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In astronomy a libration is a very slow oscillation, of a satellite as viewed from the larger celestial body around which it revolves. Used alone, the term usually refers to the apparent movements of the Moon relative to Earth. Although the Moon`s rotation on its axis is synchronously locked with its revolution around Earth, these librations permit a terrestrial observer to see slightly differing halves of the Moon`s surface at different times. This means that a total of 59% of the Moon`s surface can be observed from Earth.