“Once in a blue moon.”
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.
The phrase, “until a blue moon” developed in the 19th century, meaning never, or at least extremely unlikely. After all, they do occur.
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.
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.
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.
(Info from Nick Greene at about.com)
The next Blue Moon will be on December 31st 2009~this New Years Eve!
What are you going to do??? Fill me in!