According to popular definition, a blue moon is the second full moon in a month. But don’t expect it to be blue – the name has nothing to do with the color of our closest celestial neighbor.
A full moon occurs every 29.5 days, and most years have 12. On average, an extra full moon in a month – a blue moon – occurs every 2.5 years. The last time there was a lunar double take was in May 2007. New Year’s Eve blue moons are rarer, occurring every 19 years. The last time was in 1990; the next one won’t come again until 2028.
‘Blue moon’ is just a name in the same sense as a ‘hunter’s moon’ or a ‘harvest moon’.
The popular definition of blue moon came about after a writer for Sky & Telescope magazine in 1946 misinterpreted the Maine Farmer’s Almanac and labeled a blue moon as the second full moon in a month.
In fact, the almanac defined a blue moon as the third full moon in a season with four full moons, not the usual three.
Though Sky & Telescope corrected the error decades later, the definition caught on. For purists, however, this New Year’s Eve full moon doesn’t even qualify as a blue moon. It’s just the first full moon of the winter season.
