Verses 14-19: Then God said, "Let there be lights in the firmament of the heavens to divide the day from the night; and let them be for signs and seasons, and for days and years; and let them be for lights in the firmament of the heavens to give light on the earth"; and it was so. Then God made two great lights: the greater light to rule the day, and the lesser light to rule the night. He made the stars also. God set them in the firmament of the heavens to give light on the earth, and to rule over the day and over the night, and to divide the light from the darkness. And God saw that it was good. And there was evening and there was morning, a fourth day.
Over time, as the earth gradually cooled and the bacteria and algae did their work adding oxygen to the air, the atmosphere slowly changed. Eventually, the sky became clear enough to allow the sun, the stars and the moon to be visible from earth. Day became very different from night, as the sun’s rays were finally able to reach directly to the earth. By the end of the fourth day, the earth and its atmosphere were ready for air-breathing creatures!
Although there were no land animals or people at this early time, the description of the sun and the moon and the stars is clearly from a view point of the earth looking upward toward the heavens. The earth is the only location in the universe from which the sun and moon both appear as two great luminaries, the same size. That is because although the sun is 400 times bigger than the moon, it is also 400 times further from the earth than the moon. Hence from the earth, they both appear as the same size. During a total solar eclipse, when the Moon is between the Earth and Sun, it completely blocks the sun, allowing us to see the faint glow of the corona, the Sun's outer atmosphere. (When the Moon is at apogee, it is 11% farther from Earth than it is at perigee. This is far enough that it cannot entirely block the bright light, so eclipses which occur near apogee are not total.)
The view from the earth looking upward to see these heavenly bodies means that the atmosphere of the earth had become clear, changing from a foggy, translucent atmosphere on day three to the clear atmosphere we have today. This change occurred because of the added oxygen by the algae and plants (photosynthesizers) and because as the earth continued to cool, the vapor pressure (steam) from the oceans became much less.
According to the ancient commentaries, the sun and moon were already in the heavens since sometime in day two, but only became visible from the earth on day four. Science has confirmed this ancient understanding by telling us how the atmosphere of the earth changed in this period from fog bound to clear. The ancient commentators learned this from the phrase “God set them in the firmament of the heavens.” “Set them,” here means “made them visible.”
Note that on day four there is no mention of the further development of life. This contains a subtle piece of information. On day three we learned that life on earth started almost immediately after the crust of the earth had cooled to a temperature at which liquid water could form. Liquid water is a necessity since all life on earth is water based. Though this early crust was still quite hot, the rapid start of life conforms with recent discoveries that the earliest forms of life appear to have been thermophilic microbes. With this rapid appearance of life, one would have expected a further rapid continuing development toward animal life. Yet no such mention occurs on day four. And the fossil record follows suit. According to paleontological data, life remained microscopic for an extended period, only to burst forth in the Cambrian explosion of animal life to be discussed on day five.
Clarifying translation: God said, “Let there be luminaries in the expanse of heaven to separate between day and the night; and they shall serve as signs, and for festivals, and for days and years; and they shall serve as luminaries in the firmament of the heaven to shine upon the earth.” And it was so. And God made the two great luminaries, the greater luminary to dominate the day and the lesser luminary to dominate the night, and the stars. And God set them in the firmament of heaven to give light upon the earth, to dominate by day and by night, and to separate between the light and the darkness. And God saw that it was good. And there was evening [Erev: chaos] and there was morning [Boker: order, clarity], a fourth day.