Blue Moon?

The full moon of November arrives on this Sunday (Nov. 21, 2010) 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 the second 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.

Read more here.