Charles Ellicott Commentary Genesis 1:8

"And God called the firmament Heaven. And there was evening and there was morning, a second day." — Genesis 1:8 (ASV)

God called the firmament (the expanse) Heaven. —This is a Saxon word, meaning something heaved up. The Hebrew term probably signifies the heights, or upper regions, into which the walls of cities nevertheless ascend (Deuteronomy 1:28).

In Genesis 1:1, “the heaven” may include the abysmal regions of space. Here, however, it refers to the atmosphere around our earth, which, at a distance of about forty-five miles from the surface, melts away into the imponderable ether. The work of the second day is not described as being good, although the Septuagint adds this usual formula.

Probably, however, the work of the second and third days is regarded as one. In both, there was a separation of waters; but it was only when the open expanse reached the earth’s surface, and reduced its temperature, that water could exist in any form other than vapor. But no sooner did it exist in a fluid form than atmospheric pressure would compel it to seek the lowest level. Moreover, the cooling of the earth’s surface would produce cracks and fissures, into which the waters would descend. When these processes were well advanced, then at the end of the third day God saw that it was good.