And God said,
Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.
All Commentaries on Genesis 1:6 Go To Genesis 1
George Leo Haydock
A firmament. By this name is here understood the whole space between the earth and the highest stars. The lower part of which divideth the waters that are upon the earth, from those that are above in the clouds. (Challoner)
The Hebrew Rokia is translated stereoma, solidity by the Septuagint., and expansion by most of the moderns. The heavens are often represented as a tent spread out, Psalm. ciii. 3. (Calmet)