Earth tides

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The solid earth crust is subject to a tidal wave twice a day, the earth tides . Analogous to the ebb and flow of the oceans and large inland seas - the so-called tides or tides  - they arise from the tidal forces of the gravitation of the earth's moon and the sun on earth .

Size and measurement

The rise or fall is approximately ± 30 to ± 60 cm. It cannot be felt directly by humans. The effect, which only accounts for about one millionth of earthly gravity , can only be measured with high-precision gravimeters (extremely fine spring scales ) and special earth tide pendulums . The latter operate on the principle of a hinge : if the axis just a little out of the Lot , the door is often already half open. The vertical direction fluctuations due to the lunar orbit are not a few tenths of a degree (as, for example, with an inaccurately hinged door), but only about 0.2 ″ (0.00005 °).

Such measurements can only be carried out in locations that are completely free of shock and vibration, preferably in disused mines , caves or tunnels . These measuring stations are z. B. in Schiltach (Baden-Württemberg), Grotta-Gigante (Italy) and in Grazer Schloßberg (Austria).

Permanent tides

In the mathematical modeling of the earth's tides, constant terms arise, which are called permanent tides .

Geophysical context

The earth is not a rigid body, but reacts elastically to the gravitation of the moon and sun. The earth tides are therefore not a "movement" as in short-term earthquakes or in long-term mountain formation , but rather an oscillation ( earth spectroscopy ). The earth's body yields to such periodic forces much faster than to long-term forces such as mountain formation, the effect of which is only a few millimeters per year.

In addition to earthquake waves, the earth tides are a further, independent effect through which the properties of the earth's crust and the upper and lower mantle (e.g. viscosity or flexibility ) can be researched in earth spectroscopy.

literature

  • Wolfgang Torge: Geodesy . Second completely revised edition, Walter de Gruyter GmbH & Co KG, Berlin / New York 2003, ISBN 3-11-017545-2 .
  • J. Bartels: Geophysics II . Springer Verlag, Berlin / Göttingen / Heidelberg 1957.

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