And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters. And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so. And God called the firmament Heaven. And the evening and the morning were the second day.
Surprisingly little details are given for the events of Day 2. There seems to be at least two acts that occur (possibly distinct from each other).
1) God divides the waters into the “waters which were under” and the “waters which were above.”
2) God creates an expanse (“firmament” in the KJV) between the waters under and the waters above.
Since the details surrounding these events are so scant, we can only speculate about what these creative acts might have included. However, we must be careful to not hold out our speculation as doctrine. Some things that have been theorized seem reasonable. Some are musings. We can employ science here but we are limited since we are dealing with a unique event that wasn't observed. We can trust the clear text of the Bible but our scientific theories are suspect.
We know that God will soon be creating living things on the earth so it would not be unreasonable to suppose that He is preparing the planet to support them. The “expanse” or “firmament” is the space occupied by plants and terrestrial creatures (including us). Therefore, God possibly created the atmosphere here with its composite of various gases.
We are certain that water existed on the first day. Since water is hydrogen and oxygen (H2O), then we can say those elements were created on Day 1. If God is creating breathable air on Day 2, it is likely similar to the air we breath now. Besides oxygen, the air would have contained nitrogen, argon, carbon (as in CO2), neon, etc. So the elements of the periodic table are being created ex nihilo in the order that God has chosen to introduce them in the creation and not necessarily in the order of simple to complex as predicted by the Big Bang. More will be introduced as God creates the dry land and living creatures.
The firmament is also where weather occurs. In Genesis 2:5 we are told that it did not rain on the earth during the creation week but perhaps here God is creating clouds (we'll talk in a moment about the waters above the firmament). At the very least, God likely created the mechanisms that would govern weather in the future – things like air pressure (a necessary consequence of having created air), evaporation, etc.
The biggest controversy surrounding Day 2 is over what is meant by the “the waters above” the firmament. There are at least 3 mainstream theories:
The Mundane Explanation
The waters above could simply be clouds and the firmament is the expanse between the earth and the clouds. Such a reading fits the text well and needs little additional explanation. It should be noted that such a belief would imply that the Bible accurately described clouds as being made of water well ahead of scientific discovery.
The Canopy Theory
Some creationists believe the waters above represent a type of canopy, either of dense clouds or ice. By the way, I've often heard the canopy itself referred to as the “firmament” but the text is clear that the firmament is the expanse between the waters. Proponents of this theory believe the canopy collapsed at the time of Noah and was the source of much of the Flood waters.
This interpretation of the text carries a few difficulties. Such a canopy, for example, would obstruct our view of the stars described on Day 4. Of course, some old earth creationists suggest the sun and stars were created earlier and could not be seen. The sun could only be perceived as diffused light (described on Day 1) and it was not until Day 4 that the clouds were cleared and the sun, moon, and stars could be seen clearly.
There are also some scientific difficulties with a canopy. Of course, I never let prevailing scientific opinion trump the clear meaning of the words of the Bible. The Bible does not explicitly state there was a canopy and there is no other reference to it later in the Bible. I haven't seen a compelling argument for the existence of such a thing. I don't endorse this interpretation but I can't entirely reject it either. Perhaps clouds were more dense prior to the Flood – though not necessarily a solid dome of ice.
While most creationists do reject this idea, there are a few staunch proponents of this theory and have built elaborate models around it. Perhaps the most notable proponent is Carl Baugh who has made some dubious, scientific claims about the affects of such a canopy – things like how it was the canopy that allowed antediluvian people to live for hundreds of years.
In his “white hole cosmology,” Russell Humphreys has proposed that, when God divided the waters, He used the “waters above” as the raw material to create the rest of the universe. This position would mean that the initial earth was huge and most of it was stripped away to form everything else. Again, the Bible doesn't explicitly say this happened so I cannot say dogmatically his view is correct. It could fit the text, though, and Humphreys has made several successful predictions with this model.
Of course, it's possible for a little of each of these theories to be true. The “waters above” must mean something and since we cannot determine a precise meaning from the text, we should be careful not to wed ourselves to any particular theory as though it were scripture.
The passage ends with the usual demarcation; it was evening and morning. All the things done by God in this passage, whatever they may include, were all accomplished in a single, ordinary day.