Two way tidal lock

[vladpaln]

Main Question or Discussion Point

Can a parent body be tidally locked with a moon, so if one was standing on the planet the moon would appear in the same location in the sky day and night?

 

[DrStupid]

Can a parent body be tidally locked with a moon, so if one was standing on the planet the moon would appear in the same location in the sky day and night?

Yes, Pluto and Charon are tidally locked this way.

 

[vladpaln]

Does that mean that somewhere on the planet there is a daily eclipse??

 

[newjerseyrunner]

Does that mean that somewhere on the planet there is a daily eclipse??

If the plane of the orbit is exactly the same as the plane of the sun? Yes. The moon orbits the Earth every 30 days, but we certainly don't get a solar eclipse every month.

 

[vladpaln]

Are there any known examples where the moon orbit is locked but continues to rotate?? so instead of the moon tidaly locking the orbit it's the parent that is responsible for the tidal lock?

 

[rootone]

From an external point of view, a tidally locked moon still is rotating.
It just happens to be rotating such that it's rotation period is exactly the same as it's orbital period.

 

[vladpaln]

I don't think my question was very clear, I'm aware of two different kinds of tidal locks and I'm wondering if there is a third,
- The first is where the satellite is tidally locked to its parent (the moon), where the satellite always presents the same face to its parent as it orbits its parent
- The second is a two way tidal lock (Pluto-Charon), Charon presents the same face to Pluto but from what I understand if you are standing on Pluto Charon will always appear in the same location in the sky.
- I would like to know if there is a third kind, where the parent is tidally locked to its moon but the moon is not tidally locked to its parent, so the moon will rotate at its own rate (so over time you can see every side of the moon), but if you are standing on the parent the moon will stay in the same location in the night sky??

Is this physically possible?
If anyone is aware of an example please let me know?

 

[rootone]

That is quite unlikely unless the 'Moon' is extremely massive but tiny.
A minimal neutron star or white dwarf star orbiting a very big brown dwarf object or something.
Intuition tells me something could go badly wrong there.

 

[vladpaln]

why, from what I understand all it would take is a large moon relative to its parent (like Charon 12% of Pluto), placing the barycenter outside of the diameter of its parent, a symmetrical moon in all 3-axis (talking mass here), an asymmetrical parent creating the tidal lock, and a relativity close orbital distance (I'm sure there is a range),

someone please poke some holes in this little theory, what did I miss??