Abbas - Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs
(Blessed be He) He is the Possessor of grace He (Who hath placed in the heaven mansions of the stars) and it is said that this means: mansions, (and hath placed therein) in the heaven (a great lamp) the sun shining during the day for the children of Adam (and a moon giving light) to the Children of Adam during the night!
God, exalted be He, says: Blessed, magnified, is He Who has placed in the heaven constellations, twelve [of them]: Aries, Taurus, Gemini, Cancer, Leo, Virgo (al-sunbula), Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces. They are the mansions for the seven orbiting planets: Mars rules Aries and Scorpio, Venus: Taurus and Libra, Mercury: Gemini and Virgo, the Moon: Cancer, the Sun: Leo, Jupiter: Sagittarius and Pisces, and Saturn: Capricorn and Aquarius; and has, also, placed in it a lamp, namely, the sun, and a shining moon (a variant reading [for sirājan, ‘sun’] has the plural surujan, which would mean ‘luminous stars’). The moon is singled out for mention because of its particular merit.
Here Allah glorifies Himself and praises the beauty He created in the heavens of Al-Buruj, the giant stars, according to the view of Mujahid, Sa`id bin Jubayr, Abu Salih, Al-Hasan and Qatadah. This is like the Ayah,
(And He it is Who has put the night and the day in succession (Khilfatan),) meaning, each one comes after the other, in a never-ending alternation. When one goes the other comes, and vice versa, as Allah says:
وَسَخَّر لَكُمُ الشَّمْسَ وَالْقَمَرَ دَآئِبَينَ
(And He has made the sun and the moon, both constantly pursuing their courses) (14:33).
يُغْشِى الَّيْلَ النَّهَارَ يَطْلُبُهُ حَثِيثًا
(He brings the night as a cover over the day, seeking it rapidly) (7:54).
لاَ الشَّمْسُ يَنبَغِى لَهَآ أَن تدْرِكَ القَمَرَ
(It is not for the sun to overtake the moon) (36:40).
(for such who desires to remember or desires to show his gratitude.) means, He has caused them both to follow one another to show the times when His servants should worship Him. So whoever misses an act of worship during the night can make it up during the day, and whoever misses an act of worship during the day can make it up during the night. It was recorded in a Sahih Hadith:
(Allah spreads forth His Hand at night for the one who has done evil during the day to repent, and He spreads forth His Hand during the day for the one who has done evil during the night to repent.) Mujahid and Qatadah said: "Khilfatan means different, i.e., because one is dark and the other is light.''
Maududi - Sayyid Abul Ala Maududi - Tafhim al-Qur'an
See E.N.'s 8 to 12 of Al Hijr.
That is, the sun, as mentioned clearly in Surah Nuh: 16: "... and made the sun a lamp."
The observation and deep consideration of the wonderful phenomenon of the alternation of the day and night is a proof of Tauhid and His Providence so that man may feel grateful to Him and prostrate himself before Him in all humility.