Gibbous moon. The moon passes through a complete set of phases from new moon to new moon, once every 29.53 days (a synodic month). It is described as gibbous when more than half its disc is illuminated. The moon does not produce its own light, but it can be seen because it reflects sunlight. The dark areas or 'seas', are the remains of ancient plains of lava. The same side of the moon always faces the earth.