The Firmament: There is no escaping the literal sense.
There are two senses of Scripture, the literal and the spiritual [= allegorical, moral and anagogical senses]. [CCC 115].
The literal sense is the meaning conveyed by the words of Scripture and discovered by exegesis, following the rules of sound interpretation: "All other senses of Sacred Scripture are based on the literal." [CCC 116]
Given the preceding and the following:
a) USCCB > NAB > Gn 1:7 and note.
* [1:7] The dome: the Hebrew word suggests a gigantic metal dome. It was inserted into the middle of the single body of water to form dry space within which the earth could emerge. The Latin Vulgate translation firmamentum, “means of support (for the upper waters); firmament,” provided the traditional English rendering.
b) Genesis 1:6-8 (RSVCE)
c) Catholic Encyclopedia > Firmament (Septuagint stereoma; Vulgate, firmamentum).
d) Catholic Encyclopedia> Fathers of the Church > Hexaemeron (Basil) > Homily III > On the Firmament
Cf Bonus: THE FIRMAMENT AND THE WATER ABOVE Part I: The Meaning of raqia in Gen 1:6-8 by PAUL H. SEELY
There is no other interpretation of “the Firmament” than the literal sense.
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Ancient Hebrew Cosmology Concept