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View Full Version : Technically Speaking, Does the Moon Rotate?

Fiery Phoenix
2009-Mar-20, 06:53 PM
I am having one hell of a time trying to conceive this. After all these years of being interested in astronomy, and after the countless related books/articles I've read and documentaries I've watched, I just found out that I wasn't exactly understanding the Moon's orbit around the Earth.

First of all, I know that we only see one side of the Moon due to the fact that its day length is equal to its orbital period. Funny thing is, I don't understand what I know. I keep hearing that one face of the Moon always faces the Earth, and in plain English, when one says that one face of ball B always faces ball A, this would imply that ball B does NOT rotate, but could still orbit ball A.

So, my question is, does the Moon ACTUALLY rotate on its own axis? If one side of it apparently always faces the Earth, then how come the Moon rotates in the first place? Or does that really mean the same LIT side always faces us?

In addition to written explanation, I demand some simulation (Video, GIF, etc). My head is like exploding right now. :doh:

tony873004
2009-Mar-20, 06:59 PM
In an intertial frame it does rotate. If you stood on the moon, you would watch the stars move. In a rotating frame set to the moon's period, the moon librates as viewed from Earth. Here's an animation.
http://en.wikipedia.org/wiki/Libration

From the moon's surface, you would watch Earth trace an oval around its average position in the sky.

Argos
2009-Mar-20, 06:59 PM

speedfreek
2009-Mar-20, 07:02 PM
And here is a good visualisation. (http://www.astro.washington.edu/users/larson/Astro150b/Lectures/EarthMoon/earthandmoon.html#rotmoon)

nauthiz
2009-Mar-20, 07:04 PM
The moon does rotate on its own axis, once every 27.3 days. It also completes one orbit around the earth every 27.3 days. It's the fact that these two periods are the same that causes the near side of the moon to always be facing us.

The Bad Astronomer did a video with a good visual illustration of what's going on. You can find it here (http://blogs.discovermagazine.com/badastronomy/2007/03/04/q-ba-episode-5-spin-doctor/).

hhEb09'1
2009-Mar-20, 07:06 PM
And a five-page thread (http://www.bautforum.com/against-mainstream/84783-moon-rotation.html) in our Against The Mainstream forum, where it was debated vigorously.

Fiery Phoenix
2009-Mar-20, 07:43 PM
And here is a good visualisation. (http://www.astro.washington.edu/users/larson/Astro150b/Lectures/EarthMoon/earthandmoon.html#rotmoon)

That is excellent! Thank you very much for the link! :)

It's funny how seemingly simple stuff turns out to be more complex than expected.

Argos
2009-Mar-20, 07:49 PM
That is excellent! Thank you very much for the link! :)

The owner of that site should be granted an award. It´s not the first time on this forum it helps enlightening people on the Moon´s rotation. It´s infallible [well, some might prefer a Ferrari] :)

DaveC426913
2009-Mar-20, 09:08 PM
If the Moon were somehow knocked free of the Earth, it would contrinue to rotate about its own axis at ~27 (Earth) days per rotation even as it drifted away.

Disinfo Agent
2009-Mar-20, 09:49 PM
Rotation is relative. The Moon does not rotate as seen from Earth... but it does rotate from the point of view of an inertial reference frame centered on the Sun... or on Mars... or on the Moon itself. Remember that the Sun rises and sets on the Moon, too. How could that be, if the Moon did not rotate?

grav
2009-Mar-20, 09:51 PM
If the Moon were somehow knocked free of the Earth, it would contrinue to rotate about its own axis at ~27 (Earth) days per rotation even as it drifted away.
Is that true? If I spun a ball on a string around my head and then let go, would it continue to rotate? What about a gear on an absolutely friction free axis that I spun around on a string that is fixed to the axis? Would it remain stationary to the fixed stars while I spin the string? Would it remain stationary or rotate when I let it go?

hhEb09'1
2009-Mar-20, 09:55 PM
Is that true? If I spun a ball on a string around my head and then let go, would it continue to rotate?And it starts to wrap the string around itself :)

I should make a video. I'll add it to my list, which includes the Burn New Leaves Principle.

You have to be careful, that the mass of the string doesn't cause its own effect. But I think there's a way to demonstrate that.

cjameshuff
2009-Mar-20, 10:03 PM
Rotation is relative. The Moon does not rotate as seen from Earth... but it does rotate from the point of view of an inertial reference frame centered on the Sun... or on Mars... or on the Moon itself. Remember that the Sun rises and sets on the Moon, too. How could that be, if the Moon did not rotate?

No. Rotation is absolute, and the moon can be seen to rotate in any inertial frame. If you were floating in place of Earth and turning to watch the moon, you would not be in an inertial frame, but would instead be rotating at the same rate as the moon.

nauthiz
2009-Mar-20, 10:07 PM
You have to be careful, that the mass of the string doesn't cause its own effect. But I think there's a way to demonstrate that.

You could steal Foucault's technique (and add excitement to the demonstration) by setting the string on fire right where it attaches to the mass before you start swinging it. When the string burns through, the mass will be set free. And depending on how elastic the string is, you'll have a burning piece of string snapping back toward your face. :)

hhEb09'1
2009-Mar-20, 10:16 PM
And depending on how elastic the string is, you'll have a burning piece of string snapping back toward your face. :)Awesome. Now if we could only make it LOUD

Sam5
2009-Mar-20, 10:31 PM
So, my question is, does the Moon ACTUALLY rotate on its own axis? If one side of it apparently always faces the Earth, then how come the Moon rotates in the first place? Or does that really mean the same LIT side always faces us?

It revolves around the earth-moon barycenter. :)

hhEb09'1
2009-Mar-20, 10:34 PM
It revolves around the earth-moon barycenter. :)Which barycenter is itself moving rapidly around the sun. :)

Spaceman Spiff
2009-Mar-21, 01:08 AM
If a magic hand came down and held the Moon against rotation, ask yourself what would you see? Would you see the same face of the moon (approximately, setting aside libration) at all times?

hhEb09'1
2009-Mar-21, 01:27 AM
If a magic hand came down and held the Moon against rotation, ask yourself what would you see? Would you see the same face of the moon (approximately, setting aside libration) at all times?It would be hidden by the magic hand.

cjameshuff
2009-Mar-21, 02:33 AM
If a magic hand came down and held the Moon against rotation, ask yourself what would you see? Would you see the same face of the moon (approximately, setting aside libration) at all times?

If its rotation were stopped without changing its orbital motion, you would see all sides of the moon as it orbits the Earth. Phases would continue on as usual, so each full moon would appear rotated by about 1/12th of a full rotation.

Disinfo Agent
2009-Mar-23, 11:38 AM
No. Rotation is absolute [...]Not insofar as the presence or absence of rotation depends on whether one's reference frame is inertial or not.

[...] and the moon can be seen to rotate in any inertial frame.And I never said otherwise.

01101001
2009-Mar-23, 12:14 PM
If a magic hand came down and held the Moon against rotation, ask yourself what would you see? Would you see the same face of the moon (approximately, setting aside libration) at all times?

My magic hand would not come down from nowhere, but come up from Earth and grab that big silvery marble hard.

Why, I think it's grabbing it right now!

cjameshuff
2009-Mar-23, 02:51 PM
Not insofar as the presence or absence of rotation depends on whether one's reference frame is inertial or not.

And I never said otherwise.

You contrasted Earth's point of view with inertial frames in the sun, on Mars, and on the moon itself, implying you were comparing to an inertial frame on Earth as well...you gave no reason to make an exception for the Earth frame, and no indication you were doing so.

In any case, if you're using a rotating frame of reference, you must take into account that you are doing so. The fact that you only see one face of the moon from a co-rotating reference frame at the position of its barycenter with Earth is just an indication that it is rotating. The moon rotates, period.

Disinfo Agent
2009-Mar-23, 09:07 PM
You contrasted Earth's point of view with inertial frames in the sun, on Mars, and on the moon itself, implying you were comparing to an inertial frame on Earth as well...I was not. tony873004 had already noted, above my post, that:

In an intertial frame [the Earth] does rotate....so making such a comparison would be nonsensical. Gimme some credit!

The fact that you only see one face of the moon from a co-rotating reference frame at the position of its barycenter with Earth is just an indication that it is rotating. The moon rotates, period.LOL. Where were you during all those geocentricity threads? :D

Sam5
2009-Mar-23, 09:10 PM
Rotation is relative. The Moon does not rotate as seen from Earth... but it does rotate from the point of view of an inertial reference frame centered on the Sun... or on Mars... or on the Moon itself. Remember that the Sun rises and sets on the Moon, too. How could that be, if the Moon did not rotate?

Hi,

Are these people “rotating” on their own axis, or are they “revolving” around the axis of the turntable?

Are any of them “rotating” about their own axis at any time during the video?

hhEb09'1
2009-Mar-23, 09:15 PM
Are any of them “rotating” about their own axis at any time during the video?Sure, their own axis moves with them--just as the Earth rotates about its own axis as it goes around the Sun, the axis travels with the Earth.

Spaceman Spiff
2009-Mar-23, 09:18 PM
My thought experiment was purposefully left vague so that one might think about and discover these things. :)

Disinfo Agent
2009-Mar-23, 09:24 PM
Hi,

Are these people “rotating” on their own axis, or are they “revolving” around the axis of the turntable?

Are any of them “rotating” about their own axis at any time during the video?hhEb09'1's reply above is more insightful than the one I'm about to give, but I would say that the two axes you mentioned coincide, to a first approximation. I'd say that the people in the turntable are rotating about themselves and revolving around the axis of the turntable, and that their motion is like that of a 'static' object on the surface of the spinning Earth, relative to the Earth, although this is only an approximation.

Sam5
2009-Mar-23, 10:18 PM
I'd say that the people in the turntable are rotating about themselves and revolving around the axis of the turntable, and that their motion is like that of a 'static' object on the surface of the spinning Earth, relative to the Earth, although this is only an approximation.

Ok, this school animation tends to support your point of view:

http://www.mathsisfun.com/geometry/rotation.html

First, DON’T change the “Rotation Point” numbers, and then click on “quick rotate”.

Next, DO change BOTH “Rotation Point” numbers up to 60 or more, and then click on “quick rotate”.

Click on "refresh" of the whole page to reset the animation.

Personally, I would call only the first situation a "rotation on the object's own axis", while I would call the second situation a "revolution" of the object about the point.

Added: Could you tell me how I might be incorrect about that?

Amber Robot
2009-Mar-23, 10:30 PM
Hi,

Are these people “rotating” on their own axis, or are they “revolving” around the axis of the turntable?

Are any of them “rotating” about their own axis at any time during the video?

Yes. In fact, you can see them still continue to spin as they slide off the turntable at about t=15 seconds. And at the very end, when there's one guy left, it's pretty obvious that he's spinning.

If you can get dizzy standing on one of those things then you are definitely rotating about your own axis.

Sam5
2009-Mar-23, 10:51 PM
Yes. In fact, you can see them still continue to spin as they slide off the turntable at about t=15 seconds. And at the very end, when there's one guy left, it's pretty obvious that he's spinning.

If you can get dizzy standing on one of those things then you are definitely rotating about your own axis.

Ok, very interesting. Thanks.

If the earth is an “oblate spheroid” all around its equator because it is rotating on its axis, then shouldn’t the moon also be an “oblate spheroid” all around its equator too, if it is also rotating on its axis?

mugaliens
2009-Mar-23, 10:57 PM
Yes, and it's equitorial radius is 0.125% larger than it's polar radius. The reason it's not more has to do with the fact that the Earth rotates 1,000 mph at it's equator, which is quite a bit of centripetal acceleration going towards it's oblateness. The Moon, on the other hand, rotates at a more stately 10.35 mph.

Heck, given a good enough suit, one might be able to bound around the Moon at that speed! Put another way, you could drive around the Moon in a month and always remain in daylight.

That would be a first...

Tucson_Tim
2009-Mar-23, 10:58 PM
Moon info from wiki: (flattening due to rotation)

Mean radius 1,737.10 km (0.273 Earths)
Equatorial radius 1,738.14 km (0.273 Earths)
Polar radius 1,735.97 km (0.273 Earths)
Flattening 0.001 25

Sam5
2009-Mar-23, 11:03 PM
Yes.

Ok, that’s what I thought for about 53 years. But suddenly last week, I found out that it’s not true. I suddenly found out that the moon is oval.

http://physics.uoregon.edu/~jimbrau/BrauImNew/Chap08/FG08_11.jpg

http://www.sciam.com/article.cfm?id=flip-flop-did-the-moon-do
“(Wieczorek says that the tidal bulges on the lunar surface induced by Earth's gravity, which deform the moon into an elongated shape that helps stabilize its position, would prevent the moon from easing into synchrony at any intermediate orientation.)”

Sam5
2009-Mar-23, 11:08 PM
Another link to the moon being elongated:

http://74.125.47.132/search?q=cache:srZaqlharsIJ:physics.fortlewis.edu/astronomy/astronomy%2520today/chaisson/AT308/HTML/AT30803.HTM+moon+elongated&cd=4&hl=en&ct=clnk&gl=us

(scroll down the page at this link):
http://74.125.47.132/search?q=cache:0a__21v60RMJ:physics.uoregon.edu/~jimbrau/astr121/Notes/Chapter8.html+moon+elongated&cd=5&hl=en&ct=clnk&gl=us
The Moon's Orbit around the Earth
Orbits Earth in 27.3 days
Rotates synchronously with orbit (same side always faces Earth)
From the Earth, we see the same side of the Moon all the time
From the Moon, the Earth appears to hang fixed in one place in the sky, slowly rotating on its axis
Elongated shape of the Moon results in the synchronous orbit
Presumably, the Moon's elongated shape was created long ago by the Earth's tidal influence when the Moon was young, and the moon is now "tidally locked"

Sam5
2009-Mar-23, 11:13 PM
Yes.

Note: I responded to your original "Yes." answer, but not to your edited answer.

So, is the moon "oval" and "elongated" at the equator or not? This is news to me.

speedfreek
2009-Mar-24, 12:18 AM
Isn't the pertinent question whether the polar circumference is smaller than the equatorial circumference? Is the oval still oblate?

Sam5
2009-Mar-24, 12:29 AM
Isn't the pertinent question whether the polar circumference is smaller than the equatorial circumference? Is the oval still oblate?

I'm trying to look up that information now.

WaxRubiks
2009-Mar-24, 02:21 AM
The owner of that site should be granted an award. It´s not the first time on this forum it helps enlightening people on the Moon´s rotation. It´s infallible [well, some might prefer a Ferrari] :)

who might prefer a Ferrari?:confused:

astromark
2009-Mar-24, 07:43 AM
I, for one...!

Another interesting trivia... as the moon comes around Earth and heads toward the sun it does actually accelerate and as she goes around to be moving away from the sun 12 days later slows some... Its not a lot, but measurable... and all of this goes on every 27.3 days or so... fascinating.