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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. The tide's range reaches maximum near new and full Moons, this is called the spring tide, or just springs. The minimum range occurs near first quarter and third quarter Moons, this is called the neap tide, or neaps. Obviously when Sun and moon align tides are highest.

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 somewhere (in Finland?) has a double low tide.

It is often assumed that the tides are caused by 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: the straightforward gravitational attraction force of the Sun is considerably larger at the Earth's surface than the Moon's, but because tidal forces fall off according to an inverse cubic law, the tidal force due to the Sun is much smaller than that due to the Moon.