Day two is somewhat interesting to figure out, because words are used that aren’t typical of the way God speaks. But, God always has reasons, so it is good to try to figure out what those reasons are.
First, He gives the strange command “Let there be an expanse in the midst of the waters, and let it separate the waters from the waters.” So, what does this mean?
Recall that on the first day He made a really big ball of spinning water. This ball would be a sphere, due to the effects of its own gravity. Here, he “separates the waters from the waters” by putting an expanse between them.
It’s like He broke the sphere into a smaller ball of water, surrounded by a hollow ball of water. Between them, he put an expanse. The expanse is like a concentric sphere of nothing, between the two spheres, with the outside sphere being hollow.
On day four, we’ll see that God takes the outer (hollow) sphere and stretches it out, creating the cosmos. But, for now, He calls the expanse “heaven”. At this point, the “expanse” (KJV has “firmament”, which more accurately shows the supporting nature of it) is holding up one layer of water from the surface of the inner ball of water.
The word heaven can have several meanings. The most noteworthy for this passage are sky, the visible sky, the abode of the stars, the visible universe (different from the visible sky meaning), and Heaven with a capital H, God’s home. I don’t think it’s an accident that the word can carry all those meanings. God very frequently uses word plays and dual-definition words to get a point across. For example, Adam’s name also means earth (as in dirt), man (as in a human male) and Man (as in mankind). In the English language, different words are used, but God chose one word to mean them all. I think the word “heaven” was chosen precisely because it was ambiguous.
Why? Because God’s Word is living and active. It almost always has more than one meaning, some of which can’t be discerned by humans without deeper study.
So, I think God meant for sha mah’ yim to mean:
- The sky, both visible and invisible (we can’t see air), and
- Space (the abode of the stars), and
- God’s home, Heaven. Ever wonder why Paul says he went to the “third heaven” (2 Corinthians 12:2)? This might be telling us the answer, right here on day two.
So, we now have two chunks of water and three heavens, all
wrapped into one big ball.
 It probably was not a perfect sphere, since it was spinning, which would make it bulge at the middle. But, sphere is my word choice, not God’s, so there’s no reason it has to be perfect.