Pop quiz. What tune are we going to be playing in August/September in Astronomy land? “Blue Moon” of course! Can’t you just hear Ella Fitzgerald’s dulcet tones?
So what’s the story about blue moons? Why is the second full moon of the month referred to as “blue”? Does the moon really turn blue? Rarely apparently. Only with forest fires or volcanic eruptions. I’ll let Borgna Brunner and Anne Marie Imbornoni at infoplease.com tell you all about it.
About Blue Moons
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.
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 13th moon skewed the calendar, since there were names for only 12 moons. By identifying the extra, 13th moon as a blue moon, the ecclesiastical calendar was able to stay on track.
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.
It will be the end of August in my neck of the woods (Ireland) but if you hail from New Zealand or Kamchatka, in the Russian Far East, look to September. Make sure to get outside and take a look.
So that’s the deal about blue moons. Like and share if you found this interesting. Thanks!