The full moon, as observed from Earth. thumb|200px|Composite image of the Moon as taken by the Galileo spacecraft on 7 December 1992. The color is 'enhanced' in the sense that the CCD camera is sensitive to near infrared wavelengths of light beyond human vision. 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. At this time, as seen by viewers on 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 average lunar month is about 27.3 days long, so that the full moon falls on either the 14th or 15th of the lunar month in those calendars that start the month on the new moon. In any event, as lunar months are counted in discrete numbers of days, lunar months are said to be either 27 or 28 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 GMT). Typical monthly calendars which include phases of the moon may be off by one day if intended for use in a different time zone.

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

$D\; =\; 20.362954\; +\; 29.5305888531\; \backslash times\; N\; +\; 102.19\; \backslash times\; 10^\{-12\}\; \backslash times\; N^2$

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 non-circularity of the moon's orbit (see New moon and ). 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.

On December 12, 2008, the full moon has occurred closer to the Earth than it has done at any time for the past 15 years.