What about the planet/moon axis? Again, it’s what I would expect when talking about a specific celestial boby and would assume if I were to ask a question about a celestial body rotation.
Anyway, if you define a plane for the two first axis, you don’t have much choice left about the third one which has to be perpendicular to the other two. Well, I guess you could pick some random point on this plane to have the third axis go through, but again, doesn’t it make quite a lot of sense to pick the body’s axis of rotation? Especially when you’re precisely talking about this body’s rotation.
It’s really counter-intuitive to me that you’d rather pick the Earth as the center of the reference frame in such a thread.
What’s important about the reference frame is not its axes or center, but what vectors are arbitrarily considered at rest. If the line from Bogotá to Singapore is at rest the Earth rotates zero times in a 12-month period; if the Earth-Sun line is at rest the Earth rotates 365¼ times per year; if the Earth-Betelgeuse line (or Milky Way-Andromeda line) is at rest the Earth rotates 366¼ times per year.
Incorrect.
By definition, an object rotates on its axis, its axis being an imaginary line passing through its center. To say anything else, is wrong. The fact that it also revolves around the earth is irrelevant. From any viewpoint in the universe other than the surface of the earth, the moon clearly rotates on its axis, completely independent of the Earth.
As matter of fact, if you could remove the Earth, the Moon would continue to revolve around the sun in nearly the same path it follows now, still rotating on its axis.
Is that what that line is? I’ve been listening to that bit for coming up on 26 years, and never have been able to hear it right.
Ignorance fought!
Rotation is not relative. Otherwise, James Bond could define himself and the centrifuge arm as stationary - with the building, world and universe whirling about him - and he would have no reason to experience near-fatal forces at the hand of Hugo Drax’s henchman.
Yes — if you’re thinking of the Sir Bedevere line from Monty Python and the Holy Grail.
If, on the other hand, you’re referring to Jack Nicholson’s last line in Chinatown, then no, you’re way off.
You + Charlotte = beer
However, when Drax’s henchman sets the rest of the universe revolving around Bond, general relativistic effects produce some uncomfortable g-forces on him.
Except you can’t do that without moving things at velocities greater than c, which is one reason why rotation isn’t relative.
So if rotation is not relative, does the moon rotate or not? Should be pretty simple to answer.
It is – throw something, and see if there’s a Coriolis effect. (Err, well, it’s theoretically simple, actually going there and stuff has proven a bit tricky in the past.)
So one effect of the moon stopping its rotation would be that this effect disappears.
Concur. It’s measurable (that doesn’t mean I know the measured value)
You could also set up Foucault pendulums at various places, and measure their periods of precession. The closer you get to one of the poles, the shorter the period. When you finally reach a pole, the period there will match the rotational period of the body you’re on.
In the case of the Moon, that’s going to require a bit of patience, since the rotational period is once per 27.3 days. (Of course, you probably don’t need to wait for a full precessional cycle at each pendulum.)
Whaaaat? That’s so incorrect, I don’t know where to start. For one thing, just because you’re an object doesn’t mean you’re rotating. Second, just because you’re rotating, and there’s an axis, it doesn’t mean it’s through your center.
If I go out to the playground and grab a pole with one hand and run around the pole, would you suggest that I’m rotating around an axis along my spine? Of course not. I rotate around the pole. The pole is my axis.
Without a pole, if I just spin in place with my arms out, my hand, throughout the whole spin, always faces wrist-first toward my body. At no point in time does the wrist spin out of alignment with the rest of my hand. It’s like it’s attached to an arm that runs all the way to my body. Would you say that my fingers are rotating around a vertical axis through my wrist? My wrist around my elbow? My elbow around my shoulder? No, of course not. All those things rotate around my spine. The spine is my axis.
What about my pocket watch example? What if I just twirl a pocket watch around my its chain? My watch, throughout the whole spin, always faces noon-first toward my hand. At no point in time does noon spin out of alignment with the 6. It’s like it’s attached to a chain that runs all the way to my hand. Would you say that the watch rotates about an axis through the center of the clock face? No, of course not. It rotates around my hand.
Now for the moon. The front of the moon, throughout the whole orbit, always faces toward the Earth. At no point in time does the front spin out of alignment with the rest of the moon. It’s like it’s attached to an invisible arm that runs all the way to Earth’s center. Why would you suggest that the moon spins about it’s own center? Of course you shouldn’t. It rotates around the Earth’s axis.
If you understand why my hand doesn’t rotate around a vertical axis through my knuckles, or why my watch doesn’t rotate around itself, you’ll understand why the Moon doesn’t rotate around an axis through its own center.
Hell, by your criteria for “axis”, you could pick any point on the Moon and say the Moon rotates around it. You could pick a surface point, a point in the center, or even a point 20 feet deep in the dirt and say that that’s where the axis is. By your standard, you could pick an infinite amount of axes and they’d all be right!
One thing I hadn’t mentioned, and I thought I had, was that if the moon were tidally locked to the sun, it’d have to reverse rotational direction…or at least change speeds. Imagine that it’s in front of the Earth’s path and has an observer seeing sunrise. When the moon passes between the Earth and Sun, in order to track the sun and keep the obersver in sunrise, it’ll have to spin right. But as time goes on and the Earth moves to another quadrant around the sun, the Moon would have had to have a net spin of 90 degrees to the left. So it’ll have to wobble back and forth over the course of the month, continually changing speeds.
Surely. The question is…rotate about what?
A plane does not define two axes. You need a plane and a point. Imagine drawing a cross on a playing card and laying it on the table. You’ve defined the axes and the table as the plane, but you can still spin the card. You need to define one more axis from the plane by saying “the top of the card shall point toward this M&M” or something like that.
What people are talking about here is the Moon’s absolute rotation around its own axis. Both of these things are well defined, and independent of the Earth, the Sun, or anything else.
Define a coordinate system, any coordinate system you like, that’s fixed to the solid body of the Moon. Do some experiments on its surface — perhaps with Foucault pendulums like I suggested, or projectiles, or gyroscopes, or something else — and you can then calculate the Moon’s axis of rotation as a line (specified in your current, provisional coordinate system), as well as the direction and rate of spin around that axis.
Presumably, once you have the rotational axis, you’ll want to define a more convenient coordinate system with that as one of your cardinal axes (maybe Z), and with the center of mass at the origin. If you fix the two remaining axes (X and Y) in the body of the Moon, you’ll have a co-rotating coordinate system, which is useful for many things. You might also want to define an “inertial” coordinate system that subtracts out the rotation, which is useful for other things.
[QUOTE=Chessic Sense]
The front of the moon, throughout the whole orbit, always faces toward the Earth. At no point in time does the front spin out of alignment with the rest of the moon. It’s like it’s attached to an invisible arm that runs all the way to Earth’s center. Why would you suggest that the moon spins about it’s own center? Of course you shouldn’t. It rotates around the Earth’s axis.
[/quote]
I would say it revolves around the Earth, and it rotates (or spins) around its own axis. (More accurately, both the Earth and Moon revolve around a common point, but since the Moon’s mass is so much smaller, that point is inside the Earth.)
And I still wouldn’t say the Moon rotates or revolves around the Earth’s axis, because the Earth’s rotational axis is not the same as an axis perpendicular to the Moon’s orbital plane. Theoretically the Moon could have a polar orbit, in which case the difference would be more apparent.
If a sailor travels around the world, and returns to his home port, his wife and he have a different count for the number of days he was at sea. So, obviously, the rate of the Earth’s rotation is entirely subjective.
Tris
I’m assuming you’re joking. He’s performed an operation of rotation by going around the earth, to be taken in addition (or subtraction) to the rotatations of the Earth that both he and his wife experienced.
That’s a funny argument to make. Do you think that, if the watch spun on an axis through its center, noon and six would spin out of alignment? If not, why is the fact that it doesn’t evidence against it spinning around its center?
The simple fact is that the watch undergoes rigid rotation. The system of you spinning the watch around has a different total angular momentum depending on whether or not the watch spins around itself. Angular momentum is a vector quantity which you can break down in multiple ways, depending on the reference frame you wish to use; it’s perfectly sensible to use a decomposition consisting of the revolution of the watch around your hand, and the rotation of the watch around itself, and it’s also perfectly sensible to talk about that rotation on its own.
It’s no different with linear momenta, but perhaps easier to visualize. Any system of mass points has a total momentum, defined by the motion of its center of mass. But every mass point also has its own momentum, which it is sensible to talk about when talking about that mass point individually.
Thus, when talking about the moon, it is sensible to talk about its angular momentum, which is given by its moment of inertia and its angular velocity, the latter of which defines its angular speed and its rotational axis. This is a measurable quantity, using for instance the Foucault’s pendulum method Bytegeist has outlined.
From Wikipedia’s page on Magellan: