For the items in Super Mario World, see 3-Up Moon.
The moon is a celestial body that presumably orbits the Mushroom Planet. If it is to be thought of as similar to Earth's moon, than it reflects light from the sun and supplies light during nighttime.
The moon actually first appeared in Super Mario World, where it made a very minor cameo in the background of the Star World map. In this cameo, the moon was anthropomorphic in that it gave a smile and was shown to be in its crescent stage.
The moon, also smiling and in a crescent stage, later appeared as a playable area in Super Mario Land 2: 6 Golden Coins, in which it was one half of Space Zone. It was populated by many pigs, and this was the first time any character ever explored it; when on the moon (and anywhere in the Space Zone) Mario was forced to become Space Mario.
The moon appeared in Yoshi's Story in the background during the fight against Cloudjin. A cardboard moon with two eyes and a nose could also be seen in the background of some stages.
The Moon was also given an appearance in Luigi's Mansion, where it appeared in the background of the Observatory. Luigi, in his quest to find Mario's Star and progress in the mansion, ended up blowing the Moon up with a Meteor Ghost shot from his Poltergust 3000, which quite conveniently created a path of debris leading to the Star he was looking for. Very oddly, after this, the moon can still be seen from the outside rooms and managed to show up in other games.
In Paper Mario: The Thousand-Year Door, this is where the X-Naut Fortress is located. It can only be accessed via a cannon at Fahr Outpost or the Teleporter located in Rogueport Sewers. According to Goombella, a Goomba landed on the Moon in '69.
Many confuse this with the tiny moon in World 5-8 of Super Mario World 2: Yoshi's Island.
The moon appears in many Mario Party games. Twila, a character from Mario Party 6, could be considered the physical manifestation of the moon. Crane Game from Mario Party and Mario Party 2 also takes place on the actual moon.
Enemies (Paper Mario: The Thousand-Year Door)
Notes
- Goombella's comment about a Goomba landing on the moon in '69 is a reference to the Apollo 11 landing; the first manned landing on the real world's moon in 1969.