Moon question - Page 2

michaellxv (Michael) · #21 03-06-2010, 05:38 PM

Quote:

Originally Posted by Nesti

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.

The smaller piston has less mass and is therefore easier to move.

michaellxv (Michael) · #22 03-06-2010, 05:40 PM

Quote:

Originally Posted by adman

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.

In this example the ball and person are physically linked, the moon and earth are not.

Take a view from space above the moon such that the earth was out of your field of view. You would observe the moon rotating on its axis. It's difficult to visualise from our earth bound reference point.

Allan_L (Allan) · #23 03-06-2010, 05:40 PM

Quote:

Originally Posted by Kal

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.

Hmmm!
Interesting hypothesis.
I must admit this is the first time I have heard this, but it sounds like a valid explantion of the observed facts.

I will ponder on this for a while.

mithrandir (Andrew) · #24 03-06-2010, 05:40 PM

Quote:

Originally Posted by Kal

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.

The moon rotates on its axis. It revolves around the earth-moon barycenter.

The earth rotates on its axis. It revolves around the earth-moon barycenter.

A bit more accurately, they revolve around the sun-earth-moon barycenter.

Although it you really wanted to be pedantic, you would have to take into consideration the perturbations caused by every other object in the universe.

Allan_L (Allan) · #25 03-06-2010, 05:50 PM

Quote:

Originally Posted by michaellxv

In this example the ball and person are physically linked, the moon and earth are not.

Take a view from space above the moon such that the earth was out of your field of view. You would observe the moon rotating on its axis. It's difficult to visualise from our earth bound reference point.

I agree with Adman.

And I propose that the Earth and Moon are physically linked, by gravatational forces.

And I like the comparison to the car on the roundabout too. And they are not even linked. (physically as you put it) Or would someone like to say that the car is revoving on it axis too ?

bloodhound31 · #26 03-06-2010, 05:59 PM

Quote:

Originally Posted by Nesti

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.

Exactly! The moon is spinning on it's axis, but I agree that it's axis is not on the moon at all, but near or inside the Earth.

Regardless, it IS spinning as well as traveling a circular orbit.

I love these conversations!

Baz.

bojan · #27 03-06-2010, 06:09 PM

Quote:

Originally Posted by Nesti

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.

No they don't because of the friction, the net force is lowest for the biggest piston.

If there is no friction, all pistons start to move at the same time (of course, the biggest will move slower, but this is not the issue here).

Steffen · #28 03-06-2010, 06:11 PM

Quote:

Originally Posted by bloodhound31

Regardless, it IS spinning as well as traveling a circular orbit.

That is correct. The translational and rotational movements are independent from each other. They are superimposed. The Moon spins and has a both an angular and a linear momentum. It would preserve this momentum if the Earth disappeared in a puff of smoke by moving along on a straight line (a solar orbit really) while spinning at rate of 1/27d.

Cheers
Steffen.

michaellxv (Michael) · #29 03-06-2010, 06:13 PM

"The lunar orbit plane is inclined to the ecliptic by 5.1°, whereas the Moon's spin axis is inclined by only 1.5°" http://en.wikipedia.org/wiki/Orbit_of_the_Moon

bojan · #30 03-06-2010, 06:21 PM

I think this is irrelevant.. where the axis is.

The important thing is, Moon has rotational momentum.

If Earth, by some miraculous reason disappeared, the Moon would continue to rotate with the same rate (1 rev per 27 days).
And it would continue to orbit the Sun, of course..

Quote:

Originally Posted by mithrandir

The moon rotates on its axis. It revolves around the earth-moon barycenter.

The earth rotates on its axis. It revolves around the earth-moon barycenter.

A bit more accurately, they revolve around the sun-earth-moon barycenter.

Although it you really wanted to be pedantic, you would have to take into consideration the perturbations caused by every other object in the universe.

adman (Adam) · #31 03-06-2010, 08:27 PM

How can an object spin on an axis that is outside of itself?? Surely that is just called an orbit? The point that it orbits around is the centre of mass of the moon and the earth. Can someone explain to me how it 'spins' around this 'axis' also?

Have a look at these links....

http://www.grantchronicles.com/astro29.htm
http://www.digipro.com/Trials/moon.html

Nesti (Mark) · #32 03-06-2010, 08:30 PM

Quote:

Originally Posted by adman

How can an object spin on an axis that is outside of itself?? The point that it orbits around is the centre of mass of the moon and the earth. Can someone explain to me how it 'spins' around this 'axis' also?

Have a look at these links....

http://www.grantchronicles.com/astro29.htm
http://www.digipro.com/Trials/moon.html

Because it's not a conventional spin or a conventional axis.

Kal (Andrew) · #33 03-06-2010, 08:31 PM

Quote:

Originally Posted by bojan

If Earth, by some miraculous reason disappeared, the Moon would continue to rotate with the same rate (1 rev per 27 days).
And it would continue to orbit the Sun, of course..

I'm not so sure this is true. If the earth/moon are tidally locked in a rotation then if the earth was removed there would be no object to provide the centripetal force. The moon would in effect stop it's 27 day rotation.

(unless my analogy of swinging an object on a rope and letting go of the rope is wrong?)

adman (Adam) · #34 03-06-2010, 08:34 PM

Quote:

Originally Posted by Nesti

Because it's not a conventional spin or a conventional axis.

that's right it's an orbit...!

adman (Adam) · #35 03-06-2010, 08:35 PM

Quote:

Originally Posted by Nesti

Because it's not a conventional spin or a conventional axis.

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

Kal (Andrew) · #36 03-06-2010, 08:40 PM

Quote:

Originally Posted by adman

How can an object spin on an axis that is outside of itself?? Surely that is just called an orbit?

I was being a bit cheeky with my answer

The OP asked if the moon spins on it's axis, which it doesn't, it of course orbits the earth and is tidally locked, which is why I said it spins on an axis inside the earths crust (which is another way of saying a tidally locked orbit)

adman (Adam) · #37 03-06-2010, 08:43 PM

I think the analogy of the hammer-throw is a pretty close approximation of the earth moon setup - apart from the fact that the earth is still rotating...

mswhin63 (Malcolm) · #38 03-06-2010, 08:53 PM

Does this all mean that the earrth does not rotate but is tidally lock to the sun.

adman (Adam) · #39 03-06-2010, 09:01 PM

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?

multiweb (Marc) · #40 03-06-2010, 09:24 PM

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'