Period of rotation

The period of rotation indicates the duration put by a Astre (star, Planet, Astéroïde) to even make a turn on him. For example, the Ground has one period of Rotation of approximately 24 hours.

Periods of sidereal and synodical rotation

In the case of a star orbiting around a star, one distinguishes two rotations:

sidereal rotation, time with the end of which the planet finds the same orientation compared to surrounding stars;
synodical rotation, time with the end of which the planet finds the same orientation compared to star. It corresponds to the duration of the Jour.

The planets turning in general in the same direction on they-even as around their star, their period of synodical rotation is longer than the period of sidereal rotation. For example, the period of sidereal rotation of the Earth is 23:56 min 4 S and its synodical period is on average of 24:00 (either approximately 4 min longer): in one year, the Earth swivelled 365,2425 times in the synodical reference frame and 365,2425 + 1 time in the sidereal reference frame; one day sidereal is worth thus 365,2425/366,2425 solar day. More exactly, as the reference is not the year, but the international atomic Time, the day contains by definition 86400 S, and the average solar day is worth approximately approximately a day; the value of the period of swivelling of the Earth is nominally:

T = 86164 S + 98 ms + 903 NS + 697 PS.

This period varies approximately 1 ms over one year, 5 ms over 70 years and increases regularly on average by 1 has 2 ms per century, “because” of the distance of 3,84 m per century of the Moon. (synchronous cf Rotation).

In the Solar system, Venus constitutes an exception: retrograde planet - its rotation is carried out in the opposite direction of its revolution - has one period of synodical rotation (117 days) shorter than its period of sidereal rotation (243 days).

In the solar system, the sidereal period usually indicates the period of sidereal revolution , i.e. the return to the same position relative with the sun and the ground, concept that one should not confuse with the period of above definite rotation sidereal.

Evolution of the period of rotation

Stars

Planets

The planet lengthens slightly under the effect of the Marée: the two ends of planet are attracted with a different force because of their difference in distance to the satellite. Because of the Friction S, this lengthening is excentric compared to the satellite. It results a Couple tending from it to modify the number of revolutions which make evolve/move the period of rotation of planet, as the diagram explains it opposite. It is generally a deceleration about rotation, except when the satellite has a revolution faster than the rotation of planet and carried out in the same direction as this one. In particular the period of sidereal rotation of the ground increases because of frictions due to the movements of the tides (due to the joint action of the moon and the sun); the terrestrial loss of Angular momentum is compensated by a distance of the moon.

The change of distribution of the matter within the planet (erection of mountainous solid masses, cast iron of continental glaciers) is also likely to modify the number of revolutions.

The variation of the period of rotation of the ground influences over the duration of the day, which currently exceeds the average of 24:00 of some fractions of a second. It is thus necessary to add a intercalated Seconde regularly so that the universal Time coordinated remains fixed on the diurnal cycle.

See too

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