Once in a blue moon, it’s the usual term you can hear for a very rare activity. But, what does “Blue Moon” means? It does not literally mean that the moon turns blue, but its the second full moon in a calendar month.
August 2012 is month with two full moon, the first full moon was on August 2, and the second full moon is August 31, 2012. The time between one full moon and the next is close to the length of a calendar month. So the only time one month can have two full moons is when the first full moon happens in the first few days of the month. This happens every 2-3 years, so these sorts of Blue Moons come about that often.
The idea of a Blue Moon as the second full moon in a month stemmed from the March 1946 issue of Sky and Telescope magazine, which contained an article called “Once in a Blue Moon” by James Hugh Pruett. Pruett was using a 1937 Maine Farmer’s Almanac, but he simplified the definition. He wrote:
Seven times in 19 years there were — and still are — 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon.
When is the next Blue Moon?
The next blue moon will fall on July, 2015.
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.
This Friday evening is the last chance to had a glimpse of the Blue Moon for it will happen again in the next three years.