Full Moon. Optical image of the full Moon. A full Moon occurs when the Earth lies between the Sun and the Moon. The dark areas on the lunar surface are known as seas. This name arose from historical observations, when the Moon was wrongly thought to have liquid water on its surface. It is now known that the seas" are plains of dark lava which flowed onto the surface after most of the craters were formed. The full Moon occurs midway through the Moon's cycle, and is seen every 28 days. North is at top. For the sequence of the Moon's phases, see images R340/526-532."