Moon question

[michaellxv (Michael)]

Originally Posted by multiweb

This whole thread is starting to make my head spin . Moon's tidally locked to earth so it appears to face always the same way but for libration which I understand is due to the slightly excentric orbit. When you talk about motion or rotation you have to say in reference to what. I reckon the shape of the "orbit of the moon around the sun is something like a flower out of the 70s... without the rainbows That's spinning ... or trippin'

Yes, my head is spinning. Yes it's a rather pedantic question and debate. But I do find it interesting. Don't know what that makes me

[bojan]

Guys,
You have to understand that we are dealing here with complex movement, consisting of two components:
1) orbital component (29days)
2) rotational component (27 days)
Combine them together, and you have moon facing earth always with the same side.

So I aplologise for my erlier comment on axis passing through the barycentre odf the system Earth-Moon, it is not correct, sorry.

[michaellxv (Michael)]

Originally Posted by adman

What about an artificial satellite? Say the space shuttle. As it orbits, let's say it keeps it's underbelly oriented towards the earth - an identical situation to the moon,always keeping one face pointing to the earth. Do we say that it is rotating as it orbits the earth, or is it simply flying a curved path?

That one is even harder to visualise as the space shuttle (and other sattelites) have engines to correct their orbit as required so it not quite the same. I also keep seeing images in my head of sattelites being deliberatly spun up prior to release into orbit.

If a die was released from the ISS with no spin as such but it would have the receive orbital momentum from the ISS. Assume that is starts with the 6 facing the earth. Would it fall around the earth and keep the 6 facing the earth or would we see the other numbers as it orbited but did not spin.

6
|
v
6-> E <-6
^
|
6

or

6
|
v
5-> E <-2
^
|
1

[adman (Adam)]

Originally Posted by bojan

Guys,
You have to understand that we are dealing here with complex movement, consisting of two components:
1) orbital component (29days)
2) rotational component (27 days)
Combine them together, and you have moon facing earth always with the same side.

So I aplologise for my erlier comment on axis passing through the barycentre odf the system Earth-Moon, it is not correct, sorry.

I get the concept of something having rotational and translational components to its movement - i just don't think the moon has a rotational one

[Nesti (Mark)]

Originally Posted by adman

Seroiusly though....what do you mean by that?

Well, it's really a composites of motion-isn't it?!

The spin is incorporated in with the revolution.

Relative to our central Sun, the Moon does have a spin, but it can only come as a result of the Moons movement around the Earth; like Parallel Transport in GR which I mentioned earlier. Relative to the Moon, it would appear to have genuine spin.

But wait, there's more! An object in orbit should maintain it's orientation to the central mass, ie, the nose of a space shuttle if pointing toward Earth will remain pointed toward Earth because the geodestic pathway curves around the Earth, and the geodesic runs tangental to the curve. This is the conservation of angular momentum being conserved with the geodesic itself...or Newtons third law conserved with the curvature of spacetime...cool hey?!

[adman (Adam)]

Originally Posted by Nesti

An object in orbit should maintain it's orientation to the central mass, ie, the nose of a space shuttle if pointing toward Earth will remain pointed toward Earth because the geodestic pathway curves around the Earth, and the geodesic runs tangental to the curve.

So you would only see the 6 in the die example in post #43 above?

[Nesti (Mark)]

Originally Posted by adman

So you would only see the 6 in the die example in post #43 above?

ABSOLUTELY, if the 6 side is facing Earth and stable, the 6 face will remain facing the Earth as the ISS and dice travels around the Earth 'unless affected by an external force'. This is the preservation of our laws of motion within the geodesic pathway.

So what does that actually mean???
The dice is actually traveling in uniform motion and in a dead-straight line...which just so happens to curve around the Earth.

[mithrandir (Andrew)]

Originally Posted by Nesti

ABSOLUTELY, if the 6 side is facing Earth and stable, the 6 face will remain facing the Earth as the ISS and dice travels around the Earth 'unless affected by an external force'. This is the preservation of our laws of motion within the geodesic pathway.

So what does that actually mean???
The dice is actually traveling in uniform motion and in a dead-straight line...which just so happens to curve around the Earth.

For the 6 to remain facing the earth the die (singular noun) has to be tidally locked, so it is spinning, just like the moon. Once it is spinning it takes no force to keep it spinning.

Eventually earth will become tidally locked with the moon. They will both have the same day length, much longer than the current lunar month.

Tidal friction is slowing earth's rotation. Over the last few billion years it has already locked the moon because it is much smaller. The moon is moving away from earth to maintain the total angular momentum, thus making the lunar month longer.

Tides not only affect the oceans. The land has tidal bulges too, just much smaller because it is much less motile.

[Robh (Rob)]

Originally Posted by adman

I get the concept of something having rotational and translational components to its movement - i just don't think the moon has a rotational one

It depends on your frame of reference.
In their orbit around the Earth, both the Moon and the die/shuttle will have axial rotation relative to the distant stars. Relative to the Earth they may appear to have no axial rotation. As both Bojan and Steffen indicated, if the Earth were suddenly removed from the equation, the Moon would continue in orbit about the Sun but still have an axial rotation relative to the stars. And so would the die/shuttle.

Regards, Rob

[Nesti (Mark)]

Originally Posted by mithrandir

For the 6 to remain facing the earth the die (singular noun) has to be tidally locked, so it is spinning, just like the moon. Once it is spinning it takes no force to keep it spinning.

I don't believe it requires to be Tidally Locked to do so. So long as the Earth is uniformly round and dense around the Equator (and it pretty much is), then the geodesic pathway the die travels will be uniform. Remember, the die is moving in a dead straight line with uniform motion through the geodesic. If the geodesic is curving around the Earth, then the die does not need to spin in order for the 6 to remain facing the Earth. Tidal Lock seems to be a slow acting phenomenon like a dynamic system finding equilibrium, but geodesic pathways are always there and are instantaneous.

[michaellxv (Michael)]

Gravity between 2 bodies acts as if the masses were points of mass at the centre of gravity. Therefore I don't see how gravity is going to cause either body to necessarily start rotating.

[Steffen]

Originally Posted by michaellxv

Gravity between 2 bodies acts as if the masses were points of mass at the centre of gravity. Therefore I don't see how gravity is going to cause either body to necessarily start rotating.

It doesn't. The gravitational force between Earth and Moon isn't what makes either of them spin around their respective axes.

EDIT I should add, if you abandon the point mass assumption (because it is inaccurate and only an approximation) you will have to acknowledge the effects of tidal deformation, which results in any kind of rotation the Moon might have had initially to eventually become synchronous. If the mechanics of the Moon's genesis had not given it any kind of rotation (i.e. a pure translational path around Earth) those tidal forces would have induced a spin resulting in synchronous rotation. Hence, since Earth and Moon are not point masses you can say that gravitation (via tidal forces) can indeed induce rotation (at least in the smaller one of the two).

Cheers
Steffen.

[mswhin63 (Malcolm)]

As the moon is calculated at a rotation speed of 16.7km/hr suggests it rotates in simple terms, irrespective of tidal forces that are causing it.

http://www.newton.dep.anl.gov/askasc...9/ast99142.htm

It is still rotating on an axis.

[mithrandir (Andrew)]

Originally Posted by Nesti

I don't believe it requires to be Tidally Locked to do so. So long as the Earth is uniformly round and dense around the Equator (and it pretty much is), then the geodesic pathway the die travels will be uniform. Remember, the die is moving in a dead straight line with uniform motion through the geodesic. If the geodesic is curving around the Earth, then the die does not need to spin in order for the 6 to remain facing the Earth. Tidal Lock seems to be a slow acting phenomenon like a dynamic system finding equilibrium, but geodesic pathways are always there and are instantaneous.

If the die is not rotating once per orbit, it can not always show the 6 face to all observers on the ground.

Suppose the die was not rotating, so one of the faces always pointed at say SXDF-XCLJ0218-0510 - the most distant known galaxy as at May 10 2010. From earth it would appear to be tumbling.

[Nesti (Mark)]

Originally Posted by mithrandir

If the die is not rotating once per orbit, it can not always show the 6 face to all observers on the ground.

Suppose the die was not rotating, so one of the faces always pointed at say SXDF-XCLJ0218-0510 - the most distant known galaxy as at May 10 2010. From earth it would appear to be tumbling.

This is precisely why I said earlier that this is a composite of motions. There is a spin, but it is an apparent spin. This is actually a 'Parallel Transfer' of a body, within a geodesic, around a central mass; like a race track within a gravitational field where the outer wheels must cover a greater distance than the inside wheels; there is a consequence. The Moons spin comes as a direct relationship between it's orbital velocity and the field. The Tidal Lock is probably a feature of how the two fields (Moon and Earth) interact together.

The spin of the Die is attributed to it's orbit around the Earth, but the reason why the Die will always face the Earth (if positioned that way and left untouched) comes down to General Relativity...you see the space and time in which the Die is positioned within the geodesic pathway contains conservation of angular momentum (all laws actually) BUT, the geodesic is a curved path around the Earth, so the space spins around the Earth, not the Die (the space is bent to conform with the filed curvature)...this also means that the 6 face is being compressed and the outside face (opposite to the 6) is being stretched...the gravitational field is trying to turn it into a pie wedge. As the Die orbits the Earth the space which it occupies is rotated in sync with the curvature, so the Die isn't spinning, the space and time which the Die occupies is being warped, the net affect being tidal forces acting upon the Die and a change to the Die's orientation without any effort at all. This is also why satellite moons get crushed in orbit around gas giants to form rings of dust.

A Tidal Lock seems to be both a conservation tendency and a warping issue.

So is the Moon spinning???
I believe it has an apparent spin, but it is also stationary within the gravitational field.

[Steffen]

I think the Moon's rotation and the tidal lock can be entirely explained using conventional mechanics. No need to make the time spin

Cheers
Steffen.

[Kal (Andrew)]

Originally Posted by Nesti

So what does that actually mean???
The dice is actually traveling in uniform motion and in a dead-straight line...which just so happens to curve around the Earth.

This is a really good answer - helped my understanding and perspective

[sjastro]

Originally Posted by Nesti

This is precisely why I said earlier that this is a composite of motions. There is a spin, but it is an apparent spin. This is actually a 'Parallel Transfer' of a body, within a geodesic, around a central mass; like a race track within a gravitational field where the outer wheels must cover a greater distance than the inside wheels; there is a consequence. The Moons spin comes as a direct relationship between it's orbital velocity and the field. The Tidal Lock is probably a feature of how the two fields (Moon and Earth) interact together.

The spin of the Die is attributed to it's orbit around the Earth, but the reason why the Die will always face the Earth (if positioned that way and left untouched) comes down to General Relativity...you see the space and time in which the Die is positioned within the geodesic pathway contains conservation of angular momentum (all laws actually) BUT, the geodesic is a curved path around the Earth, so the space spins around the Earth, not the Die (the space is bent to conform with the filed curvature)...this also means that the 6 face is being compressed and the outside face (opposite to the 6) is being stretched...the gravitational field is trying to turn it into a pie wedge. As the Die orbits the Earth the space which it occupies is rotated in sync with the curvature, so the Die isn't spinning, the space and time which the Die occupies is being warped, the net affect being tidal forces acting upon the Die and a change to the Die's orientation without any effort at all. This is also why satellite moons get crushed in orbit around gas giants to form rings of dust.

A Tidal Lock seems to be both a conservation tendency and a warping issue.

So is the Moon spinning???
I believe it has an apparent spin, but it is also stationary within the gravitational field.

If the die is on a geodesic path it is not subjected to tidal forces.
What actually happens is that the Earth's rotation causes the die to deviate from it's geodesic path (the Lense-Thirring effect). This subjects the die to an external force resulting in the orbital plane of the die to tilt in the same direction as the rotation of the Earth.

This however has nothing to do with the topic.
The fact is the moon spins on it's axis. Tidal locking results in transferring some of the Earth's rotational energy to the moon. This has caused the moon's axial spin period to be equivalent to it's period of rotation around the Earth.

Regards

Steven

[adman (Adam)]

Originally Posted by Nesti

So is the Moon spinning???
I believe it has an apparent spin, but it is also stationary within the gravitational field.

This to me is the most satisfying answer so far.

[Nesti (Mark)]

Originally Posted by sjastro

If the die is on a geodesic path it is not subjected to tidal forces.
What actually happens is that the Earth's rotation causes the die to deviate from it's geodesic path (the Lense-Thirring effect). This subjects the die to an external force resulting in the orbital plane of the die to tilt in the same direction as the rotation of the Earth.

Regards

Steven

That doesn't make sense Steven. What you are saying is that a moon like Io is being shifted from it's geodesic path which creates the tidal forces rather than Io's spin subjecting Io to changing tidal forces across it's regions. If Io were locked like our moon there would be no volcanic activity as the tidal effects would be static, and that's because the regions of Io would be with respect to Jupiters field. But Io must spin because the tidal forces act around Io are squeezing it at different rates (harder closer to Jupiter), and thus creates internal heating, like heating up a squash ball. Tidal forces are present because a spinning body traveling within the geodesic will have matter which oscilates toward the field centre and then back out within a band which is the diameter of the object that is spinning. So a single point of the surface of the object would take a path much like a nail in a car tire if tracked from a stationary observer on the side of the road, It oscilates between the road surface and the tire height and travels but the band travels in a straight line...but the Die has an overall curvature (around the Earth) because it's in orbit.

Tidal forces would still be acting upon the Die regardless of whether it is spinning or not, but the effects of the tidal forces only become apparent when an object spins because the tidal forces then get to act upon different regions of an orbiting body, like energy from the Sun creates the weather because the Sun's energy gets distributed around the Earth's surface and energy like to migrate to lower regions (ignoring seasons).

The only other way would be if the orbiting body had an elliptical path, where the distance from the field centre varied...where an orbiting body would experience greater field curvature closer to Earth and less curvature farther away from Earth...this too should create internal heating as the tidal forces are changing, even if an elliptic Moon was Tidal Locked, it would still heat up because the field curvature would be more pronounced closer to the Earth.

I agree that Tidal Locking is separate to the geodesic, but the 6 face of the die would be kept facing Earth if the die is undisturbed in the geodesic...in orbit. Tidal Locking seems only to be an energy transfer within the field, like regions of higher energy must flow to regions of lower energy, and thus creates a connection between two parts of the system (like what I said about thermodynamically coupled parts...they are coupled because energy is transferring from regions of higher energy flowing to regions of lower energy); the transfer IS the coupling.