The moon you see up in the night sky tonight is a Blue Moon. But no, it's not actually the color blue. Duh, everybody knows a Blue Moon isn't actually blue. A Blue Moon means the second full moon in the same month! But, um, that's not the reason why tonight's moon is considered a Blue Moon either. Wait, what?
There are two definitions of a Blue Moon. One being the popular two full moons in a month definition and the other being the third full moon in a four full moon season. You see, we usually have three full moons per four seasons. But this summer we have four full moons and tonight's moon is the third full moon out of four. That's why it's Blue.
Enjoy it. It's supposed to reach its fullest at 9:45 PM Eastern and won't be 'Blue' again until 2015.