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Moon's face

Could someone please explain, in plain English, why we only ever see one side of the moon?

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( 25 comments — Leave a comment )
drjon
Jun. 30th, 2008 04:37 am (UTC)
Because the Moon spins around at exactly the right speed so that the same side faces us as it orbits Earth.

This is because of some Gravitational Magic.
fayanora
Jun. 30th, 2008 06:19 am (UTC)
*Blank stare*
drjon
Jun. 30th, 2008 06:45 am (UTC)
Sorry, you don't get more Plain English than my one.

I suspect you need an animated graphic to show it happening.

I will go hunt one up.
fayanora
Jun. 30th, 2008 06:49 am (UTC)
Probably. Good luck, though.
drjon
Jun. 30th, 2008 06:54 am (UTC)
(Sorry, couldn't find an animated version. And the moon's not really very egg-shaped--that's just to highlight which side is facing the earth)

The moon spins round itself once a month.

It also orbits the Earth once a month.

If you want, I can animate that graphic, which should show you what's happening more clearly.
d4b
Jun. 30th, 2008 11:14 am (UTC)
drjon
Jun. 30th, 2008 11:19 am (UTC)
Very nice phases animation. Pretty.
fayanora
Jun. 30th, 2008 07:02 am (UTC)
Ohhh.... I get it now.
dreamsarepoetry
Jun. 30th, 2008 04:57 am (UTC)
The Earth rotates, and the moon rotates around the Earth.
kengr
Jun. 30th, 2008 04:59 am (UTC)
It's tidally locked to earth.

Have Lilla stand out on the lawn. Then you stand about 10 feet away facing her. Walk in a circle around here but stay facing her.

Even though she only sees your front, you'll have turn a full 360 degrees with respect to anyvody standing somewhere else.

What's really fun are resonance locks like Mercury has. It makes 2 full rotations every 3 orbits. So the side that's facing the sun at the point where it's closest to the sun (perihelion) in orbit X, is the point farthest away from the sun at perihelion in obit X+1, and the point closest again on orbit X+2.

fayanora
Jun. 30th, 2008 06:21 am (UTC)
*Tries wrapping brain around this. Fails*
kengr
Jun. 30th, 2008 02:43 pm (UTC)
See explanation below. Also, next time you are over, read the chapter on Tidal Forces in "Indistinguishable from Magic". Dr. Forward did a good job explaining them and has diagrams that make how they work easier to understand.
d4b
Jun. 30th, 2008 11:19 am (UTC)
How/why would that "lock", then?
kengr
Jun. 30th, 2008 02:40 pm (UTC)
There's an asymmetry in the moon and Mercury (and most other bodies for that matter.
Since tidal forces vary as the *cube* of the distance from the center of the body (say, for example the tides the Earth induces in the Moon) small unevenesses in the distribution of mass lead to a lot of extra drag which will slow down a body's rotation if it is rotating faster than it's orbital period.

So the moon got locked with one face towards earth.

Mercury has a highly elliptical orbit, so it is moving at very different speeds at perihelion (fast) and aphelion (slower).

The forces work out so that the 2:3 lock is stable (rotational inertia is a lot higher than the tidals forces once you get into that resonance) so even though it might be *more* stable if it got into a 1:1 lock, the force required to get out of the 2:3 lock are high enough that it'd take a *major* jolt to change it.

It helps to remember that tidal forces pull away from the center of the orbiting body along the line joining the centers of the two bodies. They also push in along the plane at right angles to it. So the cross section in any plane that contains that line is elliptical rather than circular.

So the tidal tugs at perihelion are the same if you have the body facing either of two directions.

And yes, Earth will become tidally locked with the moon some day. But that's a long way's off.

And in the meantime, since the moon is tidally locked the rotation it is leeching from earth as it lengthens our days is making the moon spiral farther out.
(Deleted comment)
kengr
Jul. 1st, 2008 05:20 am (UTC)
Depending on how you look at it he Moon *is* another planet. It's orbit around the Sun is always concave towards the sun, never convex. Unlike all the other moons. :-)

So strictly speaking it's not orbiting Earth, it's orbiting the sun with the earth *heavily* modifying the orbit.

But in the sense it is orbiting the earth, it will be for a *long* time.
d4b
Jun. 30th, 2008 09:35 pm (UTC)
I read that twice, and still don't understand... but that's ok!

I'm sure it would make a very pretty animation, though. :-)
kengr
Jul. 1st, 2008 05:22 am (UTC)
nakednine
Jun. 30th, 2008 08:44 am (UTC)
rotation, rotation, rotation!

someday i will set up camp on the dark side of the mood.
d4b
Jun. 30th, 2008 11:19 am (UTC)
Wait... you only see one side!?...
fayanora
Jun. 30th, 2008 11:33 am (UTC)
Yeah, the ubiquitous man in the moon. What, you see more than one?
kengr
Jun. 30th, 2008 02:41 pm (UTC)
You mean the Maiden combing her hair? Or the Rabbit? (or the several other things different cultures have seen)
d4b
Jun. 30th, 2008 09:30 pm (UTC)
LOL!!

Made you pause, huh? ;-)
fayanora
Jul. 1st, 2008 11:15 am (UTC)
LOL!
wizwom
Jun. 30th, 2008 05:05 pm (UTC)
Mostly, in the end, because the moon is a solid rocky thing, rather than having a liquid core.
That means that the tides on it could always affect the same bits, and over a VERY long time, the heaviest bit came to face directly toward the Earth, and it would be very difficult to modify that now.
( 25 comments — Leave a comment )

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