The Really Strange Story Behind Sunday's Blue Moon (Joe Rao)

A "blue moon" isn't really blue, as this artist rendering suggests. The moon can appear somewhat blue if there's been a major volcanic eruption that put tons of particles in the air, but otherwise blue moons refer to multiple full moons in certain time frames. Credit: Dreamstime

Nov. 19, 2010 (SPACE.com) -- The full moon of November arrives on Sunday and will bring with it a cosmic addition: It will also be a so-called "blue moon."

"But wait a minute," you might ask. "Isn't a 'blue moon' defined as thesecond full moon that occurs during a calendar month? Sunday's full moon falls on Nov. 21 and it will be the only full moon in November 2010. So how can it be a 'blue' moon?"

Indeed, November's full moon is blue moon -- but only if we follow a rule that's now somewhat obscure.

In fact, the current "two- full moons in one month" rule has superseded an older rule that would allow us to call Sunday's moon "blue." To be clear, the moon does not actually appear a blue color during a blue moon, it has to do with lunar mechanics.

Confused yet?

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