Friday, August 31, 2012

It’s “Once in a Blue Moon” tonight

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:

Seven times in 19 years there were -- and still are -- 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon.

But the moon can turn blue, even green…

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…