Blue Moon?

On New Year's Eve, 2009, the Moon will be full, for the second time in december.

A Blue Moon?
Well, not really...

although colloquially, it is, now.

A full Moon looks full because it's directly opposite the Sun in the
sky, from our viewpoint on Earth, so its whole sunlit side faces us.
Photo by Gary Seronik / SKY & TELESCOPE magazine.

Sky & Telescope explains, with a mea culpa:

"..."In modern usage, the second full Moon in a month has come to be
called a 'Blue Moon.' But it's not!" says Kelly Beatty, Senior
Contributing Editor for SKY & TELESCOPE magazine. "This colorful term
is actually a calendrical goof that worked its way into the pages of
SKY & TELESCOPE back in March 1946, and it spread to the world from
there."

SKY & TELESCOPE admitted to its "Blue Moon blooper" in its May 1999
issue. 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.

Before 1946, a Blue Moon always meant something else. For example,
says Hiscock, sometimes it referred to an obvious absurdity. Quite a
few old songs use it as a symbol of sadness and loneliness. There's
even a cocktail called a Blue Moon; it's a mix of curacao, gin, and
perhaps a twist of lemon. And, exceedingly rarely, the Moon actually
does turn blue in our sky -- when a volcanic eruption, forest fires,
or dust storms send lots of fine dust into the atmosphere.

Our 1946 writer, amateur astronomer James Hugh Pruett (1886-1955),
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)..."

When is the Moon "blue," in a calendrical sense? According to the
Maine almanac, a "Blue Moon" is the third full Moon in a season that
contains four rather than the usual three. This type of Blue Moon can
happen only in February, May, August, or November, a month before the
next equinox or solstice. But according to modern folklore, a "Blue
Moon" is simply the second full Moon in a calendar month. This type of
Blue Moon can occur in any month except February, which is too short.
SKY & TELESCOPE illustration.

Thus myths are made.