“Blue Moon, I saw you standing alone.” Tonight is an event that only occurs once in a blue moon. When tonight’s full moon rises it will technically be a blue moon but not because it will be the second full moon in the month of August, but because of a fairly little-known role of astronomy, it will be the third full moon in a four moon season.

The blue moon isn’t really blue. The moon can look blue during a forest fire or the eruption of a volcano because of the ash that fills the sky. The moon does sometimes look like it’s red. The name Blue Moon has nothing to do with its appearance.

There are two definitions for a Blue Moon. It was originally defined as the third full moon in a four moon season. A 1943 article in the magazine "Sky & Telescope" wrongly said that a blue moon is called that because it is the second full moon in a single month. It was incorrect, but the definition stuck. Tonight’s blue moon is a technical Blue Moon, there it is not the second full moon this month..

There are many names for the Blue Moon. The Blue Moon that occurs tonight is called the Full Sturgeon Moon, the Green Corn Moon, the Grain Moon and the Full Red Moon. The full moons that occur in August have the different names because of traditions that date to both the Native Americans and European.

The Blue Moon will only be full for a short time. Even though the moon looks full on the day before and the day after it is full, there is only technically one moment when the moon is full. That will occur tonight at 9:45 p.m. eastern time.

Blue Moons are rare. After tonight, the next Blue Moon won’t be seen until 2015. The expression "once in a Blue Moon" has its roots in science. It is problematic for scientists had to predict when the distinctive full moon might take place during any given year.

by Tony Sokol