Day and night form because the earth revolves around its axis in about 24 hours (the earth's rotation in its axis). The side facing the sun has light so it is called daylight and the side facing that is in the dark so it is night time. Imagine that the earth does not revolve around itself and only one side will be facing the sun which will always have the sun and the other side will always be in the dark. Yes, the earth revolves around the sun again. So very little of the rest of the world will be exposed to the sun and night and day would be an annual event.