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).