A supermoon is the coincidence of a full moon or a new moon with the closest approach the Moon makes to the Earth on its elliptical orbit, resulting in the largest apparent size of the moon's disk as seen from Earth. The technical name is the perigee-syzygy of the Earth-Moon-Sun system. The term "supermoon" is not an astronomical one, but one that originated in modern astrology.
There are 4-6 supermoons a year on average. In fact, May 2012 presents the moon’s closest encounter with Earth since March 19, 2011, at which time the moon was a scant 380 kilometers closer to Earth. The moon won’t come as close again until August 10, 2014 – although in 2013 the moon at its closest (June 23, 2013) will lie only 36 kilometers farther away than the closest moon in 2012. Maybe this helps to illustrate that supermoons – while interesting – are fairly routine astronomical events. ~ Earth Sky
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