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Nothing demonstrates the fact that Genesis 1 is ancient cosmology and not modern science more effectively than its declaration that the firmament was solid, separating waters above from waters below.
It is this one fact more than anything else that destroys both literal and strong concordist readings of the Genesis 1 that seek to read it as a scientifically accurate account of origins.
It also shows that contemporary special creationists - both YEC and OEC - not only fail to interpret Genesis 1 properly on this point but are also ignorant of how early Christian and Jewish expositors interpreted Genesis 1. On this point we find that many accepted the solidity of the firmament.
Examples are quite easy to find:
Then God said, “Let there be a firmament in the midst of the waters, and let it be separating between the upper waters and the lower waters.” Targ. Pseu. Jon. Gen 1:6
Then God made the firmament (its thickness three finger breadths), between the sides of the heavens and the waters of the ocean. And He separated between the waters that were below the firmament and between the waters that were above, in the canopy of the firmament. And it was so. Targ Pseu. Jon. Gen 1:7
“Let there be a firmament in the midst of the waters, the middle layer of water solidified,and the nether heavens and the uppermost heavens were formed. Rab said: [God's] handiwork [the heavens] was in fluid form, and on the second day it congealed; thus Let there be a firmament means ‘Let the firmament be made strong*. R. Judah b. R. Simon said: Let a lining be made for the firmament, as you read, And they did beat the gold into thin plates…R. Simon said: The fire came forth from above and burnished the face of the firmament.” Gen. Rab. 4:2
“R. Phinehas said in R. Oshaya’s name: As there is a void between the earth and the firmament, so is there a void between the firmament and the upper waters, as it is written, Let there be a firmament in the midst of the waters, meaning, midway between them. R. Tanhuma said: I will state the proof. If it said, And God made the firmament, and He divided between the waters . . . which are upon the firmament, I would say that the water lies directly upon the firmament itself.” Gen. Rab. 4:3
“…when they had built the tower to the height of four hundred and sixty-three cubits. And they took a gimlet, and sought to pierce the heaven, saying, Let us see (whether) the heaven is made of clay, or of brass, or of iron.” 3 Apoc. Bar. 7
“But after that he makes the firmament, that is, the corporeal heaven. For every corporeal object is, without doubt, firm and solid; and it is this which “divides the water which is above heaven from the water which is below heaven.” Origen. Homily on Genesis
“If the nature of the elements is taken into consideration, how it is possible for the firmament to be stable between the waters? The one is liquid, the other solid; one is active, the other, passive.” Ambrose. Hexameron. Bk II Ch 2.48
“…’And he called the firmament, heaven.’ In a general way, He would seem to have said above that heaven was made in the beginning so as to take in the entire fabric of celestial creation, and that here the specific solidity of this exterior firmament is meant.” ibid. 2.62
“This firmament cannot be broken, you see, without a noise. It also is called a firmament because it is not weak nor without resistance…the firmament is called because of its firmness or because it has been made firm by divine power,..” ibid. 2.62
“They must certainly bear in mind that the term “firmament” does not compel us to imagine a stationary heaven: we may understand this name as given to indicate not that it is motionless but that it is solid and that it constitutes an impassable boundary between the waters above and the waters below.” Augustine. The Literal Meaning of Genesis, BkII Ch.10
If the firmament really meant 'expanse' or 'atmosphere' (and that view is flatly refuted by the fact that Genesis 1 explicitly states that sun, moon, and stars were set in the firmament) then it is curious how many different ancient expositors across both faith traditions did not seem to draw that inference.
I am indebted to both James McGrath and Robert Cargill for the examples above.