librate

Libration is the apparent wobble or oscillation of a celestial body's orientation that allows observers to see slightly more than half of its surface over time. The best-known example is the Moon. Over a lunar cycle, observers on Earth can view about 59% of the Moon’s surface, compared with 50% at any single moment.

Libration arises from three effects. Libration in longitude results from the Moon’s elliptical orbit and variable orbital speed: when the Moon moves faster or slower, the edge facing Earth shifts, revealing parts of the limb that would otherwise remain hidden. Libration in latitude comes from the Moon’s axial tilt of about 6.7 degrees relative to its orbital plane, which, combined with the orbital geometry, causes the visible limb to tilt north and south over time. Diurnal libration is due to Earth’s rotation; as observers change longitude, their viewpoint shifts slightly, producing a small daily change in the view.

These librations have practical consequences for observing and mapping the Moon. They explain why features near the lunar edge become visible at times and hidden at others, aiding lunar cartography and mission planning. Librations can be predicted from orbital mechanics and are documented in astronomical ephemerides and lunar charts. The term libration derives from Latin libratio, meaning balance or rocking, reflecting the Moon’s subtle, rocking motion. While most conspicuous on the Moon, libration is a general concept in celestial mechanics describing similar apparent motions in other bodies.