The most well-known definition of a Blue Moon is the second full Moon to occur in a calendar month.
The lunar cycle is 29.5 days long where as most months are 30 or 31 days long.
Eventually these cycles catch each other up to produce two full Moons in a month -usually on either of the first couple of days, followed by another at the very end.
This definition – also known as a “monthly Blue Moon” – is actually a fairly modern interpretation dating back just 80 years.
According to astronomers at the Royal Observatory Greenwich, external, it is actually an astronomically incorrect definition, first printed in a 1946 edition of Sky & Telescope magazine and perpetuated by the game Trivial Pursuit in the 1980s.
The more accurate definition is a “seasonal Blue Moon” which occurs when there are four full Moons in an astronomical season instead of the usual three. The third Moon of the season is known as a “Blue Moon”.
