What is a supermoon?

What is a supermoon, why do they occur and what does the term supermoon mean?

A supermoon rising behind a mountain. Credit: Mimi Ditchie Photography / Getty Images
Published: June 24, 2021 at 8:42 am
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Tonight's full Moon, 24 June 2021, will be the last so-called 'supermoon' of the year, designated a 'Super Strawberry Moon'. But what is a supermoon, when do they occur, and are they really something to get excited about?

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Supermoons occur at perigee: the point in the Moon’s orbit where it’s closest to Earth. A perigee full Moon appears a little brighter and larger than an average full Moon, and is known by some as a ‘supermoon’.

Generally speaking, however, astronomers don't tend to use these terms (likely a result of the term supermoon's astrological origins), and the technical name for a 'supermoon' is a perigee syzygy Moon.

Supermoon Over Central Illinois, by Joshua Rhoades. Equipment: Canon 6D with Sigma 100-400mm lens at 400mm, ISO400, 1/120s. The first full moon (and supermoon) of 2018 rising over Elkhart Hill and the surrounding fields in Central Illinois.
Supermoon Over Central Illinois, 1 January 2018, by Joshua Rhoades. Equipment: Canon EOS 6D DSLR camera, Sigma 100-400mm lens. Credit: Joshua Rhoades

What is a supermoon and when do they occur?

A supermoon happens when a full Moon coincides with the Moon’s closest approach to Earth along its orbit.

We get 12 or 13 full Moons each year but, due to the orientation of the Moon’s orbit, not every full Moon is a supermoon, in the same way that not every full Moon is a lunar eclipse.

The Moon's orbit around Earth

The Moon takes just over 27 days to orbit Earth on an elliptical path that takes it from its most distant point from Earth at apogee, to its closest approach at perigee.

These distances can vary between 406,712km (apogee) and 356,445km (perigee).

It also takes just over 29 days to cycle from one full Moon to the next, which you can see for yourself if you note the phases of the Moon night after night.

Meanwhile, Earth's orbit around the Sun takes 365 days and this means it takes 14 lunar cycles (411 days) to go from one full perigee Moon to the next one.

Going the distance: a comparison between April’s perigee (closest) and apogee (farthest) Moons. Credit: Alamy Stock Photo
Comparison between the perigee (closest) and apogee (farthest) Moons. Credit: Alamy Stock Photo

When does a supermoon happen?

The term supermoon refers to both perigee full and new Moons but, as the new Moon is not visible we will concentrate on the full supermoon.

The name ‘supermoon’ was first coined by astrologer Richard Nolle in 1979. He defined it as “a new or full Moon that occurs when the Moon is at or near (within 90% of) its closest approach to Earth in its orbit”.

Supermoon engulfed by clouds in Clacton-On-Sea, Essex, UK, captured by Shantelle Robinson with an Olympus E-420. Credit: Shantelle Robinson
Supermoon engulfed by clouds in Clacton-On-Sea, Essex, UK, captured by Shantelle Robinson with an Olympus E-420. Credit: Shantelle Robinson

Astronomers prefer the more precise, though perhaps less catchy, term of a perigee full Moon to describe a full Moon that occurs when the Moon’s centre is less than 360,000km from the centre of Earth.

Or a perigee syzygy full Moon, where syzygy refers to a straight-line configuration of three or more celestial bodies in a gravitational system (in this case Earth, the Moon and the Sun).

The opposite phenomenon, an apogee syzygy full Moon, is similarly dubbed a ‘micromoon’.

The difference in apparent size between apogee and perigee full Moons.
The difference in apparent size between apogee and perigee full Moons.

Is a supermoon really much bigger?

Much has been written about supermoons ‘stunning’ and ‘dazzling’ observers, and this is another reason some astronomers balk at the term.

In reality, a full Moon at perigee appears only 14% larger due to being closer to us, and only 30% brighter than an apogee full Moon.

A supermoon is also only about 7% larger and 15% brighter than an average full Moon.

A perigee full Moon (left) appears 30% brighter and 14% larger than an apogee full Moon (right). Credit: Pete Lawrence
A perigee full Moon (left) appears 30% brighter and 14% larger than an apogee full Moon (right). Credit: Pete Lawrence

Without a side-by- side comparison, it is difficult to spot the difference from one month to the next, but it is possible to record changes in the Moon's size through photography.

The difference in brightness is due to the reflected light from the lunar surface that reaches Earth being inversely proportional to the square of the distance between them.

The Moon illusion

Claims that the Moon appears larger than this are likely to be due to the ‘Moon illusion’. Indeed, the Moon does appear much larger when observed near the horizon at moonrise and moonset.

The reason for this optical illusion is still under debate, but is likely to do with the way our brains process objects close to the horizon as being larger when in proximity to buildings and trees, than when they are high in the sky with nothing to compare sizes against.

A Being 737 silhouetted against the supermoon of 14 November 2016, captured by Richard Matthews from Staffordshire, UK. Credit: Richard Matthews.
A Boeing 737 silhouetted against the Supermoon of 14 November 2016, captured by Richard Matthews from Staffordshire, UK. Credit: Richard Matthews.

Do supermoons always come in threes?

April 2021 brought the first of three consecutive supermoons in 2021, but this is not unusual.

Indeed, every 14 lunar months the series of full Moons cycles from largest to smallest and back again as the Moon orbits Earth. Perigee can last between two and five full Moons.

For a full Moon to be classed as ‘super’ it must occur when it is around 360,000km away from Earth or less, so the full Moons occurring either side commonly also fall within the supermoon category.

On rare occasions we get two or four supermoons in a row, but three is the most common.

Gallery: 7-8 April 2020 supermoon

Effects of a supermoon

Many people talk about the effects of a supermoon on us and our planet, but there is scant scientific evidence to support claims that supermoons cause earthquakes, volcanic eruptions, extreme weather and tsunamis, or floods and pestilence!

The Moon is, of course, the driving force for Earth’s tides: full and new Moons are when the Earth, Sun and Moon line up to produce extreme spring tides, but the effect of perigean spring tides only increases the tidal variations by about 5cm on average.

Supermoons have little added influence on Earth's tides. Credit: Jeff Morgan / Getty Images
Supermoons have little added influence on Earth's tides. Credit: Jeff Morgan / Getty Images

In recent years there have indeed been a handful of tsunamis and earthquakes that coincided with a supermoon but, with around three supermoons every 14 months it would be unusual if such a disaster did not coincide with a supermoon now and then.

A supermoon may not be so ‘super’, or even rare, but once you cut through the hype, it is still a great way to engage with our nearest celestial neighbour. For more on lunar-gazing, read our guide on how to observe the Moon.

Pictures of supermoons

Below is a selection of images of supermoons captured by astrophotographers and BBC Sky at Night Magazine readers. If you managed to capture one, we'd love to see it.

Don't forget to send us your images or share them with us via Facebook, Twitter and Instagram.

Super Moon over Barton-on-Sea by Jeff Brown, Highcliffe, Dorset, UK. Equipment: Canon 40D DSLR 170mm f8 1/8 ISO 1000
Super Moon over Barton-on-Sea by Jeff Brown, Highcliffe, Dorset, UK. Equipment: Canon 40D DSLR 170mm f8 1/8 ISO 1000
Super Moon by Jason Turner, Sheffield, UK. Equipment: Fujifilm Finepix s5000
Super Moon by Jason Turner, Sheffield, UK. Equipment: Fujifilm Finepix s5000
Supermoon July 12th by Michael LaMonaco, New Jersey, USA. Equipment: Celestron Nexstar 8SE, advanced VX mount, Canon 60D
Supermoon July 12th by Michael LaMonaco, New Jersey, USA. Equipment: Celestron Nexstar 8SE, advanced VX mount, Canon 60D
Harvest Supermoon by Michael LaMonaco, Kenilworth, NJ, USA. Equipment: Canon 60Da, 75-300mm Lens
Harvest Supermoon by Michael LaMonaco, Kenilworth, NJ, USA. Equipment: Canon 60Da, 75-300mm Lens
Supermoon 8 Sept 2014 by Allan Jones, Exeter, UK. Equipment: Canon EOS1100D, EFS 55-250mm lens, f5.6, 1/30, ISO400.
Supermoon 8 Sept 2014 by Allan Jones, Exeter, UK. Equipment: Canon EOS1100D, EFS 55-250mm lens, f5.6, 1/30, ISO400.
Super moon by Martin Pyott, St Andrews, Scotland, UK. Equipment: Lunt 80mm ED F7 Refractor, Celestron AZ-3 mount, Canon 1100D.
Super moon by Martin Pyott, St Andrews, Scotland, UK. Equipment: Lunt 80mm ED F7 Refractor, Celestron AZ-3 mount, Canon 1100D.
Super Moon of the 29th August 2015 by Houssem Ksontini, Tunisia. Equipment: Skywatcher 150/750 BD, Nikon D3000.
Super Moon of the 29th August 2015 by Houssem Ksontini, Tunisia. Equipment: Skywatcher 150/750 BD, Nikon D3000.
The Supermoon by Gemma Burden, Dorset, UK. Equipment: Canon EOS 7D, Skywatcher 150PL
The Supermoon by Gemma Burden, Dorset, UK. Equipment: Canon EOS 7D, Skywatcher 150PL
Super Moon by Paul, Camberwell, London, UK. Equipment: Fujifilm Bridge Camera
Super Moon by Paul, Camberwell, London, UK. Equipment: Fujifilm Bridge Camera
Supermoon over Xàtiva by José J. Chambó, Torre Cerdá, Valencia, Spain. Equipment: Canon EOS 100D, Tamron AF70-300
Supermoon over Xàtiva by José J. Chambó, Torre Cerdá, Valencia, Spain. Equipment: Canon EOS 100D, Tamron AF70-300
November 2016 Essex Super Moon by Matthew Mallett, Walton On Naze, Essex, UK. Equipment: Nikon D800, Nikon 70-200mm F2.8.
November 2016 Essex Super Moon by Matthew Mallett, Walton On Naze, Essex, UK. Equipment: Nikon D800, Nikon 70-200mm F2.8.
Supermoon by Justin Miller, Hayling Island, Hampshire, UK. Equipment: Canon 70D, Canon lens 28-300mm 3.5-5.6L, tripod.
Supermoon by Justin Miller, Hayling Island, Hampshire, UK. Equipment: Canon 70D, Canon lens 28-300mm 3.5-5.6L, tripod.
Supermoon by Justin Miller, Hayling Island, Hampshire, UK. Equipment: Canon 70D, Canon lens 28-300mm 3.5-5.6L, tripod.
Supermoon by Justin Miller, Hayling Island, Hampshire, UK. Equipment: Canon 70D, Canon lens 28-300mm 3.5-5.6L, tripod.
'Supermoon' by Katy Waters, South Shields, UK. Equipment: Canon 600d, 300mm lens.
'Supermoon' by Katy Waters, South Shields, UK. Equipment: Canon 600d, 300mm lens.
Super Moon by Dave Dowdeswell, Dorset, UK. Equipment: Nikon D800, Sigma 500mm+1.4tc+2tc + polarizer
Super Moon by Dave Dowdeswell, Dorset, UK. Equipment: Nikon D800, Sigma 500mm+1.4tc+2tc + polarizer
Supermoon by Rajneesh Prashar, Delhi, India. Equipment: Canon 700D, 18MM & 250MM
Supermoon by Rajneesh Prashar, Delhi, India. Equipment: Canon 700D, 18MM & 250MM
Cheshire's Supermoon by Tom McDermott, Northwich, Cheshire, UK. Equipment: Celestron C6-N, Celestron CG5-GT Mount, Panasonic DMC-G1 Camera
Cheshire's Supermoon by Tom McDermott, Northwich, Cheshire, UK. Equipment: Celestron C6-N, Celestron CG5-GT Mount, Panasonic DMC-G1 Camera
Supermoon of November 2016 by Houssem Ksontini, Tunis, Tunisia. Equipment: Skywatcher 150/750, Neq3-2, Nikon D5300, Processed in AS!2 and Registax
Supermoon of November 2016 by Houssem Ksontini, Tunis, Tunisia. Equipment: Skywatcher 150/750, Neq3-2, Nikon D5300, Processed in AS!2 and Registax
Super Moon by David de Cuevas, Treize Vents, France. Equipment: Canon 700D, Astrotech 66ED, StarAdventurer.
Super Moon by David de Cuevas, Treize Vents, France. Equipment: Canon 700D, Astrotech 66ED, StarAdventurer.
Super Moon and Castle by David de Cuevas, Treize Vents, France. Equipment: Canon 700D, Tamron 17/50mm.
Super Moon and Castle by David de Cuevas, Treize Vents, France. Equipment: Canon 700D, Tamron 17/50mm.
Supermoon, engulfed by clouds in Clacton-On-Sea by Shantelle Robinson, Clacton-On-Sea, UK. Equipment: Olympus E-420
Supermoon, engulfed by clouds in Clacton-On-Sea by Shantelle Robinson, Clacton-On-Sea, UK. Equipment: Olympus E-420
SuperMoon by Nick, Centereach. Equipment: Canon Eos Rebel
SuperMoon by Nick, Centereach. Equipment: Canon Eos Rebel
SuperMoon and the Sun by Nick, New York. Equipment: Celestron mini c70, Canon eos rebel
SuperMoon and the Sun by Nick, New York. Equipment: Celestron mini c70, Canon eos rebel
Supermoon Halo by Steve Brown, Stokesley, North Yorkshire, UK. Equipment: Canon 600D, 10-18mm lens, static tripod.
Supermoon Halo by Steve Brown, Stokesley, North Yorkshire, UK. Equipment: Canon 600D, 10-18mm lens, static tripod.
Full Moon at Perigee by Tom Howard, Crawley, UK. Equipment: Nikon D7000 DSLR, TS-Optics 65mm quadruplet, Nikon x1.4 Teleconverter.
Full Moon at Perigee by Tom Howard, Crawley, UK. Equipment: Nikon D7000 DSLR, TS-Optics 65mm quadruplet, Nikon x1.4 Teleconverter.
super moon 1.1.2018 by Alex Higgs, Hessle, UK. Equipment: 20x80 Binoculars, Samsung J5 Mobile Phone
super moon 1.1.2018 by Alex Higgs, Hessle, UK. Equipment: 20x80 Binoculars, Samsung J5 Mobile Phone
Supermoon Over Central Illinois by Joshua Rhoades, Elkhart, Illinois, USA. Equipment: Canon 6D, Sigma 100-400mm lens.
Supermoon Over Central Illinois by Joshua Rhoades, Elkhart, Illinois, USA. Equipment: Canon 6D, Sigma 100-400mm lens.
supermoon 1.2.2018 by Alex, Hessle, UK. Equipment: Celestron 130EQ, Samsung J5 Mobile, Phone Mount
supermoon 1.2.2018 by Alex, Hessle, UK. Equipment: Celestron 130EQ, Samsung J5 Mobile, Phone Mount
Supermoon by Jaspsal Chadha, London, UK. Equipment: TAK FSQ85, NIKON 500 DSLR, Handheld shot
Supermoon by Jaspsal Chadha, London, UK. Equipment: TAK FSQ85, NIKON 500 DSLR, Handheld shot
Super Moon Blue Moon by Nick Talbot, Telford, UK. Equipment: Mak 150, Canon 6D.
Super Moon Blue Moon by Nick Talbot, Telford, UK. Equipment: Mak 150, Canon 6D.
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This guide originally appeared in the April 2021 issue of BBC Sky at Night Magazine.

Authors

jenny winder astronomy space
Jenny WinderScience writer

Jenny Winder is an astronomy writer and broadcaster.

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