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The Moon is in synchronous rotation, meaning that it keeps nearly the same face turned away from Earth at all times. Early in the Moon's history, its rotation slowed and became locked in this configuration as a result of frictional effects associated with tidal deformations caused by the Earth. Nevertheless, small variations resulting from the eccentricity of the lunar orbit, termed librations, allow about 59 percent of the lunar surface to be viewed from Earth.
