You might have used the term ‘once in a blue Moon‘ before – a colloquial phrase to suggest that something is very rare. But just how rare, depends on your definition.
In astronomy there are two definitions of a Blue Moon; both are a type of Full Moon. Here Blue Moon is defined as either (1) the third full Moon of an astronomical season with four full Moons or (2) the second full Moon in a calendar month.
Such a Blue Moon as defined in (2) will next occur on Friday, 31-Jul-2015 at 10:43am UTC (7:43am ADT) – July/2015 therefore has two full moons; on the 1st (11:20pm ADT) and 31st of July (7:43am ADT). Continue reading DYK: Blue Moon on 31-Jul-2015 →