(01-02-2016 12:01 PM)grol Wrote: (01-02-2016 11:53 AM)GoodOwl Wrote: Most inner moons in the solar system keep one face pointed toward their central planet...
Why do most moons keep one face pointed toward their central planet?
It's called tidal Locking:
Tidal locking explanation
Tidal locking (also called gravitational locking or captured rotation) occurs when the gravitational gradient makes one hemisphere of a revolving astronomical body constantly face the partner body. This effect is known as synchronous rotation. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. For example, the same side of the Moon always faces the Earth. Usually, only the satellite is tidally locked to the larger body. However, if both the mass difference between the two bodies and the distance between them are relatively small, each may be tidally locked to the other; this is the case for Pluto and Charon. This effect is employed to stabilize some artificial satellites....
below from: Answers.com
Best Answer: This is actually the case for nearly all moons of the solar system of any significant size.
The reason for tidal locking is that gravity is significantly non-uniform across the target body (moon). The nearest part of the moon (not necessarily Earth's moon) is closer to its host planet than the farther side of the moon, and thus the nearest part of the moon experiences a greater gravitational force per unit mass. As the moon responds to the associated gravitational fields, the near side leads and the far side lags, and the entire moon stretches out as a whole. This distorts the shape of the moon along the line from planet to moon.
If enabled to swing like a pendulum, which it initially will be, a distorted moon will have its equilibrium position when the stretching line is in line with the line to the planet. Torque from the gravitational forces on this warped moon will restore it to this equilibrium position. This is where the moon will settle once the initial rotational energy dissipates internally as tidal heating. Anytime the moon swings out of this equilibrium position, a new set of tidal stresses squish and squash it, converting the rotational energy into thermal energy. The ultimate ending is that the bodies become tidally locked to one another. Usually, the satellite body tidally locks LONG BEFORE the host body does.
The sun is so far away that its gravitational forces are so uniform that the tidal influence is insignificant on the moon's motion. Earth's tidal influence is a lot more important.