Although the full moon that you'll see on Aug. 31, 2012, will look like an ordinary full moon, it was actually a bit extraordinary—a blue moon.

What is a Blue Moon?

There are in fact two definitions for a blue moon. According to the more recent definition, a blue moon is the second full moon in a calendar month. For a blue moon to occur, the first of the full moons must appear at or near the beginning of the month so that the second will fall within the same month (the average span between two moons is 29.5 days). The full Moon on Aug. 31, 2012, will be this type of blue moon; it will be the second full moon in one month.

The Other Kind of Blue Moon

The older definition, which is recorded in early issues of the Maine Farmer's Almanac, states that the blue moon is the third full moon in a season that has four full moons. Why would one want to identify the third full moon in a season of four full moons? The answer is complex, and has to do with the Christian ecclesiastical calendar