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The Amerindians of Algonquin called the full moon of May, the moon of flowers. Saturday's full moon can be called the blue-flowered moon.
But Saturday is May 18th. If a blue moon is the second full moon of a calendar month and it takes about a month (that's where the word comes from) for the moon to go through its phases, if a blue moon was not to occur only on the 30th or the 31st?
There is a second less used definition of "blue moon" – the third of four full moons of a season.
"The phrase" Blue Moon "has been around for over 400 years and has changed meaning," said Philip Hiscock, a folklorist at Memorial University of Newfoundland. "I counted six different meanings."
The sentence was originally used to describe something absurd. This then changed to mean "never".
The seasonal definition was first seen in the 1943 edition of the Maine Farmers Almanac. A 1946 article in Sky and Telescope magazine provided the monthly definition. The sentence broke down for 30 years.
Hiscock's research found a mention in the radio program "Star Date" of the 1970s using the monthly definition. The Trivial Pursuit board game also featured the monthly definition in one question. When a press service wrote on the full moons of May 1 and 31, 1988, this monthly definition was picked up by newspapers, radio stations and newsrooms.
Blue moons, whatever their definition, are certainly not common, but they are not uncommon. The definition based on the month appears approximately every 2.41 years on average, while the seasonal definition appears a little less often, about every 2.68 years on average. According to astrophysicist Neil deGrasse Tyson, blue moons are "more common than presidential elections, but no one calls them rare."
The name has very little to do with color. Expect the same white appearance as it rises higher in the sky and a more yellowish color at sunrise and sunset. All this extra atmosphere that light has to go through increases the effect of Rayleigh scattering, which diffuses blue light, leaving frequencies closer to yellow and orange.
Just as the atmosphere can make the moon rise (or set) more yellow, the moon may become bluer, but the conditions are rare. Ice crystals, sand or even water droplets in the atmosphere can disperse more red and green light leaving blue, but these conditions are rare. These particles should have a similar size, about one micron in diameter. By comparison, the most commonly observed water droplets in clouds are about five times larger.
Tony Rice is a volunteer with the NASA / JPL Solar System Ambassador Program and Software Engineer at Cisco Systems. You can follow him on Twitter @rtphokie.
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