'Supermoon' is the popular term describing a new or full moon that occurs with the Moon
at or near (within 90% of) its closest approach (perigee) to Earth in a given orbit.
In short, Earth, Moon and Sun are all in a line, with the Moon in its nearest
approach to Earth. The technical term for the event is 'perigee-syzygy'.

Photographed near Kilmarnock, Ontario, Canada.
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