Of Red and Blue Moons

August's full moon rising behind Divide Mountain

Our only full moon of August rose last night into a clear, salmon-colored twilight that slowly turned to evening blue. This month's full moon - called the "Full Red Moon" by some - was also the third full moon of this summer, which defines it as a "Blue Moon." And last night's event was the last "Blue Moon" we'll see until either 2015 or 2016, depending on which definition you learned.

In the March 1946 issue of "Sky and Telescope" magazine, author James Pruett misinterpreted a "Blue Moon" to mean the second full moon in a calendar month. His mistake probably would have gone unnoticed, except that it was repeated in January of 1980 by Deborah Byrd on her popular NPR program, "StarDate." Since then, this mistaken definition has overtaken the original meaning.


Last night's full moon behind a spur on Red Eagle Mtn. 

The original meaning of "Blue Moon" was spelled out in the May 1937 edition of the "Maine Farmer's Almanac." Full moons occur every 29.5 days on average, so there is normally one full moon sometime during each month. Dividing the year into four 3-month seasons (summer, fall, winter, spring) means that each season normally has but three full moons. But once every 2.7 years, four full moons fall within a single season. When this happens, the third full moon is called a "Blue Moon."

The next time we will have two full moons during a single month - Pruett's mistaken but popular "Blue Moon" definition - will be on July 31, 2015 (after the first full moon of that month, on July 2nd). And the next time we'll have four full moons during a single season, the third one - the original but mostly unknown "Blue Moon" definition - will occur in spring of 2016, on May 21st.

On the other side, there is also a seldom-used term, "Dark Moon." It's sometimes used to define a second new moon (vs. full moon) in a month. It's also used to describe a month without a full moon, which can only happen in February, and only about once every 19 years. Years with a full-moonless February will also have two full moons during two different months. But you'll have to wait even longer for this combination of events - the next time will be during 2018.