Objection 1. It would seem that the lights ought not to have been produced on the fourth day. For the heavenly luminaries are by nature incorruptible bodies: wherefore their matter cannot exist without their form. But as their matter was produced in the work of creation, before there was any day, so therefore were their forms. It follows, then, that the lights were not produced on the fourth day.
Objection 2. Further, the luminaries are, as it were, vessels of light. But light was made on the first day. The luminaries, therefore, should have been made on the first day, not on the fourth.
Objection 3. Further, the lights are fixed in the firmament, as plants are fixed in the earth. For, the Scripture says: "He set them in the firmament." But plants are described as produced when the earth, to which they are attached, received its form. The lights, therefore, should have been produced at the same time as the firmament, that is to say, on the second day.
Objection 4. Further, plants are an effect of the sun, moon, and other heavenly bodies. Now, cause precedes effect in the order of nature. The lights, therefore, ought not to have been produced on the fourth day, but on the third day.
Objection 5. Further, as astronomers say, there are many stars larger than the moon. Therefore the sun and the moon alone are not correctly described as the "two great lights."
On the contrary, Suffices the authority of Scripture.
I answer that, In recapitulating the Divine works, Scripture says (Genesis 2:1): "So the heavens and the earth were finished and all the furniture of them," thereby indicating that the work was threefold. In the first work, that of "creation," the heaven and the earth were produced, but as yet without form. In the second, or work of "distinction," the heaven and the earth were perfected, either by adding substantial form to formless matter, as Augustine holds (Gen. ad lit. ii, 11), or by giving them the order and beauty due to them, as other holy writers suppose. To these two works is added the work of adornment, which is distinct from perfect. For the perfection of the heaven and the earth regards, seemingly, those things that belong to them intrinsically, but the adornment, those that are extrinsic, just as the perfection of a man lies in his proper parts and forms, and his adornment, in clothing or such like. Now just as distinction of certain things is made most evident by their local movement, as separating one from another; so the work of adornment is set forth by the production of things having movement in the heavens, and upon the earth. But it has been stated above (Question 69, Article 1), that three things are recorded as created, namely, the heaven, the water, and the earth; and these three received their form from the three days' work of distinction, so that heaven was formed on the first day; on the second day the waters were separated; and on the third day, the earth was divided into sea and dry land. So also is it in the work of adornment; on the first day of this work, which is the fourth of creation, are produced the lights, to adorn the heaven by their movements; on the second day, which is the fifth, birds and fishes are called into being, to make beautiful the intermediate element, for they move in air and water, which are here taken as one; while on the third day, which is the sixth, animals are brought forth, to move upon the earth and adorn it. It must also here be noted that Augustine's opinion (Gen. ad lit. v, 5) on the production of lights is not at variance with that of other holy writers, since he says that they were made actually, and not merely virtually, for the firmament has not the power of producing lights, as the earth has of producing plants. Wherefore Scripture does not say: "Let the firmament produce lights," though it says: "Let the earth bring forth the green herb."
Reply to Objection 1. In Augustine's opinion there is no difficulty here; for he does not hold a succession of time in these works, and so there was no need for the matter of the lights to exist under another form. Nor is there any difficulty in the opinion of those who hold the heavenly bodies to be of the nature of the four elements, for it may be said that they were formed out of matter already existing, as animals and plants were formed. For those, however, who hold the heavenly bodies to be of another nature from the elements, and naturally incorruptible, the answer must be that the lights were substantially created at the beginning, but that their substance, at first formless, is formed on this day, by receiving not its substantial form, but a determination of power. As to the fact that the lights are not mentioned as existing from the beginning, but only as made on the fourth day, Chrysostom (Hom. vi in Gen.) explains this by the need of guarding the people from the danger of idolatry: since the lights are proved not to be gods, by the fact that they were not from the beginning.
Reply to Objection 2. No difficulty exists if we follow Augustine in holding the light made on the first day to be spiritual, and that made on this day to be corporeal. If, however, the light made on the first day is understood to be itself corporeal, then it must be held to have been produced on that day merely as light in general; and that on the fourth day the lights received a definite power to produce determinate effects. Thus we observe that the rays of the sun have one effect, those of the moon another, and so forth. Hence, speaking of such a determination of power, Dionysius (Div. Nom. iv) says that the sun's light which previously was without form, was formed on the fourth day.
Reply to Objection 3. According to Ptolemy the heavenly luminaries are not fixed in the spheres, but have their own movement distinct from the movement of the spheres. Wherefore Chrysostom says (Hom. vi in Gen.) that He is said to have set them in the firmament, not because He fixed them there immovably, but because He bade them to be there, even as He placed man in Paradise, to be there. In the opinion of Aristotle, however, the stars are fixed in their orbits, and in reality have no other movement but that of the spheres; and yet our senses perceive the movement of the luminaries and not that of the spheres (De Coel. ii, text. 43). But Moses describes what is obvious to sense, out of condescension to popular ignorance, as we have already said (67, 4; 68, 3). The objection, however, falls to the ground if we regard the firmament made on the second day as having a natural distinction from that in which the stars are placed, even though the distinction is not apparent to the senses, the testimony of which Moses follows, as stated above (De Coel. ii, text. 43). For although to the senses there appears but one firmament; if we admit a higher and a lower firmament, the lower will be that which was made on the second day, and on the fourth the stars were fixed in the higher firmament.
Reply to Objection 4. In the words of Basil (Hom. v in Hexaem.), plants were recorded as produced before the sun and moon, to prevent idolatry, since those who believe the heavenly bodies to be gods, hold that plants originate primarily from these bodies. Although as Chrysostom remarks (Hom. vi in Gen.), the sun, moon, and stars cooperate in the work of production by their movements, as the husbandman cooperates by his labor.
Reply to Objection 5. As Chrysostom says, the two lights are called great, not so much with regard to their dimensions as to their influence and power. For though the stars be of greater bulk than the moon, yet the influence of the moon is more perceptible to the senses in this lower world. Moreover, as far as the senses are concerned, its apparent size is greater.
Source: Summa Theologica (New Advent)