Although the full moon that occurred Tuesday, May 20, 2008, looked 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 Other Kind of Blue Moon
May 2008's blue moon qualified as such under an older definition, which is recorded in early issues of the Maine Farmer's Almanac. According to this definition, 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.
Some years have an extra full moon—13 instead of 12. Since the identity of the moons was important in the ecclesiastical calendar (the Paschal Moon, for example, used to be crucial for determining the date of
Easter), a year with a thirteenth moon skewed the calendar, since there were names for only 12 moons. By identifying the extra, thirteenth moon as a blue moon, the ecclesiastical calendar was able to stay on track.
For a fuller explanation see
http://www.inconstantmoon.com/cyc_blue.htm. For more background information on the controversy over the two definitions of blue moon, see the Sky and Telescope article,
"What's a Blue Moon?" In it they explain how the two different definitions of a blue moon came about—including their own role in introducing the second, modern definition.
A Star Rating for the Modern Blue Moon
Although Sky & Telescope calls the modern blue moon definition "trendy" and a "mistake," the fact that there is an older, preexisting (and more complicated) definition does not necessarily make it the more interesting or meaningful definition. Charting the "third full moon in four full moons" in a season isn't everyone's idea of an fascinating enterprise. The modern, "trendy" definition, however, points to an intriguing astronomical phenomenon—every so often two moons can manage to position themselves in the same month. Given that full moons occur once every 29.5 days, this is quite an accomplishment!
How Often Does a Blue Moon Occur?
Over the next 20 years there will be about 15 blue moons, with an almost equal number of both types of blue moons occurring. No blue moon of any kind will occur in the years 2011, 2014, and 2017.
The more recent phenomenon, where the blue moon is considered to be the second full moon in a calendar month, last occurred on May 31, 2007. 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.
The other, older blue moon event, which happens when there are four full moons in a season, last occured in August 2005. Since this type of blue moon is reckoned according to the seasons, it can only occur in February, May, August, or November, about a month before the equinox or the solstice.
