November 21st is a Blue Moon Event
What makes the November 21, 2010 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.
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.”
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. So get out tonight and enjoy the Blue Moon!
