Science Questions

Why do we only ever see the face of the moon?

Sun, 15th Nov 2009

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Question

Pamela asked:

Please could you help me understand why it is that we only ever see one side of the moon?

Answer

The moon is orbiting the earth and rotating exactly once per orbit, this means that one face is always facing the earth. A similar example is if you get a friend to walk in a circle while always facing the centre, they are rotating once every orbit.

The obvious question is why did this come about, is it just fluke? 

It is all because of tides, the gravity from the moon distorts the sea (and to a lesser extent the earth itself) into an ellipsoid. As the earth rotates this distortion moves around the earth froming the tides. The energy to drive the tides comes from the earth's rotation and the earth is slowing down its rotation. We know that there were 400 days in a year 250 million years ago by studying corals.

The earth also creates tides on the moon, but because the earth is about 80 times heavier than the moon they are far stronger. Originally the moon was spinning, but these immensely strong tides required so much energy that the moons rotation slowed down so much it is now always facing the earth. Essentially there are still tides - the moon is distorted, but they are locked in one place - it is said to be tidally locked.

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pamelah asked the Naked Scientists:

Hi there,

Please could you help me to understand why it is that we only ever see the face of the moon.  The explanation below has not helped me:

"The Moon's rotation is equal to its revolution period (27.3 earth days) and so the same face is turned earthward all the time."

I have tried using two spheres, bearing in mind that the Moon rotates clockwise and the Earth anti-clockwise, but am still unable to grasp the concept.

Many thanks and kind regards,
pam houghton

What do you think? pamelah, Thu, 4th Jun 2009

Put a chair in the centre of the room. Move around the chair in a circle so you are always facing it. Once you have completed one orbit around the chair, your body will also have rotated once relative to the room. lyner, Thu, 4th Jun 2009

Why is the near side of the moon cratered & why is it always facing us?

Why if the moon is rotating can we only ever see the same side?

Does The Moon's Face Sway ? Even Just a little bit ?..and other questions !

Will we eventually see the dark side of the moon?
Chemistry4me, Fri, 5th Jun 2009

Wot, no loony links? lyner, Fri, 5th Jun 2009

The moon's face does sway about quite a lot because the orbit is both slightly elliptical and inclined.  This process is known a libration

Go to  http://en.wikipedia.org/wiki/Libration to see a good video showing how the size and angle of ther moon can change over one month. Soul Surfer, Fri, 5th Jun 2009

The moon used to rotate, in relation to the earth, but it slowed down due to the earths gravity.

The earth will stop eventually as well, due to the sun's gravity. (I don't think all these wind farms are helping either)
turnipsock, Fri, 5th Jun 2009



Why? Shadow, Fri, 5th Jun 2009

It's a conservation of momentum thing. Since the windmills are winding the Earth's rotation down, he has to compensate by winding you up. Ophiolite, Sat, 6th Jun 2009

"The moon used to rotate, in relation to the earth, but it slowed down due to the earths gravity."
I doubt that. I think it's got rather more to do with the viscous drag of the tides. Bored chemist, Wed, 18th Nov 2009

Yep yor_on, Sun, 22nd Nov 2009

There is a region on the face of the moon (on the side we can see from the earth), and its so much more massive than the other parts of the moon. I think it was like 30 or 40 km. wide. And the world's gravitational pull affects this region more than the other parts of the moon. You can think the world as a hammer thrower and the moon as the hammer. Like its tied to the world with an invisible string from that point. The moon tries to escape from that gravitational pull but it cannot. Its not powerful enough.  It cannot turn its other face because this region cannot escape from this pull. And thats the reason why "The Moon's rotation is equal to its revolution period (27.3 earth days)". Thats why we always see the same side of the moon.      Novacaine, Sat, 26th Mar 2011


That is not true - the Earth's spin, the Moon's spin, and the Moon's orbit around the Earth are all anti-clockwise when seen from above the North pole. That is why you are confused. Pikaia, Sat, 26th Mar 2011

Thanks, I was just about to write this and you saved me the trouble (this is a common misconception !) syhprum, Sun, 27th Mar 2011

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