The name supermoon was coined by astrologer Richard Nolle in 1979, in Dell Horoscope magazine arbitrarily defined as:
... a new or full moon which occurs with the Moon at or near (within 90% of) its closest approach to Earth in a given orbit (perigee). In short, Earth, Moon and Sun are all in a line, with Moon in its nearest approach to Earth.
NASA image showing comparison of a supermoon (left) and a micromoon (right)
He came up with the name while reading “Strategic Role Of Perigean Spring Tides in Nautical History and Coastal Flooding” published in 1976 by NOAA Hydrologist Fergus Wood. Nolle never outlined why he chose 90%, but explained in 2011 that he based calculations on 90% of the difference in lunar apsis extremes for the solar year. In other words, a full or new moon is considered a supermoon if where is the lunar distance at syzygy, is the lunar distance at apogee, and is the lunar distance at perigee.
In practice, there is no official or even consistent definition of how near perigee the full Moon must occur to receive the supermoon label, and new moons rarely receive a supermoon label. Sky and Telescope magazine refers to full Moon which comes within 223,000 miles (359,000 km), TimeandDate.com prefers a definition of 360,000 kilometres (220,000 mi). EarthSky uses Nolle's definition comparing their calculations to tables published by Nolle in 2000.
The term perigee-syzygy or perigee full/new moon is preferred in the scientific community. Perigee is the point at which the Moon is closest in its orbit to the Earth, and syzygy is when the Earth, the Moon and the Sun are aligned, which happens at every full or new moon. Astrophysicist Fred Espenak uses Nolle's definition but preferring the label of full Moon at perigee. Wood used the definition of a full or new moon occurring within 24 hours of perigee and also used the label perigee-syzygy.
Wood also coined the less used term proxigee where perigee and the full or new moon are separated by 10 hours or less.