Genesis 1:18

And to rule over the day, and over the night,.... The one, namely the sun, or greater light, to rule over the day, and the moon and stars, the lesser lights, to rule over the night: this is repeated from Gen 1:16 to show the certainty of it, and that the proper uses of these lights might be observed, and that a just value might be put upon them, but not carried beyond due bounds:

and to divide the light from the darkness; as the day from the night, which is done by the sun, Gen 1:14 and to dissipate and scatter the darkness of the night, and give some degree of light, though in a more feeble manner, which is done by the moon and stars:

and God saw that it was good; or foresaw it would be, that there should be such lights in the heaven, which would be exceeding beneficial to the inhabitants of the earth, as they find by good experience it is, and therefore have great reason to be thankful, and to adore the wisdom and goodness of God; see Psa 136:1. See Gill on Gen 1:4.

John Gill's Exposition of the Whole Bible (1746–1763) — public domain (CC PDM 1.0). Text via the HelloAO Bible API.

This clause was omitted in the first day’s work, but is added here, because the light was then but glimmering and imperfect, which now was made more clear and complete.

Matthew Poole's English Annotations on the Holy Bible (1683–1685) — public domain. Text from a community e-Sword/MySword module.

AND TO RULE OVER THE DAY AND OVER THE NIGHT. The matter of rulership is a different matter from the function of light which He mentioned, since it includes that which He stated at first, And they shall be for signs, and seasons. Their rulership over the earth comprises the changes which they cause in it and the power of bringing about the existence and deterioration of all things in the lower world since the sun, by its rule during the day, causes the sprouting, the propagation and the growth of all the warm and dry things, while the moon by its rule increases the springs and the oceans, and all liquid and cold things. Therefore He said in a general way, And to rule over the day and over the night, because theirs is the dominion over things in the lower world.It is possible that the rulership given to them contains also a power of emanation for they are the leaders of things in the lower world, and with their power, every ruling power in nature holds sway. Thus the constellation which comes up by day rules during it, even as it is written, The sun and the moon and the stars… which the Eternal thy G-d hath allotted unto all the peoples. And this is what Scripture means when it says, He counteth the number of the stars; He giveth them all their names, likewise, He calleth them all by name. For the calling of names signifies the differentiation in their respective powers, giving to this one the power of justice and righteousness, and to that one the power of blood and the sword, and similarly all other powers, as is known in astrology. And all is done by the power of the Most High and in accordance with His Will. Therefore it says, Great is our Lord, and mighty in power, for He is greatest of all and mightiest in power over them. And similarly it says, He calleth them all by name by the greatness of His might and the strength of His power. In accord with the secret I have hinted to you, the matter of rulership is completely true.

AND TO DIVIDE THE LIGHT FROM THE DARKNESS. Rabbi Abraham ibn Ezra said: “By the coming forth of the sun at daytime and the light of the moon at night, they shall divide the light from the darkness.” (and to divide) refers to both the sun and moon, that each in coming forth separates between light and darkness. (Tur.) In my opinion, the light mentioned here refers to the day, and the darkness is the night for such are their names, as it says, And G-d called the light Day, and the darkness He called Night. from the night,” meaning, that light is to serve at daytime and darkness at night. (Tur.)Now in connection with all the works of creation, Scripture mentions the Divine command and then tells of the deed. And here too He commanded, And they shall be for lights, and then it relates, And G-d set them. He further said, And to rule over the day and over the night, meaning that one is to rule by day and the other by night, the rulership being that which He commanded when He said, And they shall be for signs, and for seasons. And now He related that the rulership of the two is not alike but instead consists of dividing the darkness from the light. The greater luminary will rule by day and light will be everywhere, even where the sun does not reach, and the smaller luminary will rule by night, and there will be darkness except that the moon will lighten its darkness. This then is the command He gave in order to divide the day from the night, as it says, And G-d divided the light from the darkness.

Ramban (Nachmanides) on the Tanakh — English translation: “Commentary on the Torah by Ramban (Nachmanides). Translated and annotated by Charles B. Chavel. New York, Shilo Pub. House, 1971-1976” (CC BY 4.0). Text via Sefaria.

ולמשול ביום ובלילה, the one, i.e. the sun by day, and the other, i.e. the moon, to govern by night.

ולהבדיל, from the time the sun rises it is light, day, and the domain of the sun, whereas from the time the stars become visible in the sky, it is nighttime, the moon’s domain. The night is called “darkness,” even though there is some light in the sky illuminating parts of the earth in a fashion.

Radak on the Tanakh — English translations: “Sefaria Community Translation” (CC0 1.0 (public-domain dedication)); “Eliyahu Munk, HaChut Hameshulash” (CC BY 4.0); “Rabbi Mike Feuer, Jerusalem Anthology” (CC BY 4.0); and “R. David Kimhi on the first book of Psalms, Translated by R.G. Finch, London, 1919” (public domain). Text via Sefaria.