Since the dawn of my existence “once in a blue moon” defined rarity of two full moons in a calendar month. More than that, it served as impetus for cosmic wonder. Tonight I discovered the Blue Moon of May 18, 2019 is “seasonal”, meaning the third of four full moons in a single season. How rare can that be?
Twelve months in a year, each month roughly based on a single lunar orbit of Earth. Twelve months, four seasons, three months per season. Fine, I get it – four full moons in a season is rare, but someone could have told me “once in a blue moon” had alternate meanings! There was one on November 21, 2010 another August 20, 2013 and May 21, 2016. After May 18, 2019 we won’t witness another seasonal blue moon until August 22, 2021.
An article published in the March 1946 issue of Sky and Telescope magazine titled Once In a Blue Moon by James Hugh Pruett, cemented blue moon in popular culture. Referring to the 1937 Maine Farmer’s Almanac, Pruett 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.”
Despite the fact Pruett goofed in reference to astronomical details of 1937 moons (there were 12 that year not 13 ), the term Blue Moon was born. Me – I’m still coming to terms with the fact I’ve lived 59 years and never considered blue moon anything other than 2 full moons in a calendar month. Lesson learned.
Link to detailed list of Blue Moon dates and designation – http://www.themoonfaqs.com/2010/01/blue-moon-dates_31.html