Moon

The Moon is a sprite that appears during night time.

The moon illuminates topmost blocks during the night in the same way that the sun illuminates them in the day, but only illuminates them to a Light level of four, rather than the sun's fifteen. You can track the moon's and sun's progress using a Clock.

Grass blocks or Saplings do not grow in moonlight, however they won't decay in moonlight either.

Phases of the Moon
The Moon goes through eight lunar phases, and changes phase every night. The phases are (in chronological order):
 * Full Moon
 * Waning Gibbous
 * Last Quarter
 * Waning Crescent
 * New Moon
 * Waxing Crescent
 * First Quarter
 * Waxing Gibbous

This allows the player to keep rough track of the passage of time, even after spending a few game days underground.

Comparison to Real-world Calendars
In the real world, the lunar cycle is 29&frac12; days, which is the basis for our months. However, most real-world calendars are dominated by the longer solar cycle, which produces our year. In Minecraft, the "lunar month" is only eight game days (closer to a week), and there is no trace of a solar cycle. One could certainly build larger calendars by grouping lunar "monthweeks", but any such system is necessarily arbitrary. Then too, animal and crop growth (which drove the development of real-world calendars) take place in game days, while their real-world equivalents require months or years.

One Minecraft lunar cycle takes 2 hours 40 minutes of play time and three lunar cycles take 8 hours of playtime. This means there are 9 lunar cycles in one real-world day (24 hours) of play. (Beds notwithstanding!)

Mobs
Hostile Mobs will spawn around dark areas of a normally generated map. When the moon is up, hostile Skeleton, Creeper, Zombie, Endermen and Spider mobs will appear and attack the player. When the sun comes back up Skeleton and Zombies will burn in the sunlight and drop items after death. Spiders will not burn in daylight, but instead become neutral during the day, only to become hostile again if attacked, when it is night, or when it falls into a dark area. Endermen will not burn in daylight, but hostile Enderman will become neutral during the day, only to become hostile again if attacked or looked upon, but will instantly teleport away and become neutral again if the Player approaches them. Creeper and Slime mobs do not burn, and will remain hostile during the day. (See also: Difficulty level.) Only one mob is ever affected by the phase of the moon:  Slimes in swamp biomes will spawn on the surface at night, in numbers proportional to the moon's fullness; most numerous at the full moon, but none at the new moon.

Stars
Together with the moon, the night sky also consists of stars. In current versions, the stars appear as bright points, and the starfield rotates as the night progresses.

Trivia

 * Night time lasts 7 minutes, plus 1.5 minutes each of sunrising and sunsetting.
 * The Moon and Sun are not visible while using the Short or Tiny render distance settings.
 * The Minecraft Moon is rotated 90 degrees. It appears to be lit from a source in the north or south, whereas it should be lit from the east or west.
 * The Minecraft Moon is fixed in the Sky relative to the sun and stars. Earth's Sun and Moon move more slowly against the stars, and the Moon moves more slowly than the sun.
 * Minecraft cannot have eclipses due to the above.
 * The Minecraft Moon can only be seen at night (unlike the real Moon), unless you are quite high above the ground.
 * If a player is viewing themselves from the front in third person, during moonrise, the moon will appear red.
 * The shadow on the moon appears to be a pixelated circle. This suggests that the moon is round, despite its square outline.
 * There is no explicit command to change the Moon's phase, but using  will advance it to the next phase.
 * The phase of the Moon influences slime spawning in the swamplands. No slimes spawn in the swamps during New Moon, most of them spawn during Full Moon. It doesn't influence spawning of slimes in designated blocks below 40th level.