You are reading a tafsir for the group of verses 56:75 to 56:76
فَلَاۤ
اُقْسِمُ
بِمَوٰقِعِ
النُّجُوْمِ
۟ۙ
وَاِنَّهٗ
لَقَسَمٌ
لَّوْ
تَعْلَمُوْنَ
عَظِیْمٌ
۟ۙ
3

Sequencing of Arguments

Preceding verses put forward rational arguments, in support of life after death, by inviting attention to Allah's infinite power. The current verses are meant to prove this fact by giving an authoritative reference, that is, the Qur'an.

Allah swears to the Greatness of the Qur’ an

فَلَا أُقْسِمُ بِمَوَاقِعِ النُّجُومِ (So, I swear by the setting places of the stars...56:75) The words لَا أُقْسِمُ 'I swear' are prefixed in the text by the particle لَا la [ no ] which is not translated in the text, because it is idiomatically prefixed to 'oath', as for example لَا وَاللہ la wallah [ No, by Allah ]. In pre-Islamic Arabic, we come across the idiomatic oath لَا وَ اَبَیکَ la wa-abik [ No, by your father ]. Some lexicologists say that the particle la is added only as an idiomatic expression [ zaidah ] which carries no sense, and others say that when the refutation of an addressee's hypothesis is intended, y la is used to signify that the assumption of the addressee is not correct, but the right thing is that which follows.

The word مَوَاقِعِ mawaqi` is the plural of مَوقع mawq' and refers to the points where or times when the stars set. Here, like in Surah -Najm وَالنَّجْمِ إِذَا هَوَىٰ "By the star when it goes down to set, [ 1] ", the oath of stars is qualified by their setting-time. The wisdom underlying this is that when the stars set, their function seems to have been cut off from the horizon, and we witness effects of their vanishing. This is the proof of their perishability and dependence on Divine power.