If you’ve been waiting for 2009′s Blue Moon — the second full moon in a calendar month, which happens only about every thirty-three months — it’s finally here, and none too soon! For the first time in 19 years, the Blue Moon will appear on December 31st, New Year’s Eve.

Will the moon actually turn blue? While the moon’s appeared blue in the past, it’s unrelated to its appearance twice monthly:

There have been times when the moon actually turned blue. When an Indonesian volcano erupted in 1883, its dust turned sunsets green and the moon blue all around the world for almost two years. In 1927, the Indian monsoons were late arriving and the extra-long dry season blew up enough dust for a blue moon. And, moons in northeastern North America turned blue in 1950 from forest fires in Sweden and Canada threw smoke particles causing the moon to appear blue.

The twice-per-month definition of a Blue Moon stems from an error made in a popular science magazine shortly after World War II:

Laurence J. Lafleur (1907-66) of Antioch College, Ohio, discussed Blue Moons in a question-and-answer column in Sky & Telescope, July 1943, page 17, citing the 1937 Maine Farmers’ Almanac as his source. It is clear that Lafleur had a copy of the almanac at his side as he wrote, since he quoted word for word the commentary on the August 1937 calendar page. This commentary notes that the Moon occasionally “comes full thirteen times in a year,” but Lafleur did not judge whether this referred to a tropical year or a calendar year. More important, he did not mention the specific dates of any Blue Moons and never said anything about two full Moons in one calendar month.

Some three years later, in March 1946, an article entitled “Once in a Blue Moon” appeared in Sky & Telescope (page 3). Its author, James Hugh Pruett (1886-1955), was an amateur astronomer living in Eugene, Oregon, and a frequent contributor to S&T. Pruett wrote on a variety of topics, especially fireball meteors. In his article on Blue Moons, he mentioned the 1937 Maine almanac and repeated some of Lafleur’s earlier comments. Then, unfortunately, he went on to say, “Seven times in 19 years there were — and still are — 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon.”

This is where the fault began, it appears: previously, the Blue Moon was when a fourth full moon appeared in any season, and it wasn’t necessarily the second full moon in any month. As American culture became more mechanized and urbanized, our definition of Blue Moon shifted from a seasonal one — the “extra moon” mixed amongst a season’s normal three full moons — to a strictly monthly-calendar-based definition. Picked up in 1980 by the radio program StarDate, the 1946 re-definition of a Blue Moon was quickly adopted in America by a population much less attached to the meaning of moons and seasons:

With two decades of popular usage behind it, the second-full-Moon-in-a-month (mis)interpretation is like a genie that can’t be forced back into its bottle. But that’s not necessarily a bad thing. Rather than argue over whether to celebrate the dawn of the new millennium on January 1st in 2000 or 2001, those with the sunniest outlooks will celebrate twice.

But a less agricultural, less moon-centered culture appears to have dropped the fourth-full-moon-in-a-season definition completely, having adopted the easier-to-track twice-monthly occurrence as our only one. A children’s almanac and Trivial Pursuit further cemented America’s adoption of the new definition:

Later, this definition of Blue Moon was also popularized by a children’s book (Margot McLoon-Basta and Alice Sigel, “Kids’ World Almanac of Records and Facts,” published in New York by World Almanac Publications, in 1985), and the board game Trivial Pursuit.

One way to enjoy two successive Blue Moons very quickly, of course, would be to travel to Asia or Australia or New Zealand, as they have two full moons in January 2010, not December 2009:

If you’re in Australia, New Zealand or Asia, your clocks and calendars will show that same moon falling on January 1, 2010. So your blue moon will come at the end of January, 2010.

Folklorist Philip Hiscock explains in his definitive article on the International Planetarium Society website that the twice-in-a-month-full-moon definition of a Blue Moon is probably here to stay:

“Old folklore” it is not, but real folklore it is.

So — have you any once-in-a-Blue-Moon activities planned for this New Year’s Eve?