The New Year's Eve blue moon. Photo: Sayyid Azim/AP.

If you looked to the night sky on New Year’s Eve, you may have wondered if the new decade was being ushered in with a full moon. Indeed, your eyes did not deceive you—and not only was the moon full, but it was blue, a feat worthy of the oft-used phrase "once in a blue moon."

For much of the world, the final night of 2009 featured a “blue moon,” the second full moon of a calendar month. Interestingly, this definition of “blue moon” most likely came about as the result of an editorial mishap, when in the 1940s, Sky & Telescope magazine mistakenly gave an incorrect explanation in an attempt to clarify exactly what “blue moon” means. Though the magazine issued a correction, the definition stuck and became part of popular culture. (This may have been partly due to the fact that the correction came a mere 50 years later.)

“Blue moon” has held other meanings, including:

• An actual blue-colored moon. The phrase is thought to have been coined following the 1883 volcanic eruption of Krakatoa; so much dust was thrown up into the atmosphere, that the moon took on a blue tinge. NASA tells us this is where the saying “once in a blue moon” comes from.
• The thirteenth full moon of a year. Each season normally has three full moons (with names like Wolf Moon [early winter], Harvest Moon [late summer], and Hunter’s Moon [early fall]). For seasons with four full moons, the third would be called Blue Moon.

A blue moon (the “second full moon of a month” version) comes around relatively frequently—about every two and a half years. But a New Year’s Eve blue moon may qualify for the saying “once in a blue moon”; the last one came in 1990, and we won’t see another until 2028.

Earthjustice wishes you a very happy new year!

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