Genesis 1:15

Look first on the firmament of heaven, which was made before the sun. Look first on the earth, which began to be visible and was already formed before the sun put in its appearance. Look at the plants of the earth, which preceded in time the light of the sun. The bramble preceded the sun. The blade of grass is older than the moon. Therefore, do not believe that object to be a god to which the gifts of God are seen to be preferred. Three days have passed.

Men ought to have been astonished and amazed not only at the arrangement of the sun and moon but also at the well-ordered movements of the stars and their unfettered courses and the timely rising of each of them; how some are signs of summer, others of winter; how some indicate the time for sowing, others the times of navigation.

"Let them serve," he says, "for the fixing of days," not for making days but for ordering the days. For day and night are earlier than the generation of the luminaries. This the psalm declares to us when it says: "He placed the sun to rule the day, the moon and stars to rule the night." How, then, does the sun rule the day? Because, whenever the sun, carrying the light around with it, rises above our horizon, it puts an end to the darkness and brings us the day. Therefore one would not err if he would define the day as air, lighted by the sun, or as the measure of time in which the sun tarries in the hemisphere above the earth. But the sun and the moon were also appointed to be for the years. The moon, when it has completed its course twelve times, measures a year, except that it frequently needs an intercalary month for the accurate determination of the seasons, as the Hebrews and the most ancient Greeks formerly measured the year. The solar year is the return of the sun from a certain sign to that same sign in its regular revolution.

For that reason the blessed Moses, inspired by the divine Spirit, teaches us with great precision, lest we fall victim to the same things as they, instead of being able to know clearly both the sequence of created things and how each thing was created. You see, if God in his care for our salvation had not directed the tongue of the biblical author in this way, it would have been sufficient to say that God made heaven and earth, the sea and living things, and not add the order of the days nor what was created first and what later.

So that they may shine in the firmament of the heaven, and give light upon the earth; and it was so. Indeed, the luminaries always shine in the firmament of the heaven, as we have said, and they flood the nearby regions with bright light, …