Why do Mercury and our Moon not spin?

[kichigai]

Does anyone know why Mercury and our Moon do not spin?

[Entropy]

The moon does spin. Its just in-sync with its orbit around the Earth so it appears not to spin to us.

[employee #416]

If I have not been mistaken, I thought we always saw one side of the moon at all times. We have never seen the other half of the moon. Wouldn't this imply that the moon does not spin? If the moon did spin, I'm sure it would be noticeable.

[enigma]

The Moon orbits the Earth.

You can tell because the side that faces the sun changes: phases of the Moon.

The Moon always has the same side facing us.

Therefore, the Moon rotates at the same rate which it orbits.

This phenomena is called 'tidal lock'.

[kichigai]

The Moon orbits the Earth.

You can tell because the side that faces the sun changes: phases of the Moon.

The Moon always has the same side facing us.

Therefore, the Moon rotates at the same rate which it orbits.

This phenomena is called 'tidal lock'.

What about Mercury?

[cepheid]

Mercury does spin. It's rotation period is once every 58.65 Earth days. That information is given here (and is probably readily available in any case):

http://www.jpl.nasa.gov/solar_system/planets/planets_index.html

[kichigai]

Great! Many thanks folks!

[Chronos]

Not fair, enigma :) You cherry-picked that one.

[Math Is Hard]

I had a dicussion with someone about this just the other day. The problem he had in thinking about the moon's tidal locking is that he assumed the moon was perfectly spherical, which it isn't. It's slightly ovoid in shape.
Just thought I would throw that out there.

[Chronos]

The moon is mainly non-spheroid because of earth's gravity.

[Math Is Hard]

The moon is mainly non-spheroid because of earth's gravity.

Well, yes. And doesn't this assist in the tidal locking phenomenon?

[Iron Sun 254]

It was once believed that Mercury did not spin and I've come across websites that still post this false information. Part of the reason for this misconception is the rotation period and orbital period are in phase so that Mercury rotates 3 times everytime it orbits the Sun twice. Even though it's rotating every other time it reaches the same point in an orbit it's facing the same direction.

[Phobos]

If I have not been mistaken, I thought we always saw one side of the moon at all times. We have never seen the other half of the moon. Wouldn't this imply that the moon does not spin? If the moon did spin, I'm sure it would be noticeable.

Take 2 coins, face up. Have the faces (Lincoln, whatever) pointing at each other. Rotate one around the other, keeping the face of the orbiting one pointing toward the center one. See how it's rotational period matches its orbital period? (1 spin per 1 orbit) That's essentially what the Moon is doing.

just my 2 cents :biggrin:

[Chronos]

Well, yes. And doesn't this assist in the tidal locking phenomenon?
Uh, er, it just dawned on me that I repeated what you had already said :blush:

[Nereid]

It was once believed that Mercury did not spin and I've come across websites that still post this false information. Part of the reason for this misconception is the rotation period and orbital period are in phase so that Mercury rotates 3 times everytime it orbits the Sun twice. Even though it's rotating every other time it reaches the same point in an orbit it's facing the same direction.Welcome to Physics Forums Iron Sun 254!

IIRC, before the 1965 radar observations (Pettengill and Dyce), astronomy books stated that Mercury's rotation was tidally locked so one hemisphere always faced the Sun, just as one side of the Moon permanently faces the Earth.

As you say, there is a tidal lock, but it's a 3:2 one, not 1:1!