charon

[pmorgan14]

Charon always stays in the same place in Plutos sky, but the moon always moves across the sky, what causes the differences?

[chakka35]

What on earth (or in the sky) are you talking about? What is 'Plutos sky'? Explain.

[jake-the-peg]

I think we're talking about spin - obital coupling otherwise known as tidal locking.

http://en.wikipedia.org/wiki/Tidal_locking

It's the tendency of orbiting bodies to match their spin to their orbit due to tidal forces.

Over time the Earth's moon has become locked to the Earth so that we only see the one face.

Pluto and Charon are twins really rather than a planet (don't start this one!) and moon. The central point of the orbit is not inside Pluto but somewhere in space between the pair. As a consequence they are locked to each other which is the key difference.