In this verse Allah (swt) swears by the sky and the star (At-Tariq).
Literally, in Arabic everything which is above you and gives you shade is considered a sky for you.
As for the sky we know, it is the heavens with the pillars that Allah (swt) has created as ceiling to this whole earth. As for the nearest one, it is above that, fixed and preserved. As Allah (swt) wills is to be as a very constructed building that we can never really comprehend exactly what it is. Had Allah (swt) willed to let us know about it, He would have informed us. But Allah (swt) willed to let it be hidden, undetermined by our senses and experiments.
However, it is sufficient for us to remember the reference of this word (sky) whenever mentioned in the Holy Qur’an.
الطَّارِق (The night comer): Allah (swt) gives us a picture of the effects and role in existence of something we do not know its essence nor its reality.
This word is derived from the verb “يطرق” meaning -in Arabic- to strike excessively causing a noise to happen. Additionally, there is a noun derived from it which is “طريق”, meaning a flattened road which people walk on a lot and their feet strike it a lot.
However, this word “الطارق” is linguistically common to refer to someone who specifically walks at night time. Because night is silent, no noise or movement is heard, so that when he reaches a destination and knock on the door, he is an unexpected arriver, causing a lot of attention.