libration

Libration is the apparent wobble or oscillation of a celestial body's orientation as seen from a particular vantage. It allows observers to glimpse regions of the object's surface that would otherwise remain hidden. On the Moon, this effect enables viewing about 59 percent of its surface over time, even though only half is visible at any instant.

Libration arises from the Moon's orbital motion and a small tilt of its rotation axis relative to its orbital plane. It appears mainly as longitudinal libration, caused by the Moon's elliptical orbit and synchronous rotation, which makes the visible western or eastern limb tilt toward or away from Earth, and as latitudinal libration, produced by the axis tilt that lifts the northern or southern limb.

Diurnal libration adds a daily, observer-based component: as Earth rotates, an observer's vantage changes slightly, producing a tiny shift in the apparent orientation. Together, longitudinal, latitudinal and diurnal librations combine to reveal different regions over time.

Beyond the Moon, libration is a general term for small oscillations in the orientation of rotating bodies under gravity. It is observed in Mercury's 3:2 spin–orbit resonance and in other satellites and asteroids, where gravitational torques drive periodic variations around an equilibrium.

In astronomy, measurements of libration help map the far side and refine models of rotation and internal structure.