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Related to tides: neap tides, spring tides
Tidesclick for a larger image
Tides: (a)(i) gravitational acceleration on Earth due to Moon, (ii) differential (tidal) acceleration found by subtracting a from other vectors on Fig. (1); (b) spring (top) and neap tides


Distortions of a planet, star, etc., produced by the differential gravitational attraction of other astronomical bodies on parts of that planet, star, etc. The Sun and Moon combine to generate two tidal bulges in the Earth's oceans, one directed toward the Moon and the other diametrically opposite. If the Earth is assumed to be spherical and to be covered with water, then the gravitational pull of the Moon will have a different force at different points on the Earth's surface (see illustration (a)(i)). The gravitational acceleration at the Earth's center is equal to the centripetal acceleration of the Earth–Moon system. If this acceleration (which is a vector quantity) is subtracted from the surface accelerations, the differential (tidal) acceleration is found (illustration (a)(ii)), showing the two tidal bulges on the Earth–Moon line.

The daily rotation of the Earth and the slower eastward revolution of the Moon in its orbit produce (usually) two high tides and two low tides every 24 hours 50 minutes. The rotation and lunar revolution are also responsible for a tidal lag between the time when the Moon crosses the local meridian and the time of high tide.

The tidal pull of the Sun is less than half that of the Moon but reinforces it at full Moon and at new Moon to produce very high, or spring tides; these tides are exceptionally high when the Earth is close to perihelion and the Moon is close to perigee (see illustration (b)). The lowest high tides, or neap tides, occur when the Moon is at quadrature. The tidal height depends on the topography of the coastline and on the area of the adjacent continental shelf. Tides are also generated in the Earth's atmosphere and in the solid Earth.

Lunar tides are raised by the Earth as a consequence of the Moon's eccentric orbit. They trigger moonquakes and transient lunar phenomena. See also tidal friction.

References in classic literature ?
We had to work fast; but before the tide came in again we had stripped her of her sails and masts, righted her, and filled her about a quarter full of rock ballast.
The red-faced man had said that the tide was ebbing through the Golden Gate.
Wheresoever the strong tide met with an impediment, his gaze paused for an instant.
We'll have happy times, for I don't suffer much, and I think the tide will go out easily, if you help me.
Herbert had sometimes said to me that he found it pleasant to stand at one of our windows after dark, when the tide was running down, and to think that it was flowing, with everything it bore, towards Clara.
And he painfully subsided into the little boat, which started, favored by wind and tide, for the coast of France.
The tide was half out, and they sailed squarely in on the sand, grounding in a row, with the salmon boat in the middle.
The favorable tide had barely three hours more to run.
And just at the same time the tide caught the brig, and threw the wind out of her sails.
She could understand it--understand the green crabs with white- bleached claws that scuttled before her and which she could see pasturing on green-weeded rocks when the tide was low.
Then he turned to Yolland, and asked, "Is there any chance of finding her, when the tide ebbs again?
A little after noon I found the sea very calm, and the tide ebbed so far out that I could come within a quarter of a mile of the ship.