Summer Blue Moon   
       
   July 27, 2015:  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."   
       
   The second full Moon of July is just around the corner. According to modern   
   folklore, it is a "Blue Moon."   
       
   https://youtu.be/XyBp-EYrwTA   
       
   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.   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.99   
    * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)