Surah 54. Al-Qamar, Ayah 1

Most of the commentators see in this verse a reference to a phenomenon said to have been witnessed by several of the Prophet's contemporaries. As described in a number of reports going back to some Companions, the moon appeared one night as if split into two distinct parts. While there is no reason to doubt the subjective veracity of these reports, it is possible that what actually happened was an unusual kind of partial lunar eclipse, which produced an equally unusual optical illusion. But whatever the nature of that phenomenon, it is practically certain that the above Qur'an-verse does not refer to it but, rather, to a future event: namely, to what will happen when the Last Hour approaches. (The Qur'an frequently employs the past tense to denote the future, and particularly so in passages which speak of the coming of the Last Hour and of Resurrection Day; this use of the past tense is meant to stress the certainty of the happening to which the verb relates.) Thus, Raghib regards it as fully justifiable to interpret the phrase inshaqqa 'l-qamar ("the moon is split asunder") as bearing on the cosmic cataclysm - the end of the world as we know it - that will occur before the coming of Resurrection Day (see art. shaqq in the Mufradat). As mentioned by Zamakhshari, this interpretation has the support of some of the earlier commentators; and it is, to my mind, particularly convincing in view of the juxtaposition, in the above Qur'an-verse, of the moon's "splitting asunder" and the approach of the Last Hour. (In this connection we must bear in mind the fact that none of the Qur'anic allusions to the "nearness" of the Last Hour and the Day of Resurrection is based on the human concept of "time".)

 The Meccan pagans challenged the Prophet (ﷺ) to have the moon split in two if he wanted them to believe in him. The moon was split, and then re-joined, as reported by several eyewitnesses, but still the pagans refused to believe, calling this miracle “sheer magic.”

See para 2 of the Introduction to S. liii. The idea of the Judgment being nigh at the beginning of this Sura connects it with the same idea at the end of the last Sara (verse 57), though the actual words used in the two cases are different.

Three explanations are given in the Mufradat, and perhaps all three apply here: (1) that the moon once appeared cleft asunder in the valley of Makkah within sight of the Prophet, his Companions, and some Unbelievers; (2) that the prophetic past tense indicates the future, the cleaving asunder of the moon being a Sign of the Judgment approaching; and (3) that the phrase is metaphorical, meaning that the matter has become clear as the moon. That the first was noticed by contemporaries, including Unbelievers, is clear from verse 2. The second is an incident of the disruption of the solar system at the New Creation: Cf. lxxv. 8-9.