Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: pamelah on 04/06/2009 13:30:02
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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?
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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.
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Why is the near side of the moon cratered & why is it always facing us? (http://www.thenakedscientists.com/forum/index.php?topic=20637.0)
Why if the moon is rotating can we only ever see the same side? (http://www.thenakedscientists.com/forum/index.php?topic=20167.0)
Does The Moon's Face Sway ? Even Just a little bit ?..and other questions ! (http://www.thenakedscientists.com/forum/index.php?topic=9511.0)
Will we eventually see the dark side of the moon?
(http://www.thenakedscientists.com/forum/index.php?topic=18674.0)
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Wot, no loony links?
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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.
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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)
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I don't think all these wind farms are helping either
Why?
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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.
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Listen to the answer to this question on our podcast. (http://www.thenakedscientists.com/HTML/podcasts/show/2009.11.15/)
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"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.
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Yep
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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.
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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.
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.
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Thanks, I was just about to write this and you saved me the trouble (this is a common misconception !)