Genesis 1:6 - Joseph Benson’s Commentary on the Old and New Testaments

Let there be a firmament This term, which is an exact translation of the word used by the Septuagint, or Greek translation of the Old Testament, by no means expresses the sense of the word used by Moses, רקיע, rakiang, which merely means extension or expansion. And as this extension or expansion was to be in the midst of the waters, and was to divide the waters from the waters, it chiefly, if not solely, means the air or atmosphere which separates the water in the clouds from that which is in and upon the earth. Thus the second great production of the Almighty was the element which is next in simplicity, purity, activity, and power, to the light, and no doubt was also used by him as an agent in producing some subsequent effects, especially in gathering the waters into one place. It is true, we afterward read of the sun, moon, and stars being set in the firmament of heaven: but the meaning seems only to be that they are so placed as only to be visible to us through the atmosphere.