Look to the sky Friday to see a "blue moon."
But don't expect to see the color blue. A blue moon refers to the rarity of multiple full moons in a month or season. will be the second in August. The first took place on Aug. 1.
Two blue moons in a month typically occurs every 2.7 years, according to the Huffington Post.
The last blue moon was on Dec. 31, 2009, and the next won't occur until 2015.
The "two moons in one month" definition of a blue moon is actually fairly modern, since the widespread adoption of the standard calender. In folklore, a blue moon was the third full moon of a season that had four.
For details on where to watch the blue moon online, click here.