In the beginning God created the heaven and the earth.
 And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.
 And God said, Let there be light: and there was light.
 And God saw the light, that it was good: and God divided the light from the darkness.
 And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.
 And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years:
 And let them be for lights in the firmament of the heaven to give light upon the earth: and it was so.
 And 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.
 And God set them in the firmament of the heaven to give light upon 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 the evening and the morning were the fourth day.
I’ve seen this topic danced around, with no particular explanation offered for the three cycles of daylight and darkness (day and night) transpiring before the appearance of the sun. For us today, who have grown up learning modern cosmology from an early age, the first explanation we hear of day and night is that it’s the result of the rotation of our spherical world; the side exposed to the sun experiencing daylight, while the opposite side falls in the sphere’s own shadow, or night.
Left to their own powers of observation, however, how might primitive peoples perceive the cycles of darkness and light? We can try forgetting about the knowledge we possess today and make some "primitive" observations of our own.
Anyone who is up before sunrise knows that the sky is already light by the time the sun appears on the horizon. Likewise, in the evening the sky remains light for a while after the sun drops below the horizon. Furthermore, entire days may pass without the sun being visible at all from the ground when the sky is cloudy or overcast, or in the presence of fog, for example. Granted, the day is not as bright without direct sunlight, but there’s still plenty of light. Inside a hut or tent or the entrance of a cave we may also be cut off from direct sunlight, but the interior can still have plenty of light, depending on the nature of any openings to the outside. Similarly, at night there are differing degrees of illumination, depending on the phase of the moon, or whether it is visible in the sky at all. The sun will cast shadows, but so does the moon. A full moon seems very bright in the night sky, yet it is nighttime nevertheless. Often the moon is visible in the daytime along with the sun, although not nearly as bright.
So, without knowledge of the real cosmological relationship between the sun and Earth, our observations might tell us that the sun merely accompanies day, without causing it, reasoning as follows:
Might we not conclude from simple observation that daytime and sunlight are two separate phenomena, just as nighttime is separate from moonlight? And just as a fire can provide light and heat, yet not change night to day, cannot the sun be merely a supplement to daylight and daytime warmth--a "greater light to rule the day," without causing the day, while the moon provides light at night without causing the night? Such cosmological insight as to refute this conclusion wouldn’t have been revealed by a deity whose accounting for the entirety of the universe beyond the sun, moon, and earth was, “…he made the stars also.”
From a primitive perspective it could very well seem that day and night could exist without need of the sun. But what’s more is that they thought the day/night cycle transpired throughout the universe, or “the firmament of the heaven.” After all, what they could see above them from where they stood constituted the entire universe. And that’s about as deep an answer as you’ll find in Genesis.
Appendix – Cosmological Discoveries Timeline
Anaximander (600 BC): "The moon is a circle nine times as large as the earth; it is like a chariot wheel, the rim of which is hollow and full of fire, as the sun also is; it has one vent, like the nozzle of a pair of bellows; its eclipses depend upon the turnings of a wheel." Yet for all that such a statement amuses us today, it was a step on the right track, away from magical explanations of reality and towards science. Anaximander is actually working by a process of analogy. He knows that heat comes from the sun, so that it is hot, like a fire, and since the moon shines, he reasons that there must be fire there too. How does it keep its circular shape, when fire is formless? It must have a rim, and we see the fire which burns inside. How is the fire maintained? By the action of a vent, which allows the passage of air like a pair of bellows. How do eclipses of the moon come about? They are circular, so it is by the action of a darkened wheel moving in front of the circular rim.
Anexagoras (450 BC): "The moon is an inhabited world. The marks on her surface are due to mountains and valleys." Here, Anexagoras has taken his form of analogy from our world. The moon is like the earth, with mountains and valleys - and this theory also manages to explain marks on the surface. Holding to this analogy, there is one inescapable conclusion: if the moon is a world like the earth, then it must be inhabited as well.
Unfortunately these strands of scientific thinking, limited as they were, did not become dominant in thinking about the moon. Instead the dominant philosophy came to be that derived from Plato and Aristotle, with an emphasis on the perfection of heavenly bodies. It was this which is at the background of Plutarch's comments on the moon, when he says: "The thing which we call the face in the moon comes from the images and shapes of the great oceans, represented in the moon as in a mirror, and the full moon herself is, for evenness, smoothness and luster, the most beautiful and pure mirror in the world."
The marks on the moon were too visible to ignore, so Plutarch (100 AD) explains them away. The moon is a vast mirror, reflecting all the flaws of the earth in her perfect, polished surface. This is a clever argument, but it must be seen as something of a backward step from earlier speculation.
The idea of a perfect moon (and other planets and sun - which together constituted the heavenly spheres) was strengthened by the teaching of the Christian Church. It came about because the Church was increasingly dependent upon Neoplatonic and Aristotelian thought (and not Christianity!) for its apologetic. This was a scientific blind alley. It was finally brought down by the triumph of the empirical tradition once more, with men like Galileo (1600), who described the moon as follows: "It is a most beautiful and delightful sight to behold the body of the moon. The moon does not possess a smooth and polished surface, but one rough and uneven just like the face of the Earth itself."
Galileo used a telescope to enlarge the "marks" on the moon. The Church tried to counter with the old argument which we have seen in Plutarch: the defect is in the telescope, or the human eye, but never in the moon. But through a certain amount of argument and a lot of luck, Galileo's interpretation was the one which prevailed.
At this point, the story seems to have been brought up to date. We recognise the viewpoint of Galileo, and it is our own. It comes as something of a shock then to hear what Kepler (1634) says about the moon: "All the creatures of the moon are either spherical or snakelike. Their bodies are covered with a thick protective layer. At evening they awake, slowly reviving under the mild earthshine and becoming completely restored during the night."
Kepler discovered mathematical relations about the planetary orbits and also held Galileo's theory to be true. So how do we explain this? As an extraordinary aberration? Not so. Kepler is simply applying our old friend, the method of analogy, once more. If there are creatures on the moon, it must be too hot in the day (like a desert) so they are nocturnal and heavily protected. They must also be able to curl up and leave the least amount of their body exposed to the sun, hence they must be either circular or snakelike.
It was, as everyone knows, the atomistic philosophers of ancient Greece who exercised perhaps the greatest influence on the modern conception of the universe.
Before the Greeks, the Babylonians and the Egyptians had already made observations, during many centuries, on the motions of the sun and of the moon with respect to the fixed stars, and knew how to predict lunar and solar eclipses. In spite of the fact that the Greeks identified the celestial bodies with gods, Anexagoras stated that the sun was like a red hot stone and that the moon was made like the earth. The Pythagoreans, at the end of the fifth century B. C., stated that the earth is spherical, Aristarchus of Samos, in the third century B. C., discovered the complete Copernican system, and Eratosthenes, in the year 200 B. C., calculated, according to Claudius Ptolemaeus, the maximum distance of the moon from the earth and the minimum distance between the sun and the earth.
Abu 'All al-Husayn ibn 'Abd Allah ibn Sina, known as Avicenna, philosopher, codifier of Aristotle and one of those who preserved and contributed to the transmission of Greek culture, stated: "Time is the measure of motion.'' In the Rasa'il, a 51-treatise encyclopedia known as the Koran after the Koran, one finds a list of distances to the planets (as a function of Earth radii) and of sizes of planets; it is stated there that space is "a form abstracted from matter existing only in the consciousness.'' But how many documents were lost or destroyed, as happened for instance in the subjugation of the magnificent pre-Columbian civilizations by the invading Spaniards in Mexico, in Central and South America?
After these systems were forgotten during the decay of later antiquity, there came the Christian medieval model of the image of the world. As expressed in Dante's Paradiso, the earth is the centre of the universe, Satan is at the centre of the earth, the heavens consist of ten concentric spheres. Everything below the moon is subject to corruption and decay; everything above the moon is indestructible. "God, the Aristotelian Unmoved Mover, causes the rotation of the Primum Mobile, which, in turn, communicates its motion to the sphere of the fixed stars and so on downwards to the sphere of the moon."
The great scientific revolution in astronomy and in physics came long after the Greeks, in the sixteenth and seventeenth centuries, with the work of Galileo and Newton, who built up the first scientific image of the universe. By discovering the laws of motion of the bodies of our daily experience and by generalizing these laws to all bodies in the universe, and by inventing the infinitesimal calculus needed for this work, Newton achieved the first great synthesis, which is the aim of modern science, in intimately correlating ideas and facts apparently strange to one another: the fall of an apple from the tree, the fall of the moon around the earth, the motion of the celestial bodies under the action of universal gravitation.