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Last updated on April 18, 2014 at 1:21 EDT

“˜Once In A Blue Moon’ This New Year’s Eve

December 29, 2009

The blue moon has long been a subject of poetry and one of those phrases that is used without quite understanding the meaning, but its presence is a scientific marker for a rotating earth that runs by its own clock, ignoring man’s calendar.

This phenomenon will happen on Thursday, as the second full moon in a calendar month appears in the night sky, an occurrence called blue moon, reported MercuryNews.com

The last time it happened was in 2007, when sky gazers saw one full moon on June 1 and another one on June 30 of the same month. The first full moon this month occurred on December 2.

And only once in every 20 years, the blue moon appears on New Year’s Eve, as it is going to do this year.

Apparently, it’s name, “blue moon”, does not have anything to do with its color, explained Conrad Jung, a staff astronomer at the Chabot Space & Science Center in Oakland. “It’s just a colloquial term, which means very, very rare,” he said.

“It’s not so much a phenomenon but an event of timing,” Jung noted. “It’s a bit esoteric, to say the least. I don’t get a lot of questions about blue moons.”

Early in the last century, a blue moon was considered the third full moon in a three-month season with four full moons, regardless of when it appeared, he said. Agrarian cultures paid more attention to full moons because their nighttime illumination could extend the working day during planting and harvest seasons, Jung explained. The September full moon, for example, is known as the harvest moon in the United States.

While a blue moon, or any other moon, may have a blue tint when observed through dust in the atmosphere from volcanic ash, Jung does not believe recent eruptions of the Mayon volcano in the Philippines will effect the color of Thursday’s blue moon.

The expression “once in blue moon” is used metaphorically to describe the rarity of an event, but former professional astronomer David Harper of Cambridge, England actually did the math.

“On average, there will be 41 months that have two full moons in every century,” he explained on his Web site. “So, you could say that “Ëœonce in a blue moon’ actually means once every two-and-a-half years.”

And for those who prefer a more exact calculation, Harper did the rest of the math, too: once in a blue moon = 1.16699016 x 10-8 hertz.

This year’s blue moon will make New Year’s Eve special, and will likely create jokes about the full moon causing bizarre behavior. It may just be an urban legend, but that has yet to stop people from making jokes about it.