"And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters." (Genesis 1:6)
Critics claim that "firmament", which suggests a firm structure, is an erroneous translation of the empty and open atmosphere. While it is tempting to try to make Genesis fit into the modern understanding of science, such practice is not honest. We only need to look at Job 37:16-18 to see that the ancients considered the atmosphere to be firm:
Dost thou know the balancings of the clouds, the wondrous works of him which is perfect in knowledge? How thy garments are warm, when he quieteth the earth by the south wind? Hast thou with him spread out the sky, which is strong, and as a molten looking glass?
This description of a "strong" "glass-like" atmosphere is available in all translations. The ESV says "hard as a cast metal mirror". The liberal NRSV makes no attempt to try to make Genesis fit into modern science, so it translates the word as "dome". The footnote in the Holman Christian Standard Bible also says the word suggests a dome that could be "beat[en] firmly".
The question is not whether "firmament" is the correct translation. It is, as it is consistent with the Old Testament understanding of a "strong" or "firm" atmosphere. The real question is, how could this understanding be reconciled with the reality of an empty and open atmosphere?
One theory is that there was once some sort of canopy over the earth, which blocked out harmful radiation which led to the prolonged lifespans of the anti-diluvian people. This is the theory of some young-earth creationists.
Another theory is that the ancients regarded the atmosphere as firm, not in density, but in permanence. Whereas forests decay and mountains erode, only the sky remains intact seemingly forever. Something that remains intact forever can be said to be "firm".
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