The transition from day to night is caused by the spinning of the Earth about its axis. We experience day when we are on the half of the Earth facing the Sun, and night once we have been spun around to the other side.
The same occurs on the moon. However, the difference is that It takes 28.5 days for the moon to spin about its axis. So one moon-day is 28.5 Earth-days long!
When we watch the phases of the moon we are actually watching the transition of moon-day to moon-night and vice-versa. Because the moon orbits the Earth in exactly the same amount of time it takes for the moon to complete a turn, the same side of the moon is always facing the Earth. (Originally the moon spun faster, but slowed down to reach this curious steady state). So you can pick a place on the moon and watch over a month as it goes from light to dark (this is called 'waning' and corresponds to moon-dusk) and then back again (waxing, which corresponds to moon-dawn) !
Sally Riordan, M.A., Management Consultant, London
'I have deep faith that the principle of the universe will be beautiful and simple.'