Translation of the verse 4 from Surah Ash-Shams : Number of verses 15 - - page 595 - Part 30.
And by the night as it conceals it (the sun);
And [by] the night when it covers it
(91:4) and by the night as it envelopes the sun; *2
*2) That is, when the night comes, the sun hides and its light remains hidden throughout the night. This state has been described, saying that the night covers up the sun, for the night actually signifies the sun's hiding behind the horizon because of which its light cannot reach that part of the earth where the night has fallen.
Source : Ash-Shams Verse 4: And [by] the night when it covers it