Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Michael Zuk on 14/12/2010 13:30:03
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Michael Zuk asked the Naked Scientists:
Hello all!
I just started listening (http://www.thenakedscientists.com/HTML/podcasts/) about a month. Keep up the fantastic work!
Why doesn't our moon rotate (spin)? I thought they all did until it dawned on me, it doesn't.
Thank you so much! Looking forward to your response. Have a great day!
Thank you.
Sincerely,
Michael Zuk
What do you think?
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But it does... it's in synchronous rotation with the earth.. you need to look at it from somewhere not Earth.
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Hi, Michael,
The moon does rotate. Its period of rotation is synchronised with its orbital period. If it did not do this, then when it was on the other side of the Earth, the opposite side of the moon would be visible.
I don't know if anyone knows how this synchronisation came about, but I suppose the moon could be slightly "heavier" in one side than the other.
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Michael, this situation is known as "tidal locking" in case you want to learn more. Most of the moons around our solar system planets are tidally locked.
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The next question has to be: how does tidal locking work?
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The gravitational attraction between the earth and moon raises a tidal bulge on the moon. The moon started out rotating faster than its ~monthly orbit so the bulge was a little in front of the direct line between the earth and the moon. This situation exerts a little more attraction tangential to the bulge in a direction that slowed the moon's rotation until it stopped and became tidally locked. This also caused the moons orbit to become larger (more distant). Had the moon been rotating slower than the orbit it would have been sped up to become tidally locked. Now, how do you suppose the earth's tidal bulge, from the moon, is affecting the earth?
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Now, how do you suppose the earth's tidal bulge, from the moon, is affecting the earth?
Following your reasoning, the moon should be slowing the Earth's ratation, very gradually.
Right?
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Bill, yes the earth's rotation is slowing, and in some very distant future the earth and moon will both be tidally locked so that they will both have a rotation period identical to the moon's orbital period, and the period will have increased even more and the moon will have receded further. I think this is the case for Pluto and its satellite. I would appreciate someone else confirming what I have written. Steve
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Some of the kinetic energy of the Earth/Moon system is being dissipated as heat, and I believe this is imposing some drag on the roatation of the Earth, so, eventually, it will stop rotating relative to the Moon. I think you are correct about the period and distance, but only because I got it wrong the last time this one came up [:D]
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Steve - I would go along with yours. On a time scale thang; the time required for the earth's day to equal one lunar month is longer than the expected lifetime of the sun. Pluto and Charon are indeed locked - although difficult to say one orbits the other, the barycentre (ie the point about which they both orbit) is actually outside surface of Pluto