A supermoon is the coincidence of a full moon or a new moon with the closest approach the Moon makes to the Earth on its elliptical orbit, resulting in the largest apparent size of the lunar disk as seen from Earth. The technical name is the perigee-syzygy of the Earth-Moon-Sun system. The term "supermoon" is not astronomical, but originated in modern astrology. The 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 such as earthquakes and volcanic eruptions, but the evidence of such a link is widely held to be unconvincing.
The opposite phenomenon, an apogee-syzygy, has been called a micromoon, though this term is not as widespread as supermoon.
The Moon's distance varies each month between approximately 357,000 kilometers (222,000 mi) and 406,000 km (252,000 mi) due to its elliptical orbit around the Earth (distances given are centre-to-centre).
A full moon at perigee is visually larger up to 14% in diameter (or about 30% in area) and shines 30% more light than one at its farthest point, or apogee.
The name SuperMoon was coined by astrologer Richard Nolle in 1979, 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.
Nolle also claimed that the moon causes "geophysical stress" during the time of a supermoon. Nolle never outlined why the 90% was chosen.
The term supermoon is not used within the astronomical community, which uses the term perigee-syzygy or perigee full/new moon. 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. Hence, a supermoon can be regarded as a combination of the two, although they do not perfectly coincide each time.
The full moon cycle is the period between alignments of the lunar perigee with the sun and the earth, which is about 13.9443 synodic months (about 411.8 days). Thus approximately every 14th full moon will be a supermoon. However, halfway through the cycle the full moon will be close to apogee, and the new moons immediately before and after can be supermoons. Thus there may be as many as three supermoons per full moon cycle.
Since 13.9443 differs from 14 by very close to 1⁄18, the supermoons themselves will vary with a period of about 18 full moon cycles (about 251 synodic months or 20.3 years). Thus for about a decade the largest supermoons will be full, and for the next decade the largest supermoons will be new.
Effect on tides
The combined effect of the Sun and Moon on the Earth's oceans, the tide, is greatest when the Moon is either new or full. At lunar perigee the tidal force is somewhat stronger, resulting in perigean spring tides. But even at its most powerful this force is still relatively weak causing tidal differences of inches at most.
As the tidal force follows an inverse-cube law, that force is 19% greater than average. However, because the actual amplitude of tides varies around the world, this may not translate into a direct effect.
There has been media speculation that natural disasters, such as the 2011 Tōhoku earthquake and tsunami and the 2004 Indian Ocean earthquake and tsunami, are causally linked with the 1–2 week period surrounding a supermoon. No evidence has been found of any correlation between supermoons with major earthquakes.
Supermoon of June 23, 2013 at
Umaid Bhawan Palace, India
The geometry of the Moon's
orbit in motion during 2014
The lunar eclipse supermoon of September 28, 2015 as seen over München, Germany
- Staff (September 7, 2014). "Revisiting the Moon". New York Times. Retrieved September 8, 2014.
- Phil Plait. "Kryptonite for the supermoon". Bad Astronomy. Discover. Retrieved August 29, 2015.
- Rachel Rice. "No Link Between 'Super Moon and Earthquakes". Discovery News. Retrieved March 20, 2015.
- What is a micromoon? Timeanddate.com, accessed 1 Oct 2015.
- "'Supermoon' coincides with lunar eclipse". BBC News. 28 September 2015. Retrieved 29 September 2015.
- Meeus, Jean (1997). Mathematical Astronomy Morsels. Richmond, Virginia: Willmann-Bell. p. 15. ISBN 0-943396-51-4.
- Plait, Phil (March 11, 2011). "No, the 'supermoon' didn't cause the Japanese earthquake". Discover Magazine. Retrieved 14 March 2011.
- Hawley, John. "Appearance of the Moon Size". Ask a Scientist (No publication date). Newton. Retrieved 14 March 2011.
- Phillips, Tony, Dr. (March 16, 2011). "Super Full Moon". Science@NASA Headline News. NASA. Archived from the original on May 7, 2012. Retrieved 22 June 2013.
- Nolle, Richard. "Supermoon". Astropro (No publication date; modified March 10, 2011). Retrieved 14 March 2011.
- Phillips, Tony (May 2, 2012). "Perigee "Super Moon" On May 5-6". NASA Science News. NASA. Retrieved 6 May 2012.
- Plait, Phil (2008). "Tides, the Earth, the Moon, and why our days are getting longer". Bad Astronomy (Modified March 5, 2011). Retrieved 14 March 2011.
- Sumich, J.L. (1996). "Animation of spring and neap tides". NOAA's National Ocean Service. Retrieved June 22, 2013.
- "Apogee and Perigee of the Moon". Moon Connection (No publication date). Retrieved 14 March 2011.
- Rice, Tony (4 May 2012). "Super moon looms Saturday". WRAL-TV. Retrieved 5 May 2012.
- Andy Bloxham (March 21, 2011). "Supermoon blamed for stranding five ships in Solent". Telegraph.co.uk. Retrieved June 22, 2013.
- Paquette, Mark (March 1, 2011). "Extreme Super (Full) Moon to Cause Chaos?". Astronomy Weather Blog. AccuWeather. Retrieved 14 March 2011.
- "Is the Japanese earthquake the latest natural disaster to have been caused by a supermoon?". The Daily Mail. March 11, 2011. Retrieved 14 March 2011.
- "Can the position of the Moon affect seismicity?". The Berkeley Seismological Laboratory. 1999. Retrieved 14 March 2011.
- Fuis, Gary. "Can the position of the moon or the planets affect seismicity?" (No publication date). U.S. Geological Survey: Earthquake Hazards Program. Retrieved 14 March 2011.
- Wolchover, Natalie (March 9, 2011). "Will the March 19 "SuperMoon" Trigger Natural Disasters?". Life's Little Mysteries. Retrieved 15 March 2011.
|Look up supermoon in Wiktionary, the free dictionary.|
|Wikimedia Commons has media related to Supermoon.|
- supermoon dates 
- Photo comparing full moon at 222,000 miles with one at 248,000 miles
- Richard Nolle's definition
- About the second brightest Supermoon to grace the night skies, Blabberpost.
- CBC News, Gallery of the March 19, 2011 supermoon, viewed from across Canada (Dead link as of 2014.08.11)
- BBC News, Gallery of shared pictures
- Amazing 21 photos of the Rare Super Moon from New York City, NY, USA