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| The Blue Moon in the Dubai sky tonight |
Tonight is a “Blue Moon” night and you can use the expression "Once in a Blue Moon" à gogo today.
The saying is used for a rare,
absurd or impossible event, for when was the last time you saw the moon turn
blue?
According to
modern folklore, a Blue Moon is the second full moon in a calendar month.
Usually months have only one full moon, but occasionally a second one sneaks
in. Full moons are separated by 29 days, while most months are 30 or 31 days
long; so it is possible to fit two full moons in a single month. This happens
every two and a half years, on average.
The Farmers' Almanac defined “Blue Moon” as an extra full moon that occurred in
a season; one season was normally three full moons. If a season had four full
moons, then the third full moon was named a “blue moon.”
The idea of a Blue Moon as the second full moon in a month
stemmed from the March 1946 issue of Sky and Telescope
magazine, which
contained an article called “Once in a Blue Moon” by James Hugh Pruett. Pruett
was using a 1937 Maine Farmer’s Almanac, but he simplified the definition and wrote:
But the moon can turn blue, even
green…
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| A 1888 lithograph of Krakatoa's eruption (Wikipedia) |
In 1883, Indonesian volcano Krakatoa
exploded. Scientists liken the blast to a 100-megaton nuclear bomb. Fully 600
km away, people heard the noise as loud as a cannon shot. Plumes of ash rose to
the very top of Earth's atmosphere. And the moon turned blue.
Krakatoa's ash is the reason. Some of the ash-clouds were
filled with particles about 1 micron (one millionth of a meter) wide -- the
right size to strongly scatter red light, while allowing other colors to pass.
White moonbeams shining through the clouds emerged blue, and sometimes green. Blue moons persisted
for years after the eruption.
Other less potent volcanoes have turned the moon blue, too.
People saw blue 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.
So the key to a blue moon is having lots of particles
slightly wider than the wavelength of red light (0.7 micrometer) -- and no
other sizes present, which is rare. But volcanoes sometimes produce such
clouds, as do forest fires. Ash and dust clouds thrown into the atmosphere by
fires and storms usually contain a mixture of particles with a wide range of
sizes, mostly smaller than one micrometer, and they tend to scatter blue light.
This kind of cloud makes the moon turn red; thus red moons are far more common
than blue moons.
Although it will most probably not be blue tonight, we can
still enjoy our second full moon this month, maybe while listening to Frank
Sinatra…
