Once In a Blue Moon

A Blue Moon can be the second full moon in a calendar month. Or it can be the third of four full moons in a single season. The November 21, 2010 Blue Moon is the third of four full moons between the September 2010 equinox and December 2010 solstice.

The next Blue Moon will be Sunday, November 21, 2010. Informed sky watchers know that a Blue Moon of this sort – one that can be scheduled ahead on the calendar – is not likely to be blue in color. So what makes the November 21 full moon a Blue Moon?

In the 21st century, according to folklore, the name Blue Moon has two meanings. A Blue Moon can be the second full moon in a calendar month. Or it can be the third of four full moons in a single season. The November 21, 2010 Blue Moon is the third of four full moons between the September 2010 equinox and December 2010 solstice.

Watch for November 21 Blue Moon near Pleiades star cluster

So don’t be misled by the photo above. Although certain-sized particles of dust or smoke can cause a moon to look blue in color, the sorts of moons people commonly call Blue Moons aren’t usually blue. For more about truly blue-colored moons, click here.

Every month has a full moon, and, most of the time, the names coincide with particular months or seasons of the year. By either definition, the name Blue Moon accounts for times when there happen to be more full moons than is convenient.

Second full moon in a month. In recent decades, many people have begun using the name Blue Moon to describe the second full moon of a calendar month.

The time between one full moon and the next is close to the length of a calendar month. So the only time one month can have two full moons is when the first full moon happens in the first few days of the month. This happens every 2-3 years, so these sorts of Blue Moons come about that often.

When is the next Blue Moon, according to this first definition? August 31, 2012.

The idea of a Blue Moon as the second full moon in a month stemmed from the March 1946 issue of Sky and Telescope magazine, which contained an article called “Once in a Blue Moon” by James Hugh Pruett. Pruett was using a 1937 Maine Farmer’s Almanac, but he simplified the definition. He wrote: “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.”

EarthSky’s Deborah Byrd happened upon a copy of this old 1946 issue of Sky and Telescope in the stacks of the Peridier Library at the University of Texas Astronomy Department in the late 1970s. Afterward, she began using the term Blue Moon to describe the second full moon in a calendar month on the radio. Later, this definition of Blue Moon was also popularized by a book for children by Margot McLoon-Basta and Alice Sigel, called “Kids’ World Almanac of Records and Facts,” published in New York by World Almanac Publications, in 1985. The second-full-moon-in-a-month definition was also used in the board game Trivial Pursuit.

Can there be two blue moons in a single calendar year? Yes. It last happened in 1999. There were two full moons in January and two full moons in March and no full moon in February. So both January and March had Blue Moons.

The next year of double blue moons is coming up in 2018.

Third full moon of four in a season. The Old Farmer’s Almanac defined a Blue Moon as an extra full moon that occurred in a season. One season – winter, spring, fall, summer – typically has three full moons. If a season has four full moons, then the third full moon may be called a Blue Moon.

When is the next Blue Moon, according to this second definition? November 21, 2010.

In recent years, a controversy has raged – mainly among purists – about which Blue Moon definition is better. The idea of a Blue Moon as the third of four in a season is older than the idea of a Blue Moon as the second full moon in a month. Is it better? Is one definition right and the other wrong? After all, this is folklore. So the folk get to decide, and, in the 21st century, both sorts of full moons have been called Blue.

As the folklorist Phillip Hiscock wrote in his comprehensive article Folklore of the Blue Moon: Old folklore it is not, but real folklore it is.

So enjoy the November 21, 2010 Blue Moon!

earthsky.org