We find that it is traveling in a circle - or very nearly a

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 every 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.
此处的going和前面的traveling是对等的,即we find that it is traveling in a circle round the earth, We find that it is going completely round （the earth）,or (more exactly, we find that it is going round the earth once every27 1/3 days).
该句为find后接宾语从句,其中宾语从句由3个句子构成,即.,.,or.
going实际应该是it is going,即作谓语动词.round为介词,引起短语round the earth,整个短语做状语.