My pal Chris Soprych suggested I do something for Wednsday's Blue Moon. Problem was: I always thought a blue moon was when you have two full moons in a single month. That IS the definition now, but it wasn't always. Once upon a time, a blue moon happened when you had four full moons in a single season. The blue moon was the third of the four. The definition shifted because of a mistake made in 1946 by a writer for Sky & Telescope magazine. So I explained the two tips of blue moons and showed how often they happen. Which is a lot more often than you'd think: Sometimes, twice in a single year. Illustrated with wonderful file photos by ace O.C. Register photographer Bruce Chambers. Aug. 20, 2013.