Synchronous Rotation
The synchronous rotation is a phenomenon which occurs when a Natural satellite orbit near its Planet and which has as consequence that the Period of rotation of the satellite is synchronized with its Period of revolution so that, seen planet, it always presents the same face to him. It is also said that these periods are in resonance. Synchronous rotation does not relate to only the natural satellites of planets but any object in orbit around an other.
Description
Attraction nelle Gravitation between two bodies produces a Force of tide on each one of them, stretching them in the direction of the axis planet-satellite. If the bodies in question are sufficiently flexible and that the force of tide is sufficiently strong, these bodies will be slightly deformed. As the majority of the moons and all the astronomical bodies of big size are spherical under the action of their own gravity, the action of the forces of tide returns them slightly prolate S (cigaroïdes).
In the case of the systems moon-planet, this lengthened form is unstable. Let us suppose that the satellite rotates more quickly than around its planet, and than its planet rotates more quickly than the satellite does not turn around it (the phenomenon will be the same one if the opposite is true, only the signs will be reversed). Pulled by the rotation of the satellite, the protuberances raised by the tide of planet will be found in advance; the gravitational force of planet will then exert a couple on each protuberance, which will cause to slow down the rotation of the satellite in the case of the protuberance located on the planetary side, and to accelerate it in the other. However, the back protuberance (external) being slightly further away from planet than the protuberance before (interior), its couple will be less, and the Net effect will be a deceleration of the rotation of the satellite. If the protuberances are fluid (tides oceanic or atmospheric), they will move around the moon in order to remain in the axis of planet, which will induce a couple by the means of the friction. The Net effect is primarily the same one.
The Angular momentum orbital of the satellite, by the same set of forces and couples, increases precisely the more so as its rotational angular momentum decreases.
At the same time, the tides raised by the satellite on its planet will seek to synchronize the rotation of the latter with the revolution of its satellite; the most important effect, however, is that the protuberances of planet will exert a couple Net on the satellite which will cause to accelerate it on its orbit, and thus to force it to move away. In the case of the system the Ground-Moon, the distance separating them increases 3.84cm per annum (study by Lunar Laser Ranging).
If the revolution of the satellite is faster than the rotation of the planet (what is the case of Phobos around Mars, as well as several of the interior moons of Uranus), the forces of tide of planet will decrease the orbital ray of the satellite, until it damages on its surface.
In the case of the small moons, of irregular form, these forces will tend to align more the main roads of the moon with the orbital ray, and the smallest axis with the normal with the orbit.
Examples of objects in synchronous rotation
Most of the moons of the Solar system are in synchronous rotation with their planet, because they orbit with short distances from their planet and the force of tide increases quickly with the reduction in this distance (the gravitational gradient is proportional contrary to the cube of the distance). Moreover, Mercure is in synchronous rotation with the Sun in a 3:2 report/ratio. More subtly, Venus is in synchronous rotation with the Earth, so that all the times where Venus is in lower conjunction, Venus present the same one vis-a-vis the Earth. The forces of tide implied in synchronization Venus-Ground are extremely weak and it may be that it is only one coincidence.
Charon, satellite of Pluto, is in synchronous rotation with this last: like the the Moon with the Ground, it always presents the same face to him. But, contrary to the latter, Charon evolves/moves on the Geostationary orbit of Pluto. Thus, in addition to the fact of always presenting the same face, Charon thus appears motionless in the sky of Pluto.
In general, any object which orbits for long periods near an other object much more massive is likely to be in synchronous rotation with this one. It is supposed that the binary stars close are mutually in synchronous rotation. In the same way, it is thought that the planets extrasolaires which were detected near their star are in synchronous rotation with this one.
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