Why doesn't the Moon Spin?

**RichD** · Feb7-04, 04:28 PM

Why doesn't the Moon spin on its axis?

If someone is going to answer this with "Why should it" then Why does the Earth (and many planets) spin on their axis?

I'm sure someone told me the answer once, but I can't remember, and it's bugging me...

**GRQC** · Feb7-04, 04:38 PM

Originally posted by RichD
Why doesn't the Moon spin on its axis?

If someone is going to answer this with "Why should it" then Why does the Earth (and many planets) spin on their axis?

I'm sure someone told me the answer once, but I can't remember, and it's bugging me...

It does spin (with a period of about 28 days), that's why we always see the same side as it revolves around the earth.

**Janus** · Feb7-04, 05:29 PM

To anticipate your next question of "Why does the moon rotate in the the same time as it orbits the Earth?", the answer is "tides".

Just as the Moon causes tides on the Earth, the Earth exerts a tidal force on the Moon. This force tends to stretch the moon out online the line joining it to the Earth.

At one time, the moon spun at a much higher rate then it does now. But the action of the Earth's tidal force on it caused friction which slowed it down unitl it reached the state iti is in now where it always faces one side towards the Earth.

(Actually, the moon kind of rocks back and forth in a motion called Libration. This is caused by the fact that the Moon does not have a perfectly circular orbit, and therefore its rotation will lag behind or speed ahead a little of its orbit at different points. )

The Moon's tides are also slowing the Earth's rotation, but since the Earth is some 81 times more massive that the Moon, it will still take some few billion years before the Earth always presents one side to the Moon.

**RichD** · Feb8-04, 12:14 PM

I really should have thought about the 28 day period, that makes perfect sense!

The anticipated answer was very interesting though, thanks for that.

**eagleone** · Feb8-04, 03:31 PM

Originally posted by Janus
The Moon's tides are also slowing the Earth's rotation, but since the Earth is some 81 times more massive that the Moon, it will still take some few billion years before the Earth always presents one side to the Moon.

Well that want happen, because our friend Moon is going to leave us before that date. Reason: tides again.

**Janus** · Feb8-04, 06:35 PM

Originally posted by eagleone
Well that want happen, because our friend Moon is going to leave us before that date. Reason: tides again.

While the Moon is receding due to this process, it will never leave Earth orbit completely because of it. The Earth should become tidally locked with the Moon by the time the Moon's orbit has increased by about 1.6 times its present distance and its oribtal period has increased to about 2 months.

In order for the Moon to break free of the Earth and follow it own independent orbit around the Sun, it would have to recede to more than 2.4 times its present distance.

But it is true that the Earth will in all likelyhood never become tidally locked with the Moon, but for another reason.

Before it happens, our Sun will expand into a Red giant, engulfing both the Earth and Moon.

**the_truth** · Feb10-04, 07:51 AM

"It does spin (with a period of about 28 days), that's why we always see the same side as it revolves around the earth."

Wrong.

Earth, during it's formation didn't have a moon, but there was a massive collision of some sort sending tons of earth into space. The debris orbitted the earth and beganm to form the moon. Whilst the moon was forming, not only was the debris affected by the forming moon, but by the earth's gravity. As a result one side of the moon is heavier and this has 'calibrated' the moon's orbit so that it always faces the earth.

The moon does 'spin' in a way. Though in reality it wobbles negligibly from side to side undergoing simple harmonic motion.

I think.

**jjalexand** · Feb10-04, 08:07 AM

I have noticed that the period of rotation of the moon pretty well matches the average rotational period of the sun, something like 28.35 days I think. I think they match to within a couple of percent.

The Sun has an 'average' period because it is fluid, and the north and south polar regions don't rotate at the same angular rate as the equator.

It seems unlikely to be a coincidence. The moon's period may have gradually tuned itself to the sun's, possibly via (earthly?) tidal mechanisms. Sorry if this is well known.

**Phobos** · Feb10-04, 10:20 AM

Originally posted by the_truth
"It does spin (with a period of about 28 days), that's why we always see the same side as it revolves around the earth."

Wrong.

Earth, during it's formation didn't have a moon, but there was a massive collision of some sort sending tons of earth into space. The debris orbitted the earth and beganm to form the moon. Whilst the moon was forming, not only was the debris affected by the forming moon, but by the earth's gravity. As a result one side of the moon is heavier and this has 'calibrated' the moon's orbit so that it always faces the earth.

The moon does 'spin' in a way. Though in reality it wobbles negligibly from side to side undergoing simple harmonic motion.

I think.

the_truth
While you are correct that the moon probably formed as a result of a massive collision during the Earth's early formational period, you are incorrect about the moon not spinning. Janus' explanation is correct.

Visualize it with 2 coins (both face-up). Now have one coin "orbit" the other, keeping the face toward the second coin at all times. You'll see that, in order to do this, it spins on its axis once per revolution around the other coin.

You are also correct that the moon does have a "wobble", called libration. Here's an awesome movie clip of the effect...
http://antwrp.gsfc.nasa.gov/apod/ap010218.html

It's not quite "negligible" (except of course to the casual observer) as it allows us to view about 59% of the moon.

**the_truth** · Feb10-04, 02:43 PM

eek, I said 'Wrong.' in an arrogant manner.

:(

During the formation of the moon, centripetal acceleration would have negated any effects of gravity the earth would have on the forming moon. Also the original rotation of the moon would have prevented it's elliptical orbit from making the moon a 'pear' shaped body.

I noticed this a few moments after I got up and walked away from the computer.

**tyroman** · Feb10-04, 04:38 PM

Try this;

Assume the moon is a sphere and you could suspend it from its geometric center (the center of its core); in a uniform gravitational field it would hang earth-side-face down.

Whatever the moon's origin (capture by or collision with the earth) it is unlikely to have remained molten long enough to become absolutely homogeneous (mass distributuon wise) and therefore its center of mass does not correspond with its geometric center.

A line from the center of the earth would pass through the moon's center of mass on its way to the moon's geometric center. If the moon's two centers DID correspond, then the moon could spin at any rate on any axis it chose.

Kepler's laws relate to the centers of mass of astonomical objects... not their geometric centers.

**chroot** · Feb10-04, 05:07 PM

Originally posted by the_truth
As a result one side of the moon is heavier and this has 'calibrated' the moon's orbit so that it always faces the earth.

That's also incorrect. As Janus explained, the cause is tidal locking. Many other bodies in the solar system exhibit the same behavior, for the same reason.

- Warren

**Janus** · Feb10-04, 06:15 PM

Originally posted by jjalexand
I have noticed that the period of rotation of the moon pretty well matches the average rotational period of the sun, something like 28.35 days I think. I think they match to within a couple of percent.

The Sun has an 'average' period because it is fluid, and the north and south polar regions don't rotate at the same angular rate as the equator.

It seems unlikely to be a coincidence. The moon's period may have gradually tuned itself to the sun's, possibly via (earthly?) tidal mechanisms. Sorry if this is well known.

The Sun's period of rotation is 24.6 days

The Moon's sidereal period(fixed star to fixed star) is 27.32 days, this is a difference of 10% not just a couple of percent.

The Moon's synodic period(full moon to full moon) is 29.53 days for a difference of 20%

And even this situation is temporary. As the Moon is moving slowly away from the Earth, its period of revolution is getting longer.

So yes, it is just a coincidence, and not even a very good one at that.

**Janus** · Feb10-04, 06:47 PM

Just a few points on the moon's Librations. There are four: libration of Latitute, libration of longitude, diurnal libration and physical libration. Only the last, and smallest of these is due to any actual "wobble" by the Moon itself.

Libration of latitude is due to the Moon's equatorial inclination to its orbit. We alternately see the Moon's North pole and South pole depending upon which point is is at in its orbit when we see it. (Much the way the Sun shines alternately on the North and South pole of Earth over the course of a year.)

Libration of longitude is due to the fact that while the moon's rotation rate is constant, it orbital speed is not. As the Moon approaches perigee it speeds up in its orbit, and as it approaches apogee it slows down. As a result, the Moon's rotation(which remains constant) lags behind and speeds ahead during the course of a month. This allows us to see an additional few percent of the Moon's surface.

Diuranl Libration is due the fact that over the course of time from moonrise to moonset we see the Moon from different angles as the Earth rotates. Thus allows us to see about 2% more of the Moon.

Physical Libration is due to the the fact that the moon is not perfectly spherical(it is slighty elongated towards the Earth), and that the librations of latittude and longitude cause the Earth's gravity to tug slightly off-center of this elongation. This actually causes a small alternating change in the Moon's rotation. This libration only amounts to about a mile on the Moon's surface.

**jjalexand** · Feb10-04, 07:45 PM

I think you are referring to the Sun's sidereal period, whereas I was referringto it's synodic period.

If you take the synodic period (which is the natural one to look at from the point of view of it's effect on earthly and lunar issues, I think you will find it is very close to 27.32, probably 27.35 (I may well have mis-remebered it as 28.35 instead of 27.35)

You mention both of the moons periods, why ignore the two solar periods, and pick the wrong one to boot?

**jjalexand** · Feb10-04, 08:15 PM

The following website gives the solar (presumably synodic) period of rotation period as 27.3 days (left hand column near bottom)

http://www.oarval.org/SSen.htm