The second set of four works on the last three days corresponds to the set of four on the first three. Thus we have the creation of light and of the luminaries; the firmament separating the upper from the lower waters, and the birds which fly across the firmament and the fish in the sea; the appearance of the land and creation of land animals; finally the creation of herbs and fruit, and the creation of man, who till the Flood subsists entirely upon these.
The heavenly bodies are described as they appear to us. hence the stars are a mere appendix to the two great lights, added almost as an after-thought, possibly by some scribe or reader. The plain meaning of the passage is that the lights were created on the fourth day, not that they had been created before and only then became visible! They are attached to the firmament, and serve as lamps for the earth. They also regulate the festivals and other occasions, secular as well as sacred, and the divisions between day and night, and they determine the length of the year. They serve, moreover, as signs, perhaps in the astrological sense as foreshadowing the future. But they are not to be worshipped, nor are they even represented here, as often in Scripture, as animated beings (Genesis 1:21 *).