Why the period of rotation and revolution of moon is same?

[shanu_bhaiya]

Period of rotation and revolution of moon is same (w.r.t. distant star), that's why we can only view only one face of the moon.

Cosmological fact or reasonable science?

 

[DaveC426913]

Cosmological fact and reasonable science.

Tidal locking. Quite common.

Pluto : Charon.
Mercury : Sun.
Earth : Moon.
Many Jovian and Saturnian moons.

 

[mgb_phys]

And inevitable fate.

Any two bodies in orbit aound each other are going to lose energy so that their rotations synchronise like this.
The effect is especially efficient of you have surface liquid to form tides and so is called tidal friction or tidal locking. Both obects rotations slows but since the Earth is so much heavier than the moon most of the effect is on the moon's rotation.
Pluto and it's moon charon are similair sizes and so both have locked facing each other.

The Earth is also slowly loosing energy to the Sun through this effect and so the length of the day on Earth is also slowly changing and eventually the Earth will tidally lock with the sun so that only one face is facing the sun.

 

[DaveC426913]

And I'm pretty sure that, in principle, if a body were perfectly round (no mountains, valleys or sloshing water), perfectly rigid (so that it stayed round) and of symmetrical density (round even internally) tidal locking would not occur.

I'm not suggesting this can happen, I'm just illuminating the properties that cause it. i.e. that tidal locking is all about the asymmetrical shapes of bodies and resulting friction.

 

[mgb_phys]

Yes that's true, although tidal torques would tend to create bulges so it would have to be perfectly spherical and inifinitely rigid.

 

[shanu_bhaiya]

Period of rotation of moon is about 57.3 days and of revolution is 88 days.
Maybe it's not a synchronous rotation. Please explain why?

I want to also ask that isn't it have to matter with the "center of figure" and "center of mass"? Is assymetry causing tidal locks and would it not happen if we've symmetrical objects?

 

[mgb_phys]

I think those are the values for mercury - which is being sllowly tidally locked to the sun, it used to be thought it already was and had a hot side and cold side!
The moon's orbital period is 27.3 days.

In theory completely solid ( no surface water or liquid core ) and completely spherical objects wouldn't experience tidal torques because there is nothing to grip onto if you like!
In practice the tides may be enough to create bulges on an existing object.

 

[DaveC426913]

In theory completely solid ( no surface water or liquid core ) and completely spherical objects wouldn't experience tidal torques because there is nothing to grip onto if you like!

No, not even in theory.

Granting the discovery of planet Cueball, it will still get tidally locked. As I pointed out, the third requirement is perfect rigidity, which cannot exist.
Round as it is, Cueball is still not perfectly rigid, and will distort under the tides just like any other body, and then the tides will have something to grab onto.

 

[shanu_bhaiya]

Can it be said that if the center of figure and center of gravity aren't coincident then we have a tidal lock?

 

[mgb_phys]

No, you can have the centre of mass in the centre of a non-spherical object.
You need two things for tidal locking, some non spherical bulge for the tidal torque to grip onto (as Dave said in the real world the tides are strong enough to distort a real object to create the bulges) and it needs to be in orbit with another object.

The centre of rotation of the Earth-moon system (the barycentre) is not at the centre of the Earth because of the mass of the moon it is a few 100km closer to the moon. Similairly the centre of the Earth sun system is a small distance from the centre of the sun. This is not caused by tidal locking.

 

[DaveC426913]

I do not know what "center of figure" means.

 

[Nesk]

I do not know what "center of figure" means.

My guess is geometrical center, i.e. the point that would be the COM if the mass density was uniform throughout the body under consideration.

 

[shanu_bhaiya]

I do not know what "center of figure" means.

Oh, yes "center of figure" is the geometrical center or a symmetrical point by the virtue of shape of the body.

 

[DaveC426913]

Well, a lemon-shaped body will have centre of figure and centre of mass at the same point, yet it will most definitely experience tidal locking.