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The tide is the regular rising and falling of the ocean's surface caused by the Moon's gravity (and to a lesser degree the Sun's).

The maximum water level is called high tide; the minimum level is low tide. At any given point on the ocean, there are normally two high tides and two low tides each day. On average, high tides occur 12 hours 24 minutes apart. The 12 hours is due to the Earth's rotation, and the 24 minutes to the Moon's orbit.

The height of the high and low tides (relative to mean sea level) also varies. Around new and full Moon, the tidal forces due to the Sun reinforce those of the moon. The tide's range is at its maximum: this is called the spring tide, or just springs. When the Moon is at first quarter or third quarter, the forces due to the Sun partially cancel out those of the Moon. At these points in the Lunar cycle, the tide's range is at its minimum: this is called the neap tide, or neaps.

The exact time and height of the tide at a particular coastal[?] point is also greatly influenced by the local topography. There are some extreme cases: Southampton in the United Kingdom has a double high tide caused by the flow of water around the Isle of Wight, and Portland has a double low tide. Also there is little tide in the Mediterranean due to the narrow connection with the ocean.

It is often assumed that the tides are simply the Moon's gravitational force pulling the oceans' water toward itself, but this is wrong. Were it so, there would only be one high tide every 24 hours (imagine the water around the Earth with a single bulge pointing towards the Moon). Instead, the tide is caused by tidal forces, which are due to the difference in gravitational attraction on the near and far sides of a body. The tidal force produces two bulges: one pointing towards the Moon, and one pointing away. This is also why the Moon is the major cause of the tides: at the Earth's surface the straightforward gravitational attraction of the Sun is considerably larger than that of the Moon, but the difference in the Moon's grativational force from the near side of the Earth to the far side is much greater than the Sun's.

The tidal forces fall off according to an inverse cubic law: the gravitational forces themselves are proportional to the inverse square of distance, and the significance of a difference in distance falls inversely with distance. The much greater distance of the Sun makes its tidal forces on the Earth much smaller than the Moon's.

Tides also affect the shape of the Earth itself, not just its oceans. These "land tides" are not as pronounced as the ocean tides, however, due to the reduced flexibility of Earth's crust and mantle. Land tides are also delayed about two hours relative to ocean tides due to this stiffness.

See also: coastal erosion storm tide

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