Blue Moon

Today there will be a blue moon – the second full moon within one calendar month. This doesn’t happen very often – once every few years. The next one will be August 2012, with the full moons falling either side of the London Olympics.

This time, there will also be a partial lunar eclipse.

Those reading from Australia don’t get their full moon until January 1st, which makes January the blue moon month for them.

I read about this first on Wikipedia by accident quite some time ago, and put it in my diary to look out for when it happens next.

This piece on Yahoo News goes on to explain that it’s not really a significant occurrence for real astronomers:

A full moon occurs every 29.5 days, and most years have 12. On average, an extra full moon in a month — a blue moon — occurs every 2.5 years. The last time there was a lunar double take was in May 2007. New Year’s Eve blue moons are rarer, occurring every 19 years. The last time was in 1990; the next one won’t come again until 2028.

Blue moons have no astronomical significance, said Greg Laughlin, an astronomer at the University of California, Santa Cruz.

“`Blue moon’ is just a name in the same sense as a `hunter’s moon’ or a `harvest moon,'” Laughlin said in an e-mail.



One comment on “Blue Moon

  1. RogerBW says:

    Ah, but this seems to have been based on a mistake. The March 1946 issue of _Sky and Telescope_ started it, with an article called “Once in a Blue Moon”: “Seven times in 19 years there were — and still are — 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon.”

    But actually James Hugh Pruett was misinterpreting what he’d remembered from the 1937 Farmers’ Almanac. That (naturally enough) gave the dates of full moons, but “blue moon” was used in a very specific and different sense. Moons in a _season_ were named “early”, “mid” and “late” (e.g. “late summer moon” and so on); if there were four moons in a (roughly three month) season, the third one was named “blue” (and the fourth one was “late”).

    (If you want to work this out for yourself:

    (1) locate solstices and equinoctes.

    (2) locate nearest new moon to each; these lunations (i.e. periods from these until next new moon) are the “early” moons.

    (3) the lunations immediately following these are the “mid” moons.

    (4) the lunations immediately before the “early” moons are the “late”
    moons of the previous season.

    (5) the remaining lunations, if any, are the “blue” moons.

    In 2010 there’s a blue moon of this sort in autumn, 6 November – 4 December.)

    The Almanac had been doing it this way since the 19th century (and still does). Clearly on this basis there will be about one blue moon per two or three years. (29.30509 days per lunation on average; 351.67068 days for twelve lunations; 365.2425 days per year on average; 13.57182 days of extra lunation = 0.46310895 extra lunations per year.)

    More details on the history of the error here:

    Roger (nitpicking ex-rat)

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