Home PageAstronomy ArticlesStars PageAstronomy Links


Is the November 21, 2010 Full Moon Truly A Blue Moon?


Yes, the Full Moon falling on November 21, 2010 is a Blue Moon - at least by one of definition of the term. It doesn't come close to matching the popularity of the more modern definition of the Blue Moon as the second of two Full Moons to occur in the same calendar month. The November 2010 Full Moon is still a Blue Moon, but for a different reason.

Third of Four Full Moons In One Season

full moonIn an ironic twist, the newer and more famous 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 period of time between a solstice and an equinox - or vice versa. Most of the time, there are only three Full Moons in one season.
Photo on right, courtesy of Juan Jonston's photostream
Apparently, the modern 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 it referred to an extra Full Moon in a calendar month, rather than an extra Full Moon in a season.

Thirteen Full Moons in Which Yearly Period?

If you have 13 Full Moons between successive December solstices, then four Full Moons will appear in one season during that time span. However, if you have 13 Full Moons in one calendar year, then you're assured of having two Full Moons in one calendar month in that year. Whereas the Maine Farmer's Almanac based the Blue Moon on the former, Pruett thought it was the latter.

Why Not the Fourth Full Moon?

The November 2010 Full 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 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 equinoxes and solstices. 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 most popular definition of the term, but both types of Blue Moon refer to that odd extra Full Moon, whether it be found in a season or the calendar month. Watch as the November 21, 2010 Blue Moon - the third of the season's four Full Moons - shines boldly from dusk till dawn!

copyright 2010 by Bruce McClure


Can you tell me the full moon names?

Blue Moon & the Metonic Cycle


October 2010 Feature * December 2010 Feature