A
full moon is the
lunar phase that occurs when the
Moon is completely illuminated as seen from the
Earth. This occurs when the moon is in
opposition to the
Sun (when it is on the opposite side of the Earth from the Sun; more precisely, when the
ecliptic longitudes of the Sun and Moon differ by 180 degrees). This means that the hemisphere of the Moon that is facing the Earth (the
near side) is almost fully illuminated by the Sun and appears round (while the
far side is almost completely unilluminated).