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  • Summer Blue Moon -

  • Presented by Science@NASA

  • When someone says

  • 'Once in a Blue Moon,'

  • you know what they mean:

  • Rare, seldom, even absurd.

  • This year it means 'the end of July.'

  • For the second time this month,

  • the Moon is about to become full.

  • There was one full Moon on July 2nd,

  • and now a second is coming on July 31st.

  • According to modern folklore,

  • whenever there are two full Moons in a calendar month,

  • the second one is 'blue.'

  • This definition of a Blue Moon is a recent thing.

  • If you told a person in Shakespeare's day

  • that something happens 'once in a Blue Moon'

  • they would attach no astronomical meaning to the statement.

  • Blue moon simply meant rare or absurd,

  • like making a date for 'the Twelfth of Never.'

  • Since then, however,

  • its meaning has shifted.

  • The modern definition sprang up in the 1940s.

  • In those days

  • the Maine Farmer's Almanac

  • offered a definition of Blue Moon so convoluted

  • many astronomers struggled to understand it.

  • It involved factors such as

  • ecclesiastical dates of Easter and Lent,

  • tropical years,

  • and the timing of seasons

  • according to the dynamical mean sun.

  • Aiming to explain blue moons to the layman,

  • Sky & Telescope published an article in 1946 entitled

  • 'Once in a Blue Moon.'

  • The author James Hugh Pruett (1886-1955)

  • cited the 1937 Maine almanac

  • and opined that the 'second [full moon] in a month,

  • so I interpret it,

  • is called Blue Moon.'

  • This was not correct,

  • but at least it could be understood.

  • And thus the modern Blue Moon was born.

  • Most Blue Moons look pale gray and white,

  • just like the Moon you've seen on any other night.

  • Squeezing a second full Moon into a calendar month

  • doesn't change its color.

  • Nevertheless,

  • on rare occasions the Moon can turn blue.

  • A truly-blue Moon

  • usually requires a volcanic eruption.

  • Back in 1883, for example,

  • people saw blue moons almost every night

  • after the Indonesian volcano Krakatoa

  • exploded with the force of a 100-megaton nuclear bomb.

  • Plumes of ash rose to the very top of Earth's atmosphere,

  • and the Moon ... it turned blue!

  • Krakatoa's ash was the reason.

  • Some of the plumes were filled with particles 1 micron wide,

  • about the same as the wavelength of red light.

  • Particles of this special size strongly scatter red light,

  • while allowing blue light to pass through.

  • Krakatoa's clouds thus acted like a blue filter.

  • People also saw blue-colored Moons in 1983

  • after the eruption of the El Chichon volcano in Mexico.

  • And there are reports of blue Moons

  • caused by Mt. St. Helens in 1980

  • and Mount Pinatubo in 1991.

  • Forest fires can do the same trick.

  • A famous example is the giant muskeg fire

  • of Sept. 1953 in Alberta, Canada.

  • Clouds of smoke containing micron-sized oil droplets

  • produced lavender suns and blue Moons

  • all the way from North America to England.

  • At this time of year,

  • summer wildfires often produce smoke

  • with an abundance of micron-sized particles-

  • just the right size to turn the Moon truly blue.

  • On the other hand,

  • maybe it will turn red.

  • Often, when the Moon is hanging low,

  • it looks red for the same reason that sunsets are red.

  • The atmosphere is full of aerosols

  • much smaller than the ones injected by volcanoes.

  • These aerosols scatter blue light,

  • while leaving the red behind.

  • For this reason,

  • red Blue Moons are far more common than blue Blue Moons.

  • Sounds absurd?

  • Yes, but that's what a Blue Moon is all about.

  • Step outside at sunset on July 31st,

  • look east, and see what color presents itself.

  • For more rare and colorful occurrences,

  • on Earth and beyond,

  • stay tuned to science.nasa.gov

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サイエンスキャスト夏のブルームーン (ScienceCasts: Summer Blue Moon)

  • 104 9
    Jeng-Lan Lee に公開 2021 年 01 月 14 日
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