supermoon, also spelled super Moon, also called perigee-syzygy Moon, a full moon that occurs when the Moon is at perigee (the closest point to Earth in its orbit). The Moon is typically about 12 percent (or about 43,000 km [27,000 miles]) closer to Earth at perigee than at apogee, and thus a full moon at perigee would be about 25 percent brighter than one at apogee.
The term supermoon was coined by American astrologer Richard Nolle in 1979 to describe both a new and a full moon occurring at or near (within 10 percent of) perigee. However, supermoon came to denote the more-restrictive meaning of a full moon at perigee. Nolle claimed that a supermoon would cause an increase in severe weather and earthquakes, but no such connection has been found.
The astronomical term for this phenomenon is perigee-syzygy Moon. The alignment of the Sun, the Moon, and Earth that occurs at both new and full moon is known as syzygy.