The lunar phase or Moon phase is the shape of the Moons directly sunlit portion as viewed from Earth.The lunar phases gradually change over a synodic month (about 29.53 days) as the Moons orbital positions around Earth and Earth around the Sun shift. The visible side of the moon is variously sunlit, depending on the position of the Moon in its orbit.The Moons regular phases make it a very convenient timepiece, and the periods of its waxing and waning form the basis of many of the oldest calendars. Tally sticks, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon. The 30day month is an approximation of the lunar cycle.
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