No. 2 - Blue Moon

The popular definition of a "blue moon" is the second full moon in a calendar month. Since a full moon occurs every 29 1/2 days, if there is a full moon on the 1st or 2nd day of a month, there is a good chance there will be a second full moon, or blue moon, that month.

The more traditional definition is when an extra full moon occurs in a season. Each season - winter, spring, summer and fall - would typically have three full moons. However, if a season had four full moons, then the third, not the fourth, full moon is called the blue moon.

Blue moons occur about once every two and a half years, which is the origin of the saying "once in a blue moon."

The last blue moon (popular definition) occurred on December 31, 2009 and the next will not appear until August 2012.

And a blue moon is not blue in color.