No, the moon won't actually be blue -- and no, it's not the second full moon in August.
Tuesday's full moon will be the the first "seasonal blue moon," meaning it's the third of four full moons in a season, in nearly three years.
The official definition comes from the August 1937 page in the Maine Farmers' almanac, explaining a Blue Moon is when "one of the four seasons would contain four full moons instead of the usual three."
"There are seven Blue Moons in a Lunar Cycle of nineteen years," continued the Almanac. "In olden times the almanac makers had much difficulty calculating the occurrence of the Blue Moon and this uncertainty gave rise to the expression 'Once in a Blue Moon.'"
And if you thought a Blue Moon meant the second full moon in a month, you're not alone: The common misconception was started when James Hugh Pruett defined, wrongly, a Blue Moon in an article in the March 1946 issue Sky & Telescope magazine:
"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."
Pruett's misinterpretation was cemented when Deborah Byrd used it in an episode of NPR's StarDate on Jan. 31, 1980, and has since been considered an alternate definition.
And yes, a moon can sometimes actually appear blue, usually when a volcanic eruption or a bad fire temporarily changes the composition of the atmosphere.
In 1883, the Indonesian volcano Krakatoa exploded, sending ash high into the air. The particles in the ash cloud were just the right size -- about 1 micron in diameter -- to scatter red light, allowing white moonlight to shine through clouds to appear blue, or sometimes green, for years worldwide after the eruption.
It happened again after the eruption of Mount St. Helens in Washington State in 1980 and Mount Pinatubo in the Philippines in 1991.
And in 1950, fires in Alberta, Canada went from smoldering to a major conflagration, putting large amount of smoke into the air. Oily droplets were just about the right size to scatter red and yellow light and make the sun appear blue or lavender.
After this month's blue moon, the next won't come until May 21, 2016.
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