'Blue Moon' falling on Nov. 21 according to Norwood astronomer
Thursday, November 18, 2010 - 6:24 am


The Full Moon falling on Nov. 21 is a Blue Moon by at least by one definition of the term. It doesn't match a 20th century definition of the Blue Moon: the second of two Full Moons to occur in the same calendar month.

In an ironic twist, the newer definition of Blue Moon is thought to have originated from the less well known and somewhat antiquated definition: the third of four Full Moons to fall in one season. A season refers to the time period between a solstice and an equinox, or vice versa. Most of the time, there are only three Full Moons in a season.

Apparently, the latest definition first took root with the publication of James Hugh Pruett's "Once In a Blue Moon" article in the March 1946 issue of Sky & Telescope magazine. Pruett's unwittingly creative misinterpretation of the "original" definition of Blue Moon, as used by the Maine Farmer's Almanac, led him to believe that it referred to an extra Full Moon in a calendar month, rather than an extra Full Moon in a season.

The November 2010 Blue Moon counts as a "seasonal" Blue Moon because it's the third of four Full Moons to fall in between the September 2010 equinox and the December 2010 solstice. Some people ask why the fourth Full Moon of the season isn't called the Blue Moon. It's because almanac makers gave each Full Moon a certain name, depending on its place relative to the solstices and equinoxes. The extra Full Moon threw a monkey wrench into Full Moon nomenclature, but it was easier to save appearances by calling the third of the season's four Full Moons a Blue Moon.

The November 2010 Blue Moon doesn't represent the recent definition of the term, but both of those Blue Moon definitions refer to that odd extra Full Moon, whether it be found in a season or the calendar month.

Watch as the November 21 Blue Moon - the third of the season's four Full Moons - shines boldly from dusk till dawn!

Bruce McClure of Norwood is a freelance astronomer and a frequent contributor to "StarDate" and "Earth and Sky" radio programs. His web site is idialstars.com.