We had two Blue Moons in 2018, one on January 31 and the second on March 31. Both the January and March 2018 Blue Moons were blue in name only. Both were the second of two full moons to fall within a single calendar month. The next Blue Moon will be blue in name only, too, but the rules for it are a bit different. The Blue moon of May 18, 2019 will be a seasonal Blue Moon, that is, the third of four full moons in one season.
In recent years, people have been using the name Blue Moon for these two different sorts of moons: second of two full moons in a calendar month, or third of four full moons in a single season.
Someday, you might see an actual blue-colored moon. Meanwhile, the moon you’ll see Saturday night won’t look blue, and blue-colored moons in photos – like the ones on this page – are made using special blue camera filters or in a post-processing program such as PhotoShop.
Let’s talk about seasonal Blue Moons first. A year has 12 months, and months – or “moonths” – have lengths more or less based on a single orbit of the moon around Earth. What we call a season – winter, spring, summer, fall – typically lasts three months, and typically has three full moons.
If a season has four full moons, then the third full moon may be called a Blue Moon, according to the old Maine Farmer’s Almanac. There was a Blue Moon by this definition on November 21, 2010, another on August 20-21, 2013, and another on May 21, 2016.
The next seasonal Blue Moon is May 18, 2019.
The best-known and most popular definition of Blue Moon is that it describes the second full moon of a calendar month. By this definition, there was a Blue Moon on July 31, 2015, and then – of course – the recent ones on January 31, 2018 and March 31, 2018.
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 two to three years, so these sorts of Blue Moons come about that often.
Very rarely, a monthly Blue Moon (second of two full moons in one calendar month) and a seasonal Blue Moon (third of four full moons in one season) can occur in the same calendar year. For this to happen, you generally need 13 full moons in one calendar year and 13 full moons between successive December solstices.
This will next happen in the year 2048, when a monthly Blue Moon falls on January 31, and a seasonal Blue Moon on August 23.
Then 19 years later, in the year 2067, there will be a monthly Blue Moon on March 30, and a seasonal Blue Moon on November 20. In this instance, there are 13 full moons between successive December solstices – but only 12 full moons in one calendar year and no February 2067 full moon.
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 referring to the 1937 Maine Farmer’s Almanac, but he inadvertently 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.
Had James Hugh Pruett looked at the actual date of the 1937 Blue Moon, he would have found that it had occurred August 21, 1937. Also, there were only 12 full moons in 1937. You need 13 full moons in one calendar year to have two full moons in one calendar month.
However, that fortuitous oversight gave birth to a new and perfectly understandable definition for 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.
Today, it has become part of folklore. As the folklorist Philip Hiscock wrote in his comprehensive article Once in a Blue Moon:
“Old folklore” it is not, but real folklore it is.
Can a moon be blue in color? Yes, but it’s very rare to see a blue-colored moon. You need unusual sky conditions – certain-sized particles of dust or smoke – to create them.
Blue-colored moons aren’t predictable. So don’t be misled by the photos above. The sorts of moons people commonly call Blue Moons aren’t usually blue.
Bottom line: A blue-colored moon is rare. But folklore has defined two different kinds of Blue Moons, and moons that are Blue by name have become pretty common. The next Blue Moon is May 18, 2019.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.