The gravity of the moon on the earth is one of the causes of the tide phenomenon. The orbit of the moon around the earth is synchronous. The so-called synchronous rotation is not strict. Because the orbit of the moon is elliptical, when the moon is at perihelion, its rotation speed can not catch up with the revolution speed. Therefore, we can see the east of the moon is 98 degrees east longitude. On the contrary, when the moon is at the distant sun, its rotation speed is faster than the revolution speed. Therefore, we can see the west of the moon is 98 degrees west longitude. This phenomenon is called libration. Because the orbit of the moon inclines to the equator of the earth, the polar region will shake about 7 degrees when the moon moves in the starry sky. This phenomenon is called libration. Moreover, since the moon is only 60 earth radius away from the earth, if the observer observes from the moon to the moon, the observation point will have a displacement of the earth diameter, which can be seen in the area with a moon longitude of 1 degree. This phenomenon is called libration.
严格来说,地球与月球围绕共同质心运转,共同质心距地心4700千米(即地球半径的2/3处)。由于共同质心在地球表面以下,地球围绕共同质心的运动好像是在“晃动”一般。从地球北极上空观看,地球和月球均以迎时针方向自转;而且月球也是以迎时针绕地运行;甚至地球也是以迎时针绕日公转的。
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