TCampbell wrote in post #18158273
Libration occurs because the Moon's orbit isn't a circle, but an ellipse. The distance between the Earth & Moon changes by about 10% each Lunar month.
In an elliptical orbit, objects pick up speed as they approach the point nearest to the object they are orbiting (generically that point is called a periapsis but since the body is orbiting the Earth it gets the special name "perigee" -- if it were orbiting the Sun it would get a special named called a perihelion) and the object will lose speed as it gets to the farthest point in the orbit (generically called an apoapsis, but given the special name apogee since the body is orbiting the Earth.)
Meanwhile, the Moon is tidally "locked" so that one face of the moon is always toward the Earth. But to pull this off, the moon "rotates" exactly once as the moon orbits the Earth exactly once. (one revolution per one orbit). Since the orbital velocity changes... the rotational speed would also need to change in order for viewers on Earth to see *precisely* the same face of the moon. Since changing a rotational speed would be very difficult (it would require a great deal of energy since the moon has a lot of mass), the rotational speed of the Moon basically does _not_ change even though it's orbital speed _does_ change.
This creates the effect called libration that you're seeing... it allows us to sneak a peak just behind the Moon's horizon as the moon appears to (from our point of view) oscillate from side-to-side as it orbits.
Libration occurs because the Moon's orbit isn't a circle, but an ellipse. The distance between the Earth & Moon changes by about 10% each Lunar month.
In an elliptical orbit, objects pick up speed as they approach the point nearest to the object they are orbiting (generically that point is called a periapsis but since the body is orbiting the Earth it gets the special name "perigee" -- if it were orbiting the Sun it would get a special named called a perihelion) and the object will lose speed as it gets to the farthest point in the orbit (generically called an apoapsis, but given the special name apogee since the body is orbiting the Earth.)
Meanwhile, the Moon is tidally "locked" so that one face of the moon is always toward the Earth. But to pull this off, the moon "rotates" exactly once as the moon orbits the Earth exactly once. (one revolution per one orbit). Since the orbital velocity changes... the rotational speed would also need to change in order for viewers on Earth to see *precisely* the same face of the moon. Since changing a rotational speed would be very difficult (it would require a great deal of energy since the moon has a lot of mass), the rotational speed of the Moon basically does _not_ change even though it's orbital speed _does_ change.
This creates the effect called libration that you're seeing... it allows us to sneak a peak just behind the Moon's horizon as the moon appears to (from our point of view) oscillate from side-to-side as it orbits.
The above is a correct description of libration, but libration is not what has happened in the two images that Charles posted. Instead, all that's happened is that the Moon has rotated relative to the ground as it moved across the sky (as indicated by Pulsar123 above).
Cheers,
Stephen
