John Calvin 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)

Let them be for lights. It is well to repeat again what I have said previously: that it is not philosophically discussed here how great the sun is in the heavens, or how great or how small the moon is, but rather how much light comes to us from them. For Moses here addresses our senses, so that the knowledge of God's gifts, which we enjoy, may not slip away.

Therefore, to understand Moses' meaning, it is pointless to soar above the heavens; let us only open our eyes to behold this light which God kindles for us on the earth. By this method (as I have previously observed) the dishonesty of those who criticize Moses for not speaking with greater precision is sufficiently rebuked.

For as was fitting for a theologian, his concern was for us rather than for the stars. Nor, in truth, was he ignorant of the fact that the moon did not have sufficient brightness to enlighten the earth unless it borrowed from the sun; but he considered it sufficient to declare what we all can plainly perceive—that the moon is a provider of light to us.

That it is, as the astronomers assert, an opaque body, I concede 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 necessarily be a fiery body.

It therefore follows that it is also luminous; but since it does not have sufficient light to penetrate to us, it borrows what it lacks from the sun. He calls it a lesser light by comparison, because the amount of light it emits to us is small compared with the infinite splendor of the sun.