Radak on Genesis 1:3

יהי אור, אף שלא היו המאורות ברקיע השמים עד יום רביעי, מכל מקום נבראו ביום ראשון עם הגלגלים, והכל נברא ברגע אחד, אך לא יצאו פעילתם לפעלם על פני תבל, כי אם כל אחד ואחד ביומו; וגם האור לא החחיל לשלח ניצוציו לגדל צימחי ארץ, ולהיות עשב מזריע זרע עד אשר אמר השם שיהיו מאורות ברקיע השמים להאיר על הארץ ולעשות פעולתם בעולם הארצי. יום, מלת יום היא זמן היות השמש על הארץ לבד, וכן "כל היום ההוא וכל הלילה" (שמות י׳:י״ג) ופעמים כולל היום והלילה, כמו "ויהי ערב ויהי בקר יום אחד" ויאמר יום על העת, כמו "ביום הכותי כל בכור" (במדבר ח'): ויאמר אלוקים יהי אור ויהי אור. The light was already in the heaven, seeing that the luminaries were created together with the planets. Daniel 12,3 refers to this when he compares the radiance of the knowledgeable people in the future to the radiance of the expanse of the sky, describing the latter as זהר הרקיע. In Exodus 24,10 the visions experienced by the elite of the Israelites is compared to וכעצם השמים לטוהר, “as pure as the radiant light of the heavens.” What had been lacking was only that this light had not illuminated the face of the deep as this had not been equipped to reflect such light as yet. Seeing that the directive concerned only the creatures in the “lower universe,” G’d did not introduce His creative activities with the words: יהי שמים, “let there be a heaven,” but the Torah wrote simply: “He created heaven, etc., ברא.” Once the various planets were placed in orbit to perform their orbits [I suppose that the sun only is meant, Ed.] every 12 hours the “lower” part of the universe would be in darkness. At the end of the first period of 12 hours, G’d issued the directive: “let there be light”, i.e that light should replace the darkness in that location. When the wind blows over the water it dries some of the surface of the waters enabling the waters to be illuminated faintly by the daylight above. The primordial light had not become distinct, strong, until the fourth day.
Possibly, the words זהר הרקיע describe the light by means of which the heaven itself shines, radiates light, as we have written. In that event, the light described here refers to a different period of 12 hours, a whole day comprising 24 hours. This light would have become missing on the first day from the “lower regions” of the universe. With the absence of this light darkness would have ensued in that region. During the first three days of the creative period, the only reason for such absence of light from part of the universe would be the fact that it was the will of the Creator that it be so. This can be so only if we adopt the view that the light prevailing during the first three days of that period was none other than the “natural” light radiated by the רקיע, the firmament. If, on the other hand, we assume that the light under discussion is the light of the sun, which up until then had shone weakly, seeing that the atmosphere had not been conditioned to let its rays traverse it without hindrance, and that as a result of this during these three “days” total darkness pervaded the universe, there can be no question that these regions had not been conditioned to receive the light from heaven at all, not even a weak diffused kind of light. Total darkness would have prevailed during these “days.” It follows that the reason for this total darkness during these three “days” must have been none other than G’d’s will, His pleasure that it be so. Clearly, although G’d saw that the light “was good,” He decreed in His wisdom that light not be present at all times in all locations. The reason is that the universe needs darkness also, why else would G’d have created it as we know from Psalms 104,20 תשת חשך ויהי לילה, “You bring on darkness and it is night.”
Maimonides writes (chapter 30, second part of Moreh) that the principal cause of the coming into existence and the destruction of celestial forces such as the planets is light and darkness respectively, both of which are forces to which these phenomena are drawn. The constant change from extremes of heat and cold subject these celestial bodies to wear and tear. Climates are affected through the change from light to darkness, as well as the properties of the planets themselves. These changes reflect the will of higher forces, i.e. G’d, Who wishes rain, change of climate to occur on earth at certain times. Darkness is the natural state of the “lower regions” of the universe, light acting as something reviving these regions. The scholar Ibn Ezra writes that the light of which our verse speaks originated in the regions above the רוח, the wind (spirit) operating within the atmosphere. When the Torah wrote ויאמר, commonly translated as “He said,” this is merely a figure of speech, just as when David wrote in Psalms 33,6 בדבר ה' שמים נעשו, “that the heavens were created by the word of G’d.” These formulations are used by the Torah to describe expressions of G’d’s will.