Full moon

The full moon is the lunar phase in which the entire hemisphere of the Moon visible from Earth is illuminated by the Sun, producing a bright, circular disc in the night sky. This event occurs when the Earth lies directly between the Sun and the Moon, placing the Moon at the phase of opposition in the ecliptic coordinate system. Although commonly associated with a whole night of brilliance, the full moon is, in fact, an instantaneous astronomical moment at which waxing illumination ends and waning begins.

Orbital Mechanics and Timing

The full moon recurs roughly once every synodic month, averaging 29.53 days. Irregularities in the Moon’s orbit, including its eccentricity and gravitational interactions with the Earth and Sun, cause the exact timing of full moons to vary by up to thirteen hours from the mean value. Time zone differences and cultural conventions about when a day begins may introduce further discrepancies into calendar calculations, especially within lunar and lunisolar systems.
Because a lunar month contains a whole number of days, lunar calendars include months of either twenty-nine or thirty days. The date of the full moon often falls on the fourteenth or fifteenth day of such calendars, depending on whether the month is reckoned from first visibility of the new moon or from astronomical conjunction.
Supermoons—full moons occurring near perigee, when the Moon is closest to Earth—appear brighter and larger than average and typically occur three to four times per year.

Visibility and Observational Characteristics

Only about half of all full moons are seen at night from a single location; the remainder occur when the Moon is below the horizon during local daytime. Because the Moon’s orbit is inclined by approximately 5.14° to the ecliptic, it is seldom perfectly opposite the Sun in three-dimensional space. A perfectly aligned full moon occurs only during a lunar eclipse, when the Moon passes through Earth’s shadow.
For observational astronomy, the full moon is suboptimal. Bright moonlight diminishes contrast in the night sky, making faint stars and nebulae harder to observe. On the lunar surface, shadows disappear under direct sunlight, reducing the visibility of features such as craters, ridges and mountain chains.
Many almanacs list full moons by precise timing in Coordinated Universal Time (UTC), while general-purpose calendars may show an apparent variation of up to a day depending on the reader’s location.

The Lunar Phase Cycle

The complete sequence of lunar phases spans the 29.5-day synodic cycle and consists of eight principal stages:

• New Moon
• Waxing Crescent
• First Quarter
• Waxing Gibbous
• Full Moon
• Waning Gibbous
• Last Quarter
• Waning Crescent

These phases reflect the changing geometry between the Earth, Moon and Sun and the varying amount of the Moon’s illuminated hemisphere visible from Earth. Crescent phases are slender and curved, while gibbous phases show more than half but less than full illumination.

Mathematical Approximation of Full Moon Dates

Approximate dates and times of full moons can be calculated using standard astronomical formulae. One such equation estimates the number of days, d, since 1 January 2000 (Terrestrial Time):
d = 2451550.26 + 29.530588861 × N + 0.000000102 × N²
where N is the number of synodic months since the first full moon of 2000. A small additional correction accounts for differences between Terrestrial Time and Universal Time. These calculations assume a circular lunar orbit; real full moon times may differ by up to about 1.45 hours due to orbital eccentricity.
The Moon’s apparent size and age at full phase vary over a cycle of just under fourteen synodic months, sometimes referred to as the full moon cycle.

Lunar Eclipses

Lunar eclipses occur only during a full moon, when the Earth casts its shadow upon the Moon. Because the Moon’s orbit is inclined relative to the Earth’s orbital plane, most full moons pass above or below Earth’s shadow. A lunar eclipse occurs only when full phase coincides with either the ascending or descending node of the lunar orbit.
These events occur approximately every six months and are paired with solar eclipses, which happen about two weeks earlier or later at the opposite node. During a total lunar eclipse, the Moon may appear red due to Rayleigh scattering and atmospheric refraction of sunlight—a phenomenon popularly referred to as a blood moon.

Folklore, Tradition and Cultural Associations

Cultural traditions worldwide attach deep significance to the full moon.

• In Buddhism, the Vesak festival—commemorating the birth, enlightenment and death of the Buddha—occurs on the full moon of the Vaisakha month.
• In Arabic, the term badr denotes the full moon and, by extension, the bright “white days” in which the full moon is observed.
• In Neopaganism, the symbol of the Triple Goddess incorporates a full moon flanked by waxing and waning crescents.

Popular folklore frequently links full moons to insomnia, madness, increased crime and supernatural transformations such as lycanthropy. Scientific studies, however, provide no consistent evidence that full moons significantly affect human behaviour. For example, contrasting studies published in the British Medical Journal in 2000 found opposite results regarding the frequency of dog bite admissions during full moons.

Full Moon Names and Seasonal Terminology

Traditional cultures often named full moons according to seasonal activities or environmental conditions. Although month names in modern calendars no longer derive from lunar cycles, several traditional full moon names remain familiar:

• Harvest Moon: the full moon closest to the September equinox. It is noted for rising soon after sunset for several consecutive evenings, providing extended twilight illumination during the harvest season.
• Hunter’s Moon: the full moon following the Harvest Moon, associated with hunting in late autumn.
• Blue Moon: used in modern contexts to refer either to the second full moon in a calendar month or, historically, the third full moon in a season containing four.

Atmospheric scattering often gives low-rising full moons a golden or reddish tint, particularly in autumn. Dust, pollution and humidity modify this effect, producing a range of visual hues.
Festivals such as the Mid-Autumn Festival in East Asia celebrate the seasonal full moons of September or October, reflecting long-standing cultural and agricultural ties to the lunar cycle.