Full Moon

The Full Moon is the phase at which the Moon stands opposite the Sun in the sky, so that its entire near face is lit and the disc appears whole and bright. It rises around sunset and sets around sunrise, visible all night. The full moon fixes the timing of many festivals, Passover, Easter, and the mid-autumn and other observances across Asia are set by it, and it is the point at which a lunar eclipse can occur, when the Earth passes exactly between Sun and Moon. Traditionally the full moon is read as a time of culmination, fulfilment, and heightened intensity. These are the astronomical facts of the phase together with the meanings traditions attach to it; the full disc is simply the whole sunlit hemisphere of the Moon turned toward the Earth at opposition.

A full moon occurs roughly once every twenty-nine and a half days, but because that synodic period does not divide evenly into the solar year, most years contain twelve full moons while some contain thirteen, and when a second full moon falls within a single calendar month, a purely calendrical coincidence with no astronomical significance of its own, it has been called a blue moon in English usage since the twentieth century. In Wiccan and related modern pagan practice the full moon is marked by the Esbat, a monthly rite distinct from the eight solar Sabbats of the wheel of the year, and is traditionally associated with the Mother aspect of a threefold goddess figure, standing between the Maiden of the waxing crescent and the Crone of the waning phase in a scheme first popularised in the twentieth century rather than inherited from antiquity.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with Wicca: A Guide for the Solitary PractitionerScott Cunningham, The Oxford Companion to the Year: An Exploration of Calendar Customs and Time-ReckoningBonnie Blackburn and Leofranc Holford-Strevens

Full moon is opposition, elongation a hundred and eighty degrees, the Earth between the Sun and the Moon and the whole visible disc lit. Like the new moon it is an instant rather than a night, and the Moon appears full to the eye for a day either side of it.

The phases are produced by the changing angle between the Sun, the Earth and the Moon, and the quantity that defines them is elongation, the Moon's angular distance from the Sun as seen from the Earth. The complete round from one new moon to the next is the synodic month, about 29.53 days, and it is longer than the sidereal month of about 27.32 days in which the Moon returns to the same place among the stars, because the Earth has moved along its orbit in the meantime and the Moon must travel further to catch up.

A lunar eclipse can only occur at full moon, and only when the opposition falls near a node of the Moon's orbit, which is the same condition that governs solar eclipses at conjunction. Because the two conditions are linked, eclipses come in seasons, and the interval at which the pattern repeats, the saros of about eighteen years and eleven days, was known to Babylonian astronomers and used for prediction.

The full moon is the anchor of a great many calendars and observances. Passover and Easter are tied to it through their computations, the Panchanga names the day Purnima and reckons the fifteenth tithi as ending at it, and in several Hindu calendrical conventions the month itself begins or ends at the full moon rather than at the new.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with The Indian CalendarRobert Sewell and Sankara Balkrishna Dikshit, The Oxford Companion to the Year: An Exploration of Calendar Customs and Time-ReckoningBonnie Blackburn and Leofranc Holford-Strevens