CAPE CANAVERAL, Florida -- On August 31st, 2012, there is going to be a Blue Moon. The phrase "Once in a Blue Moon" is commonly known to mean something rare and offbeat because of the rare occurrence of a Blue Moon.
A Blue Moon is the second full moon in a calendar month. 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. NASA says this happens every two and a half years, on average.
But will it really be blue? Probably not. The date of a full moon, all by itself, doesn't affect the moon's color. The moon on August 31st, 2012 will be pearly-gray, as usual.
People saw blue moons almost every night when the Krakatoa volcano exploded in 1883 with the force of a 100-megaton nuclear bomb. Plumes of ash rose to the very top of Earth's atmosphere. Some of the ash-clouds were filled with particles about 1 micron 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.
NASA says the key to a blue moon is having in the air lots of particles slightly wider than the wavelength of red light (0.7 micron)--and no other sizes present. This is rare, but volcanoes sometimes spit out such clouds, as do forest fires.
In the western U.S., there will be wildfires burning on August 31st. If any of those fires produce ash or oily-smoke containing lots of 1-micron particles, the Blue Moon there could be blue. More likely, it'll be red. Ash and dust clouds thrown into the atmosphere by fires and storms usually contain a mixture of particles with a wide range of sizes. Most are smaller than 1 micron, and they tend to scatter blue light. This kind of cloud makes the Moon turn red. NASA says that red Blue Moons are far more common than blue Blue Moons.
Image Credit: NASA (blue enhanced)