Tide

The tide is the regular rising and falling of the ocean's surface caused by changes in gravitational forces external to the Earth. The main changing gravitational field is due to the Moon while a lesser field is caused by the Sun.

Tides & fluids

Tides and tidal effects happen in general whenever a mass with some volume moves in a gravitational field that is not uniform. This is, they always happen. For example, in one way or the other, all objects moving in space will see some form of tidal forces. By acting on an ideal rigid body, by definition tides will not deform the body. Many bodies which are moving within the solar system, for example, are not rigid but merely balls of gas or fluids, hovering in empty space (Sometimes they have a very thin solid crust). Tidal forces generate pressure differences between different volumes within such objects, and thus generate material currents on or within such bodies. The following argument applies in general to all such bodies, but the discussion here is restricted to a simplified Earth - Moon system (the sun also generates tides in real life, which are about half as strong as the moon's tides).

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The moon's tidal effects generate an acceleration field at the surface regions of the earth which point in its direction or the opposite direction. This field is equivalent in strength to the weight of one tenth of a microgram per kilogram material. In other words, each kilogram of material at the surface of the earth experiences an "upward" force that is equivalent to the weight of one tenth of a microgram.

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It is perfectly clear that nothing starts to move upward because of this. What happens instead, especially within fluids, is a change in the statical pressure within the fluid, because the masses on top lose a little bit of weight. There will be a pressure difference to neighbouring regions, and a material current will start to flow into this regions, until the pressure difference due to tide is balanced by a higher level of the fluids surface.

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In the earth's oceans, the secondary effects of the material currents amplify the tidal effects by as much as a factor of 20. An equipotential surface of the ocean in a tide region would be 2 ft (60 cm) above normal level, but some coastlines experience tides of 40 ft (12 m) or more.

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It is important to notice that pressure differences and thus material currents are not only generated in the earth's oceans, but in the interior of the earth as well.

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By the MHD effect, the material currents generated by the tides will also affect the earth's electromagnetic field. This is seen in real life.

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The tides continously excite (seismic) waves within the earth which can be measured by seismology.

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