Seasons
Plant life on this planet of ours depends on day and night, and seasons of growth followed by seasons of dying back. Plant life has emerged here fully adapted to these patterns, so their explanation comes next:
God said, "Let there be lights in the expanse of the sky to divide the day from the night; and let them be for signs to mark seasons, days, and years; and let them be for lights in the expanse of the sky to give light on the earth"; and it was so. God made the two great lights: the greater light to rule the day, and the lesser light to rule the night. He also made the stars. God set them in the expanse of the sky to give light to the earth, and to rule over the day and over the night, and to divide the light from the darkness. God saw that it was good.
We understand now pretty well what goes on in the solar system: the elliptical orbit of our planet round the sun, rotating round its own axis at an angle to the path of its solar orbit. This is how we get days and nights, summer and winter. Trees respond to day and night. To quote Wikipedia: 'As part of the carbon cycle, plants, algae, and cyanobacteria use light energy to photosynthesize carbohydrate from carbon dioxide and water, with oxygen produced as a waste product. However, photosynthesis cannot occur in darkness and at night some carbon dioxide is produced by plants during respiration.' Acting as an enormous 'carbon sink', trees soak up carbon dioxide from the air, producing life-giving oxygen in return. In fact, a medium-sized tree generates the same amount of oxygen as each one of us needs to breathe.
Many plants have the annual sequence, as every gardener knows, of springtime growth and autumnal dying back. And some marine organisms benefit from the regular covering and uncovering of sea water provided by the tides. So our myth, having introduced plant life, needed to explain this vital component of their cycles.
It is a fascinating 'coincidence' that the moon and the sun appear to be exactly the same size. If you see the sun through enough mist to make it possible to give it your attention, it appears to be just like a full moon. The reason? The sun is 400 times larger than the moon, and its distance from the earth is 400 times more than the distance of the moon.
So we can offer:
"To provide the daily and annual variations that are essential to the respiration and cyclic nature of plant life, the sun and moon were arranged to give night and day, and the pattern of seasons."