Tuesday's blue moon a rare one

Tuesday night, as the sun sets and the moon rises, we're in for something rare: a blue moon.

The moon itself won't change colors so why is it called a 'blue moon'? Well, it's complicated.

There are two definitions for blue moon. One defines it as the second full moon to occur in a calendar month. Usually, we get just one full moon per month. The last one was August 31, 2012, and the next one will be July 2015.

However, Tuesday's blue moon is a "seasonal blue moon", meaning it's the third of four full moons in a season.

The first definition is generally assumed to be the definition of blue moon, but the second one is the more accurate one.

On a side note, if you really want to see a blue moon, you'll have to look at it after a major volcano erupts or after a major forest fire. The ash in the air acts as a filter, turning it blue.