Does the Moon Rotate?

[jtb]

jtb wrote:Viscount, my point in the wiki demonstration is that if tidal locking is true, viewed from Earth, a non-rotating moon would look exactly like one rotating twice per orbit.

I'm not understanding your idea the Moon "rotating twice per orbit looking the same."

http://en.wikipedia.org/wiki/Tidal_locking
The wiki demonstration image of tidal locking on the right is an illustration of a non-rotating moon. If observed from the center, all sides of the moon are visible once per orbit.

The left illustration is of the moon rotating once per orbit. Only one side of the moon is visible from the center.

If rotating twice per orbit, center observer would see all sides of the moon once per orbit: the same as a non-rotating moon.

Observed from Earth, there may be a way of differentiating between a non-rotating moon and one rotating twice per orbit, but, I am unaware how that could be done.

Perhaps there is an astrophysicist reading this that could explain.

[viscount aero]

jtb wrote:Viscount, my point in the wiki demonstration is that if tidal locking is true, viewed from Earth, a non-rotating moon would look exactly like one rotating twice per orbit.

I'm not understanding your idea the Moon "rotating twice per orbit looking the same."

http://en.wikipedia.org/wiki/Tidal_locking
The wiki demonstration image of tidal locking on the right is an illustration of a non-rotating moon. If observed from the center, all sides of the moon are visible once per orbit.

The left illustration is of the moon rotating once per orbit. Only one side of the moon is visible from the center.

Ok I follow that.

If rotating twice per orbit, center observer would see all sides of the moon once per orbit: the same as a non-rotating moon.

I'm beginning to see what you mean--sort of maybe.

Being that the Moon already rotates once per orbit (that you don't believe in), it shows only one face to the observer from Earth. However were it to show all sides once per orbit, were it to rotate on its axis faster than it orbits the Earth, then to an Earth observer it would be technically rotating twice per orbit.

As a side issue, this is also true of all the planets around the Sun: The planets are actually rotating on their axes not only due to their inherent "day" spin rates but also because their orbits themselves create a rotation--a "compound" rotation.

Observed from Earth, there may be a way of differentiating between a non-rotating moon and one rotating twice per orbit, but, I am unaware how that could be done.

Perhaps there is an astrophysicist reading this that could explain.

I explained it above even though you won't really agree with me.

Keep in mind that the animation on the right hand side on the Wiki page actually proves a non-rotating Moon condition even though to an Earth-based observer all sides are revealed. Seeing "all sides" doesn't automatically mean the body is rotating about its axis. This is a large reason why this topic sits badly with some people. The phenomena being discussed here appear counter-intuitive.

[nicho247]

http://encyclopedia2.thefreedictionary. ... f+the+Moon : "Optical libration in longitude occurs because the moon revolves about the earth nonuniformly while it rotates about its axis at a constant angular velocity."

http://earthsky.org/space/how-much-of-t ... #longitude : " Libration in longitude is the moon’s east-west wobble. This sort of libration is a product of the moon’s elliptical (elongated) orbit. Although the moon’s rotation, or spin, goes at a nearly constant rate, its orbital speed varies, going fastest at perigee (moon’s closest point to Earth) and slowest at apogee (moon’s farthest point from Earth)."

http://www.phy6.org/stargaze/Smoon4.htm : "The rotation rate of the Moon is pretty steady. However, the orbiting motion of the Moon is not steady--at some times it advances faster than average, some times it is slower."

Hi Chrimony,

I love the links.

The first makes a statement about libration with a diagram depicting the moons orbit. This is more explanatory, not so much actually providing evidence. Also, I found this disclaimer amusing "The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased." Note, it also doesn't say the move has any rotation, just the rotation is at a constant value (could be zero).

The third makes some good cases explaining the different types of libration, but I don't see how they refute my statements or reinforce yours. The section you quoted out of the link just states the moons speed appears to vary relative to its elliptical orbit.

Can you invoke libration in longitude without elliptical orbits?

Now, the second link....provides an excellent youtube video (the hilarious comments for the video mirror this forum thread).

Please note, this is discussing relative axial rotation, and has nothing to do with absolute axial rotation.

I'd like to breakdown the video into segments because I think they not only provide hypothetical what-if's, but they provide a visual demonstration relative to the observer (a bird's eye view from above the earth), a critical point to revisit. Also note, there are a few things wrong (the orbit, size of the bodies, earth isn't rotating, earth's north pole appears to be the Caribbean, the moons orbit speed isn't varying, etc.) but the focus is on two things, (1) the orbit around the earth and (2) the moons axial rotation. Lastly, this video is in support of the moon's relative axial rotation, however I believe it does the opposite.

Section 1: 00:05 to 00:30 : The moon with no axial rotation (relative to the observer)


The moon orbits the earth with no axial rotation, those on earth would no longer see a near side, instead the moon would appear to rotate clockwise (and viewers would see all sides of the moon).

Section 2: 00:30 to 00:57 : The moon with high axial rotation (relative to the observer)

The moon orbits the earth with axial rotation, those on earth would no longer see a near side, instead the moon would appear to rotate counter clock-wise (and viewers would see all sides of the moon).

Section 3: 00:58 to 2:19 : The moon with lower axial rotation (synchronous) (relative to the observer)

The moon orbits the earth with synchronous rotation, those on earth only see the near side, and moon does not appear to rotate (viewers only see the near side).

Again, relative to the observer above the earth, there are examples of the moon having (1) no axial rotation, (3) low (counter-clockwise) axial rotation, and (2) high (counter-clockwise) axial rotation. However, from the view on the earth, the moon would have (1) clockwise axial rotation, (3) no axial rotation, and (2) counter-clockwise axial rotation, respectively.

The moon's relative axial rotation depends on where you viewing it from, hence relative.

Now, if we are to state the moon rotates regardless of frame of reference(absolute), this would have to be proved or demonstrated. Libration in longitude doesn't prove this, it is just an artifact of the elliptical orbit, and it would be present in all of the above hypotheticals with different axial rotations (clockwise or counter) because the elliptical orbit was constant.

Nick

[viscount aero]

Now, if we are to state the moon rotates regardless of frame of reference(absolute), this would have to be proved or demonstrated. Libration in longitude doesn't prove this, it is just an artifact of the elliptical orbit, and it would be present in all of the above hypotheticals with different axial rotations (clockwise or counter) because the elliptical orbit was constant.

Nick

It is demonstrated and proved by the animations. Looking for something else beyond that is useless. Relative to this or that observer is irrelevant. It is an absolute geometric relationship between the Earth and Moon.

[jtb]

Being that the Moon already rotates once per orbit (that you don't believe in), it shows only one face to the observer from Earth. However were it to show all sides once per orbit, were it to rotate on its axis faster than it orbits the Earth, then to an Earth observer it would be technically rotating twice per orbit.

As a side issue, this is also true of all the planets around the Sun: The planets are actually rotating on their axes not only due to their inherent "day" spin rates but also because their orbits themselves create a rotation--a "compound" rotation.

I believe that the moon rotates once per orbit about Earth; not about its axis: it has no "day" spin. Orbital rotation is the "compound" rotation you mention.

Earth's tidal locking ratio would be 1:366. 366 rotations per orbit: one "compound" rotation and 365 "day" spins. The "compound" rotation has to be subtracted from 366 to obtain the number of "day" spins.

Apply the same logic to the lunar tidal locking ratio 1:1. One "compound" rotation and zero "day" spins.

Keep in mind that the animation on the right hand side on the Wiki page actually proves a non-rotating Moon condition even though to an Earth-based observer all sides are revealed.

We actually can't prove anything. All we can do is provide overwhelming evidence to support our theory until new evidence is discovered supporting another theory.

Seeing "all sides" doesn't automatically mean the body is rotating about its axis.

Agreed. This quote is in reference to a center observer and an orbiting object. As I stated before, an axis is an imaginary axle, and to function as an axle, it has to be attached to something. If attached to the center of orbit, a point on the axle always faces the direction of motion and seeing all sides means the object is rotating.

If the axle is attached to an external perpendicular force, a point on the axle always faces the same direction, for example, south. The center observer sees all sides of an orbiting, non-rotating object.

But again, with this scenario, the center observer can't distinguish between a non-rotating object and one rotating twice per orbit.

[chrimony]

The third makes some good cases explaining the different types of libration, but I don't see how they refute my statements or reinforce yours. The section you quoted out of the link just states the moons speed appears to vary relative to its elliptical orbit.

It supports what I said. That libration in longitude is a result of a constant spin and a varying orbital velocity. You have given no explanation, despite repeated requests, as to why an elliptical orbit would result in libration in longitude without a spinning Moon.

Can you invoke libration in longitude without elliptical orbits?

Of course not, because it's a result of the elliptical orbit and a body with a constant spin. The real question is can you explain it without constant spin using Newtonian mechanics that matches all the evidence. I can come up with a calculation using the standard model that matches what we see. Where is your predictive model?

Now, the second link....provides an excellent youtube video (the hilarious comments for the video mirror this forum thread).

The same ignorance and bad analogies are on display, yes. However, libration is never mentioned, which completely blows the rigid body analogies out of the water.

The moon's relative axial rotation depends on where you viewing it from, hence relative.

That's like saying the stars can be seen whirling about us every 24 hours, relatively. I could also say the Moon is oscillating back and forth, relatively. It's garbage physics.

Now, if we are to state the moon rotates regardless of frame of reference(absolute), this would have to be proved or demonstrated. Libration in longitude doesn't prove this, it is just an artifact of the elliptical orbit, and it would be present in all of the above hypotheticals with different axial rotations (clockwise or counter) because the elliptical orbit was constant.

You're just repeating your unpredictive position ad nauseam. If you want to question the scientific position that's been around since Newton, come up with some science. Show us a working model.

[viscount aero]

Being that the Moon already rotates once per orbit (that you don't believe in), it shows only one face to the observer from Earth. However were it to show all sides once per orbit, were it to rotate on its axis faster than it orbits the Earth, then to an Earth observer it would be technically rotating twice per orbit.

As a side issue, this is also true of all the planets around the Sun: The planets are actually rotating on their axes not only due to their inherent "day" spin rates but also because their orbits themselves create a rotation--a "compound" rotation.

I believe that the moon rotates once per orbit about Earth; not about its axis: it has no "day" spin. Orbital rotation is the "compound" rotation you mention.

Earth's tidal locking ratio would be 1:366. 366 rotations per orbit: one "compound" rotation and 365 "day" spins. The "compound" rotation has to be subtracted from 366 to obtain the number of "day" spins.

Apply the same logic to the lunar tidal locking ratio 1:1. One "compound" rotation and zero "day" spins.

I see your logic. But the day spin of the Moon is not actually zero (a subtractive value per your math).

The Moon's day and orbit are the same event.

Your logic misses a variable by not taking into account the same-face to Earth condition (rotating) versus a multi-faced/360º condition of a non-rotating Moon as seen in the animations.

Hence it is a geometric relationship.

This geometric relationship is such that the orbit itself creates a spin and is one and the same with the day spin. The Moon's day is equal to its orbital period about the Earth.

[jtb]

If a 1:1 ratio means one rotation per orbit, a 1:2 ratio would be 2 rotations per orbit, and so on until we reached 1:366. A distant observer would see 365 24 hour days and 1 day lasting 1 year.

It would be like a horse on a 1 mile race track. A person in the stands sees all sides of the horse once every mile-- 1:1 ratio. If at the 1/4 mile mark the horse makes a 360 degree turn and continues to the finish line, a person would see all sides of the horse twice per orbit: one rotation lasting a mile and one of short duration--1:2 ratio.

I guess the question is, is the day lasting 1 year and the horse's rotation lasting 1 mile real or an optical illusion? My guess would be that to the person at the center of the orbit it is an illusion; to the distant observer: real.

[viscount aero]

If a 1:1 ratio means one rotation per orbit, a 1:2 ratio would be 2 rotations per orbit, and so on until we reached 1:366. A distant observer would see 365 24 hour days and 1 day lasting 1 year.

It would be like a horse on a 1 mile race track. A person in the stands sees all sides of the horse once every mile-- 1:1 ratio. If at the 1/4 mile mark the horse makes a 360 degree turn and continues to the finish line, a person would see all sides of the horse twice per orbit: one rotation lasting a mile and one of short duration--1:2 ratio.

I guess the question is, is the day lasting 1 year and the horse's rotation lasting 1 mile real or an optical illusion? My guess would be that to the person at the center of the orbit it is an illusion; to the distant observer: real.

Well at least we're sort of meeting half-way with these concepts. You raise interesting and intelligent points.

[nicho247]

Hi Chrimony,

I took a break from posting for a while because I didn't think this topic was going any where. We appear to be at a stalemate, where we both view the same thing, and have two different opinions. I am unable to sway you from your dogma despite my analogies and examples, and vice versa.

To follow up where we have left off....

libration in longitude is a result of a constant spin and a varying orbital velocity. You have given no explanation, despite repeated requests, as to why an elliptical orbit would result in libration in longitude without a spinning Moon.

Can you invoke libration in longitude without elliptical orbits?

Of course not, because it's a result of the elliptical orbit and a body with a constant spin.

I don't believe spin is part of the observation, libration in longitutude would be viewable with any spin, CCW, CW, or no spin. Why is spin mandatory or critical for libration in longitude to occur?

The real question is can you explain it without constant spin using Newtonian mechanics that matches all the evidence. I can come up with a calculation using the standard model that matches what we see. Where is your predictive model?

I haven't stated the moon doesn't have constant spin. I've stated it appears to have no relative to earth, axial rotation (a constant), and that I don't know if it has absolute axial rotation because it hasn't been measured.

You're just repeating your unpredictive position ad nauseam.

Like-wise.

Going back to my examples, I'm curious to your position on how the moon would be viewed from earth if it didn't have axial rotation in your point of view (absolute or relative). I provided 3 examples of relative CCW axial spin, no axial spin (synchronous), and CW axial spin, with how each of three would be viewed from earth and another vantage point. PS (all examples have libration in longitude).

Nick

[chrimony]

I am unable to sway you from your dogma despite my analogies and examples, and vice versa.

My position isn't based on dogma. It's based on evidence and established science (libration in longitude and Newtonian mechanics).

I don't believe spin is part of the observation, libration in longitutude would be viewable with any spin, CCW, CW, or no spin. Why is spin mandatory or critical for libration in longitude to occur?

Because it explains what we see using simple Newtonian mechanics.

I haven't stated the moon doesn't have constant spin. I've stated it appears to have no relative to earth, axial rotation (a constant), and that I don't know if it has absolute axial rotation because it hasn't been measured.

I've bolded the key. Taking that position, the Moon appears to oscillate back and forth (libration in longitude). For this point of view to be valid, you need a force to cause this oscillation. Something you will never do, because you can't, is to draw a diagram of the Earth and Moon showing how they move based on Newtonian mechanics (forces) that yield an oscillation that makes sense without a constantly spinning Moon. The position you are arguing is akin to epicycles.

You're just repeating your unpredictive position ad nauseam.

Like-wise.

I've bolded the key word. You make no statements that yields calculable results. Instead you handwave about relative axial rotation. My position is the mainstream one, which makes predictions based on Newtonian mechanics which can be confirmed through observation. If you want to be taken seriously, stop handwaving and draw the diagram. Show the forces that yields calculable predictions.

[jtb]

The reason for the confusion of whether or not the moon is rotating is that an axis is an imaginary axle. A real axle is attached to something other than the object mounted on it. Also, whether stationary or moving, if a point on the axle is continually aligned with a point on the mounted object, the object is not rotating. A point on a real axle can either point in the direction of motion, or, always point in the same direction. If the former, the mounted object rotates about the axle. If the latter, the axle rotates within the object.

Modern theory maintains that a point on the moon's axle, or axis, always faces the same direction—south for instance. That means there has to be an external force on the axle causing it to continually face the same direction, and the axle is rotating within the object; not the object rotating about the axle. This theory would be correct if it was stated that the axis rotates within the moon, rather than the moon rotates on its axis.

As a result of this method being used, a distant observer (sidereal) must subtract one rotation per orbit to know the true number of rotations. A distant observer sees the moon rotate once per orbit so one rotation has to be subtracted. Similarly, a distant observer sees 366 Earthly days per orbit, or year, and must subtract one day to get 365 calendar days.

[nicho247]

My position isn't based on dogma. It's based on evidence and established science (libration in longitude and Newtonian mechanics).I've bolded the key.

Hi Chrimony,

I had given this thread a break since I didn't feel I would be able to sway you with logic. I will try again. Note, I had referenced multiple instances of sources stating libration in the longitude does not equate to rotation. It equates to an elliptical orbit.

Here is a new one.

Libration in longitude. Libration in longitude is the moon’s east-west wobble. This sort of libration is a product of the moon’s elliptical (elongated) orbit. Although the moon’s rotation, or spin, goes at a nearly constant rate, its orbital speed varies, going fastest at perigee (moon’s closest point to Earth) and slowest at apogee (moon’s farthest point from Earth).

Here is another new one.

Libration of longitude is an effect of the Moon's varying rate of travel along its slightly elliptical orbit around the Earth. The Moon travels faster when it is at its closest to Earth, and its slowest when it is farthest away. Its rotation on its own axis is more regular, the difference appearing again as a slight east-west "no" oscillation.

Why does libration in the longitude = moon rotation, again? These references say otherwise. My prediction is any object orbiting another with an elliptical path will have libration in the longitude, regardless if it is spinning clockwise, counterclockwise, or not at all. Rotation is not prerequisite of this effect.

My position is the moon's relative rotation to earth is 0 (synchronous rotation), and it's absolute rotation unknown. It is not rotating relative to earth, hence the near side. Consider the following examples....

What would the moon appear to us if...

(A) The moon is currently rotating counter-clockwise and the Moon's rotation speed was faster? The moon would appear to us no longer in a synchronous rotation and is rotating(counter-clockwise) in the sky as it orbits.

(B) The moon is currently rotating counter-clockwise and the Moon's rotation speed was slower? The moon would appear to us no longer in a synchronous rotation and is rotating(clockwise) in the sky as it orbits.

Given, the viewer on earth could see two different moons one rotating (A)counter-clockwise, and the other rotating (B)clockwise all while the moon's actual rotation direction hasn't changed, nor it's orbit, nor the earths, etc. This is my point. Absolute Rotation <> Relative Rotation, as in example (B), the absolute rotation is counter-clockwise, but the relative rotation to earth is clockwise.

Nick

[chrimony]

I had given this thread a break since I didn't feel I would be able to sway you with logic. I will try again.

Why don't you respond to the challenge that prompted you to reopen this thread, instead of repeating arguments that I have already responded to? Show me your Newtonian diagram with objects and forces labeled, one that yields predictions that can be tested against observations. This is a science board. I believe in the mainstream position because it's a predictive model. You want to sway me, show your diagram.

[jtb]

Lunar longitudinal libration is the ability of observers on Earth to see a little more than one half of the moon's surface due to a change in distance caused by its elliptical orbit. To simulate longitudinal libration, hold a curved object with printing on it at arm's length and then at close range. You see a little more of the surface of the object when it is further away.