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goobgubgoob
2012-Sep-15, 12:08 PM
The moon surely rotates around the earth's axis. Not it's own. But this is just another way of saying that it orbits the earth.

You never seen an earth rise or earth set if you are on the moon.

What is the logical difference between the moon, and space station. Nobody would say that a space station is rotating on it's axis.

tusenfem
2012-Sep-15, 12:30 PM
Unfortunately you are wrong, but it is an often enough mistake.
In order for the moon to keep the same side pointed at the Earth during her orbit around the Earth, she must make a full rotation around her own axis in one orbit.
Go to the moon and look up at the stars, and you will see that they also turn through the firmament, just like on Earth, it just takes a bit longer.
Indeed, you will see no Earthrise, and that is because the moon's orbital period and rotional period are the same.

antoniseb
2012-Sep-15, 12:34 PM
We get questions like this a few times a year. It is sometimes hard to find the right way to get the asker to conceptualize it correctly, but let me try this. Suppose that you are observing the sky from the Sea of Moscow (middle of the far side). Over the course of 27.3 days you see the entire sky revolve around you, including having sunrises and sunsets. You'd never see the Earth, and never think to yourself that there isn't some rotary motion going on with a 27.3 day frequency. If you decided on the Copernican (as opposed to Ptolemaic) model, you'd assume the Moon was rotating... and you'd be right.

goobgubgoob
2012-Sep-15, 12:38 PM
Go to the moon and look up at the stars, and you will see that they also turn through the firmament, just like on Earth, it just takes a bit longer.
[/quote]

In order for the stars to move through the firmament, all you need is for the moon to be rotating. You don't need it to be rotating about it's own axis. The moon is rotating, but surely around the axis of the earth.
If the earth always had one face towards the sun, the stars would still be moving. We would still be rotating. But not on the earth's axis.


Indeed, you will see no Earthrise, and that is because the moon's orbital period and rotional period are the same.

but that's because by 'rotational period' you actually mean 'orbital period'. It's not like it is some remarkable coincidence that they are exactly the same.

goobgubgoob
2012-Sep-15, 12:39 PM
the fact that the moon is rotating doesn't mean that it is rotating on it's own axis.

pumpkinpie
2012-Sep-15, 01:21 PM
You can try an experiment. You are the Moon, and choose something to be Earth that you can walk around. As you do so, stay facing Earth. When you get halfway around the circle you'll see you are still facing Earth but are now facing the other side of the room. You've rotated!

Strange
2012-Sep-15, 01:25 PM
If the earth always had one face towards the sun, the stars would still be moving. We would still be rotating. But not on the earth's axis.

Wrong. If the earth did not rotate on its axis, the stars in the sky would be fixed and the sun would rise and set once per year.

goobgubgoob
2012-Sep-15, 01:29 PM
You can try an experiment. You are the Moon, and choose something to be Earth that you can walk around. As you do so, stay facing Earth. When you get halfway around the circle you'll see you are still facing Earth but are now facing the other side of the room. You've rotated!

but everybody agrees that the moon does rotate. The question is rotating around which axis. I say it is the earth's, and not it's own

Strange
2012-Sep-15, 01:30 PM
You can try an experiment. You are the Moon, and choose something to be Earth that you can walk around. As you do so, stay facing Earth. When you get halfway around the circle you'll see you are still facing Earth but are now facing the other side of the room. You've rotated!

Perhaps a better demonstration (as goobgubgoob seems to think there is a difference between rotating around its own axis and rotating around the centre of the orbit) would be to walk around the object representing the earth while always facing the same direction (e.g. looking at the door of the room).

In this case you would not be rotating. But if you hold a piece of paper with an arrow on it and keep that pointing at the "Earth" you will find you need to keep rotating the sheet of paper as you go round.

goobgubgoob
2012-Sep-15, 01:30 PM
Wrong. If the earth did not rotate on its axis, the stars in the sky would be fixed and the sun would rise and set once per year.

so you are saying that if one side of the earth always faced the sun you would still say that the earth is rotating on it's own axis?

goobgubgoob
2012-Sep-15, 02:05 PM
It seems like you are saying, that anything that is not situation A below, is rotating on it's own axis.

When we say that the earth rotates on it's own axis, we mean situation C.
Would we say the earth is rotating on it's own axis, if it was situation B?

IF so, what is the logical difference between the movement of the moon, and the movement of an object on the surface of the earth? If the moon is rotating about it's axis, wouldn't you also have to conclude that your house is rotating about it's own axis?


17524

Strange
2012-Sep-15, 02:56 PM
Take all the pink dots in your figure B and lay them in a straight line. You will see they are rotating.

goobgubgoob
2012-Sep-15, 03:00 PM
yes. they are rotating, but by the same logic, so is your house, or a man made satellite if that is not a good example.

The black lines in B converge in the centre of the earth. Surely that is how the 'axis' is defined.

grapes
2012-Sep-15, 03:12 PM
When we say that the earth rotates on it's own axis, we mean situation C.
Would we say the earth is rotating on it's own axis, if it was situation B?
Yes, we would. :)

I would also say that it is rotating on the axis through the (central object). Physically, dynamically, they are equivalent. Both are true.

ETA: you do agree that it is not rotating in A, right?

grapes
2012-Sep-15, 03:19 PM
yes. they are rotating, but by the same logic, so is your house,
Yes, and large weather systems--which controls the direction of hurricanes. :)

goobgubgoob
2012-Sep-15, 03:22 PM
ETA: you do agree that it is not rotating in A, right?

yes. The black lines would not even begin to converge.

antoniseb
2012-Sep-15, 03:29 PM
Going back to the OP, another thing you might want to consider is libration (http://en.wikipedia.org/wiki/Libration). This is essentially the apparent wiggling of the Moon as seen from the Earth because it is not in a perfectly circular orbit around the Earth, but does rotate in a many orders of magnitude more nearly perfect constant rate on its axis.

goobgubgoob
2012-Sep-15, 03:39 PM
using this definition of rotating about it's own axis, is there anything that isn't doing so?

R.A.F.
2012-Sep-15, 04:15 PM
so you are saying that if one side of the earth always faced the sun you would still say that the earth is rotating on it's own axis?

Yes...because that is exactly what it would be doing.

Nereid
2012-Sep-15, 04:42 PM
Going back to the OP, another thing you might want to consider is libration (http://en.wikipedia.org/wiki/Libration). This is essentially the apparent wiggling of the Moon as seen from the Earth because it is not in a perfectly circular orbit around the Earth, but does rotate in a many orders of magnitude more nearly perfect constant rate on its axis.

This is a good way to illustrate the fact that it's not the Earth's axis that the Moon is rotating around, but it's (the Moon's) own.

As the Moon's orbit is inclined to the equatorial plane of the Earth, there would be no libration - of the north-south kind - if the Moon rotated on the Earth's axis. But there is, so it can't (the Moon cannot be rotating on the Earth's axis).

The east-west libration may also 'prove' this point, but it would be a much subtler case to make ...

wd40
2012-Sep-15, 04:44 PM
Going by visual observation only, if as an ancient "Lunacentric" without a telescope you decided on a Lunar version of the Ptolemaic model (ie that the Moon was the stationary center of the universe), would one have any difficulty reconciling the observed motions of the stars, Earth and Sun with the idea of an absolutely non-rotating, non revolving, non-translating Moon?



If you decided on the Copernican (as opposed to Ptolemaic) model, you'd assume the Moon was rotating... and you'd be right.

utesfan100
2012-Sep-15, 04:56 PM
I suppose that there is some ambiguity in the concept of a rotation axis. Usually we decompose the motion of the Moon's into its orbital motion and a rotation about its axis. It is also possible to decompose any rotation and translation into a pure rotation about a fixed axis.

For a simplified model of the Moon, perfectly tidally locked with a circular orbit, this would result in a rotation axis perpendicular to the Moon's orbital plane and through the Earth-Moon barycenter, not the rotation axis of the Earth. This axis, however, is not a constant of motion. We can define an instantaneous axis and examine how it varies with time.

The eccentricity of the actual orbit will cause the instantaneous axis to rotate around this axis, and the Moon's axial tilt to orbit of 7 degrees would cause this instantaneous axis to precess like the axis of a spinning top. Both effects have a period of roughly once per month.

grapes
2012-Sep-15, 04:58 PM
using this definition of rotating about it's own axis, is there anything that isn't doing so?

Not that we know of.

antoniseb
2012-Sep-15, 05:01 PM
... would one have any difficulty reconciling the observed motions of the stars, Earth and Sun with the idea of an absolutely non-rotating, non revolving Moon? I *did* specify that the observer was on the far side of the Moon, so the apparent movement of the Earth would not come into the picture.

utesfan100
2012-Sep-15, 05:03 PM
We can define an instantaneous axis and examine how it varies with time.
If we examine the instantaneous axis in a like manner for a circular Earth orbit with no axis tilt we would find that the instantaneous axis is roughly 410,000 km away from the Earth in the direction of the Sun, or just barely past the lunar orbit.

Hornblower
2012-Sep-15, 06:08 PM
Why do people say the Moon rotates on its own axis? They do so because it is a mathematically useful way of analyzing this compound motion. Treating it as a vector combination of rotation about the body's axis and a translational motion of the body along an orbital path simplifies the calculation of such things as its angular momentum. The naysayers on this topic appear to arbitrarily be treating rotation with respect to the orbital center as the only valid definition of rotation in the special case of a body in a closed orbit. I see no compelling mathematical reason for doing so.

R.A.F.
2012-Sep-15, 06:57 PM
Unfortunately you are wrong, but it is an often enough mistake.

Actually it is a mistake made WAY too often...the last thread regarding this was only 4 months ago.

pzkpfw
2012-Sep-15, 08:30 PM
Often (I think) people have this idea partly (mainly?) because of the tidal locking. They compare tidally locked Moon to a ball on a string, being swung around by a person, as though the Moon were being "pulled" toward Earth.

(In the ball and string case, I imagine the person is the centre of rotation of the ball+string system. The ball itself, has it's own axis it rotates at.)

More usefull, is the realisation that the Moon wasn't always tidally locked to the Earth. It once rotated at a "different" speed. In that situation, where the Moon orbits Earth and also spins at it's own rate, is it useful to say it rotates around Earth? I'd say no, it becomes clear that it's rotating around its own axis while also orbting Earth *. Now as it's spin speed slowly changes, as it becomes tidally locked, does it's centre of rotation suddenly "jump" from it's own axis to the Earth? I'd say that's silly. It spins around its own axis and also orbits Earth, and these two now "match". They did not always match, but the Moon always had its own rotation plus its orbit of Earth.

* After all, would someone say the Earth doesn't spin around its own axis, but merely "spins" around the Sun?

Ara Pacis
2012-Sep-15, 08:54 PM
I think the problem here is semantics. Rotate is what an object does when it spins. Revolve is what an object does when it moves around something else in an orbit.

Thus,
The Moon rotates around it's own axis, meanwhile it's also revolving around the Earth.

The moon has achieved tidal lock, which means that it's Rotation is synchronous with its Revolution, meaning they both take about ~29 days to come full circle. In other words, it rotates at a speed that allows the same face to turn towards the Earth as it move around the Earth.

If the moon were connected to the Earth with string, we might say it rotates on the Earth's axis because it would then be said to be a physical part of the Earth that moves with it. However, the moon actually revolves a lot slower than the Earth rotates (~709 hours vs. 24 hours, synodic), and it wiggles a bit and moves closer and farther away in it's orbit as well as moving up and down compared to the Earth's equator (it more closely follows the ecliptic), so for that and other reasons we consider the moon to be a separate object and rotating with respect to its self against the larger perspective of the sun and stars.

Still, one might simply say that the Earth's axis of rotation coincides with the moon's axis of revolution. However, that's not technically true, since the moon (and Earth) actually revolves around the barycenter, which is the fulcrum-like point of gravitational equilibrium between the masses of the two. The barycenter, while still within the surface of the Earth, is not at the same location as the Earth's axis of rotation. Moreover, the axis of revolution through the barycenter would be at an angle different from the angle of the Earth's axis of rotation. Furthermore, if we decide to consider that close enough to be truthy, then we may also consider that since the Earth also revolves around the barycenter (once per ~29 days), that it is also revolving around the moon), but we don't do that because it's technically untrue and even more confusing.

Jeff Root
2012-Sep-15, 09:19 PM
goobgubgoob,

As I said before, Gerald Kelleher (Oriel36) is the one
person you need to talk with. You should be able to
find him online somewhere.

Questions for you:

An astronaut is in space, always facing the same direction,
not rotating. There is a rotating object in front of him.

How can he determine whether it is rotating on an axis?

If the object is rotating on an axis, how can the astronaut
determine the location of that axis?

-- Jeff, in Minneapolis

speedfreek
2012-Sep-15, 10:09 PM
If the Earth were to suddenly and inexplicably disappear, and the Moon headed off in the direction it was going when the Earth disappeared, I am assuming the Moon would continue to rotate as it travelled through the Solar System and found its own orbit around the Sun.

Or only ever so slightly more realistically, if the Earth were suddenly destroyed such that it blew apart, the expanding cloud of debris would continue to orbit the Sun as it spread out, and the path of the Moon would be determined by the gravity of that debris until it was so thinly spread out that the Moon was simply just another object orbiting the Sun. Again, I am assuming the Moon would continue to rotate.

Centaur
2012-Sep-15, 11:05 PM
The Moon’s equatorial diameter is slightly greater than its polar diameter due to the apparent centrifugal force resulting from its rotation. If an astronaut were to place a Focault pendulum on the Moon, I’ll bet its plane of oscillation would appear to twist. That would confirm axial rotation just as it does on Earth.

While the Moon’s orbital angular velocity varies considerably over the course of a month in step with the Moon’s changing distance from Earth, the Moon’s rotational angular velocity around its own axis remains virtually constant. Thus we witness the effect of longitudinal libration as we alternately see further around the lunar eastern and western limbs. This indicates that the Moon's orbital revolutions and axial rotations are not continuously synchronized; it’s only over a significant length of time that their average periods become equal.

To take the OP's argument a step further, it could be said that the Moon orbits the Sun, since its heliocentric orbit is continuously concave toward the Sun. In that case the Earth and Moon simply perturb each other. While that further shifts the reference points, it does not eliminate the physical evidence that the Moon rotates about its own axis at a constant rate as expected for a rigid body unaffected by friction.

Tobin Dax
2012-Sep-16, 12:13 AM
I think the problem here is semantics. Rotate is what an object does when it spins. Revolve is what an object does when it moves around something else in an orbit.
I was going to post to make this very statement. IMO, using rotation to only describe spinning about an axis (and not to describe following a circular path around an object) helps to show that these are two distinct motions.


(Edit: changed about to around, for clarity.)

Jens
2012-Sep-16, 12:55 AM
Thus,
The Moon rotates around it's own axis, meanwhile it's also revolving around the Earth.


And in fact, it's unnecessary to state "rotates around its own axis," because rotation is always around its own axis. No other possibility exists.

Jens
2012-Sep-16, 12:58 AM
using this definition of rotating about it's own axis, is there anything that isn't doing so?

To add to what Grapes wrote, no, there isn't. And actually there couldn't be. Because not rotating is just one point along an infinite number of possibilities. An object can rotate at an infinite amount of speeds, and 0 is just one. So the possibility of the rotation of an object being perfectly 0 is 0%, so mathematically it won't happen (unless the universe if infinitely large, in which case I don't know how the statistics work).

Ara Pacis
2012-Sep-16, 01:47 AM
And in fact, it's unnecessary to state "rotates around its own axis," because rotation is always around its own axis. No other possibility exists.

Good point. Sometimes redundancy helps, sometimes it hinders explanation.

grapes
2012-Sep-16, 02:01 AM
And in fact, it's unnecessary to state "rotates around its own axis," because rotation is always around its own axis. No other possibility exists.
You can have multiple axes, usually "its own axis" refers to one through its center of mass, but it could rotate about other axes. Even, st the same time.

goobgubgoob
2012-Sep-16, 02:05 AM
Don't you think that...
http://i.picasion.com/pic58/5ded022904b96eba22a2fbc5c5d0ee29.gif (http://picasion.com/)

...is significantly different to...

http://i.picasion.com/pic58/565cb56e158caea252cbc72a7f905fa5.gif (http://picasion.com/)

?
Isn't one rotating about an internal axis, and the other about an external axis?

Jens
2012-Sep-16, 02:11 AM
Well sure they are different, but they are both rotating, and to me rotating is rotating, so it doesn't matter whether the object is moving in space or not. In your second picture it is going in a circular motion while rotating. You could also have the picture move in a straight line downward, and ask if it is the same. There are other movements involved, but the way I see it is that I can see it rotating at 90 degrees in each case, so they are rotating in an equivalent way.

Jens
2012-Sep-16, 02:22 AM
You can have multiple axes, usually "its own axis" refers to one through its center of mass, but it could rotate about other axes. Even, st the same time.

You may be talking about a changing rotation over time? Euler's theorem on rotation says that a rotating object can only have a single axis of rotation. There is always a single axis around which an object rotates, and the axis can be moving, of course. For a two-dimensional object this would be a point. So I see the issue the OP is raising as a case of a moving axis. It could move in a straight line, or could move in a circle around another object, but still the axis remains, and that's what I mean when I say that it can only rotate in one way.

goobgubgoob
2012-Sep-16, 02:38 AM
it doesn't matter whether the object is moving in space or not. In your second picture it is going in a circular motion while rotating. You could also have the picture move in a straight line downward, and ask if it is the same.

But i'm rotating it about the pink blob. It's not that it is rotating anyway, and then being moved in arbitrary directions. Isn't the pink blob in the second GIF the axis?

Jeff Root
2012-Sep-16, 02:45 AM
goobgubgoob,

Answering these two questions will lead directly to the
answer to your question.

An astronaut is in space, always facing the same direction,
not rotating. There is a rotating object in front of him.

How can he determine whether it is rotating on an axis?

If the object is rotating on an axis, how can the astronaut
determine the location of that axis?

-- Jeff, in Minneapolis

goobgubgoob
2012-Sep-16, 02:54 AM
How can he determine whether it is rotating on an axis?

if he sees more than one face of the object, whilst the background remains static.


If the object is rotating on an axis, how can the astronaut
determine the location of that axis?

observe the object over the time it takes for it to do a 360 degree rotation. The axis is on the average position of the object over that time.

i thought about it the first time you asked, but then started drawing pictures of a moving 3d helix and started going a bit mental

grapes
2012-Sep-16, 02:59 AM
You may be talking about a changing rotation over time? Euler's theorem on rotation says that a rotating object can only have a single axis of rotation.

Euler's theorem? This one?
http://en.wikipedia.org/wiki/Euler's_rotation_theorem

It doesn't say that at all! :)

Clearly, Sir Patrick is rotating about his nose in both diagrams (at least, according to me and you) but also clearly if his face were printed on a card in the second diagram, with the pink blob at the center of the card, it would be rotating around the (external) pink blob.


There is always a single axis around which an object rotates, and the axis can be moving, of course. For a two-dimensional object this would be a point. So I see the issue the OP is raising as a case of a moving axis. It could move in a straight line, or could move in a circle around another object, but still the axis remains, and that's what I mean when I say that it can only rotate in one way.Clearly not true, no matter what Euler says. :)

pzkpfw
2012-Sep-16, 03:04 AM
But i'm rotating it about the pink blob. It's not that it is rotating anyway, and then being moved in arbitrary directions. Isn't the pink blob in the second GIF the axis?

No, in the second gif there are two axis. You didn't draw the one in the middle of the face, but that doesn't mean it isn't there.

In space, free to "float" under the influence of gravity, the orbit of the Moon around Earth is a separate motion to the revolution of the Moon around its own axis.

Just as Earth rotates around its own axis while also orbiting the Sun.

Ara Pacis
2012-Sep-16, 06:59 AM
Don't you think that...
...is significantly different to...
?
Isn't one rotating about an internal axis, and the other about an external axis?

The top image is rotating around the pink mark. The second image is revolving around the pink mark. The second image is also rotating around an unmarked point.

This is assuming that the white-space is intended to represent a disconnection instead of a white card with both an image and a pink spot printed on it.

Jeff Root
2012-Sep-16, 11:29 AM
How can he determine whether it is rotating on an axis?
if he sees more than one face of the object, whilst the
background remains static.
Okay, this is a good start. I get at least one important
piece of information from that answer: You would say
that if an object is rotating, it has an axis of rotation.
It was possible that you would say that a rotating object
does not necessarily have an axis of rotation. That would
be okay, too, but then we would have had to try to figure
out when a rotating object has an axis of rotation and
when it doesn't.




If the object is rotating on an axis, how can the astronaut
determine the location of that axis?
observe the object over the time it takes for it to do a
360 degree rotation. The axis is on the average position
of the object over that time.
Okay, I see what you have in mind. I'm not saying that I
agree with it, but it looks completely reasonable.

I need to make an animation, too. I thought I could use
one I already had. It will take longer than I thought.

-- Jeff, in Minneapolis

grapes
2012-Sep-16, 11:29 AM
This is assuming that the white-space is intended to represent a disconnection instead of a white card with both an image and a pink spot printed on it.But you can see how that could be a legitimate interpretation?

We just have to convince everyone that the first is also legitimate.

ShinAce
2012-Sep-16, 12:15 PM
Here's the March thread about the same thing:
http://cosmoquest.org/forum/showthread.php/129845-Moon-movements-!!!
with pretty graphics!

Let's assume the moon doesn't rotate about its own axis. Now, if tidal forces are constantly 'pushing' the moon into a further, and hence longer, orbit, how can it be that it's not rotating? It changes orbit around the earth, but manages to remain facing us all the time! To do that, you need to adjust your orbital period about your own axis.

That's exactly what the moon does. As its orbit around the earth changes, it also changes its time to revolve. Tidal forces are what causes them to line up.

profloater
2012-Sep-16, 12:55 PM
greetings Goobgubgoob,
If you look at your diagrams and include the sun, the moon is also in orbit around the sun but is also spinning. If you were to view with a telescope from, say, Jupiter, you would see the moon going around the earth, the earth going round the sun, and as you watched you would see all the "faces" of the moon, so you would conclude the moon is spinning and you would note that it was at the same rate as the orbit around the Earth, a locked spin.

Paul Wally
2012-Sep-16, 03:13 PM
observe the object over the time it takes for it to do a 360 degree rotation. The axis is on the average position of the object over that time.

i thought about it the first time you asked, but then started drawing pictures of a moving 3d helix and started going a bit mental

The position of the object has nothing to do with its rotation. An object rotates if it's spinning around it's own axis. Picture a bicycle wheel suspended with an axle. You can then see whether the wheel is rotating by marking a spot on the rim (say 6-o-clock). It is then clear that if the wheel is not spinning, the axle can still move around freely. Now picture the axle moving clockwise around in a large circle with the wheel also spinning clockwise. For a tidally locked analogy: While the axle moves from 12 o-clock to 1 -o-clock on the big circle, the mark on the wheel rotates from 6 o-clock to 7 o-clock. There are two different circular motions: The spin of the wheel around it's own axle and the movement of the axle around the center of the big circle.

profloater
2012-Sep-16, 03:26 PM
If the moon were rigidly attached to the Earth and you treated the whole as a single body, that single body rotates about approximately the Earth centre, but the Moon is not attached, it is a free body with forces acting on it. The tidal forces have tidally locked the rotation of the moon but it is still treated as a free body.

trinitree88
2012-Sep-16, 03:58 PM
Goog. Note: 27.321582 days to rotate
27.321582 days to revolve
If you were compiling statistics about planetary bodies or their moons, and you found that two stats were identical to 8 significant figures.....that should catch your eye as "something is up here". Tidal locking has been known for decades...but computer simulations indicate that the effect is not forever, and other resonant ratios will occur. pete


SEE:http://en.wikipedia.org/wiki/Moon

djellison
2012-Sep-16, 04:16 PM
but everybody agrees that the moon does rotate. The question is rotating around which axis. I say it is the earth's, and not it's own

It is rotating on its own axis, and revolving around the earth - and a similar rate (as the moon is tidally locked)

Ara Pacis
2012-Sep-16, 05:05 PM
But you can see how that could be a legitimate interpretation?

We just have to convince everyone that the first is also legitimate.

If the Earth and moon were connected, then it might make sense in the context of a single object, but since they're not connected that context cannot be obtained without using one's imagination.

BTW, since the moon isn't in a perfectly circular orbit, would the barycenter also vary in its distance from the Earth's axis and surface?

agingjb
2012-Sep-16, 06:08 PM
The moon rotates. Perhaps "on its own axis" doesn't add a lot to this.

Ara Pacis
2012-Sep-16, 06:48 PM
Maybe a simpler way to say it is that the moon rotates on its own axis because the Earth isn't part of the moon.

Jeff Root
2012-Sep-16, 08:33 PM
The moon rotates. Perhaps "on its own axis" doesn't add
a lot to this.
That has been my assertion in the recent past. I think it will
continue to be the main point of my argument when I can put
together an animation to illustrate it. At the moment I tend
to agree: Saying "on its own axis" doesn't add anything that
is needed, and could give the misleading idea of "on an axle".
Like my globe, which rotates on an axle.

-- Jeff, in Minneapolis

ShinAce
2012-Sep-17, 03:30 PM
That has been my assertion in the recent past. I think it will
continue to be the main point of my argument when I can put
together an animation to illustrate it. At the moment I tend
to agree: Saying "on its own axis" doesn't add anything that
is needed, and could give the misleading idea of "on an axle".
Like my globe, which rotates on an axle.

-- Jeff, in Minneapolis

Don't bother making graphics, they've been done. Actually, right here on baut:
http://cosmoquest.org/forum/showthread.php/129845-Moon-movements-

The first four graphics in that thread give you a reference to speak about your thoughts.

I personally have one major problem with rotation. In the Newton's bucket(or Mach principle) thought experiment, I don't see how rotation can exist in an empty universe. You need the so called 'fixed stars' to measure it. In our case, we can use the CMBR.

I claim that the moon rotates about its own axis, about the earth, about the sun, and about the galactic center. It's motion with respect to the fixed stars has 4 components, maybe more if our local cluster is spinning too. Now, we can measure it using a doppler shift of the CMBR as measure from opposites sides of the moon but pointing in the same direction. In that sense, the 'moon orbiting about its own axis' is very real and absolute.

Then we come full circle. Absolute motion when we hold relativity so dear. Now my head hurts again.

Jeff Root
2012-Sep-17, 10:47 PM
ShinAce,

What I plan to illustrate is something different from any
animation I've seen.

My understanding is that Einstein tried to incorporate
Mach's principle into relativity, but wasn't able to.
I don't know whether the result is that rotation is in
some sense "absolute" or not, but I generally say that
it is. It may not be right, but at least it is clear. :)

The Moon's motion, as viewed from Earth, is probably
the most complex of any celestial object. Lots of
different motions all combine together. I expect that
Jan Meeus has published a complete list, but I don't
have any of his books.

-- Jeff, in Minneapolis

pzkpfw
2012-Sep-17, 11:14 PM
I was trying to come up with an analogy for visualising this. And got (probably not "new")...

(Danger: imprecise language used for, er, effect. This is simple analogy, not accurate scientific equivalent)

Get a cup of coffee. Stir the coffee so it's spinning around in the cup.
Now hold the cup out at arms length and turn around.
The cup is going around you, and the coffee is is going around the cup. They are independant motions


Now stir the coffee so it goes around the cup once per ten seconds. Watch the water go around and around a while.
Now hold the cup out at arms length, and turn around once per ten seconds (in the appropriate direction).
The cup is now going around you, and the water is going around the cup. A bubble of milk may stay facing you, but the coffee is still spinning.


Or another way to see it: try spinning the coffee in the cup at various different speeds, in both directions. Hold out the cup and turn around.
At all times, the water spins 'round the cup, while the cup also goes around you.
Why is the one speed for water in the cup, where it appears to always face you, the "one special speed" where you'd say it "isn't rotating"?

ShinAce
2012-Sep-18, 01:22 AM
My understanding is that Einstein tried to incorporate
Mach's principle into relativity, but wasn't able to.
I don't know whether the result is that rotation is in
some sense "absolute" or not, but I generally say that
it is. It may not be right, but at least it is clear. :)

I've understood the opposite. Mach's principle was on Einstein's mind, and he saw it in General Relativity as what we now call the Lense-Thirring effect. Indeed he felt that GR incorporated the principle.

However, whether that means rotation is absolute or not is still a matter of debate. See the Sagnac effect. It neither proves nor disproves relativity vs ether.

If the moon isn't rotating, that means a Foucault pendulum would always trace over the same line. I guess it's time to get back to the moon, we have an experiment to conduct! Likewise, there should be no coriolis force. Personally, I would bet the other way around. The pendulum would trace over a circle if you gave it the time it needed.

wd40
2012-Sep-18, 01:49 PM
Page 33 of this downloadable pdf file has an article by a geocentrist, apparently a real professor, claiming that Foucault's Pendulum may not be that reliable at detecting rotation!
http://www2.zshare.ma/s99xl6cnx0re



If the moon isn't rotating, that means a Foucault pendulum would always trace over the same line. I guess it's time to get back to the moon, we have an experiment to conduct! Likewise, there should be no coriolis force. Personally, I would bet the other way around. The pendulum would trace over a circle if you gave it the time it needed.

NEOWatcher
2012-Sep-18, 03:29 PM
Page 33 of this downloadable pdf file has an article by a geocentrist, apparently a real professor, claiming that Foucault's Pendulum may not be that reliable at detecting rotation!
http://www2.zshare.ma/s99xl6cnx0re

No... I am not going to load special download software just to get some document.
So; What is he a professor of?

Although; it may not matter. If the guy is a geocentrist, then he's already got a bias against the standard model and I wouldn't trust what he says.

wd40
2012-Sep-18, 03:46 PM
No... I am not going to load special download software just to get some document.
If you click the correct download button on the page & have Adobe, it downloads in seconds. Please please do it so that you can refute this fellow's math.

What is he a professor of?
He says Computer Science at Cleveland State University

Hornblower
2012-Sep-18, 03:57 PM
Let's get the focus of this discussion back on the OP issue. As I see it, the OP is not denying a motion that would cause a Foucault pendulum to drift with respect to the Moon's surface, or cause Coriolis effects. He simply is objecting to the mathematical practice of deconstructing the observed motion into a rotational component about the Moon's center and a translational component of that center along the orbital path, for reasons which have nothing to do with the mathematical usefulness of that practice. I stand by my prior remarks on this topic.

profloater
2012-Sep-18, 05:05 PM
I agree with Hornblower although I cannot find a previous post of his. The OP objects to the convention. (In mechanism analysis it is possible to refer any rotation to a distant centre wherever you like and it is simple maths to recalculate the moment of inertia etc.) It is only because the rotation of the moon is such that it appears to be fixed to the earth as if there was a structural linkage that the OP objection has any merit. i.e it is a special case.

Hornblower
2012-Sep-18, 05:08 PM
I agree with Hornblower although I cannot find a previous post of his. The OP objects to the convention. (In mechanism analysis it is possible to refer any rotation to a distant centre wherever you like and it is simple maths to recalculate the moment of inertia etc.) It is only because the rotation of the moon is such that it appears to be fixed to the earth as if there was a structural linkage that the OP objection has any merit. i.e it is a special case.My bold. Post #26 in this thread.

profloater
2012-Sep-18, 05:17 PM
My apologies, I skimmed quickly through all the posts looking for that horn but Obviously too quickly. To add yet another tangent, if you swing a (initially still) gyroscope flywheel around an axis parallel to its rotor, it will not spin up, it stubbornly refuses, but if it is spinning a little to begin with there is a sequence of applied torques that can make it spin faster and faster, despite frictionless bearings. Only the former part is relevant to the OP but the latter is, I find, more interesting.

Centaur
2012-Sep-19, 09:39 PM
A good test of a scientific model is prediction. The altitudes and azimuths of stars as viewed from the lunar surface can be easily predicted, if it is assumed that the Moon rotates on its axis at a virtually constant rate. Attempting to predict those stellar positions by starting with knowledge of the Moon’s varying orbital speed around the Earth and the long-term average equality with its axial rotation speed, requires a theory to explain what would then seem to be the Moon’s apparent rotational oscillations that produce longitudinal librations. But assuming that the Moon rotates about its axis at a virtually constant rate, while revolving around the Earth at an inconstant rate, results in a far simpler model for predicting stellar positions as viewed from the Moon that's in sync with dynamical theory. Occam’s Razor is often helpful in such matters.

As an aside, while the use together of both the words axis and rotate may indeed create a redundancy based on modern astronomical jargon, in the scientific past the words rotate and revolve were synonymous and remain so in general usage. In this current discussion a little otherwise unnecessary verbiage can reduce the potential for confusion.

Reality Check
2012-Sep-20, 12:13 AM
Although; it may not matter. If the guy is a geocentrist, then he's already got a bias against the standard model and I wouldn't trust what he says.
Actually it is even worse - this is an article from the Biblical Astronomer, a creationist journal!

Centaur
2012-Sep-20, 04:35 AM
Actually it is even worse - this is an article from the Biblical Astronomer, a creationist journal!

In a strictly kinematic context we can make any point the center of a coordinate system. In some contexts it is convenient to make that point the center of the Earth, i.e. geocentrism. In our daily lives we see things from our own perspective and see ourselves as the center, i.e. physical egocentrism. In another context the Sun or the solar system barycenter may be the most convenient center, i.e. heliocentrism. Of course we can extend possibilities to the galactic center or beyond. Positions can be mathematically translated from one coordinate system to another, if we are only concerned about the kinematics and not the dynamics. For instance we may calculate the position of Ceres within a Sun centered system, and then translate that to the view from an observer’s location so he knows how to aim his telescope.

After acknowledging the work of Copernicus, Galileo and Kepler, Isaac Newton developed dynamical theories of motion and gravitation that explained in the simplest possible ways how solar system bodies move and interact. Lunar librations can be most simply explained by assuming that the Moon rotates about its axis at a constant rate, while revolving around the Earth at a variable rate. The fact that tidal locking leads to these two rates averaging the same value in the long run is irrelevant.

The simple dynamical theories of Newton led to an industrial revolution and allowed us to live quite differently from people in Newton’s time. We’ve moved beyond serendipity and beyond trial & error, and have some confidence that conceptions based on scientific laws will actually work. Unfortunately, there still seem to be people in this world with certain vested interests who want to convince us that books written in ancient times by people ignorant of modern science have an authority that must be respected and require no proof. So we see them coming to forums like this with half baked ideas, perhaps trying to convince themselves more than us that nonsense is reality.

Tog
2012-Sep-20, 10:26 AM
This is an odd argument. I've seen the ones that say the moon does not rotate, but this time the moon does rotate, just not around it's own axis. Let's look at that a minute.

Take 2 objects A is big and is basically fixed in space. B is smaller, goes around A, and spins around some axis.

1-If B spins 1 time per day, and takes 10 days to go around A, B spins 0.1 times per orbit. Is it spinning on its own axis?
2-If B spins 10 times per day, and takes 1 day to go around A, B spins 10 times per orbit. Is it spinning on its own axis?
3-If B spins 1 time per day and takes 1 day to go around A, B spins 1 time per orbit. Is it spinning on its own axis?

If you say "no" to #3, then you must say "no" to #1 and #2.

Saying "no" to 3, but "yes" to the others will need some explaining since it would mean that B spins about its own axis right up to the moment the times synchronize. At that instant, the axis of rotation jumps to the center of A.

That would bring up other questions, like, how exact does the rotation need to be to move the center of rotation? Would a rotation of 0.999 times per revolution be enough to do make it shift?

Or does it make more sense that the revolution and the rotation are different aspects that don't share a common axis, yet do share a common object.

NEOWatcher
2012-Sep-20, 11:51 AM
Here's a strange way to look at it, if I can explain it right.

We all know it's a matter of perspective of where we are looking from. But; with the moon we have an additional clue.
The librations have to move around something. That something is it's axis. So; there is a rotational axis even if you say there is no "spin" from the view of the Earth.

grapes
2012-Sep-20, 12:35 PM
This is an odd argument. I've seen the ones that say the moon does not rotate, but this time the moon does rotate, just not around it's own axis. Let's look at that a minute.

Take 2 objects A is big and is basically fixed in space. B is smaller, goes around A, and spins around some axis.

1-If B spins 1 time per day, and takes 10 days to go around A, B spins 0.1 times per orbit. Is it spinning on its own axis?
2-If B spins 10 times per day, and takes 1 day to go around A, B spins 10 times per orbit. Is it spinning on its own axis?
3-If B spins 1 time per day and takes 1 day to go around A, B spins 1 time per orbit. Is it spinning on its own axis?

If you say "no" to #3, then you must say "no" to #1 and #2. Not necessarily. It's certainly possible that they could say, in #2, B is spinning on its own axis nine times, and around A once.

ShinAce
2012-Sep-20, 01:15 PM
Not necessarily. It's certainly possible that they could say, in #2, B is spinning on its own axis nine times, and around A once.

Or they could just say that A isn't fixed, but B is. In that case, the moon isn't spinning, and the earth isn't going around the moon either. Instead, the earth seems to spin on itself while all the other planets, sun, and stars are going around in a ~28 day cycle. So in this view, if we say the moon doesn't rotate about its own axis, then the rest of the universe does, at a fixed rate... (edit: Except the earth. From the non rotating moon view, the earth librates a bit, but otherwise does not wander around the sky. So this implies that if the moon doesn't rotate, the earth doesn't move much.)

It seems much more natural to say the universe isn't spinning(See Mach principle for reasons why) but the moon does. This would get rid of the mysterious force of a spinning universe.

chrlzs
2012-Sep-20, 01:45 PM
Forgive my shallow, perhaps hasty and simplistic observation, but what on earth (or moon) is the point of the discussion? It seems all protagonists agree completely about what is observed and what is actually happening, and it's all about nomenclature from differing frames of reference..

I would sum up the situation by observing that the earth does NOT rotate (well, not from where I'M STANDING, it doesn't..).

And yes, that is a pretty pointless statement, but hopefully you get my drift..

grapes
2012-Sep-20, 01:51 PM
Forgive my shallow, perhaps hasty and simplistic observation, but what on earth (or moon) is the point of the discussion? It seems all protagonists agree completely about what is observed and what is actually happening, and it's all about nomenclature from differing frames of reference..

I would sum up the situation by observing that the earth does NOT rotate (well, not from where I'M STANDING, it doesn't..).

And yes, that is a pretty pointless statement, but hopefully you get my drift..Actually, we're all in complete agreement that you'd be wrong. :)

Everyone here seems to agree that the moon rotates, mostly because of the drift of the background stars. From that point of view, the earth rotates too.

wd40
2012-Sep-20, 04:53 PM
The claim was made that astronauts on the Moon saw the Earth rotate.

But is this not analogous to the astronauts sitting on a horse on a carousel? As they go round and round on the carousel, they look toward the center where the engine and supports are bolted to the ground. To them the engine seems to be turning on its axis while they stand still. The engine has visually been "proven" to rotate and the platform of the carousel has been "proven" to stand still, because the astronauts from their horses saw all sides of the central engine as it turned.

grapes
2012-Sep-20, 04:57 PM
The claim was made that astronauts on the Moon saw the Earth rotate.

But is this not analogous to the astronauts sitting on a horse on a carousel? As they go round and round on the carousel, they look toward the center where the engine and supports are bolted to the ground. To them the engine seems to be turning on its axis while they stand still. The engine has visually be "proven" to rotate and the platform of the carousel has been "proven" to stand still, because the astronauts from their horses saw all sides of the central engine as it turned.It is not analogous, because over the course of a single earth day, the moon does not move much in its orbit. In other words, the astronauts *could* see the earth rotate, without they with the moon moving much at all.

The earth would appear four times bigger than the moon appears to us. Because the earth is four times bigger than the moon. :)

grapes
2012-Sep-20, 05:00 PM
It seems much more natural to say the universe isn't spinning(See Mach principle for reasons why) but the moon does. This would get rid of the mysterious force of a spinning universe.That's not an issue, though, in this case.

NEOWatcher
2012-Sep-20, 05:50 PM
It is not analogous, because over the course of a single earth day, the moon does not move much in its orbit.
Right, to fix improve the analogy, the center hub of the carousel would be spinning at roughly 28 times the speed of the horses. So, even if the carousel stops, the horse sees the hub spinning quite fast.

But; it does boil down to...

In other words, the astronauts *could* see the earth rotate, without they with the moon moving much at all.

grapes
2012-Sep-20, 05:56 PM
Right, to fix improve the analogy, the center hub of the carousel would be spinning at roughly 28 times the speed of the horses. So, even if the carousel stops, the horse sees the hub spinning quite fast.
And you probably should unbolt it from the ground first :)

Ara Pacis
2012-Sep-20, 06:41 PM
Even if you call revolution rotation for the sake of changing the frame of reference, it doesn't change the fact that the axis used by the moon is not the Earth's axis of rotation. It's different by a few thousand kilometers and several degrees of tilt. If we want to say that this is also revolved about by the Earth and call that rotation in the frame of reference we've been using, then it is a shared axis which would not exist without both objects.

wd40
2012-Sep-20, 08:35 PM
Is there a source that Michelson's 1931 dying wish to his daughter was that his experiment be performed on the Moon? If he was convinced of Relativity and the Moon going round the Earth/Earth going round the Sun, why would he make such a request?

Swift
2012-Sep-20, 08:42 PM
Is there a source that Michelson's 1931 dying wish to his daughter was that his experiment be performed on the Moon? If he was convinced of Relativity and the Moon going round the Earth/Earth going round the Sun, why would he make such a request?
wd40

Stop it. This is not proper behavor for most threads on BAUT and it certainly isn't for Q&A. This isn't your thread (you didn't start it) and this has nothing to do with the OP. If you want to ask other questions, such as this one, go start your own threads.

AriFeannor
2015-Jul-03, 05:49 PM
This is an odd argument. I've seen the ones that say the moon does not rotate, but this time the moon does rotate, just not around it's own axis. Let's look at that a minute.

Take 2 objects A is big and is basically fixed in space. B is smaller, goes around A, and spins around some axis.

1-If B spins 1 time per day, and takes 10 days to go around A, B spins 0.1 times per orbit. Is it spinning on its own axis?
2-If B spins 10 times per day, and takes 1 day to go around A, B spins 10 times per orbit. Is it spinning on its own axis?
3-If B spins 1 time per day and takes 1 day to go around A, B spins 1 time per orbit. Is it spinning on its own axis?

If you say "no" to #3, then you must say "no" to #1 and #2.

Saying "no" to 3, but "yes" to the others will need some explaining since it would mean that B spins about its own axis right up to the moment the times synchronize. At that instant, the axis of rotation jumps to the center of A.

That would bring up other questions, like, how exact does the rotation need to be to move the center of rotation? Would a rotation of 0.999 times per revolution be enough to do make it shift?

Or does it make more sense that the revolution and the rotation are different aspects that don't share a common axis, yet do share a common object.

I know this is an old thread but I don't see why no one caught this. In example 1, if B spins 1 time per day and takes 10 days to go around A, B spins(or rotates) not 0.1 times per orbit(or revolution) but 10.

DALeffler
2015-Jul-08, 02:57 AM
The moon surely rotates around the earth's axis. Not it's own. But this is just another way of saying that it orbits the earth.

You never seen an earth rise or earth set if you are on the moon.

What is the logical difference between the moon, and space station. Nobody would say that a space station is rotating on it's axis.

How I see it is that the space stations ground track is different for each orbit: if the earth were not spinning on its axis, the space station would fly directly over the same ground track every orbit.

Likewise, a satellite in polar orbit around the moon should expect to eventually directly pass over (as the moon spins/rotates under the sat) the entire lunar surface if the moon is spinning on its own axis.

Right?

Grashtel
2015-Jul-08, 03:57 AM
How I see it is that the space stations ground track is different for each orbit: if the earth were not spinning on its axis, the space station would fly directly over the same ground track every orbit.

Likewise, a satellite in polar orbit around the moon should expect to eventually directly pass over (as the moon spins/rotates under the sat) the entire lunar surface if the moon is spinning on its own axis.

Right?
I wouldn't hold your breath waiting for a reply, goobgubgoob was last active in 2012

Jens
2015-Jul-08, 04:03 AM
How I see it is that the space stations ground track is different for each orbit: if the earth were not spinning on its axis, the space station would fly directly over the same ground track every orbit. Likewise, a satellite in polar orbit around the moon should expect to eventually directly pass over (as the moon spins/rotates under the sat) the entire lunar surface if the moon is spinning on its own axis.

Right?

I think you are right on both counts, but I'm not sure how that relates to the topic of the thread. Whether the ISS is rotating on its own axis or not (it is) would not depend on whether the earth is rotating.

DaveC426913
2015-Jul-08, 08:15 PM
The clinching argument to this is the Moon's libration.

The Moon does NOT always keep the same face to us. From our viewpoint it seems to oscillate back and forth over the month.
Similarly, if you pointed a laser from the Moon's surface at the Earth, it would not stay pointed at the Earth - it would drift to the left for a week, then come back and drift off the the right for a week.


This is because the Moon is in an elliptical orbit - thus its angle to Earth's centre - is changing constantly over a month.
Yet its rotation about its own axis over a month is constant.


That eloquently and conclusively demonstrates that its rotation is not the same as its revolution (even if they do synchronize monthly).