I seem to remember from my childhood, reading from a Guinness book of world records, that actual blue moons do occur and that the last recorded one was when Krakatoa blew it's top :-) the current convention to call the thirteenth full moon in any given year a 'Blue Moon' is fairly recent (last century) and is not at all rare because it is eminently predictable! There are approximately 12 moons every year... 'Approximate' being the definitive word here as lunar cycles are only 29 days :-) this means there will be a 'Blue' or thirteenth full moon every 3 years, it also means there are years with TWO months that have two full moons! the last was 1999 in January and March of that year(February lost its moon!) and the next two are 2018 followed by 2037- I don't know if there is a name for these moons but they are more rare than blue moons!