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What Is a Supermoon?
Mike Boening, an urban photographer and photography instructor in Detroit, Michigan took this photo on Dec. 13, 2016.
Credit: Mike Boening
A supermoon happens when the full moon coincides with the moon's closest approach to Earth in its orbit. Supermoons make the moon appear a little brighter and closer than normal, although the difference is hard to spot with the naked eye. The last supermoon (the only one of 2017) happened on Dec. 3.
It kicked off a trilogy of consecutive supermoons; the next ones happen on Jan. 1 and Jan. 31. The Jan. 31 supermoon will also coincide with a lunar eclipse best visible in the western United States, the Pacific and eastern Asia.
The term "supermoon" has only been used in the past 40 years, but it received a slew of attention in late 2016 when three supermoons occurred in a row. The supermoon of November 2016 was also the closest supermoon in 69 years, although a closer supermoon will rise in the 2030s. [Supermoon Secrets: 7 Surprising Big Moon Facts]
How a supermoon happens
The moon's orbit around the Earth is not a perfect circle. It has an average distance of 238,000 miles (382,900 km) from Earth, but its apogee and perigee — the closest and farthest approaches from Earth — change every lunar month.
"The main reason why the orbit of the moon is not a perfect circle is that there are a lot of tidal, or gravitational, forces that are pulling on the moon," said NASA's Noah Petro, deputy scientist of the Lunar Reconnaissance Orbiter mission, in a 2016 Space.com interview.
He added that the different gravities of the Earth, sun and planets all have an effect on the moon's orbit. "You have all of these different gravitational forces pulling and pushing on the moon, which gives us opportunities to have these close passes."
The moon, or supermoon, is seen as it sets over the Martin Luther King Jr. Memorial on Monday (Nov. 14) in Washington.
Credit: NASA/Aubrey Gemignani
A supermoon needs two key ingredients to occur. The moon needs to be at its closest approach, or perigee, to the Earth in its 27-day orbit. The moon also needs to be at the full phase, which happens every 29.5 days when the sun fully illuminates the moon. Supermoons only happen a few times a year (at most) because the moon's orbit changes orientation while the Earth orbits the sun — that's why you don't see a supermoon every month.
The moon will appear as much as 30 percent brighter and 14 percent larger than usual, but it's very hard to spot the difference with the naked eye. "That's not enough to notice unless you're a very careful moon-watcher," Sky & Telescope magazine senior editor Alan MacRobert said in a 2016 statement.
The supermoon may look especially large to you, however, if it's very close to the horizon. But that has nothing to do with astronomy and everything to do with how the human brain works. This effect is called the "moon illusion" and may arise from at least a couple of different things. Scientists suggest that perhaps the brain is comparing the moon to nearby buildings or objects, or perhaps our brain is just wired to process things on the horizon as bigger than things in the sky.
Astrological origins
The term "supermoon" didn't originate in astronomy, but in astrology — a pseudoscientific tradition that studies the movements of celestial objects to make predictions about human behavior and events. The term was first mentioned in a 1979 article for Dell Horoscope magazine by Richard Nolle, according to Astronomy.com. Nolle defined a supermoon as "a new or full moon which occurs with the moon at or near (within 90 percent of) its closest approach to Earth in a given orbit", without explaining where he obtained the 90 percent figure.
But it wasn't until the last few years that the term received more attention. A search on Google Trends reveals that starting from 2004, the word "supermoon" was not used often until at least 2011. Interest in the supermoon hit a high in November 2016, when Earth experienced the largest supermoon in 69 years. Further, the term appears to be more popular in certain areas of the world — principally Southeast Asia and North America — with some lesser interest in places such as Europe or India.
Recent astronomical terms such as "supermoon" or "black moon" (the second new moon in a month) could create a perception of "false events" among the public, Cincinatti Observatory outreach astronomer Dean Regas told Space.com in 2016. But Regas, who also co-hosts the PBS program "Star Gazers," said the "supermoon" term is a great public outreach term for astronomy that could have other benefits beyond the event itself.
"It's a great way to get the public interested," he said of the supermoon. "It's something that they can relate to and they can go out and actually see."
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Notable supermoons
On Sept. 27, 2015, the supermoon coincided with a lunar eclipse. Lunar eclipses occur when the Earth is exactly between the sun and the moon. The moon shines brown-red because the only light it receives is reflected from Earth. The previous supermoon eclipse happened in 1982, and the next one is in 2033.
The end of 2016 saw three supermoons in a row in October, November and December. But it was the Nov. 14 one that got the most attention because it was the closest supermoon in recent memory. The moon's perigee was 221,524 miles (356,508 kilometers) from Earth, making it the closest full moon to Earth in 69 years — specifically, since the supermoon of Jan. 26, 1948.
)An even closer full moon took place in January 1912; it was roughly 100 kilometers nearer to Earth than in November 2016. But skywatchers who are around in November 2034 will get a special treat, as that moon will be even closer than both the 1912 and 2016 moons.)
Supermoon
A supermoon is a full moon or a new moon that approximately coincides with the closest distance that the Moon reaches to Earth in its elliptic orbit, resulting in a slightly larger-than-usual apparent size of the lunar disk as seen from Earth.[1]The technical name is the perigee syzygy of the Earth–Moon–Sun system or more simply full (or new) Moon at perigee.[a] The term supermoon is astrological in origin, and has no precise astronomical definition.[2]
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.[3]
The opposite phenomenon, an apogee syzygy, has been called a micromoon,[4] though this term is not as widespread as supermoon. Also, some do not consider new Moons near perigee as supermoons.
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[hide]Occurrence[edit]
Of the possible 12-13 full or new Moons each year, 3-4 usually may be classed as a supermoon as commonly defined.
The most recent supermoon occurrence was on January 2, 2018, and the next one will be on January 31, 2018.[5] The one on November 14, 2016 was the closest supermoon since January 26, 1948, and will not be surpassed until November 25, 2034.[6] The closest supermoon of the century will occur on December 6, 2052.[7]
The oscillating nature of the distance of the full and new moons is due to the difference between the synodic and anomalistic months.[5] The beat period between the two is roughly 14 synodic months.
Occasionally, a supermoon coincides with a total lunar eclipse. The most recent occurrence of this was in September 2015, while the next one will be in January 2018.[8]
Appearance[edit]
A full moon at perigee appears roughly 14% larger in diameter than at apogee.[9] Many observers insist that the moon looks bigger to them. This is likely due to observations shortly after sunset when Moon is near the horizon and the moon illusion is at its most apparent.[10]
While the moon's surface luminance remains the same, because it is closer to the earth the illuminance is about 30% brighter than at its farthest point, or apogee. This is due to the inverse square law of light which changes the amount of light received on earth in inverse proportion to the distance from the moon.[11] While a typical summer full moon at temperate latitudes provides only about 0.05-0.1 lux, a supermoon directly overhead in the tropics could provide up to 0.36 lux.[12]
Definitions[edit]
The name supermoon was coined by astrologer Richard Nolle in 1979, in Dell Horoscope magazine arbitrarily defined as:
Nolle told Atlantic magazine that he created the term because “‘perigee syzygy’ is something that the average sixth-grader can’t handle, and that’s the average reading capability of the American adult.”[14] Though literacy tests in the United States consistently place the mean level at 9th grade or higher.[15]
The came up with the name while rading “Strategic Role Of Perigean Spring Tides in Nautical History and Coastal Flooding” published in 1976 by NOAA Hydrologist Fergus Wood.[16][17] Nolle never outlined why he chose 90%,[2] but explained in 2011 that he based calculations on 90% of the difference lunar apsisextremes 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 the apogee, and is the lunar distance at the perigee, and is the lunar distance at the apogee. [18][19]
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.[20][21]
The term perigee-syzygy or perigee full/new moon is preferred in the scientific community.[22] 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.[5] Wood used the definition of a full or new moon occurring within 24 hours of perigee and also used the label perigee-syzygy.[17]
Wood also coined the less used term proxigee where perigee and the full or new moon are separated by 10 hours or less.[17]
Effects on Earth[edit]
Nolle also claimed that the moon causes "geophysical stress" during the time of a supermoon. There has also 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.[23][24] A large, 7.5 magnitude earthquake centred 15 km north-east of Culverden, New Zealand at 00:03 NZDT on November 14, 2016, also coincided with a supermoon.[25][26] These claims have been refuted by scientists.[27][28][29][30]
Scientists have confirmed that the combined effect of the Sun and Moon on the Earth's oceans, the tide,[31] is greatest when the Moon is either new or full.[32] and that during lunar perigee, the tidal force is somewhat stronger,[33] resulting in perigean spring tides. However, even at its most powerful, this force is still relatively weak,[34] causing tidal differences of inches at most.[35]
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