On Friday 31st of August 2012 there will be a Full Blue Moon.
So you may be wondering why that particular Full Moon is called a Blue Moon.
According to modern folklore: The Blue Moon is the 2nd Full Moon in a calendar month. Usually a month has only one Full Moon. Full Moons are separated by 29 days while most months have 30 or 31 days so it's possible to have 2 Full Moons in one month. The only month which can't have 2 Full Moons is February.
According to seasonal folklore: Each season contains 3 months and each month has a Full Moon. There are 3 Full Moons for Winter, 3 Full Moons for Spring, 3 Full Moons for Summer, and 3 Full Moons for Fall. All these seasonal moons add up to 12. But occasionally one season has 4 Full Moons instead of 3. The 4th Seasonal Full Moon is the Blue Moon.
Astronomical there's nothing really special about a Full Blue Moon. The Full Blue Moons happen on average once every 2.7 years.
There have been instances in the historical past that the Full Moon was really colored blue.
On 26 August 1883 an Indonesian Volcano name Krakatoa exploded violently. The volcanic explosion threw tons of ash high up into the upper atmosphere. The ash clouds were moved around the world by the Jet Stream. From late 1883 through 1885 folks around the world did see a Blue Colored Full Moon.
Several other volcanoes have caused Blue Color Full Moons: Mount St Helens in Washington State in 1980, Mount El Chichon in Mexico in 1983, and Mount Pinatubo in the Philippines in 1991. The Blue Colored Full Moons were usually seen locally after the eruption of the volcanoes. None of the reported Blue Colored Full Moons were as wide spread worldwide as the ones reported after Krakatoa
Occasionally very large forest fires can produce enough smoke and ash to turn the Full Moon blue color. On 23 September 1950 Alberta, Canada had several very large forest fires. These large fires produced massive amount of smoke and ash that went up into the upper atmosphere. Two days later Ontario Province, Canada and much of the East Coast of the USA reported a Blue Colored Full Moon.
Written by Gary T. Nowak
Vermont Astronomical Society