Full moon is when the moon is at its brightest.

Full moon is a lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. The full moon is when the three celestial bodies are aligned as close as possible to a straight line. At this time, as seen by viewers on Earth, the hemisphere of the Moon that is facing the Earth (the near side) is fully illuminated by the Sun and appears round. Only during a full moon is the opposite hemisphere of the Moon, which is not visible from Earth (the far side), completely unilluminated.

Characteristics of the full moon.

Composite image of the full moon as taken by the Galileo spacecraft on 7 December 1992.

Although it takes only 27.322 days on average for the Moon to complete one orbit around the Earth (the sidereal month), as a result of the Earth's orbital motion around the Sun it requires about two additional days for the Earth, Moon and Sun to acquire the same relative geometry. So on average the number of days between two similar phases (e.g. between one full moon and the next full moon) is about 29.531 days. This period is referred to as a lunation, lunar month, or synodic month. The actual number of days in a lunation can vary from about 29.272 to 29.833 because the velocities of the Moon and of the Earth are not constant in their elliptic orbits, and because of gravitational interactions with other bodies in the Solar System.

Because the month of February has only 28 days (or 29 in a leap year), there have been a few occasions during which this month has been without a full moon. In particular, there was no full moon in February of 1866, 1885, 1915, 1934, 1961 or 1999. In these years, there were instead either two full moons in January, March, or both (as in 1999). In the leap year of 1972, there was a full moon on February 29.

The date and time of a specific full moon (assuming a circular orbit) can be calculated from the equation:

D = 20.362954 + 29.5305888531 × N + 102.19 × 10-12 × N 2

where D is the number of days since 1 January 2000 00:00:00 UTC, and N is an integer number of full moons, starting with 0 for the first full moon of the year 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 noncircularity of the Moon's orbit. The age and apparent size of the full moon vary in a cycle of just under 14 synodic months, which has been referred to as a full moon cycle.

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.

Eclipses of the full moon.

A full moon is the only time when a lunar eclipse is possible. At this time, it is possible under certain circumstances for the Moon to move through the shadow cast by the Earth. However, because of an approximately 5º tilt of the orbital plane of the Moon with respect to the orbital plane of the Earth (the ecliptic), the Moon usually passes to the north or south of Earth's shadow during a full moon.

Folklore of the full moon.

Full Moons are traditionally associated with temporal insomnia, insanity (hence the terms lunacy and lunatic) and various magical phenomena such as lycanthropy. Psychologists, however, have found that there is no strong evidence for effects on human behaviour around the time of a full moon . They find that studies are generally not consistent, with some showing a positive effect and others showing a negative effect. In one instance, the December 23, 2000 issue of the British Medical Journal published two studies on dog bite admission to hospitals in England and Australia. The study of the Bradford Royal Infirmary found that dog bites were twice as common during a full moon, whereas the study conducted by the public hospitals in Australia found that they were less likely. Psychologists point out that there is a difference between correlation and causation. The mere fact that two events happen at the same time doesn't mean that there is a cause and effect relationship between the two.

Many neopagans hold a monthly ritual called an Esbat at each full moon, while some people practicing traditional Chinese religions prepare their ritual offerings to their ancestors and deities on every full and new moon.

Calendars for the full moon.

The Hindu, Thai, Hebrew, Islamic, Tibetan, Neopagan, Celtic, and the traditional Chinese calendars are all based on the phases of the Moon. None of these calendars, however, begin their 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.

In the Gregorian calendar, the date of Easter is the first Sunday after the ecclesiastical full moon which occurs after the ecclesiastical vernal equinox. In this context, the date of the full moon (together with the date of the vernal equinox) is calculated not according to actual astronomical phenomena, but according to a calendrical approximation of these phenomena.

In the Chinese calendar, the Mid-Autumn Festival falls on the full moon of the eighth month, whereas the Lantern Festival falls on the first full moon of the year.

Full moon names.

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 (see article at 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 21st, and the Lenten Moon would be the last moon on or before March 21st. 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 Native Americans in the northern and eastern United States, other common names, and Hindu names. Note that purnima or pornima is Hindi for full moon.

Full Moon Names.
MonthEnglish NamesNative American NamesOther Names UsedHindu Names
January Old Moon Wolf Moon Moon After Yule, Ice Moon Paush Purnima
February Wolf Moon Snow Moon Hunger Moon, Storm Moon Magh Purnima
March Lenten Moon Worm Moon Crow Moon, Crust Moon, Sugar Moon, Sap Moon, Chaste Moon Holi
April Egg Moon Pink Moon Sprouting Grass Moon, Fish Moon, Seed Moon, Waking Moon Hanuman Jayanti
May Milk Moon Flower Moon Corn Planting Moon, Corn Moon, Hare's Moon Buddha Purnima
June Flower Moon Strawberry Moon Rose Moon, Hot Moon, Planting Moon Wat Purnima
July Hay Moon Buck Moon Thunder Moon, Mead Moon guru Purnima
August Grain Moon Sturgeon Moon Red Moon, Green Corn Moon, Lightning Moon, Dog Moon Narali Purnima, Raksha bandhan
October Harvest Moon Hunter's Moon Travel Moon, Dying Grass Moon, Blood Moon Kojagiri or Sharad Pornima
November Hunter's Moon Beaver Moon Frost Moon, Snow Moon Kartik Pornima
December Oak Moon Cold Moon Frost Moon, Long Night's Moon, Moon Before Yule Margashirsha Pornima

The Blue moon.

The origin of the folkloric term "Blue Moon" is complicated, because its meaning has changed over time. Modern practice is to name a full moon a blue moon if it is the second of two full moons to occur in the same calendar month. The original meaning of blue moon was the third full moon in a season when there were four Full Moons in that season.

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