Earth tide
Earth tide (also known as solid-Earth tide, crustal tide, body tide, bodily tide or land tide) is the displacement of the solid earth's surface caused by the gravity of the Moon and Sun. Its main component has meter-level amplitude at periods of about 12 hours and longer. The largest body tide constituents are semi-diurnal, but there are also significant diurnal, semi-annual, and fortnightly contributions. Though the gravitational force causing earth tides and ocean tides is the same, the responses are quite different.
Ocean tidal loading[edit]
In coastal areas, because the ocean tide is quite out of step with the Earth tide, at high ocean tide there is an excess of water above what would be the gravitational equilibrium level, and therefore the adjacent ground falls in response to the resulting differences in weight. At low tide there is a deficit of water and the ground rises. Displacements caused by ocean tidal loading can exceed the displacements due to the Earth body tide. Sensitive instruments far inland often have to make similar corrections. Atmospheric loading and storm events may also be measurable, though the masses in movement are less weighty.
Effects[edit]
Seismologists have determined that microseismic events are correlated to tidal variations in Central Asia (north of the Himalayas); see: tidal triggering of earthquakes.
Volcanologists use the regular, predictable Earth tide movements to calibrate and test sensitive volcano deformation monitoring instruments; tides may also trigger volcanic events.[4][5]
The semidiurnal amplitude of terrestrial tides can reach about 55 cm (22 in) at the equator which is important in geodesy using Global Positioning System, very-long-baseline interferometry, and satellite laser ranging measurements.[6][7] Also, to make precise astronomical angular measurements requires accurate knowledge of the Earth's rate of rotation (length of day, precession, in addition to nutation), which is influenced by Earth tides (see also: pole tide).
Terrestrial tides also need to be taken in account in the case of some particle physics experiments.
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For instance, at the CERN or the SLAC National Accelerator Laboratory, the very large particle accelerators were designed while taking terrestrial tides into account for proper operation. Among the effects that need to be taken into account are circumference deformation for circular accelerators and also particle-beam energy.
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In other astronomical objects[edit]
Body tides also exist in other astronomical objects, such as planets and moons. In Earth's moon, body tides "vary by about ±0.1 m each month."[11] It plays a key role in long-term dynamics of planetary systems. For example, it is due to body tides in the Moon that it is captured into the 1:1 spin-orbit resonance and is always showing us one side. Body tides in Mercury make it trapped in the 3:2 spin-orbit resonance with the Sun. [12] For the same reason, it is believed that many of the exoplanets are captured in higher spin-orbit resonances with their host stars. [13]