פּירוש אויף בראשית 1:18

ולמשול ביום ובלילה, “and to rule during the day and the night.” One would rule by day, the other by night. The moon even when visible by day, hardly “rules.” Or, in the words of the moon itself, quoted in the Talmud Chulin folio 60, “who needs light at noontime?”

ולהבדיל בין האור ובין החושך, between the period when light begins and the period when darkness begins. Sunrise is the period when light begins, whereas the clear visibility of the stars in the evening is the period when the darkness begins.

ולמשול ביום ובלילה, in order to bring new creatures into life in the “lower” regions of the universe, i.e. these living creatures, as opposed to the plants, required more than the light G’d had created on the first day. Seeing they are more important and more sophisticated beings they require exposure to better light in order to flourish.