Charles Ellicott Commentary Genesis 1:15

"and let them be for lights in the firmament of heaven to give light upon the earth: and it was so." — Genesis 1:15 (ASV)

To give light. —This was to be from now on the permanent arrangement for the bestowal of what is an essential condition for all life, plant 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 established as they are now, and shone regularly and clearly in the bright sky.