A supermoon is a full moon or a new moon that nearly coincides with perigee—the closest that the Moon comes to the Earth in its elliptic orbit—resulting in a slightly larger-than-usual apparent size of the Moon as viewed from Earth. The technical name is a perigee syzygy (of the Earth–Moon–Sun system) or a full (or new) Moon around perigee. Because the term supermoon is astrological in origin, it has no precise astronomical definition.
The real association of the Moon with both oceanic and crustal tides has led to claims that the supermoon phenomenon may be associated with increased risk of events like earthquakes and volcanic eruptions, but no such link has been found.
The opposite phenomenon, an apogee syzygy or a full (or new) Moon around apogee, has been called a micromoon.
Definitions
The term supermoon is attributed to astrologer Richard Nolle while reading "Strategic Role Of Perigean spring tides in Nautical History and Coastal flooding" published in 1976 by NOAA Hydrologist Fergus Wood. 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.
Nolle
Nolle described the concept in a 1979 edition of Dell Horoscope including both full and new moons, but has never outlined why he chose 90% nor has provided a definitive formula for determining if a given full or new moon is "super". The basic 1979 definition read: Nolle amended his definition in 2000 specifying the distance of a given full or new moon be judged against 90% of the mean distance of perigees. Nolle (incorrectly) referenced
In 2011, Nolle added apogees to consideration explaining that he based calculations on 90% of the difference in lunar apsis extremes for the solar year. EarthSky analyzed Nolle's tables and described the updated definition as 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. Nolle based those the mean apsis extremes referencing (incorrectly) the Wikipedia article on the subject arriving at:
Nolle also added the concept of extreme supermoon in 2000 describing the concept as any new or full moons that are at "100% or greater of the mean perigee".
Espenak
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 on full moons occurring "within 90% of its closest approach to Earth in a given orbit" over Nolle's calculations based on the closest of all orbits during the solar year. Wood used the definition of a full or new moon occurring within 24 hours of perigee and also used the label perigee-syzygy.
Other definitions
Sky and Telescope magazine chose a definition of .
TimeandDate.com prefers a definition of .
EarthSky uses Nolle's definition comparing their calculations to tables published by Nolle in 2000.
Wood also coined the less used term proxigee where perigee and the full or new moon are separated by 10 hours or less.
Occurrence
Of the possible 12 or 13 full (or new) moons each year, usually three or four may be classified as supermoons, as commonly defined.
The most recent full supermoon occurred on September 29, 2023, and the next one will be on August 19, 2024.
The supermoon of November 14, 2016, was the closest full occurrence since January 26, 1948, and will not be surpassed until November 25, 2034.
The closest full supermoon of the 21st century will occur on December 6, 2052.
The oscillating nature of the distance to the full or new moon is due to the difference between the synodic and anomalistic months. The period of this oscillation is about 14 synodic months, which is close to 15 anomalistic months. So every 14 lunations there is a Full Moon nearest to perigee.
Occasionally, a supermoon coincides with a total lunar eclipse. The most recent occurrence of this by any definition was in May 2022, and the next occurrence will be in October 2032.
Appearance
A full moon at perigee appears roughly 14% larger in diameter than at apogee. Many observers insist that the Moon looks bigger to them. This is likely due to observations shortly after sunset when the Moon appears near the horizon and the Moon illusion is at its most apparent.