libration

Any of several periodic rocking motions of an orbiting body. The various librations of the Moon enable, over time, about 59% of the Moon's surface to be seen from Earth. Libration in latitude, a north-south nodding, is caused by the tilt of the Moon's rotation axis relative to its orbital plane. The average up/down latitude wobbling is 5.13°, corresponding to the Moon's orbital inclination with respect to the ecliptic, though perturbations by the Sun can add a further +/- 0.9°. Libration in longitude arises from the difference between the Moon's varying orbital velocity and its constant rotation rate. Without any other factors it would average 6.29°, but the Sun's contribution pushes the peak oscillation in longitude to 7.75°. Diurnal libration is an optical rather than a physical libration, amounting to less than 1°, and stems from the fact that, because of Earth's rotation, we view the Moon from different angles at moonrise and moonset. Libration happens when an orbiting body is locked in a synchronous rotation; Mercury shows it, as do some other moons in the Solar System.

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• CELESTIAL MECHANICS

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