The moon, sun and stars are visible at different time of the day or night.
This one is relatively simple and built into normal game play.
The Minecraft Wiki has a excellent breakdown of just how long each minute of game play actually equals in “real time” as well as a video showing what day looks like in the game.
While this is not quite as realistic as our normal days, (the sun and moon can never been seen at the same time) there are very different parts of the day. There is a sunrise, the moon always goes in the same direction, and eventually there is a sunset and as the sun goes away, the moon comes up on the opposite horizon and travels with stars across the sky until it is time for the sun comes back up.
Students can easily observe this and should be able to discuss this concept.