PARIS, Dec 31 (AFP) -- For only the second time in nearly two decades, December will end with Earth illuminated by a "Blue Moon," the name given to the second full moon appearing in a single month.

For New Year's revelers in the Western hemisphere, the calendar will not quite have turned into the new decade when the perfectly round orb lights up the night sky.

Not since 1990 has the world had the opportunity to observe on a mass scale the compounded impact of a Blue Moon and bubbly.

The double full moon phenomenon happens on average every 2.7 years, with the most recent in May 2007, and the next set for August 2012.

This month, the moon was also at its maximum on December 2.

Why Blue? For no particular reason, according to Space.com, a popular science website based in the United States.

"If there's been a recent volcanic eruption that poured significant ash into the upper atmosphere, it is possible for the moon to take on a blue tint," noted the site's editorial director Robert Roy Britt.

"That's not expected tonight," he added, as if by way of apology.

It also turns out that the term "blue moon" is a misnomer that can be traced back to an editorial blooper 65 years ago in the normally irreproachable magazine Sky and Telescope.

The original meaning was the third full moon in a season with four, a more common occurrence.

There is also a cocktail -- curacao, gin and a twist -- by that name, and a turn of the (20th) century expression meaning something absurd.

And, of course, there are those things, like a double full moon, that only happen once in a....