Tonight’s’ full moon sets the stage for a Blue Moon on July 31st. “Blue Moon” refers to a second full moon in a calendar month. Our moon follows a 19 year loop called the Metonic Cycle – every 19 years phases of the moon recur on or near the same calendar date. Nineteen years has 228 months with 235 full moons, meaning 7 of those 228 months have a blue moon. Sometimes February’s short number of days produces 8 blue moons in a Metonic Cycle (February 2018 won’t have a full moon, pushing the extra moon to another calendar month)
Popular use of the term is credited to a 1946 issue of Sky and Telescope magazine. Author James Hugh Pruett penned an article “Once In A Blue Moon”, Pruett inadvertently screwed up finer details when referencing the 1937 Maine Farmer’s Almanac – nevertheless “once in a blue moon” was born.
The Maine Farmers Almanac described blue moons as an extra full moon in a “season”. Each season – spring, summer, fall, winter typically has 3 full moons, when a 4th happens, the 3rd moon of that season becomes the blue moon. By this rule the next one falls on May 21, 2016. Although two distinctly different definitions exist, most people subscribe to the monthly club.
It’s possible to have 2 blue moons in a calendar year, the next time is January and March of 2018, followed by January and March 2037. Sometimes a rare year has both monthly (2 full moons in a month) and seasonal (3rd full moon of 4 in a season) – don’t hold your breath, it will be 2048 before the monthly in January, seasonal in August.