Full Moon

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 longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.

Only during a full moon is the opposite hemisphere of the Moon, which is not visible from Earth far side, completely un-illuminated. As a lunar month is about 29.531 days long, 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, the period between full moons can be either 29 or 30 days.