登月 is a separable verb (离合词) meaning 'to land on the moon' or 'to achieve a moon landing'. It follows the pattern Verb + Object: 登 ('to ascend/to land') + 月 ('moon'). When you need to add a measure word, aspect particle, or complement, 登 and 月 split apart — for example, 登上了月球 ('landed on the moon'), 登过月 ('have landed on the moon before'), or 登一次月 ('land on the moon once'). The object 月 always stays as the second element; you never put anything after 月.