Supermoon is not an astronomical term. It was coined in 1979 by the astrologer Richard Nolle, who defined it and predicted it would coincide with earthquakes and storms.

Astronomers say perigee syzygy, which is why nobody says it. The word sat unused for thirty years and then went everywhere after the 2011 Japanese earthquake was wrongly linked to one. The disaster prediction has been tested and fails; the word has entered dictionaries and NASA's own press releases, which is a remarkable career for a piece of astrological jargon.