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Lunar day
A lunar day is the period of time it takes for the Earth's Moon to complete one full rotation on its axis with respect to the Sun. Equivalently, it is the time it takes the Moon to make one complete orbit around the Earth and come back to the same phase. It is marked from a new moon to the next new moon.
With respect to the stars, the Moon takes 27 Earth days, 7 hours and 43 minutes 12 seconds to complete its orbit;〔(【引用サイトリンク】title=Month )〕 but since the Earth-Moon system advances around the Sun in the meantime, the Moon must travel further to get back to the same phase. On average(mean), this synodic period lasts 29 days, 12 hours, 44 minutes and 3 seconds.〔 This is an average figure, since the speed of the Earth-Moon system around the Sun varies slightly during a year, due to an imperfect circularity of its orbit, variances in orbital velocity, and a number of other periodic and evolving variations about its observed, relative, mean values, due to the gravitational perturbations of the Sun and a vast, complex universe.
As a result, daylight at a given point on the Moon would last approximately two weeks from beginning to end, followed by approximately two weeks of night.
==Alternate usage==
The term "lunar day" may also refer to the period between moonrises in a particular location on Earth. This period is typically slightly longer (50 minutes) than a 24-hour Earth day, as the Moon revolves around the Earth in the same direction as the Earth's axial rotation.〔("Animation of a lunar day" ). NOAA. Retrieved 1 May 2012.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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