When I first heard the term "Blue Moon" I wasn’t exactly sure what it meant.  I looked it up somewhere and found out it was when there was a 2nd full moon in the same calendar month.  Now I hear there is a different definition of a Blue Moon.

"Once upon two blue moons" before 2009 ends

2009-12-30 13:20:00 From SyFi News website

Before the year ends, sky gazers would have already witnessed the rare phenomenon of two full moons in just one month, which is also termed as "blue moon".

After watching the full moon on December 2nd this year, avid sky watchers would witness the celestial phenomenon, also referred as "blue moon", for the second time on December 31, that is, on New Year’s Eve.

Many people use the expression "once in a blue Moon" to mean something that occurs rarely and many people might be tempted to call December 31st’s full Moon a "Blue Moon" too.

"In modern usage, the second full Moon in a month has come to be called a ‘Blue Moon’, but it’s not!" said Kelly Beatty, Senior Contributing Editor for SKY and TELESCOPE magazine.

"This colorful term is actually a calendrical goof that worked its way into the pages of SKY and TELESCOPE back in March 1946, and it spread to the world from there," she added.

Canadian folklorist Philip Hiscock and Texas astronomer Donald W. Olson had helped the magazine’s editors figure out how the mistake was made, and how the two-full-Moons-in-a-month meaning spread into the English language.

In 1946, writer, amateur astronomer James Hugh Pruett made an incorrect assumption about how the term had been used in the Maine Farmers’ Almanac – which consistently used "Blue Moon" to mean to the third full Moon in a season that contained four of them, rather than the usual three.

By this definition, there is no Blue Moon in December 2009; instead, the last one was in May 2008, and the next happens in November 2010.

But now, the concept of a Blue Moon as the second full Moon in a month, as well as it being the third full Moon in a season with four, are both listed now as definitions in the American Heritage Dictionary

By either definition, Blue Moons happen about once every 2.7 years on average.

The last occurrence of two full Moons in one month was in May 2007 and the next one will be in August 2012.

Due to the wrong definition, people would believe that they have seen the "blue moon" on December 31.

The newer, "wrong" definition is simpler and handier for most people to grasp and use.

"That’s how the English language shifts. You can’t beat back the tide," said SKY and TELESCOPE Senior Editor Alan MacRobert. "Not when the Moon is pulling the tide," he added. (ANI)