When it comes to blue moons, one thing is certain: No matter which definition you’re using, they don’t happen very often.
Now, about that definition.
If you’re content with the increasingly accepted modern definition, there’s a blue moon tonight.
It’s the second full moon in a month. It isn’t blue. And it looks and acts just like any other full moon. It just happens to fall on a certain calendar day. The last such blue moon was New Year’s Eve 2009, and the next will be July 2015, according to Sky and Telescope magazine.
But count Sky and Telescope among entities not content with the aforementioned definition.
“It’s wrong!” cries an article on the magazine’s website. “At least if you’re a stickler about these things.”
The colorful term is actually a “calendrical goof” that worked its way into the magazine back in 1946, then ballooned, senior contributing editor Kelly Beatty writes. A contributor made an incorrect assumption about the Maine Farmers’ Almanac definition, which used “Blue Moon” to describe the third full moon in a season containing four.
By that definition, there’s no blue moon tonight. The last one was in November 2010, and the next one will be August 2013.
But even the almanac’s definition is questionable.
“What’s interesting is that we use it as if it was longstanding folklore, and it’s really not,” said Barbara Anthony-Twarog, Kansas University professor of physics and astronomy. “Only in the last half-century or so have people decided to give that the title of a blue moon.”
Once upon a time, once in a blue moon simply referred to something that happened extremely rarely.
We’ve got a hunch that usage came from a phenomenon that actually made the moon appear blue: volcanic eruptions violent enough to shoot plumes of ash to the top of Earth’s atmosphere.
One such instance was the 1883 eruption of the Indonesian volcano Krakatoa, according to NASA Science News. The moon, full crescent or otherwise, appeared blue for years. Less forceful eruptions, such as Mount St. Helens in 1980, spawned reports of blue moons, too.
If thinking about astronomy happens once in a blue moon for you, folks who study it for a living will take multiple definitions in stride.
“I think anything that encourages people to actually pay attention to the sky is probably a plus,” Anthony-Twarog said.
— Features reporter Sara Shepherd can be reached at 832-7187. Follow her at Twitter.com/KCSSara.