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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