Genesis 1:15

15. Let them be for lights It is well again to repeat what I have said before, that it is not here philosophically discussed, how great the sun is in the heaven, and how great, or how little, is the moon; but how much light comes to us from them. For Moses here addresses himself to our senses, that the knowledge of the gifts of God which we enjoy may not glide away. Therefore, in order to apprehend the meaning of Moses, it is to no purpose to soar above the heavens; let us only open our eyes to behold this light which God enkindles for us in the earth. By this method (as I have before observed) the dishonesty of those men is sufficiently rebuked, who censure Moses for not speaking with greater exactness. For as it became a theologian, he had respect to us rather than to the stars. Nor, in truth, was he ignorant of the fact, that the moon had not sufficient brightness to enlighten the earth, unless it borrowed from the sun; but he deemed it enough to declare what we all may plainly perceive, that the moon is a dispenser of light to us. That it is, as the astronomers assert, an opaque body, I allow to be true, while I deny it to be a dark body. For, first, since it is placed above the element of fire, it must of necessity be a fiery body. Hence it follows, that it is also luminous; but seeing that it has not light sufficient to penetrate to us, it borrows what is wanting from the sun. He calls it a lesser light by comparison; because the portion of light which it emits to us is small compared with the infinite splendor of the sun.

John Calvin's Commentaries (1540-1563) — public domain. Text from the Calvin Translation Society English edition (1840s-50s) via the Christian Classics Ethereal Library (ccel.org). Calvin's coverage is partial (roughly two-thirds of the Bible); uncommented books and verses are honest gaps, never fabricated. The 19th-century editor's footnotes and remarks are omitted so the notes are faithfully Calvin's own voice.

(15) To give light.—This was to be henceforward the permanent arrangement for the bestowal of that which is an essential condition for all life, vegetable and animal. As day and night began on the first day, it is evident that very soon there was a concentrating mass of light and heat outside the earth, and as the expanse grew clear its effects must have become more powerful. There was daylight, then, long before the fourth day; but it was only then that the sun and moon became fully formed and constituted as they are at present, and shone regularly and clearly in the bright sky.

A Bible Commentary for English Readers, edited by Charles John Ellicott (1878–1897) — public domain. Text from a community e-Sword module.

And let them be for lights in the firmament of the heaven,.... To continue there as luminous bodies; as enlighteners, as the word signifies, causing light, or as being the instruments of conveying it, particularly to the earth, as follows:

to give light upon the earth; and the inhabitants of it, when formed:

and it was so: these lights were formed and placed in the firmament of the heaven for such uses, and served such purposes as God willed and ordered they should.

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

והיו למאורות AND LET THEM BE FOR LIGHTS — They shall serve also this purpose, namely, to give light to the world.

Rashi on the Tanakh — English translations: “The Judaica Press complete Tanach with Rashi, translated by A. J. Rosenberg” (CC BY 4.0); “Pentateuch with Rashi's commentary by M. Rosenbaum and A.M. Silbermann, 1929-1934” (public domain); “The Metsudah Tanach series, Lakewood, N.J” (CC BY 4.0); and “The Book of Joshua, Metsudah Publications, 1997” (CC BY 4.0). Text via Sefaria.

AND THEY SHALL BE FOR LIGHTS IN THE FIRMAMENT OF THE HEAVEN TO GIVE LIGHT UPON THE EARTH. He added here that their light should reach the earth since it is possible for the light to be seen in the heavens and perform all mentioned functions without lighting upon the earth. Hence He said that it be for lights in the firmament of the heaven, directed toward the earth and shining upon it.

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.

והיו, they should also serve as luminaries, i.e להאיר על הארץ, to provide illumination on earth. Prior to the fourth “day” the sun and moon had already been sources of light for the earth. Suspending these luminaries in the sky on the fourth ”day” only made them visible to creatures who possessed a sense of sight. Their light also had an effect on all of these creatures, each according to their physical constitution.

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.