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Summer Blue Moon -
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Presented by Science@NASA
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When someone says
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'Once in a Blue Moon,'
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you know what they mean:
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Rare, seldom, even absurd.
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This year it means 'the end of July.'
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For the second time this month,
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the Moon is about to become full.
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There was one full Moon on July 2nd,
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and now a second is coming on July 31st.
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According to modern folklore,
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whenever there are two full Moons in a calendar month,
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the second one is 'blue.'
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This definition of a Blue Moon is a recent thing.
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If you told a person in Shakespeare's day
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that something happens 'once in a Blue Moon'
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they would attach no astronomical meaning to the statement.
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Blue moon simply meant rare or absurd,
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like making a date for 'the Twelfth of Never.'
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Since then, however,
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its meaning has shifted.
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The modern definition sprang up in the 1940s.
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In those days
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the Maine Farmer's Almanac
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offered a definition of Blue Moon so convoluted
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many astronomers struggled to understand it.
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It involved factors such as
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ecclesiastical dates of Easter and Lent,
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tropical years,
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and the timing of seasons
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according to the dynamical mean sun.
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Aiming to explain blue moons to the layman,
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Sky & Telescope published an article in 1946 entitled
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'Once in a Blue Moon.'
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The author James Hugh Pruett (1886-1955)
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cited the 1937 Maine almanac
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and opined that the 'second [full moon] in a month,
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so I interpret it,
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is called Blue Moon.'
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This was not correct,
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but at least it could be understood.
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And thus the modern Blue Moon was born.
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Most Blue Moons look pale gray and white,
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just like the Moon you've seen on any other night.
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Squeezing a second full Moon into a calendar month
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doesn't change its color.
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Nevertheless,
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on rare occasions the Moon can turn blue.
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A truly-blue Moon
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usually requires a volcanic eruption.
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Back in 1883, for example,
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people saw blue moons almost every night
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after the Indonesian volcano Krakatoa
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exploded with the force of a 100-megaton nuclear bomb.
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Plumes of ash rose to the very top of Earth's atmosphere,
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and the Moon ... it turned blue!
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Krakatoa's ash was the reason.
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Some of the plumes were filled with particles 1 micron wide,
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about the same as the wavelength of red light.
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Particles of this special size strongly scatter red light,
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while allowing blue light to pass through.
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Krakatoa's clouds thus acted like a blue filter.
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People also saw blue-colored Moons in 1983
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after the eruption of the El Chichon volcano in Mexico.
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And there are reports of blue Moons
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caused by Mt. St. Helens in 1980
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and Mount Pinatubo in 1991.
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Forest fires can do the same trick.
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A famous example is the giant muskeg fire
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of Sept. 1953 in Alberta, Canada.
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Clouds of smoke containing micron-sized oil droplets
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produced lavender suns and blue Moons
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all the way from North America to England.
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At this time of year,
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summer wildfires often produce smoke
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with an abundance of micron-sized particles-
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just the right size to turn the Moon truly blue.
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On the other hand,
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maybe it will turn red.
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Often, when the Moon is hanging low,
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it looks red for the same reason that sunsets are red.
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The atmosphere is full of aerosols
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much smaller than the ones injected by volcanoes.
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These aerosols scatter blue light,
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while leaving the red behind.
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For this reason,
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red Blue Moons are far more common than blue Blue Moons.
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Sounds absurd?
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Yes, but that's what a Blue Moon is all about.
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Step outside at sunset on July 31st,
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look east, and see what color presents itself.
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For more rare and colorful occurrences,
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on Earth and beyond,
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stay tuned to science.nasa.gov