Once in a blue moon

So lots of news webpages are talking about us seeing out the year 2009 with a “Blue Moon”, and lots of idiots get the wrong idea, and think it means the moon is actually blue, and even post that they can see it turning blue, when it is doing no such thing.

But, as it happens, it is not just the public who are in error, the news media have got it wrong too. They are using a popular, but totally incorrect definition of Blue Moon; that is “any month where there are two full moons in the same month”. By this reckoning, a blue moon will occur any time the full moon coincides with the beginning/end of a month with 30 or 31 days. This will occur reasonably often, and not as rarely as you would think when considering the phrase “once in a blue moon”.

The correct definition of a Blue Moon is when a season contains 4 full moons, instead of the normal 3. When this occurs, the third full moon of the season is the blue moon. So although we have had 2 full moons in December, neither is the Blue Moon. This happens every 2.7 years, the last Blue Moon was in May 2008, and the next Blue Moon will not be until November 2010.

It now seems that the false definition is now more popular than the correct one. But it doesn’t make it right.


  1. January 1, 2010

    You’re letting the facts get in the way of a good story again, aren’t you? 🙂

    • chris
      January 1, 2010

      Indeed. 🙂

      In fact, what I found most frustrating was not the news sources who simply got it wrong, but those who headlined “2009 ends in a Blue Moon!!!!!” then, in the article itself, essentially say “actually, it’s not, but we can fill a few column inches with this, and then cover ourselves with a note at the end”.

      Of course, the average punter will read no further than the first paragraph, which is why the idea that “two full moons in a month” is a Blue Moon continues to propogate.

  2. January 1, 2010

    Thanks. I’ve been puzzling about the arithmetic — since there are 12.34 full moons in a year, and only 12 months, doesn’t that mean that there a month of the year with two full moons (and therefore a season with 4 full moons) with two full moons roughly once every three years? By whichever definition, that’s not actually all that unusual.

    I still favour the definition of when the moon actually does look blue, since that is indeed rare. If it ever happens at all…

    • chris
      January 2, 2010

      There will definitely be a season with 4 full moons – and therefore a Blue Moon – every 2.7 years. However, it is possible for there to be two full moons in a month more often than that, and without there being 4 in a season.

      Take for example, the year 1999 where both January and March contained two full moons, but February contained none, and the entire Winter Season (from late December 1998 to mid-March 1999) only had 3 full moons – the second full moon of March was in Spring.

      As for the December just gone, there was a full moon on December 2nd, and a second on December 31 (the one that prompted this posting). But neither was a Blue Moon – the December 2nd full moon was the last full moon of Autumn, and the December 31st full moon was the first of Winter.

      Note that, also, the opposite is true – you can have 4 full moons in a season, and not need to have 2 in a month. Autumn 2010 runs from September 23 to December 21. In that period, there are 4 full moons:

      September 23
      October 23
      November 21
      December 21

      but only one full moon in each month.

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