Full moon is a lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent (ecliptic) longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.^[1]

Lunar eclipses can only occur at full moon, where the moon's orbit allows it to pass through the earth's shadow. Lunar eclipses do not occur every month because the moon sometimes passes above or below the earth's shadow. Lunar eclipses can occur only when the full moon occurs near two nodes of the orbit, either the ascending or descending nodes. This causes eclipses to only occur about every 6 months, and often 2 weeks before or after a solar eclipse at [[new moon] at the opposite node.

As seen from Earth, the hemisphere of the Moon that is facing the earth (the near side) is almost 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.

The time interval between similar lunar phases—the synodic month—averages about 29.53 days. Therefore, in those lunar calendars in which each month begins on the new moon, the full moon falls on either the 14th or 15th of the lunar month. Because lunar months have a whole number of days, lunar months may be either 29 or 30 days long.

Characteristics

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.^[2]

Formula

The date and approximate time of a specific full moon (assuming a circular orbit) can be calculated from the following equation:^[3]

${\displaystyle d=20.362955+29.530588861\times N+102.026\times 10^{-12}\times N^{2))$

where d is the number of days since 1 January 2000 00:00:00 in the Terrestrial Time scale used in astronomical ephemerides; for Universal Time (UT) add the following approximate correction to d:

${\displaystyle -0.000739-(235\times 10^{-12})\times N^{2))$ days

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.^[4] See New moon for an explanation of the formula and its parameters.

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.

Folklore

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 behavior around the time of a full moon.^[5] They find that studies are generally not consistent, with some showing a positive effect and others showing a negative effect. In one instance, the 23 December 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.

Calendars

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.^[6][7]

Biblical Holy Days

The third book of the Torah, Leviticus, clearly mandates three major holy days on the full moon: PASSOVER, 1st day of UNLEAVENED BREAD, and the 1st day of SUKKOT (Feast of Booths).

The Biblical mandate for the celebrations of PASSOVER and the first day of UNLEAVENED BREAD is in chapter twenty-three in verses four through seven:

These are the set feasts of the LORD, even holy convocations, which you shall proclaim in their appointed season. In the first month, on the fourteenth day of the month in the evening, is the LORD's Pesach PASSOVER. On the fifteenth day of the same month is the feast of matzah UNLEAVENED BREAD to the LORD. Seven days you shall eat matzah UNLEAVENED BREAD. In the first day you shall have a holy convocation. You shall do no regular work.

^[8]

The Biblical mandate for the holy day of the first day of SUKKOT is also in chapter twenty-three in verses thirty-three through thirty-five:

The LORD spoke to Moshe, saying, "Speak to the children of Yisra'el, and say, 'On the fifteenth day of this seventh month is the feast of booths for seven days to the LORD. On the first day shall be a holy convocation: you shall do no regular work....'"

^[9]

For further clarification, it should be noted that biblical months always begin at the first sighting of the new moon and that biblical days begin at sundown: thus the evening of the 14th day and the 15th day of the biblical month is the full moon.

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 (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.^[10] 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.

The blue moon

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.^[11]

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.