Libration

Libration is an apparent back-and-forth oscillation in a celestial body's orientation or position as seen from another body. It reveals changing views of a body even when its rotation is synchronized with its orbit.

Optical libration: An apparent change in the visible portion of a synchronously rotating body's surface caused by viewing geometry. The Moon’s elliptical orbit and changing viewing angle let observers see slightly different longitudes over time.
Lunar cartography: The measurement and mapping of the Moon's surface features. Combining observations across lunar librations extends maps beyond the hemisphere visible at one time.
Galileo Galilei: An Italian astronomer and physicist whose telescopic observations transformed early modern astronomy. Galileo described the Moon’s apparent oscillations in his early telescopic observations.
Rotation: The turning of a body around an internal axis. Libration may look like rotation changing, but optical libration is produced by viewpoint rather than bodily turning.
Physical libration: A real oscillation in a celestial body's rotation around its equilibrium orientation. Unlike optical libration, this involves actual changes in the body's rotation.
Lunar far side: The hemisphere of the Moon that generally faces away from Earth. Libration exposes narrow strips of the far side along the Moon’s apparent limb.
Johannes Hevelius: A seventeenth-century astronomer known for detailed observations and maps of the Moon. His lunar studies helped document changing views of the Moon’s surface.
Precession: A gradual change in the direction of a rotating body's axis. Precession changes an axis over time, whereas libration is an oscillation around an orientation.
Synchronous rotation: A rotational state in which a body's rotation period equals its orbital period. The Moon’s synchronous rotation sets the baseline against which its apparent libration is noticed.
Lunar limb: The apparent edge of the Moon's disk as seen from Earth. Libration shifts which surface features lie at the visible edge.

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