Not even once in a blue moon

I'm reissuing this post from way back in 2009, as the media, including the BBC (which ought to know better) have been going on about there being a blue moon last month. No, there wasn't. See below.

I've seen lots of Twittering over the last few days suggesting that there is going to be a blue moon tonight. Sorry, guys, there isn't.

Take it away Terry Moseley of the Irish Astronomical Association:
There has been a false idea circulating that this will be a 'Blue Moon' because it's the second Full Moon in a month! That erroneous description started when Sky & Telescope magazine wrongly interpreted an old New England Farmer's Almanac as calling the 2nd Full Moon in a month a 'Blue Moon'. They later admitted that they had got it wrong, and published a correction, but not everyone saw the retraction.

A 'Blue Moon' means a very rare and unpredictable event, and it arose after the great Krakatoa volcanic eruption in 1883 blew so much fine volcanic ash into the upper atmosphere that for a while the moon did sometimes appear blue. But that was almost a one-off event, and so the term 'once in a blue moon' means 'hardly ever'. Whereas there are actually two full moons in the same month every few years or so!

So it's not going to be a 'Blue Moon', and in fact if anything, it will appear partly red!
Quick update after some comments on Facebook - I've checked in the OED, and they have a reference to 'blue moon' being used in this way in 1821, so the Krakatoa event isn't the origin of the term. It doesn't change the fact that tonight isn't a blue moon, though. 

Image from Wikipedia


  1. Brian

    I just thought you'd like a longer explanation of the origin of "blue moon" for which I have raided the archives of World Wide Words by Michael Quinion (

    His article traces the origin of the phrase to 1528 and also sets out the nature of how "Blue Moon" came to get its current (incorrect) association with two full moons a month.

    Happy New Year

  2. That's interesting - thanks, Ian. Someone should tell the OED, as they always like to have the earliest usage...


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