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I have a 1609 Catholic Bible that infallibly defines "A firmament" (Genesis 1:6, "And God said: Let there be a firmament made amidst the waters: and let it divide the waters from the waters") as follows:
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"Chap. 1. Ver. 6. A firmament. By this name is here understood the whole space between the earth, and the highest stars. The lower part of which divideth the waters that are upon the earth, from those that are above in the clouds."
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Consequently, the firmament is not any solid or hard material but rather it is the zone or space between the earth and the highest stars which modern man would call the atmosphere and outer space. The water that is divided is the separation between liquid water as it occurs on the surface of the earth and vaporous water as it occurs in clouds in the sky. The ancient Bible authors had no way of understanding that the principal limit of water vapor is the earth's atmosphere. But today, we know that even in outer space, water can exist albeit in a very sparse and expanded form, with much distance between each water molecule without confines of ambient air pressure.