When the moon appears perfectly round in the sky, it is known as a “full moon,” indicating that the disc of the moon is full or complete. Full moons happen on average every 29 days, and advances in astronomy have allowed scientists to carefully predict their patterns. Numerous traditions and superstitions are associated with the full moon in almost every culture on Earth, especially when the moon becomes eclipsed, as will happen now and then.

The appearance of a full moon is caused by the orbit of the moon around the Earth, and through the Earth's orbit around the sun. As the moon moves behind the Earth, so that the Earth is between it and the sun, more and more of the moon will appear illuminated in a cycle called “waxing.” When the moon is all the way behind the Earth, the sun's light will completely illuminate the visible side of the moon, making it look like a large white disc in the sky. As the moon moves around to the front of the Earth again, the visible disc shrinks, in a process referred to as waning, until the moon appears totally dark because the illuminated portion faces away from Earth.

Although the moon generally takes a little over 27 days to completely orbit the Earth, it takes another few days for the Earth, moon, and sun to be in the proper alignment to create a full moon. As a result, the lunar cycle takes around 29 days in total. Since the lunar cycle is such a present and obvious part of life, it lent its name to the word “month,” which is derived from “moon.”

Occasionally, two full moons will appear in the same calendar month. This is known as a “blue moon,” and it is quite rare. Some people consider blue moons to be highly auspicious, because of their rarity. More rarely, the month of February will have no full moon, since it is usually only 28 days long. When this happens, January or March will be marked with a blue moon. The full moon is also periodically eclipsed, thanks to its position behind the Earth. When the three planetary bodies reach the perfect alignment, the Earth's shadow will cover the moon partially or totally.

The appearance of the moon from Earth is referred to as a “lunar phase,” reflecting the cyclical nature of the moon's appearance. Many cultures have historically tracked lunar phases, and associated certain superstitions and beliefs to them. The new moon, for example, is often linked with crime and questionable activity, since the night is dark, allowing people to participate in activities without detection. The full moon, on the other hand, has been associated with insanity, although studies have suggested that the full moon does not have influence on human or animal behavior.