Genesis 1:6 (BSB)

And God said, “Let there be an expanse between the waters, to separate the waters from the waters.”

From Genesis 1. Also in the ESV.

Commentary on Genesis 1:6

• John Calvin (Reformed), Calvin's Commentaries on Genesis 1:6: 6 Let there be a firmament “Sit extensio.” In the next verse he changes the word to “expansio”. “Fecit expansionem.” — “He made an expanse.” The work of the second day is to provide an empty space around the circumference of the earth, that heaven and earth may not be mixed together.
• Matthew Henry (Presbyterian), Matthew Henry's Complete Commentary on Genesis 1:6-8: We have here an account of the second day's work, the creation of the firmament, in which observe, 1. The command of God concerning it: Let there be a firmament, an expansion, so the Hebrew word signifies, like a sheet spread, or a curtain drawn out.
• John Gill (Reformed Baptist), Exposition of the Old and New Testaments on Genesis 1:6: And God said, let there be a firmament in the midst of the waters,.... On which the Spirit of God was sitting and moving, Gen 1:2 part of which were formed into clouds, and drawn up into heaven by the force of the body of fire and light already produced; and the other part left on the earth, not yet gathered into one place, as...
• Jamieson-Fausset-Brown (Reformed), Commentary Critical and Explanatory on the Whole Bible on Genesis 1:6-8: SECOND DAY. (Gen 1:6-8) firmament--an expanse--a beating out as a plate of metal: a name given to the atmosphere from its appearing to an observer to be the vault of heaven, supporting the weight of the watery clouds. By the creation of an atmosphere, the lighter parts of the waters which overspread the earth's surface were drawn up and suspended in the visible heavens, while...