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Animation of the Moon as it cycles through its phases, as seen from the Northern Hemisphere. The apparent wobbling of the Moon is known as libration. The apparent change in size is due to the eccentricity of the lunar orbit.


The lunar phase or phase of the moon is the shape of the illuminated (sunlit) portion of the Moon as seen by an observer on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing positions of the Moon and Sun relative to the Earth. The Moon and the Earth are tidally locked, therefore the same lunar surface always faces Earth. This face is variously sunlit depending on the position of the Moon in its orbit. Therefore, the portion of this hemisphere that is visible to an observer on Earth can vary from about 100% (full moon) to 0% (new moon). The lunar terminator is the boundary between the illuminated and darkened hemispheres. Each of the 4 lunar phases is roughly 7 days (~7.4 days) each but varies slightly due to lunar apogee and perigee. Aside from some craters near the lunar poles such as Shoemaker, all parts of the Moon see around 14.77 days of sunlight, followed by 14.77 days of "night" (the "dark side" of the Moon is a reference to radio communication darkness, not visible light darkness).


Lunar phase sections
Intro   The phases of the moon    The calendar    Calculating phase    Effect of parallax    Misconceptions    See also    References    External links   

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