Howdy folks,
Prepare to be mooned--blue mooned. This Saturday, July 31, a "blue moon" rises. Of course, you probably won't see even a hint of blue color, just your average full moon. So what gives? What's so blue about a moon that isn't? Heck, astronomers even say that blue moons aren't rare. And folklorists say they're not what everyone thinks they are.
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Today's Knowledge Blue Moons That Are (and Blue Moons That Aren't)
As any budding astronomer can tell you, a "blue moon" is the second full moon in a calendar month. It isn't really blue. It's just a name. But why, of all things, do we call a white moon "blue"?
The First Recorded Blue Moon
Actually, there are truly blue moons. In 1883, an Indonesian volcano erupted with the force of a 100-megaton nuclear bomb. With a roar heard nearly 400 miles (over 600 kilometers) away, Mount Krakatoa belched a column of volcanic gas and ash into the atmosphere. That night, the moon rose blue over Indonesia.
Particles from Krakatoa's ash, not more than a micron wide, made it happen. These micron-wide particles were exactly the right size to scatter red light, while allowing other colors, such as blue, to pass. The result: a blue sphere hanging in the sky. Different sized particles filtered other colors and caused different effects.
In fact, for several years after the eruption, there were reports from all over the globe of red moons, green moons, and, yes, even more blue moons. What's more, some of the sunsets following the eruption were such a blazing red that people actually called on firemen to drown the optical illusion. The eruptions of Mount St. Helens and Mount Pinatubo produced the same anomaly. So have forest fires.
Changing Hue
Still, none of this explains why we call the second full moon in a month a blue moon. The phrase is old, even if the modern meaning is not. Before Shakespeare penned a word of Hamlet, the English knew little couplets like:
If they say the moon is blewe, We must believe that it is true. Eventually, a 19th-century almanac put this metaphorical moon in the sky. According to the Maine Farmers' Almanac, a blue moon occurred whenever a season had four full moons instead of three. It was common to give moons seasonal names during this time, so you had harvest moons, fruit moons, and egg moons, too.
Follow That Moon
It didn't stop there. In 1946, Sky & Telescope magazine published an article that misinterpreted the Maine almanac's seasonal definition, making the blue moon the second full moon in a month instead of the fourth full moon in a season. The magazine soon adopted this new meaning. (They 'fessed up to their mistake in 1999.)
It took the modern media machine, however, to put blue moons on the tip of everyone's tongue. Starting in 1986, the Genus II edition of Trivial Pursuit told a whole generation of trivia buffs that blue moons were the second full moon in a calendar month. Their source? A 1985 children's book, Facts and Records. Its source? No one knows.
In 1999, the blue moon's fate was sealed through extraordinary lunar happenings. There were two full moons in January and March and none at all in February. The media had a field day, talking over and over about the "blue moons" of January and March. It was blue moon mania. The new definition stuck. Sure, the blue moon was no longer blue, and a long way from its Indonesian home. But transformations like that can occur--once in a blue moon.
Will Bilbrey July 29, 2004
Want to learn more? Look at a moon map http://nssdc.gsfc.nasa.gov/planetary/ lunar/moon_landing_map.jpg
