If the skies are clear, revelers will get an eyeful of the second full moon of the month—commonly called a blue moon. A blue moon isn't actually blue—as commonly defined, the name reflects the relative rarity of two full moons in a month and is linked to the saying "once in a blue moon."

"There is nothing scientific about it, and it has no astronomical significance," Mark Hammergren, a staff astronomer at the Adler Planetarium in Chicago, Illinois, said in 2009.

"But I believe it does give us some insight into history and makes us think of how our calendar system has derived from motions of objects in the sky."

(Take a moon myths quiz.)

The popular definition of a blue moon