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Just How Rare Is a Blue Moon?

Not as rare — or as old — as you might think

Friday morning marks August’s second full moon, a blue moon. While the term blue moon dates back hundreds of years, its meaning of the second full moon in a single month was crafted in the 20th century. Its early usage you might hear in the phrase I’ll believe that when the moon is blue.
    In 1937, the Maine Farmer’s Almanac refered to a blue moon as the third of four full moons in any three-month season. For the Almanac, the blue moon was a placeholder in the colloquial names of the year’s full moons. For instance, August’s full moon is the Corn Moon and September’s the Harvest Moon. Without the Blue Moon, Friday’s full moon would be the Harvest Moon, throwing the off the whole system.
    Then in 1946, an article on the moon in Sky and Telescope cited the Almanac’s usage of the blue moon phrase but simplified its meaning to be the second full moon in any given month. By either definition, this Friday is our blue moon, with a full moon already on July 1 and August 1 and another on September 29.
    Blue moons are uncommon, but they are not rare. A blue moon is a result of the disconnect between our solar calendar and the moon’s 291⁄2-day orbit around the earth. Every 32 months or so, a 30- or 31-day month hosts two full moons, working out to a cycle of seven blue moons every 19 years. Our next blue moon comes July 31, 2015, so you won’t have to puzzle over it again any time soon.
    As the night sky darkens, the bright light directly overhead is Vega, one of the three stars in the Summer Triangle. Look for Deneb to its east and Altair to its south.
    Mercury hovers above the east-northeast horizon with the approach of sunrise. Brilliant Venus is 45 degrees higher, and Jupiter almost that much higher again. In the evening, Mars and Saturn appear in the west-southwest as the sky darkens.