Genèse 1:6

"Let there be a firmament:" Let the firmament harden. Since, even though the heavens were created on the first day, they were still wet, and they solidified on the second from the scolding of the Holy One, blessed be He, in his saying, "Let there be a firmament." And this is what is written (Job 26:11), "The pillars of the heavens were shaking" the entire first day and on the second, "they were astounded by his scolding," like a person that is confounded and stops by the scolding of someone threatening him.

"In the midst of the waters:" In the middle of the waters, as there is a space between the higher waters and the firmament just like there is [one] between the firmament and the waters on the earth. Behold, you have learned that they are suspended by the word of the King.

And the Lord said, Let there be an expanse in the midst of the waters, and let it separate between the waters above and the waters beneath.

“And He said”, the Gaon said regarding the firmament (raki’a) things that could not be; rather its explanation is “extended”, like “they were extended” (vayerak’u) Exodus 39:3, “I shall extend them” (erka’em Samuel II 22:43, and similarly, “and He stretched them [i.e., the heavens] out like a tent for dwelling”. Isaiah 40:22. And how profound is the word of the one who says that the edges of the sun are with the edges of the waters of the oceans. Now this firmament is the atmosphere, for when the light coalesced over the earth and a dry wind from the earth, the blade/flame was changed and became the firmament. And thus did it say in a Psalm Psalms 104:2 “He stretches the heavens like a curtain… the one who makes roofbeams of water”. And it mentions the clouds and the wind and the earth element, which is higher than the water, and thus it writes, “for He founded it upon the seas” Psalms 24:2, “the One Who extends the earth over the waters" Psalms 136:6 and similarly, anyone who goes to the sea is called, “goes down”. The meaning of “the One Who calls to the waters of the sea” Amos 5:8 – so that they should ascend, referring to clouds, and afterwards, “and He pours them out”.

LET THERE BE A FIRMAMENT. G-d now said that the substance which had come into being first — that which He created from nought — should be a firmament, stretched as a tent in the midst of the waters, separating between waters and waters. It is possible that this is what the Rabbis intended by their saying, “Rav said, ‘The heavens were in a fluid form on the first day, and on the second day they solidified.’ Rav thus said, ‘Let there be a firmament means let the firmament become strong.’ Rabbi Yehudah the son of Rabbi Shimon said, ‘Let the firmament become like a plate, just as you say in the verse, And they did beat — [vayerak’u contains the same root as raki’a, firmament] — the gold into thin plates.’”

IN THE MIDST OF THE WATERS. This means in the center of the waters, between the higher waters and the lower waters. For there is the same distance between the firmament and the waters upon the earth as between the higher waters and the firmament. Thus you may infer that the upper waters are suspended in space by the command of G-d. Thus it is explained in Bereshith Rabbah and in Rashi’s commentary. This is part of the process of creation [which those who know it are obliged to conceal]; so do not expect me to write anything about it, as the subject is one of the mysteries of the Torah, and the verses in their plain meaning do not require such an interpretation since Scripture itself did not go into it at length, and to give the interpretation is forbidden even to those who know it, and so much the more to us.

ויאמר אלוקים יהי רקיע, after the first day had been concluded with its morning, G’d issued a new directive.

רקיע, the word (noun) is a participle describing a creature such as עשיר which means the same as מעושר, “someone who has been equipped with wealth,” or the word חסיד, used to describe a pious individual. This noun is also a derivative from the word חסד, loving kindness, i.e. a חסיד is a creature equipped with this attribute of loving G’d. Now the universe was to be equipped with a horizon. Seeing that from the earth until the highest regions of the heaven there was an unbroken region filled with water, G’d now separated these waters in the “middle,” by making this firmament, expanse of atmosphere we call horizon. This רקיע appears as if stretched from east to west and from north to south effectively separating the “upper” layers of water from the “lower” ones. Seeing that G’d did not assign any specific function to the waters until the third “day” when He told the waters to congregate in a specific region so that the dry land beneath it should become visible, the expression “it was good,” is not mentioned in connection with the second day. This is compensated for on the third “day,” when this statement “it was good” appears twice in the Torah’s report. It appears after the waters complied with G’d’s directive, and after the earth, i.e. the dry land, produced the vegetation including fruit-bearing trees which G’d had commanded it to produce.

בין מים למים, so that half the waters would remain in the “upper” regions and half in the “lower” regions.

יהי רקיע בתוך המים, let there be טבע, i.e. guidelines, rules governing the function of what is below and what is above the primordial waters. Just as there are such parameters surrounding our world, i.e. the globe we call “earth,” so there should be similar guidelines governing what is below the atmosphere and earth itself. The region in which air exists should become clearly defined. [the gaseous nature of air should not be allowed to escape into all directions of the universe beyond our globe so that creatures on earth would die from the lack of air. In other words, the atmosphere should be “enclosed” whether by gravitational or other forces is immaterial. Ed.] At the same time, air should be given enough space within which “to breathe,” so to speak.

ויאמר אלוקים יהי רקיע, actually this firmament, horizon, already existed, seeing that it was the atmosphere, the air, just as the Torah speaks in verse 20 of ועוף יעופף על הארץ “and the birds are to fly above the earth,” adding: על פני רקיע השמים “on the face of the firmament spanning the heavens.” What then do the words יהי רקיע mean? They mean that this רקיע should now become something substantial. It should now become capable of sustaining the atmosphere, the air. In other words, this רקיע, which up until this point had been above the waters and had therefore been very wet and weak, impeding the light from above it from properly penetrating to the earth below, should now function constructively.
This רקיע did not become “strong” until the earth itself had become dry land, i.e. on the third “day.” Only at that point did the light encounter something solid, i.e. a hard place in the globe so that it condensed into becoming as if frozen due to its proximity to this earth. The רקיע was made on the second “day,” and the words ויאמר אלוקים יקוו המים ותראה היבשה, a directive (verse 9) issued on the third “day,” refers to an existing phenomenon. There are a number of such. This is nothing unusual. This is also the view expressed by Ibn Ezra, who writes in his commentary on verse 9 that “this paragraph is inextricably linked to the one preceding it,” refuting the view that the “horizon” was not created until the second day. But as soon as the wind dried out the earth on the first day the earth became hard enough (dry), and the powerful heat was transformed into what we know as the atmosphere, heat retreating upwards to the region of the “upper” waters. Proof that the atmosphere, רקיע, had already existed before the second day is Genesis 2,1 ביום עשות אלוקים שמים וארץ, “on the day G’d had made heaven and earth.” When something is hidden, not concentrated and its particles have not been defined, it does not yet qualify for the expression בריאה, “something which has been created.” This is what Ibn Ezra wrote, and his comments are perfectly correct.

בתוך המים, as far as this expression is concerned, it was necessary, seeing that the atmosphere itself was a component of the waters, [just as the Jewish people had been part of the Egyptian population prior to the Exodus so that G’d in redeeming them has spoken of taking גוי מקרב גוי, “one nation out of the very midst of the same nation” (Deut. 4,34. Ed.]