It's once in a long blue moon

The old phrase "once in a blue moon" has a special meaning tonight. When the full moon rises tonight it will be the second time this month we have had a full moon.

The last time it happened on New Year's Eve was 1990. And it is being predicted that it won’t be repeated until 2028.

The rising of a moon twice in the same calendar month is a rare happening. The moon circles the earth once every 29.5 days. The last time the full moon rose was on December 2.

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."

Is there really such a thing as a blue moon?

In 1883, an Indonesian volcano named Krakatoa exploded. 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 volcanos have have turned the moon blue, too. People saw blue moons in 1983 after the eruption of the El Chichon volcano in Mexico. And there are reports of some caused by Mount 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 send out such clouds, as do forest fires.

The use of the phrase blue moon to indicate an astronomical phenomenon 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 definition used most often 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.

Whether you use the newer definition or the one from the Maine Farmer’s Almanac, a blue moon, while not common, happens regularly. 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 blue moon was in 1999. The next will happen in 2018.