Most plainly, a blue moon means seldom. Practically never. It's shorthand for an event that happens so infrequently you might as well wait for the big white pumpkin in the sky to change color.
But the meaning and roots of the phrase are tangled up in error and dispute. Pick your own explanation and raise a glass to Earth's lonely satellite.
The most common explanation, accepted by both NASA and the American Heritage Dictionary, defines a blue moon as two full moons in a single month -- a phenomenon that occurs every 2.5 years on average. This month, the big round orb appears on Dec. 2 and Dec. 31, making this the first blue moon on New Year's Eve since 1990.
Trouble is, this definition stems from a mistaken report in Sky & Telescope magazine, printed in 1946. In that article, the writer attributed the two-full-moon story to the Maine Farmers Almanac. More than 50 years later, the magazine owned up to the error and corrected it, but the story endures.
Here's what that farmers almanac meant to say: Normally, you get 12 full moons a year. A blue moon means a 13th full moon -- or the third full moon in any particular season that includes four. Naming the third moon blue, rather than the fourth, would help keep the farmers from getting their seasons out of whack. September, for example, brings what is sometimes called the corn moon.
This echoed the ancient practice of using the appearance of full moons to set the date of important religious observances, such as Easter. Designating the additional full moon as "blue," kept the ecclesiastical calendar on track.
Confused? Don't fret. For a physics professor, Thursday night's blue moon doesn't even count.
A real blue moon means the moon actually turns blue, and there is such a phenomenon, said Michael Paesler, head of the physics department at North Carolina State University.
For this to happen, he explained, the air must contain particles that are larger than usual; the particles scatter red light and give the moon a bluish tint. But they must be between 0.5 and 0.7 microns in size, or about a millionth of a meter -- an incredible rarity.
So a true blue moon doesn't come around every other year. It happened when Krakatoa erupted in 1883 and killed 36,000 people. As for this New Year's Eve moon, Paesler said with a touch of disdain, "This is a numerological blue moon."
found here
Author