Moon question

[Allan_L (Allan)]

Does the Moon rotate on its axis?

Or is it a captured movement of the Earth's influence.

I know it always presents the same face to the Earth, but I believe there may be differences of opinion on this area of lunar physics.

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?

[Nesti (Mark)]

Originally Posted by Allan_L

Does the Moon rotate on its axis?

Or is it a captured movement of the Earth's influence.

I know it always presents the same face to the Earth, but I believe there may be differences of opinion on this area of lunar physics.

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?

I believe it rotates at the same rate as it revolves [around the earth]. According to modern genesis theories, Earth was hit by a large body. This blew a large chunk off which became the Moon. The spin rate was conserved (conservation of angular momentum) in the matter which formed the Moon. So the spin rate for the Moon and that of Earth should be similar.

As for this statement, "a captured movement of the Earth's influence"...the Moon is most definitely captured by the Earth's influence, likewise, the Earth is captured by the moon's influence. This extends to all matter - and energy - in the entire Solar System, Milky Way, all Galaxies...everything that has mass...and if you subscribe to Einstein's equations, that must include energy too (energy momentum tensor).

There is only one Spacetime and we are all embedded in it. It's features are governed by both the distribution of mass and energy as well as what that mass and energy are doing. Mass and energy literally create the spatial and temporal separation, but an "influence", from one body to another, is also governed by the degree of spatial and temporal separation (distance and time).

In modern jet engines, large sections may be thermodynamically coupled; you cannot affect one without affecting all coupled parts, even though they are not physically connected, and yet they spin freely. Likewise, you cannot change the mass, velocity, spin etc (the Energy Momentum Tensor) without affecting the spatial and temporal properties of Spacetime (the field), not just because they are balanced, but because they are the same system. Energy/Mass, and, Space/Time, are not separable, they are mutually supportive...you could almost say they are different properties of the same thing.

[michaellxv (Michael)]

It rotate's on its own axis. Like the earth its axis of rotation is tilted from the plane of its orbit. This creates the libration which allows us to see more than half of is surface.

Interesting question.

[adman (Adam)]

The moon does not spin on its axis at all....

Adam

[Nesti (Mark)]

Originally Posted by adman

The moon does not spin on its axis at all....

Adam

If you stood on the surface of the dark side of the moon, you would see the stars move, around and around...how is that not spinning body?

[bloodhound31]

Use your mobile phone on the desk like a toy car and drive it around and around in circles. Drive around in ever decreasing circles until you are spinning on the spot, rotating on an axis.

Just because you are driving around something, does not mean you are not spinning yourself. It just means you are moving in an orbital path as you are spinning. Look out the window of a real car doing the same thing, and you will notice the objects around you move around and around.

Now, hold your mobile phone on the desk and just draw circles with it, without turning it. It is just on an orbital path without spinning, and if you were inside a real car doing this, the view outside your window would not change.

Baz.

[wasyoungonce (Brendan)]

Shee's fella's is Tidal Lock.

[Nesti (Mark)]

Forget what we see the Moon doing from the perspective of the Earth...what matters is what is seen from the perspective of the Moon itself...the Moon sees a reoccurring and revolving sky pattern. That is spin.

So, Baz, the Moon is like your mobile phone traveling in an orbital path, and we, at the circle's centre, only get to see one side of the mobile phone, while the rest of the universe gets to see all sides of the mobile phone (Moon).

Like Parallel Transport.

[Nesti (Mark)]

Originally Posted by wasyoungonce

Shee's fella's is Tidal Lock.

LOL...Tidal Lock is the phenomenon...so what causes Tidal Lock???

This'll be good...

[wasyoungonce (Brendan)]

Probably a natural energy level settling, an interplay of Newtons & Keplers laws of the bodies.

Each body exerts gravitational energy on each other..the bigger mass wins the smaller mass looses energy & attains a stable orbit.

Moon has a solid core and this may also add to the effect of tidal locking.

[mswhin63 (Malcolm)]

They both, Moon spins on it axis every 27 days and it rotates around a captured earth gravity every 27 days. The end result is we never see the dark side of the moon.

A small AWSA training session explained something else that the Moons rotation is increasing and Earth Rotatation is slowing ever slow slightly so that in a few million or billion years there will be catchup of some sort.

[Nesti (Mark)]

Originally Posted by wasyoungonce

Probably a natural energy level settling, an interplay of Newtons & Keplers laws of the bodies.

Each body exerts gravitational energy on each other..the bigger mass wins the smaller mass looses energy & attains a stable orbit.

Moon has a solid core and this may also add to the effect of tidal locking.

I've seen something similar in hydraulics, where there are 3 pistons with different surface areas, but connected to a common manifold which slowly increases the pressure until a single piston moves, however;

1. The 1sq inch piston has 10lbs on top of it
2. The 10sq inch piston has 100lbs on top of it
3. The 100sq inch piston has 1000lbs on top of it

If you dial up the pressure, which piston overcomes it's supported weight first. Common sense would say they all act at the same time...not so!

Conservation of energy somehow makes the 1sq inch piston move first.

The locking of smaller mass/energy to larger mass/energy isn't surprising.

I actually miss sequenced hydraulics...hydraulics in general is fascinating.

[michaellxv (Michael)]

It's tied up with conservation of energy and the way the oceans slosh about on the surface of the earth. With each orbit a small amout of energy is transfered from the earth to the moon.

Since Apollo the distance to the moon has been observed to increase by a cm per year. The earths rotation is subsequently slowing and each day is slightly longer.

Eventually a day on earth will be the same as an orbit of the moon and the moon will only ever see one face of the earth.

[Nesti (Mark)]

Originally Posted by michaellxv

It's tied up with conservation of energy and the way the oceans slosh about on the surface of the earth. With each orbit a small amout of energy is transfered from the earth to the moon.

Since Apollo the distance to the moon has been observed to increase by a cm per year. The earths rotation is subsequently slowing and each day is slightly longer.

Eventually a day on earth will be the same as an orbit of the moon and the moon will only ever see one face of the earth.

This universe gets more interesting by the day...Does the transfer or energy translate to a change in mass, or does gravitational radiation emit that off???

Got a reference for that?

[michaellxv (Michael)]

Originally Posted by Nesti

This universe gets more interesting by the day...Does the transfer or energy translate to a change in mass, or does gravitational radiation emit that off???

Got a reference for that?

This is a good explanation.

http://en.wikipedia.org/wiki/Orbit_of_the_Moon

[Nesti (Mark)]

Originally Posted by michaellxv

This is a good explanation.

http://en.wikipedia.org/wiki/Orbit_of_the_Moon

Cheers!

[Kal (Andrew)]

Originally Posted by Allan_L

Does the Moon rotate on its axis?

no - the moon does not rotate on it's axis. It rotates on an axis which is 380,000 km away, inside the earths crust.

[bojan]

Originally Posted by Nesti

I've seen something similar in hydraulics, where there are 3 pistons with different surface areas, but connected to a common manifold which slowly increases the pressure until a single piston moves, however;

1. The 1sq inch piston has 10lbs on top of it
2. The 10sq inch piston has 100lbs on top of it
3. The 100sq inch piston has 1000lbs on top of it

If you dial up the pressure, which piston overcomes it's supported weight first. Common sense would say they all act at the same time...not so!

Conservation of energy somehow makes the 1sq inch piston move first.

The locking of smaller mass/energy to larger mass/energy isn't surprising.

I actually miss sequenced hydraulics...hydraulics in general is fascinating.

Hmmm

So, each piston sits in it's own cylinder and everything is connected with manifold to the same pump... and they are not connected otherwise (like on a same crank-shaft ?

The relative load on each piston is 1, 10, 100x.

Intuitively, one would think that the largest piston would start move because the force on it's head is the largest..

If we assume there is not friction between pistons and cylinders, all the pistons should start to move simultaneously...

If we take into account the friction, then we have an area between cylinders and pistons which is causing the friction, and the relative sizes of that area is proportional to the product of lengths of the piston (assumed the same for all of them) and circumference of said piston.. which will be proportional to the SQRT of piston head area.

So I think the biggest piston will start to move first, because the relative friction of the widest piston is lowest.


And, yes, the Moon rotates, one rot per ~27 days.
Where that axis is.. it is actually passing through the gravity center of Earth-Moon system.. I think Andrew is spot on here :-)

[adman (Adam)]

Originally Posted by Nesti

what matters is what is seen from the perspective of the Moon itself...the Moon sees a reoccurring and revolving sky pattern. That is spin.

Not really. You get the same effect driving a car around and around a roundabout. Re-occurring and revolving scenery, the car always presents the same side to the centre of the roundabout - but the car is not spinning on its axis. It has forward, but curved motion.

Imagine this - the olympic sport of hammer throw. Big heavy person swinging a ball on the end of a chain. Once it gets up to speed they rotate around their common centre of mass. The ball always presents the same face to the thrower, but it is not spinning on its axis. A tiny observer sitting on the ball's surface would get revolving/re-occurring scenery.

[Nesti (Mark)]

Originally Posted by bojan

Hmmm

So, each piston sits in it's own cylinder and everything is connected with manifold to the same pump... and they are not connected otherwise (like on a same crank-shaft ?

The relative load on each piston is 1, 10, 100x.

Intuitively, one would think that the largest piston would start move because the force on it's head is the largest..

If we assume there is not friction between pistons and cylinders, all the pistons should start to move simultaneously...

If we take into account the friction, then we have an area between cylinders and pistons which is causing the friction, and the relative sizes of that area is proportional to the product of lengths of the piston (assumed the same for all of them) and circumference of said piston.. which will be proportional to the SQRT of piston head area.

So I think the biggest piston will start to move first, because the relative friction of the widest piston is lowest.


And, yes, the Moon rotates, one rot per ~27 days.
Where that axis is.. it is actually passing through the gravity center of Earth-Moon system.. I think Andrew is spot on here :-)

All pistons would require 10psi to get their respective loads to move. Negate the friction from the seals and the pistons should move at the same time, but apparently they don't.