The cosmological sense is coined in the Vulgate in imitation of LXX στερέωμα "firm or solid structure", which in turn translates Hebrew רקיצ. Some have viewed this as a mistranslation and the modern New American Standard Bible translates this as "expanse" - based on a modern scientific bias. However, ancient Hebrews - including those who translated the Hebrew Bible into Greek (the Septuagint or LXX) - understood this term to refer to solid material. Hence, the LXX στερέωμα "firm or solid structure." The Hebrew רקיע, raqia, refers to a hard substance that has been hammered out by crafstmen (Exodus 39:3; Isaiah 40:19; compare Job 37:18, Exodus 24:10, Ezek. 1:22). Ancient Hebrews, in line with their Mesopotamian neighbors, understood the "firmament" or "sky" as a dome, over which was a layer of water, under which rotated the sun, moon, and stars (Gen. 1:6-8, 14-15). The Genesis creation account is accommodated to this pre-scientific understanding of the sky as a dome, with a layer of cosmic waters above and the stellar constellations below.

http://biologos.org/uploads/projects/godawa_scholarly_paper_2.pdf http://faculty.gordon.edu/hu/bi/ted_hildebrandt/OTeSources/01-Genesis/Text/Articles-Books/Seely-Firmament-WTJ.pdf