I’ve always wondered what they were talking about when I was told “That’ll happen ‘once in a Blue moon”. Then I heard that we were going to see one on the 31st of July 2015!
A blue moon? Are you telling me that I’m going to see the Moon turn blue? I’ve seen eclipses and other phenomena, but this I have to see!! It’s not that a full moon rising over the Sea of Cortez isn’t an awesome sight. Trust me, it is. We usually have one full moon per month, right? The second full moon to occur within a given month is called a Blue Moon. The next calendar blue moon occures on August 31, 2012. Other factors such as forest fires, volcanic eruptions, and plain old air pollution can cause the moon to take on a bluish glow. This type of thing happened in 1991 when Mt Pinatubo blew its top in the Philippines. In the 1950’s forest fires in western Canada caused the same effect in some places, but these had nothing to do with the Blue Moons that we refer to here. Mathematically, a blue moon occurs, on average, about every 32 months, but in 1999, three occurred in a two month span!
The first record of the term was in 1824, but its official use began in 1932 with a reference in the Maine Farmer’s Almanac. They considered a blue moon as the fourth full moon in a three month season. The next seasonal blue moon occurs August 21, 2013. In 1946 Sky and Telescope magazine misinterpreted this to mean the second full moon in a given month. Several other sources continued to express the same misinterpretation through the 1980’s and that became the definition accepted today. Can there be two blue moons in a single calendar year? Yes. It last happened in 1999. There were two full moons in January and two full moons in March and no full moon in February. So both January and March had Blue Moons. The next year of double blue moons is coming up in 2018. Click here to see the US Navy Chart of Lunar Phases