We find that it is traveling in a circle - or very nearly a circle - round the earth, going completely round once a month, or, more exactly, once very 27 1/3 days.

我们发现月球环绕地球的轨道总是圆周形--或很近似圆周形。每一个月，或者更确切些说是每27又1/3天就环绕地球转一圈。