The full moon is the phase of the moon that occurs when the Moon lies on the opposite side of Earth from the Sun. The moon as seen from the surface of the earth is fully illuminated by the sun at this time, presenting a "full" round disc to viewers on earth. As always, only half the total surface of the moon is illuminated.
A full moon is the only time when a lunar eclipse is possible; at that time the moon may move through the shadow cast by the earth. However, because of the tilt of the moon's orbit around the earth relative to the earth's orbit around the sun, the moon may pass above or below the shadow, so a lunar eclipse does not occur at every full moon. Full moons are generally a poor time to conduct astronomical observations, since the bright reflected sunlight from the moon overwhelms the dimmer light from stars.
The average time of full moon number N is approximately:
- D = 20.362954 + 29.5305888531 × N + 102.19 × 10-12 × N 2
where D is the number of days (and fractions) since 1 January 2000 00:00:00 TT, and N is an integer.
To obtain this day expressed in UTC (world clock time) for future events (N > 0), apply the following approximate correction:
- -0.000739 - 235 × 10-12 × N 2
The true full moon may differ from this by more than 14 hours, due to periodic perturbations. The long-term error of the formula is approximately 1 × cy2 seconds in TT, and 11 × cy2 seconds in UT (cy is centuries since 2000); see the new moon page for a detailed explanation.
The age and apparent size of the full moon vary on cycle of just under 14 synodic months, which is called the full moon cycle.
Full moons are traditionally associated with insanity (hence the terms lunacy and lunatic) and with various unusual phenomena such as lycanthropy.
Neopagans hold a monthly ritual called an Esbat at each full moon.
The traditional Chinese calendar is based on the phases of the moon. The full moon is always the middle of a month. The Mid-Autumn Festival falls on the full moon of the eighth month. The Lantern Festival falls on the first full moon of the year. Many religious Chinese people prepare their ritualistic offerings to their ancestors and deities on every full moon and new moon.
Full Moon Names
||Other Names Used
||Crow Moon, Crust Moon, Sugar Moon, Sap Moon
||Sprouting Grass Moon, Egg Moon, Fish Moon
||Corn Planting Moon, Milk Moon
||Rose Moon, Hot Moon
||Thunder Moon, Hay Moon
||Red Moon, Green Corn Moon
||Corn Moon, Barley Moon
||Travel Moon, Dying Grass Moon
||Long Nights Moon
These are the traditional names given to each month's Full Moon by Native Americans in the northern and eastern United States. The Moon was used to track the seasons. (From Farmer's Almanac )
The Blue Moon
The origin of the term "Blue Moon" is steeped in folklore, and its meaning has changed and acquired new and interesting meanings and nuances over time. The earliest known recorded usage was in 1528, in a pamphlet entitled Rede Me and Be Not Wrothe: "Yf they say the mone is belewe, we must believe that it is true". This implies the expression had a meaning of something that was absurd, and bears close resemblance to another moon-related adage first recorded in the following year "They woulde make men beleue ... that ye Moone is made of grene chese".
In modern terms, the event known as a blue moon is related to the western calendar system. A blue moon is the second of two full moons to occur in the same calendar month. Blue moons occur infrequently (thus the saying once in a blue moon to denote a rare event), because the length of the calendar month in this system is close to the length of the period of the moon's phases (synodic month). They are not impossible, because every month except February is longer than this period by 1 or 2 days. The next blue moons (based on UTC) will be on June 30, 2007; and December 31, 2009.
The original meaning of blue moon was the third full moon in a season when there were four full moons in that season: this had to do with church holy days related to the last or first full moon of a season (like Easter). This usage had been almost entirely forgotten, and the original meaning was uncovered only when researchers for Sky & Telescope magazine noticed that the Maine Farmer's Alamanac from 1829 to 1937 reported blue moons that did not fit the first meaning of the term above. (See What's a Blue Moon?)
Visibly blue moons are rare events. They can be caused by smoke or dust particles in the atmosphere, such as happened after forest fires in Sweden in 1950 and Canada in 1951 and, notably, after the eruption of Krakatoa in 1883, which caused blue moons for nearly two years.