Figure: A brief explication of the lunar tidal locking and the lunar libration

Image 1 Caption: A not-to-scale diagram illustrating the lunar phases (i.e., the phases of the Moon) over the course of a lunar month = 29.530588861 days (mean J2000) ≅ 29.53059 days (mean J2000 to 7 digits) ≅ 29.5 days. For further explication of the lunar phases, see Moon file: moon_lunar_phases.html.

In this figure, we consider the lunar libration, a mostly apparent rocking motion of the Moon as seen from Earth.

Further explication:

1. See the NASA video NASA: Moon Wobble, October 20, 2011, ID:10836 | 2:26: Authoritative: The lunar phases, lunar tidal locking , and lunar libration are shown. Good for a minute or so for the classroom.

2. From the NASA video, you see that from Earth we see only the near side of the Moon. This is due tidal locking (AKA synchronous rotation) which causes the Moon's orbital period and on-its-axis rotational period to be the same on average, and thus causes the Moon to always turn one side toward the Earth. This side we call the near side of the Moon.

   The side of the Moon we do not see from the Earth is the far side of the Moon. The far side was first seen by the Soviet space program probe Luna 3, 1959 Oct07.

3. For the physical origin of the tidal locking, see Moon file: tidal_locking_origin.html.

4. However, as the NASA video also shows there is a rocking motion of Moon. This is lunar libration.

   The lunar libration is due to the observer seeing slightly different hemispheres as the Moon and Earth move in space.

5. There are actually three effects that add up to the overall apparent (or optical) lunar libration (see Wikipedia: Lunar libration). Remember "apparent" in astro jargon means as seen from Earth.

   

   The most easy to understand is the latitudinal libration. Because of the Moon's orbital inclination (i.e., orbital tilt from the ecliptic plane) of 5.145°, the center-to-center Earth-Moon line touches the Moon at different lunar latitudes: we see further north on the Moon when the Moon is below the ecliptic plane and further south then it is above the ecliptic plane. See Image 2 and Image 3 below for more on the latitudinal libration of the Moon.

6. Image 2 Caption: Image 2 A diagram illustrating the explication of the latitudinal libration of the Moon.

   

7. Image 3 Caption: The Moon at full moons with extremely different libration orientations (which are mostly the latitudinal libration it seems) scaled for equal Moon diameter. The Mare Humboldtianum is not visible for the full moon on the right-hand side of Image 3.

8. Most of the lunar libration is apparent, and so mostly there is no no rocking relative to the Earth-Moon system center-of-mass inertial frame.

   But there is a small physical libration due to astronomical perturbations which will not explicate (see Wikipedia: Libration: Physical libration).

   Note also, relative to Earth-Moon system center-of-mass inertial frame, the Moon's axis (axial tilt about 1.53° relative to the ecliptic) precesses with a period of 18.6 years. The axial tilt relative to the Moon's own orbital pole is about 6.68°. (See Google AI question: What is the moon's axial tilt relative to the ecliptic pole and how fast does it vary?) This precession gives a very slow small physical libration effect.

9. Because of the lunar libration we do see a 59 % of the Moon's surface from the Earth (see Wikipedia: Tidal locking: Occurrence: Earth's Moon), but only very nearly 50 % at one time, of course.