurnal" tides since they have periods of roughly 1/2 day.   The inclination of the earth's spin axis to the plane of the moon's revolution about the earth and the earth's revolution about the sun creates in addition weaker "diurnal" tides with periods of roughly 1 day. The amplitude of the semidiurnal lunar high earth tide is about 1 m at the equator, about twice that of the solar tide.  We do not feel these earth tides due to their very large horizontal scales (many 1000's km).

The fluid ocean also experiences the TGFs. Unlike the simple tidal budges created in the earth's crust, ocean tides have complex spatial patterns due to the complicated shapes and topographies of the different ocean basins. In general, however, ocean tides at any spot consist of a mixture of semidiurnal and diurnal tides.  The world's largest semidiurnal tides exist in the Bay of Fundy (maximum high tide ~12-15 m), where the Bay of Fundy/Gulf of Maine acts as a coupled hydrodynamic system which is forced near its own resonant frequency by the semidiurnal tide in the western North Atlantic Ocean.  Similar very high tides are found in other coastal areas (e.g., the Amazon and the Patagonia shelves) where the regional topography creates a near-resonant response to the adjacent deep ocean tide.


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Last updated 17 June 1999


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