Today is special. It is a red letter day. Well … no … actually it’s a blue moon! So anything could happen — allegedly.

The mostly used definition of a blue moon is where there are two full moons in a calendar month. But that it appears is a more modern definition, the older one being applied where there were four full moons in a season. Various older belief systems give each of the three normal full moons in a season a name. Where there are four full moons the third of the four is called a blue moon so that the last may keep it’s “correct” name and rightful place in the season.

Which might suggest to you that blue moons aren’t that rare. And you’d be right. They occur every 2-3 years (actually 7 times in the moon’s 19-year Metonic cycle), because of the mismatch between the 28 day lunar cycle and months of 30 or 31 days in our solar calendar.

Curiously it seems no-one really knows why it is called a blue moon, but it almost certainly isn’t because the moon suddenly becomes Smurf-coloured for the day. Smurf-coloured moons can happen but only as a result of significant atmospheric pollution, like the eruption of Krakatoa in 1883.

One theory for the name is that “blue” in this context is derived from the Old English word belewe meaning “betray” which was used to describe “false” moons entering the calendar. Well I suppose that’s possible, but given that the earliest known English reference dates from only 1524 it is perhaps unlikely.

Well, anyway, enjoy the last day of summer. In London it is bright and sunny but Autumnally cool, which is actually rather nice in what in the UK has been the wettest summer for 100 years. And if the sky is clear this evening go and bathe in the light of the blue moon! Sadly you’ll have to provide your own Blue Moon Cocktail.

You can find more on Blue Moons at:
Wikipedia : Blue Moon
Wikipedia : Full Moon Names
Wikipedia : Metonic Cycle
Jodrell Bank : Night Sky in August
And in various news stories, eg. here