An associate of mine has the charming habit of preemptively attacking people with counterintuitive facts, in order to correct them when they take the bait. “Say, would you agree that .999~ and 1 are not exactly equal?” he’ll say, and then sit there, eyes wide, waiting to strike. Since I read the Straight Dope, I didn’t fall for that one, but he did manage to catch me out the other day by asking if the moon rotates. I said I didn’t think it did, at which point he retaliated with an armload of references to the contrary, including a quote from a NASA website.
On the one hand, I’m certainly not going to argue with NASA about what the moon does. Having said that, I’ll be darned if I can wrap my head around their explanation. The site states: *“Over the eons, the gravitational forces of the earth have slowed down the moon’s rotation about its axis until the rotational period exactly matches the revolution period about the earth.” * NASA advises using a pair of softballs to illustrate the principle to oneself. However, I’ve tried this experiment, and still don’t grasp the concept. It seems to me that if I drilled a hole in a softball and threaded a rope through it, then swung the rope around my head, the softball would orbit me in the same manner as the moon does the earth, one side always facing toward me. But I don’t see how it makes sense to describe the softball as rotating on its axis while simultaneously revolving around me. Doesn’t rotation about an axis suggest that opposite sides of an object are traveling in opposite directions?
But the opposite sides of the Moon are travelling in opposite directions! Don’t sit on the Earth to watch it. Park yourself in orbit somewhere between the Earth and Mars and take a time-lapse film of the Moon. Lo and behold, it’s rotating once every ~28 Earth days.
But to an outside observer, the opposite sides of my bola-softball are going to appear to be traveling in opposite directions too as I whirl it around (assuming I don’t clock them in the head with it by accident). This is what confuses me, since it seems to imply that the property of rotation is entirely dependent on the observer’s perspective rather than any actual physical motion of the body in question. Wouldn’t a hypothetical Martian observer really be observing the overall rotation of the Earth-moon system as a whole, rather than the rotation of the moon itself on its axis?
Get a tennis ball, place it on a table. Now in the other hand, get another tennis ball, and hold it on the table. Now “orbit” the second tennis ball around the first one while keeping the same face towards it at all times. You’ll notice, that to do this, you have to rotate the hand holding the orbiting ball. If you do it without rotating your hand (and therefore the ball), the side facing the first tennis ball is constantly changing.
If you consider the moon and the earth in isolation, you can consider either one to be stationary and the other to be orbiting around it. If you consider the moon to be stationary and the earth to be in orbit around it, it becomes apparant that the moon is rotating slowly to keep one face towards earth at all times.
Well, look at it another way. Go sit on the Moon and watch the Sun and the stars whirl around you (“whirl” in the sense of taking a month to go round). Or go sit on your bola-softball and watch the distant landscape spinning round. Either of those should convince you that you are rotating.
Not convinced yet? Set up a Foucalt’s pendulum on the Moon. Or fire a missile from the Pole to the Equator and watch how the track is skewed by Coriolis. Admittedly the Coriolis effect is small; the lunar equator is only going around at a paltry (does the math) 9mph or so, roughly 1% of the speed of an object on Earth’s equator, but the effect should be measurable.
So if the pull of the Earth’s gravity were suddenly negated in some fashion, and the moon were released from its orbit and flew off into space in a straight line, would it still be rotating on its axis once every 28 days? It seems to me that if I were spinning my bola-softball around, and suddenly let the rope go, the softball would not display any tendency to spin at the same rate as it flew away from me; the side facing away would stay there.
Does rotation require energy? Right now the moon’s rotational period exactly matches its revolution period around Earth. Will it continue to slow down?
The moon would carry on rotating if the Earth somehow vanished. Your softball on a rope has the slight complication of the drag of the rope itself, which would introduce too much messy stuff into the mechanics for the rotation of the ball to be all that visible. But if you could come up with some means of uncoupling the rope from the ball, you might see such an effect.
No, it would spin at the same rate. That would be an interesting experiment, no?
Your previous comment about “the property of rotation is entirely dependent on the observer’s perspective rather than any actual physical motion of the body in question” is sorta true, obviously. If you traveling around the earth once per day (like a geosynchronous satellite) you’d never the earth rotate either. We know the earth rotates, once per 23h 56m, relative to the stars. There’s even a centrifugal bulge at the equator of about 20km.
Relative to the sun, the earth rotates once in about 24hr, naturally.
The moon is tidally locked to the earth. Tides are slowing the earth down, and the moon is also slowing down as it does.
I think the key flaw in the softball-on-a-string analogy is that there is a difference between a softball being bound to your hand by a string and the moon being bound to the earth by gravity. While it’s true that the string in your analogy generates the centripetal force necessary to create the “orbit” of the softball around your hand, the string introduces a factor that is not present in the earth-moon system - the string’s attachment point to the ball. This attachment restricts the rotational motion of the ball. Imagine trying to model an earth-moon system in which the moon is revolving around the earth and at the same time rotating relatively quickly (say, once every day rather than every 28). Clearly this can’t be done without a pretty ingenious string.
The string does, as you say, force the ball to rotate just so it has the same face inward at all times, but this is due to the fact that the centripetal force here is exerted at a point on the ball’s surface. From the point of view of the earth, the gravitational force exerted on the moon to generate its orbit is exerted at its center of mass - in no way inhibiting rotational motion of the moon. [Although, as I see on preview, RM Mentock has correctly pointed out that tidal forces are (slowly) slowing the periods of rotation.]
This should more properly be “From the point of view of the center of mass of the earth-moon system…” which is more correct, clunkier, and ultimately doesn’t change the argument. Carry on.
The moon always keeps the same side facing the earth. Inthis drawing of one circuit of the moon around the earth the dark half has obviously faced all directions. So the moon must have rotated once on its axis for one revolution around the earth.
From the point of view of someone standing on the surface of the Earth, at the center of the Earth with X-ray vision (and a really good pressure/temperature suit), or at the gravitational center of the Earth-Moon doublet, the Moon keeps one face turned to the Earth at all times.
But from any other vantage point, it does in fact rotate. Another thought experiment: getting a really good temperature-and-pressure suit and SPF-1,000,000,000 sunblock, position yourself “on the surface of the sun,” i.e., on the edge of the chromosphere, with ability to observe the Moon from there, using a telescope made of materials that won’t melt at that temperature of course. As you observe it in orbit, it will turn, in sequence, so that every portion of it is visible to you, over a 28.5-day period.
And some more nitpicking: People here have been using 28 days for the length of the moon’s siderial day, but it’s actually closer to 27 days (27.3).
Right, a body’s apparent rotation rate is going to be different if you view it from a rotating or revolving system.
But it’s the sideral period that really matters physically (I’ll argue), since that’s the body’s intrinsic rotation period with respect to the fixed stars. So for example, to the extent a body deforms from its rotation, becoming an oblate spheroid, the amount of that oblateness will depend on its siderial rotation rate. (Not that the Moon is very oblate; it’s quite the sphere. But take a look at Jupiter sometime.) This is also the rate to use if you want to calculate the apparent “centrifugal force” reducing your effective weight when you stand on the body’s surface.
Constant rotation in a frictionless environment needs no additional energy to maintain itself. A rotating solid body in the middle of deep space would keep on rotating forever, in principle.
However, gravity’s tidal forces induce friction, and with it a loss of rotational angular momentum. This has happened and continues to happen for both Earth and Moon, which gradually but relentlessly slow each other down. (I think the Earth’s day used to be only 18 hours long, back when it was new.)
If there’s enough time left before the Sun destroys us all — and I don’t remember now whether that’s the case — then the Earth and Moon will both become tidally locked to each other, with the Moon orbiting much further away, and with both bodies’ rotation periods synced to the orbital period.
The moon’s pull on the earth also causes a tidal bulge in the earth pointed nearly at the moon. The bulge follows the moon and the earth rotates through it. It is analogous to the flat spot on a tire where the tire contacts the road. That spot stays in one place while the tire rotates through it. This constant deformation and relaxing of the matter of the earth generates heat, just like the tire gets hot, and that takes energy out of the earth’s rotation so that the earth is gradually slowing its rotation.
In addition, the earth’s gravitational bulge is pulled slightly ahead of pointing directly at the moon because of the earth’s rotation. This results in a pull on the moon which increases is orbital velocity. So at any time the moon is going slightly too fast for its orbital distance from the earth. As a result it continually moves slightly further away from the earth as a rate, I think, of about 1 cm/year.
Since the moon is tidally locked to the earth its rotation rate on its own axis is slow as the earths rotation on its axis slows. And the fact that the moon is gradually receding means that its period is gradually getting longer which further slows its rotation around its own axis.
So yes, it will continue to slow down. Evenentually, assuming the sun doesn’t explode first, the earth could become tidally locked to the moon and always present the same face to the moon. Whether is does that or not depends upon how strong the moon’s tides are on earth. As the moon recedes the tides become weaker and the earth’s tidal bulge becomes less and the slowing of the earth’s rotation less. Calculation of the results of these conflicting effects is beyond me.
