chrimony wrote: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 It might be outdated or ideologically biased."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