Another possible explanation is to understand by the firmament that was made on the second day, not that in which the stars are set, but the part of the atmosphere where the clouds are collected, and which has received the name firmament from the firmness and density of the air. "For a body is called firm," that is dense and solid, "thereby differing from a mathematical body" as is remarked by Basil (Hom. iii in Hexaem.). If, then, this explanation is adopted none of these opinions will be found repugnant to reason. Augustine, in fact (Gen. ad lit. ii, 4), recommends it thus: "I consider this view of the question worthy of all commendation, as neither contrary to faith nor difficult to be proved and believed."
Reply Obj. 1: According to Chrysostom (Hom. iii in Genes.), Moses prefaces his record by speaking of the works of God collectively, in the words, "In the beginning God created heaven and earth," and then proceeds to explain them part by part; in somewhat the same way as one might say: "This house was constructed by that builder," and then add: "First, he laid the foundations, then built the walls, and thirdly, put on the roof." In accepting this explanation we are, therefore, not bound to hold that a different heaven is spoken of in the words: "In the beginning God created heaven and earth," and when we read that the firmament was made on the second day.
We may also say that the heaven recorded as created in the beginning is not the same as that made on the second day; and there are several senses in which this may be understood. Augustine says (Gen. ad lit. i, 9) that the heaven recorded as made on the first day is the formless spiritual nature, and that the heaven of the second day is the corporeal heaven. According to Bede (Hexaem. i) and Strabus, the heaven made on the first day is the empyrean, and the firmament made on the second day, the starry heaven. According to Damascene (De Fide Orth. ii) that of the first day was spherical in form and without stars, the same, in fact, that the philosophers speak of, calling it the ninth sphere, and the primary movable body that moves with diurnal movement: while by the firmament made on the second day he understands the starry heaven. According to another theory, touched upon by Augustine [*Gen. ad lit. ii, 1] the heaven made on the first day was the starry heaven, and the firmament made on the second day was that region of the air where the clouds are collected, which is also called heaven, but equivocally. And to show that the word is here used in an equivocal sense, it is expressly said that "God called the firmament heaven"; just as in a preceding verse it said that "God called the light day" (since the word "day" is also used to denote a space of twenty-four hours). Other instances of a similar use occur, as pointed out by Rabbi Moses.
The second and third objections are sufficiently answered by what has been already said. _______________________
SECOND ARTICLE [I, Q. 68, Art. 2]
Whether There Are Waters Above the Firmament?
Objection 1: It would seem that there are not waters above the firmament. For water is heavy by nature, and heavy things tend naturally downwards, not upwards. Therefore there are not waters above the firmament.
Obj. 2: Further, water is fluid by nature, and fluids cannot rest on a sphere, as experience shows. Therefore, since the firmament is a sphere, there cannot be water above it.
Obj. 3: Further, water is an element, and appointed to the generation of composite bodies, according to the relation in which imperfect things stand towards perfect. But bodies of composite nature have their place upon the earth, and not above the firmament, so that water would be useless there. But none of God's works are useless. Therefore there are not waters above the firmament.
On the contrary, It is written (Gen. 1:7): "(God) divided the waters that were under the firmament, from those that were above the firmament."