It was the largest full moon in more than 18 years. A SuperMoon occurs when the Earth, Moon and Sun are all in a line, with Moon in its nearest approach to Earth. SuperMoons can cause tidal forces that are 18% greater than average. A SuperMoon is 12% larger and brighter than an average full moon. See the last photo for a view of the moon as it appeared on 3/19/2011 at 8:23PM EDST.