A supermoon, photographed on May 5, 2012, from Mazatlan, Mexico.
A supermoon, also known as a perigee full moon, is the Moon when seen from the Earth at the biggest and brightest that it can possibly appear. During a supermoon occurrence, the Moon appears up to 14% bigger and 30% brighter than when it is furthest from the Earth.
Supermoons happen when the Moon is both full (i.e. on the opposite side of the Earth from the Sun) and also at a minimum distance from the Earth in its elliptical orbit – when it is just 356,400 kilometers (221,457 miles) away, compared to its mean distance of 384,000 kilometers (238,606 miles). The last supermoon occurred on May 5, 2012, when the accompanying photo was taken.
The term 'supermoon' derives from modern astrology and is not used within the astronomical community, which uses the terms perigee full moon or perigee-syzygy moon.