Rotation of the Earth

The number of rotations of the Earth on itself is of 365,2425 per annum approximately (Gregorian Calendrier), that is to say 366,2425 days sidereal (rotation compared to the celestial frame of reference). As the Earth is not rigorously a solid, the concept number of revolutions angular must be carefully definite by geodesists and astronomers, then measured. This service is carried out by the IERS ( International Earth Rotation Service ), which puts on line all the known data.

The number of revolutions oscillates in an irregular way. During the time of rotation (length of the day), one observes mainly a seasonal variation from approximately 1 ms and changes decennial (between 10 and 70 years) about 5 ms. Moreover, the action of the the Moon on the rising of the tide produces a retarding torque which induces a secular increase in the duration of the day from approximately 2 ms/siècle and a distance of the Moon of 3,84 cm/an (synchronous cf Rotation).

Compared to stars the axis of rotation sweeps in 25800 years a cone centered on the poles of the plan of the ecliptic (the axis perpendicular to the terrestrial orbit), whose half angle point angle - obliqueness of the ecliptic - is worth approximately 23°26'. This movement, the precession, superimpose small periodic oscillations, nutations, whose principal one, in 18.6 years, has an amplitude of 20". Precession-nutation is caused by the joint gravitational action of the Moon and the Sun on the equatorial pad of the Earth, and of this fact is the subject of relatively precise models, giving the space position of the axis of rotation to the millisecond of degree near.

The axis of rotation osiclle also in the Earth of 1" at most, mainly at the 430 days and 365 days periods.

The pole of rotation is the intersection of the axis of rotation with the surface of the northern hemisphere. At present the direction of the pole in space as in the Earth is measured with a precision which confines 0,1 millisecond of degree = 0,0001".

The difficulty of defining the rotation of the Earth

If a swarm of bees is considered, its translatory movement is easy to define, but its rotation movement is less! The Earth not being a solid, speech of rotation of the Earth could thus seem an ineptitude.

Nevertheless, one knows well the rotation of the Tectonic plate S; what makes it possible to establish a convention, known as Tisserand, to solidify a ITRS (theoretical definition) then to fix a ITRF (practical realization, obviously sullied with error). Then, it any more but does not remain to take account of the winds, wet or not (aeronomy); solar thermal tide, atmospheric tide; oceanic tides; interactions Ocean-Atmosphere; terrestrial tide; interactions CAT (Ground-Atmosphere-Ocean): for example the altitude of Brest varies much and that of Rennes much less; oceanic currents; movements of the coat (the postglacial rebound enormously moved the pole towards the Greenland!) ; movements of seed of the core and core and finally of the interaction core-coat.

Earth thus " solidifiée" by these different " reduction-of errors-systématiques" , it any more but does not remain to study the action of the stars on this solid, located by its angles of Euler (cf Rotation).

The length of the day

The " longueur" vector rotation, is

\ Omega = \ frac {2 \ pi} {T}

gives the length of the stellar day T, i.e. the time interval which separates two consecutive passages from same star to the meridian line. It is close to the sidereal day, the time interval which separates two consecutive passages from the vernal point (animated precession) to the meridian line. However one uses the longor of the solar day LOD = 1.002 737.811.911 354 48 T (of English Length off Day), which is the time interval separating two consecutive passages from the " sun moyen" with the meridian line. The LOD is worth 86400 S TAI (the atomic day) with a margin of more or less 5 ms. International Earth Rotation Service (http://hpiers.obspm.fr/eop-pc) daily determines the variation of the LOD with the atomic day. This variation presents a seasonal variation about the ms, on which are superimposed longer-term between 10 and 70 years, reaching 5 ms and not easily foreseeable oscillations. According to the common theory, these oscillations are caused by the coupling between the fluid core and the coat. On scales of time even larger, the deceleration of terrestrial rotation becomes prevalent and involves an increase length of the day of 2 - 4 ms the every two hundred years.

The length of the day is measured thanks to technique GPS and the shootings laser on artificial satellites with a precision of about 0,020 ms = 20 microseconds.

The bond hereafter makes it possible to visualize the variation length of the day at the atomic day since January 2000 until the week in progress.

The herpolhodie

It is the angular movement of the vector rotation in space (more precisely the celestial frame of reference). It is characterized by its Précession of period 26.000 years, already detected by Hipparque, and its Nutation. There exist irregularities, because the Earth is not a solid.

The polhodie

Of pole hodos (the way): It is the movement of the vector rotation in the terrestrial frame of reference (ITRF: international terrestrial refers frame , correctly definite (the plate tectonics of a few centimetres per annum obliges on such a moving ground to have a precise convention if one wants to detect a movement with the millimetre near). At present, this movement is raised very precisely by satellite Géodésie and the VLBI.

See too

Service of the Rotation of the Earth of the Observatory of Paris
International Earth Rotation Service and Refers Frame (IERS)
terrestrial Orientation observee by VLBI (Observatory of Paris)
Géodésie and its gate
synchronous Rotation
Period of rotation
ITRF

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