Why do Mercury and our Moon not spin?

In summary, Mercury and our Moon both spin, but their rotation is synchronized with their orbit around their respective planets. This is known as tidal lock. The Moon always has the same side facing us due to this synchronization, while Mercury rotates three times for every two orbits around the Sun. Initially, it was believed that Mercury did not spin, but this was later disproven by radar observations.
  • #1
kichigai
Does anyone know why Mercury and our Moon do not spin?
 
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  • #2
The moon does spin. Its just in-sync with its orbit around the Earth so it appears not to spin to us.
 
  • #3
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.
 
  • #4
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'.
 
  • #5
enigma said:
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?
 
  • #6
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
 
  • #7
Great! Many thanks folks!
 
  • #8
Not fair, enigma :) You cherry-picked that one.
 
  • #9
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.
 
  • #10
The moon is mainly non-spheroid because of Earth's gravity.
 
  • #11
Chronos said:
The moon is mainly non-spheroid because of Earth's gravity.

Well, yes. And doesn't this assist in the tidal locking phenomenon?
 
  • #12
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.
 
  • #13
employee #416 said:
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:
 
  • #14
Math Is Hard said:
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:
 
  • #15
Iron Sun 254 said:
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!
 

1. Why does Mercury have a slow rotation compared to Earth?

Mercury has a slow rotation because it is located very close to the Sun. The gravitational pull of the Sun on Mercury is much stronger than the pull of Earth on its own moon. This tidal force causes Mercury to have a slower rotation as it is constantly being pulled towards the Sun.

2. How does the distance between Mercury and the Sun affect its rotation?

The closer an object is to the Sun, the stronger the Sun's gravitational pull is on that object. This means that Mercury, being the closest planet to the Sun, experiences a greater tidal force that slows down its rotation.

3. Why does the Moon always show the same side to Earth?

The Moon's rotation is synchronized with its orbit around the Earth, meaning it takes the same amount of time for the Moon to rotate on its axis as it does to orbit around the Earth. This is due to the tidal forces exerted by the Earth on the Moon, causing the Moon's rotation to slow down and eventually become locked in this synchronized rotation.

4. How does the Moon's distance from Earth affect its rotation?

Similar to Mercury, the Moon's distance from Earth also affects its rotation. The closer an object is to Earth, the stronger the Earth's gravitational pull on that object. This results in a stronger tidal force on the Moon, slowing down its rotation and causing it to become synchronized with its orbit around Earth.

5. Can the rotation of Mercury and the Moon change in the future?

Yes, it is possible for the rotation of Mercury and the Moon to change in the future. The tidal forces acting on these objects can change over time due to various factors, such as changes in their orbits or the gravitational forces of other celestial bodies. However, any changes in their rotation would likely occur over a long period of time and would be difficult to observe in a human lifetime.

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