The Moon, or Luna, is the nearest celestial body to the planet Earth, and its only natural satellite. It was the source of great wonderment and mystery for many ancient civilizations, and some developing cultures still worship the moon to this day. Because the moon orbits the Earth every 28 or 29 days, many ancient calendars were based on it, and that is where we derive the English work month.
As the Moon passes through its orbit, the shadow of the Earth causes it to change shape in the sky. These changes are called phases. A full moon is a phase in which the entire moon is visible, whereas a new moon is when the Earth's shadow completely covers the moon, making it invisible. When two full moons are seen in the same month (which is extremely rare), it's called a blue moon. In 1969, Neil Armstrong became the first man to walk on the Moon. Since that time, NASA has been improving its space-traveling technology, and hopes to someday create a Lunar Outpost.
The phase of the moon was also important during a later adventure, Blizzard of the Blue Moon.