Lunar Phases
Lunar Libration
Librations refer to the ways in which observers on Earth can “peek around the edges” of the Moon, revealing areas that would otherwise remain hidden due to its synchronous rotation. There are three key geometrical librations. First, the Moon’s rotation axis is tilted by 6.5° relative to the Earth-Moon line, allowing viewers to see small portions of the Moon’s polar regions at different points in its orbit. This is analogous to how the Earth’s axial tilt affects seasonal sunlight exposure on its poles, though the Moon’s tilt reveals only a small extra area of its surface.
The second geometrical libration arises from the Moon’s elliptical orbit around Earth. As the Moon speeds up or slows down depending on its proximity to Earth, its orbital motion becomes uneven compared to its steady rotation. This allows observers on Earth to see slightly more of the Moon’s eastern or western edges, adding around 7.7 degrees to the visible area. The unevenness occurs because the Moon’s speed varies as it approaches and recedes from Earth, in accordance with Kepler’s laws of planetary motion. This variation lets astronomers gain additional longitudinal coverage over time.
A third type of libration occurs due to the Earth’s rotation. As the Earth rotates, observers at different locations or hemispheres can view slightly different angles of the Moon, increasing the visible surface area. This effect is subtle but measurable, especially at the lunar distance of 60 Earth radii. While it only shifts the viewing angle by about 1°, it is enough to expand the amount of the Moon that is observable from Earth. These geometrical librations collectively allow astronomers to see roughly 59% of the Moon’s surface over time, despite its synchronous rotation.
In addition to geometrical librations, the Moon also experiences physical librations, which involve actual pendulum-like wobbles and nodding motions. These physical librations are much smaller, with a primary pole-to-pole nodding of about 1.5° and a longitudinal oscillation of only a fraction of an arc minute. These subtle motions are detectable through laser signals bounced off reflectors placed on the Moon by Apollo missions and Soviet landers. Moreover, many other moons in the solar system, particularly those of Jupiter and Saturn, exhibit similar synchronous rotation, keeping the same face toward their parent planets due to gravitational locking.
(ChatGPT summary of https://pwg.gsfc.nasa.gov/stargaze/Smoon4.htm)