Convention this week passed a resolution referencing "natural day." Important to understand this as the technical term it is. Here is Luther's explanation from his Genesis Commentary:
Moses' additional statement, "to divide day and night," refers to the distinction of the natural and the artificial day, which is also customary among the astronomers. For above he said: "Evening and morning became one day." There he is speaking of the natural day, which consists of twenty-four hours, during which the primum mobile revolves from east to west. When God says here: "That they divide the day and the night," He is speaking of the artificial day, during which the sun is above the horizon.