What is a Blue Moon?

You know that saying "once in a blue moon?" Well, prepare yourselves, because thanks to some clear skies, you'll see one tonight!

What is a blue moon? Well, if you're expecting to see the moon to turn blue, you'll be disappointed.

A blue moon is the term for either a second full moon in one month or the third full moon out of four in a single season. (The reason for this is because there are usually three moons in a season, for example, the early summer moon, midsummer moon, and late summer moon. The extra one throws a kink in the plans, so the third one is called the blue moon so that the fourth can still be called the late moon.)

Generally lunar cycles line up enough with our calendars so that is one full moon every month. About every 2-3 years, the timing doesn't exactly work out, so there is an 'extra' full moon.

This also means that tonight's blue moon will be the last until July 2015.

One of these is clearly in order for tonight:

Cheers!