Strictly speaking, the earth and the moon revolve around a common center of mass, 4700 kilometers (2 / 3 of the earth's radius). Since the common center of mass is below the earth's surface, the movement of the earth around the common center of mass seems to be "shaking". When viewed from the north pole of the earth, both the earth and the moon revolve in the clockwise direction; moreover, the moon also revolves around the earth in the clockwise direction; even the earth revolves around the sun in the clockwise direction.
很多人不明白为甚么月球轨道倾角和月球自转轴倾角的数值会有这么大的变化。其实,轨道倾角是相对于中心天体(即地球)而言的,而自转轴倾角则相对于卫星(即月球)本身的轨道面。在这个定义习惯很适合一般情况(例如人造卫星的轨道)而且是数值相当固定的,但月球却非如此。
Many people don't understand why there is such a big change in the inclination of the moon's orbit and rotation axis. In fact, the orbit inclination is relative to the central object (i.e. the earth), while the rotation axis inclination is relative to the orbit surface of the satellite (i.e. the moon). In this definition, the custom is very suitable for the general situation (such as the orbit of artificial satellite) and the value is quite fixed, but the moon is not.
月球的轨道平面(白道面)与黄道面(地球的公转轨道平面)保持着5。145396°的夹角,而月球自转轴则与黄道面的法线成1。5424°的夹角。因为地球并非完美球形,而是在赤道较为隆起,因此白道面在不断进动(即与黄道的交点在顺时针转动),每6793。5天(18。5966年)完成一周。期间,白道面相对于地球赤道面(地球赤道面以23。45°倾斜于黄道面)的夹角会由28。60°(即23。45°+5。15°)至18。30°(即23。45°-5。15°)之间变化。同样地,月球自转轴与白道面的夹角亦会介乎6。69°(即5。15°+1。54°)及3。60°(即5。15°-1。54°)。月球轨道这些变化又会反过来影响地球自转轴的倾角,使它出现±0。00256°的摆动,称为章动。
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