When God set lights in heaven their purpose was to divide: night and day, signs, seasons, days, and years.
At creation was darkness and God created light making for day and night - - "Evening and Morning" - - but there were no "lights". By using the lights (Sun, Moon and Stars) men would be able to organize time into days (as of the days of the month), and years, and seasons. With just light and dark they could not do that.
Dividing of the waters may be the establishment of an atmosphere. What we see as the firmament (Sky) occurs because of the atmosphere being held close to the earth by gravity. It could also mean the first existence of the different states of matter, as in solid, liquid, gas. Without any water vapor and "lights" there would be no "firmament" to perceive - - only blackness of space. When God made a firmament maybe he "split" water vapor from liquid water. The "gathering together" of waters may have resulted as heaver liquid water descended and land was pushed up from below.
When standing on the earth, air, water vapor and lights appear to be stretched out as a dome overhead. As we progress up through it the gas gets thinner until it contributes no longer to the appearance of a firmament. The fact that this is no solid surface, as a dome stretched out over the earth, may say something about the nature of the difference between the physical and spiritual.
