这颗星球每围太阳绕行一周，它的椭圆形轨道的中轴就变化一次。
The mainaxis of the planet's orbital ellipse shifts each time it goes round the sun.
月球绕行地球一周只需二十天，地球的一天也只有十八小时长。
It took the Moon only twenty days to circle the Earth, and Earth's day was only eighteen hours long.
月球绕行地球一周只需二十天，地球的一天也只有十八小时长。
It took the Moon only twenty days to circle the Earth, and Earth's day was only eighteen hours long.
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