Why is the ninth month called September?
mid-13c., from Latin firmamentum "firmament," literally "a support or strengthening," from firmus "firm" (see firm (adj.)), used in Vulgate to translate Greek stereoma "firm or solid structure," which translated Hebrew raqia, a word used of both the vault of the sky and the floor of the earth in the Old Testament, probably literally "expanse," from raqa "to spread out," but in Syriac meaning "to make firm or solid," hence the erroneous translation.
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 (Gen. 1:7). The _raki'a_ supported the upper reservoir (Ps. 148:4). It was the support also of the heavenly bodies (Gen. 1:14), and is spoken of as having "windows" and "doors" (Gen. 7:11; Isa. 24:18; Mal. 3:10) through which the rain and snow might descend.