A full moon is often thought of as an event of a full night's duration. This is somewhat misleading, as the Moon seen from Earth is continuously becoming larger or smaller (though much too slowly to notice with the naked eye). Its absolute maximum size occurs at the moment expansion has stopped, and when graphed, its tangent slope is zero. For any given location, about half of these absolute maximum full moons will be potentially visible, as the other half occur during the day, when the full moon is below the horizon. Many almanacs list full moons not just by date, but by their exact time as well, usually in Coordinated Universal Time (UTC)). Typical monthly calendars that include phases of the moon may be off by one day if intended for use in a different time zone.
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.
On 12 December 2008 the full moon occurred closer to the Earth than it has done at any time for the past 15 years.
The date and approximate time of a specific full moon (assuming a circular orbit) can be calculated from the following equation:
where N is the number of full moons since the first full moon of 2000. The true time of a full moon may differ from this approximation by up to about 14.5 hours as a result of the non-circularity of the moon's orbit. See New moon for an explanation of the formula and its parameters.
Full Moons are traditionally associated with temporal insomnia, insanity (hence the terms lunacy and lunatic) and various "magical phenomena" such as lycanthropy. Dr. Ehrehreigh from Brigham Young University a world renowned lunologist says in her book "The Moon: Psychological Effects on Every Day" "The Moon has always been a part of the human existence, you can see hieroglyphics showing the sacrifice by fire on the the days of a full moon. In independent studies across the United States the murder rate increased by thirty four percent."
The Hindu, Thai, Hebrew, Islamic, Tibetan, Mayan, Neo-pagan, Germanic, Celtic, and the traditional Chinese calendars are all based on the phases of the Moon. None of these calendars, however, begins its months with the full moon. In the Chinese, Jewish, Thai and some Hindu calendars, the full moon always occurs in the middle of a month.
It is traditional to assign special names to each full moon of the year, although the rule for determining which name will be assigned has changed over time (e.g., the blue moon). An ancient method of assigning names is based upon seasons and quarters of the year. For instance, the Egg Moon (the full moon before Easter) would be the first moon after March 21, and the Lenten Moon would be the last moon on or before March 21. Modern practice, however, is to assign the traditional names based on the Gregorian calendar month in which the full moon falls. This method frequently results in the same name as the older method would, and is far more convenient to use.
The following table gives the traditional English names for each month's full moon, the names given by Algonquian peoples in the northern and eastern United States, other common names, and Hindu and Sinhala names. Note that purnima or pornima is Sanskrit for full moon, which has also become the Malay word for full moon purnama. Full moon days are sacred according to Buddhist tradition and called Poya in Sinhala, the dominant language of the Buddhist majority of Sri Lanka.
|Positional name||Associated Month||English names||Algonquian names||Other names used||Hindu names||Sinhala (Buddhist) names|
|Early Winter||January||Old Moon||Wolf Moon||Moon After Yule, Ice Moon||Paush Poornima||Duruthu Poya|
|Mid Winter||February||Wolf Moon||Snow Moon||Hunger Moon, Storm Moon, Candles Moon||Magh Poornima||Navam Poya|
|Late Winter||March||Lenten Moon||Worm Moon||Crow Moon, Crust Moon, Sugar Moon, Sap Moon, Chaste Moon, Death Moon||basanta (spring) purnima, dol purnima (holi)||Medin Poya|
|Early Spring||April||Egg Moon||Pink Moon||Sprouting Grass Moon, Fish Moon, Seed Moon, Waking Moon||Hanuman Jayanti||Bak Poya|
|Mid Spring||May||Milk Moon||Flower Moon||Corn Planting Moon, Corn Moon, Hare's Moon||Buddha Poornima||Vesak Poya|
|Late Spring||June||Flower Moon||Strawberry Moon||Honey Moon, Rose Moon, Hot Moon, Planting Moon||Wat Poornima||Poson Poya|
|Early Summer||July||Hay Moon||Buck Moon||Thunder Moon, Mead Moon||Guru Purnima||Esala Poya|
|Mid Summer||August||Grain Moon||Sturgeon Moon||Red Moon, Green Corn Moon, Lightning Moon, Dog Moon||Narali Poornima, Raksha bandhan||Nikini Poya|
|Late Summer||September||Corn Moon||Harvest Moon||Barley Moon||Bhadrapad Poornima||Binara Poya|
|Early Fall||October||Harvest Moon||Hunter's Moon||Travel Moon, Dying Grass Moon, Blood Moon||Kojagiri or Sharad Purnima, lakshmi puja||Vap Poya|
|Mid Fall||November||Hunter's Moon||Beaver Moon||Frost Moon, Snow Moon||Kartik Poornima||Il Poya|
|Late Fall||December||Oak Moon||Cold Moon||Frost Moon, Long Night's Moon, Moon Before Yule||Margashirsha Poornima||Unduvap Poya|
The term "blue moon" traditionally referred to an extra moon in a season: if a season had four full moons (rather than the more common three), then the third of the four moons was known as a blue moon.
A mistaken definition, that the second full moon in a calendar month is known as a blue moon, became common in parts of the U.S. during the second half of the twentieth century due to a misinterpretation of the Maine Farmers' Almanac in the March 1946 Sky & Telescope magazine; this was corrected in 1999.
Since there are on the average 12.37 full moons in a year, a "blue moon" must occur on the average every 2.7 years (7 times in the 19 years of the Metonic cycle), by either definition.