from the Vulgate firmamentum, which is used as the translation of the Hebrew raki'a

This word means simply “expansion.” It denotes the space or expanse like an arch appearing immediately above us. They who rendered raki'a by firmamentum regarded it as a solid body. The language of Scripture is not scientific but popular, and hence we read of the sun rising and setting, and also here the use of this particular word. It is plain that it was used to denote solidity as well as expansion. It formed a division between the waters above and the waters below (Genesis 1:7). The raki'a supported the upper reservoir (Psalm 148:4). It was the support also of the heavenly bodies (Genesis 1:14), and is spoken of in Genesis as having “windows” and “doors” (Genesis 7:11; Isaiah 24:18; Mal. 3:10) through which the rain and snow might descend.

More information