Moon question

[adman (Adam)]

in my own mind I waver between thinking that it is a completely semantic argument based around what you define as spin, and being adamant that there is no spin....

[Steffen]

Originally Posted by adman

in my own mind I waver between thinking that it is a completely semantic argument based around what you define as spin, and being adamant that there is no spin....

You can of course postulate that the universe spins around Moon's axis. But in the sense of classical mechanics the Moon spins around its axis and has a rotational momentum that it will preserve if whatever force made it spin in the first place goes away.

Cheers
Steffen.

[sjastro]

Originally Posted by Nesti

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.

I'm not saying that at all.
Only point masses on geodesic paths are not subjected to external forces (excluding gravity). Since Io is not a point mass the geodesic "passes" through the centre of the moon and intersects at two points on Io's surface. All other points on Io's surface and interior not being on the geodesic are subjected to tidal forces. The Lense-Thirring effect is negligible compared to this.
Since Io rotates on it's axis the tidal forces act at different points on the moon. No difference of opinion here.

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.

Our moon is tidal locked. This doesn't mean that the moon doesn't rotate on it's axis. If Io was tidal locked it would still exhibit volcanic activity for the reasons you have given, because it is still rotating.

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).

You don't need spin for tidal forces to become apparent. Going back to the Io example, if Io didn't spin there would be a more pronounced elongation of the moon. The realignment of this elongated axis to the orbital axis of the Jupiter/Io system due to Jupter's gravity would impart spin on Io. In this case tidal locking would have a very opposite effect.

Regards

Steven

[bloodhound31]

Originally Posted by multiweb

This whole thread is starting to make my head spin .

That's gonna make it hard to find the axis of the moon.....first you will have to find the axis of your head....

[Nesti (Mark)]

Originally Posted by sjastro

Only point masses on geodesic paths are not subjected to external forces (excluding gravity). Since Io is not a point mass the geodesic "passes" through the centre of the moon and intersects at two points on Io's surface. All other points on Io's surface and interior not being on the geodesic are subjected to tidal forces. The Lense-Thirring effect is negligible compared to this.
Since Io rotates on it's axis the tidal forces act at different points on the moon. No difference of opinion here.

Oh thank God for that!

Originally Posted by sjastro

Our moon is tidal locked. This doesn't mean that the moon doesn't rotate on it's axis. If Io was tidal locked it would still exhibit volcanic activity for the reasons you have given, because it is still rotating.

I’m not saying that the Moon must not rotate, because it does…I’ve been saying it spins all along. I have also been saying that it is not the intuitive type of spin we are accustomed to.

Sorry, I didn’t say, and should have said that if Io was to show Jupiter only one side (like our moon). That confused the point.

Originally Posted by sjastro

You don't need spin for tidal forces to become apparent. Going back to the Io example, if Io didn't spin there would be a more pronounced elongation of the moon. The realignment of this elongated axis to the orbital axis of the Jupiter/Io system due to Jupter's gravity would impart spin on Io. In this case tidal locking would have a very opposite effect.

Yes, I know, I did mention that point when talking about the Die. I wrote “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”...to clarify that better, the sidea are being squeezed, the back face is being pulled, but the front face is being pulled harder...for our Earth, this translates to a high tide closest to our Moon, low tides on the sides, and a not so high tide on the far side away from the Moon.

Earth
↑
→ ● ←
↓↓




↑↑↑
→ ● ←
↑↑

Moon


Right, I'm off for a lie-down now.

Cheers
Mark

[sjastro]

Thanks Mark.

The term "tidal locked" is a major point of confusion as it doesn't mean the obvious.

Regards

Steven

[Nesti (Mark)]

Originally Posted by sjastro

Thanks Mark.

The term "tidal locked" is a major point of confusion as it doesn't mean the obvious.

Regards

Steven

I don't know what causes it, I'm just assuming it's gravity's version of resonance; maximum amplitude at an optimal frequency for a given body. Or like a ball that always rolls down to the lowest potential...a local minima.

Cheers

[Steffen]

Yes, I'm sure synchronous rotation is the least energetic state for the whole system.

Cheers
Steffen.

[el_draco (Rom)]

Originally Posted by Allan_L

Picture a leaf caught in a whirlpool. It may always present the same face to the centre of the whirlpool, but could you really say that leaf is rotating on its axis?

What do you reckon?

No you could not say this. The leaf would rotate aound the centre of the whirlpool. If it was rotating on its axis, it would spin independently of the whirlpool.

Likewise, the moon "rocks" on its axis whilst orbiting the barycentre of the earth/moon system. The earth rotates on its axis and is not locked to facing the earth/moon system barycentre...

[mithrandir (Andrew)]

Originally Posted by sjastro

The term "tidal locked" is a major point of confusion as it doesn't mean the obvious.

It's 35 years since I've done any physics, so any astrophysicist reading can correct me, with explanations.

Any two bodies orbiting each other have tidal bulges. If either one is spinning on its own axis at a speed other than one where both objects always show the same side to each other, the tidal bulges will move on that object. This movement requires/releases energy. The energy has to come from somewhere and the only place it can come from is the individual object's rotation about its axis or the revolution of both objects around the barycentre.

The moon has used up it's store of rotational energy. The moon's tides have locked its face on earth.

The earth is still losing rotational energy to tidal friction and is slowing. Some of this energy is lost as heat. Some is acting to send the moon further away. Eventually earth's face will lock on the moon.

Originally Posted by el_draco

Likewise, the moon "rocks" on its axis whilst orbiting the barycentre of the earth/moon system. The earth rotates on its axis and is not locked to facing the earth/moon system barycentre.

Libration comes from the moon's orbit not being a circle, and the plane of the moon's orbit not being aligned with the plane of earth's orbit. If the moon was not tidally locked we would see the whole surface.

[Astro78]

Just as I was going to reply to the contrary, a picture of the rising earth from the moon comes to mind. It therefore must be in the same 28 day sync as earth

hang on we cannot have the earth rising without any axial rotation from the moon....cancel

edit - there's either wobble in its orbit or it spins on it axis but back and forth and never fully around. Or the pictures were fakes

what a little gem of a Q.

[Nesti (Mark)]

Originally Posted by Astro78

Just as I was going to reply to the contrary, a picture of the rising earth from the moon comes to mind. It therefore must be in the same 28 day sync as earth

hang on we cannot have the earth rising without any axial rotation from the moon....cancel

edit - there's either wobble in its orbit or it spins on it axis but back and forth and never fully around. Or the pictures were fakes

what a little gem of a Q.

Sorry but, the ‘Earth Rise’ as seen from the Apollo 8 crew and in photographs were taken in orbit (as with all other Lunar missions). The rise of the Earth came as a result of the motion of the spacecraft around the Moon, not the Moon’s motion. So Apollo 8 witnessed 10 Earth Rises as it did 10 orbits of the Moon.

The Moon does of course have a Sunrise, albeit a very long one. The Apollo missions took advantage of the extended Lunar Sunrises too, the shadows cast across the Lunar surface offered greater depth perception for the crew. As a Lunar day is 29.5 Earth days the shadows barely move throughout the entire descent. Because of the lengthy day, no Apollo crew ever witnessed a Lunar Night while on the surface, and was the reason why they used sun shades during rest periods.