Although the full moon that occurred in the western hemisphere on Thursday, May 31, 2007, looked like an ordinary full moon, it was actually a bit extraordinary—it was a blue moon.

What is 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). May 2007 had two full moons: the first on May 2, the second on May 31—that second full moon was called the blue moon. In the Eastern Hemisphere, the full moon in question occurred on June 1. For that half of the world, the blue moon will be on June 30, 2007.

How Often Does a Blue Moon Occur?

Over the next twenty years there will be a total of 17 blue moons. No blue moon will occur in the years 2006, 2011, 2014, and 2017. Two full moons in one month may occur in any month out of the year except for February, which is shorter than the lunar cycle.

Twice in a Blue Moon

The rare phenomenon of two blue moons occurring in the same year happens approximately once every 19 years. 1999 was the last time a blue moon appeared twice, in January and March. The months of the double blue moons are almost always January and March.