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.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.
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!
Image from Wikipedia