You have probably heard this expression before. It usually means not very often. But, is there really such a thing?
Well, yes, but it’s probably not what you may think, and it’s definitely not what it used to be.
According to David Wilton’s fabulous Word Origins web site, the phrase Blue Moon probably started with an anonymous poem from 1528, Read me and be not wrothe, For I say no things but truth:
- "If they say the moon is blue,
"We must believe that it is true."
In 1883, an Indonesian volcano named Krakatoa exploded. Scientists liken the blast to a 100-megaton nuclear bomb. Fully 600 km away, people heard the noise as loud as a cannon shot. Plumes of ash rose to the very top of Earth's atmosphere. And the moon turned blue.
Krakatoa's ash was the reason. Some of the ash-clouds were filled with particles about 1 micron (one millionth of a meter) wide--the right size to strongly scatter red light, while allowing other colors to pass. White moonbeams shining through the clouds emerged blue, and sometimes green.
Blue moons persisted for years after the eruption. People also saw lavender suns and, for the first time, noctilucent clouds. The ash caused "such vivid red sunsets that fire engines were called out in New York, Poughkeepsie, and New Haven to quench the apparent conflagration," according to volcanologist Scott Rowland at the University of Hawaii.
Other less potent volcanos have turned the moon blue, too. People saw blue moons in 1983, for instance, after the eruption of the El Chichon volcano in Mexico. And there are reports of some caused by Mt. St. Helens in 1980 and Mount Pinatubo in 1991.
The key to a blue moon is having in the air lots of particles slightly wider than the wavelength of red light (0.7 micron)--and no other sizes present. This is rare, but volcanoes sometimes spit out such clouds, as do forest fires.
The use of the phrase blue moon to indicate an actual astronomical phenomenon first started in 1932 with the Maine Farmer’s Almanac. It’s definition was a season with four full moons rather than the usual three, where the third of four full moons would be called a "blue moon." Since seasons are established by the equinoxes and solstices and not calendar months, it is possible for a year to have twelve full moons, one each month, yet have one season with four.
That definition mutated into the one most quoted today when in 1946, an article in an astronomy magazine by amateur astronomer James Hugh Pruett misinterpreted the Maine rule to mean two full moons in one month. This definition seems to have stuck, despite its error, possibly thanks to being picked up by the Trivial Pursuit game. As we’ve seen previously, as in the case of seeing the Great Wall of China from space, the writers of Trivial Pursuit are capable of making errors.
Whether you use the newer definition or the one from the Maine Farmer’s Almanac, a blue moon, while not common, happens on a regular basis. Either way, they occur approximately 7 times in a 19 year period.
Much less common is a double blue moon (2 in one year). That only happens once in the same 19 year period. They occur in January and March, thanks to the short month, February. The last double we saw was in 1999. The next will happen in 2018.
So, will you ever see a blue moon? In astronomical terms, it is very likely. If you hope to see a full moon which is the actual color blue, that is less likely, but possible, especially during forest fire season. But, if you don’t want to do something, don’t put it off until a blue moon. You may be at it sooner than you’d like.
- To learn more about other full moons, check out our Named Moons.