Thrice in a Blue Moon

Once should be enoughWhat's up with all these full moons? Around here, this month has two full moons, but in some places it happens next month, and not until July in other locations. To further complicate matters, some call the second full moon in one month a blue moon, although by tradition the phenomenon was always taken to mean the third full moon out of four in a particular season. How can this be allowed, and who deserves the punishment for all this discrepant information?

Ironically, the venerable Sky and Telescope magazine had quite a bit to do with it. As a 1999 article explained, a mistake made by the author of an article that appeared in the same magazine in 1946 actually led to the creation of the two-full-moons-in-a-month definition, which has now become so popular that it isn't likely to go away again, ever. The error was the result of a misinterpretation of a page from the 1937 Maine Farmers' Almanac, which based the timing of seasons on idealized celestial motion in order to achieve seasons of equal duration. Sometimes this yielded the same blue-moon date as the more accurate modern methods, and sometimes not. You can see the graphic results for yourself at Blue Moon - four Full Moons in a season, and read all about the Sky and Telescope fiasco in the brief Blue Moon - what's the real definition? article at the same site.

As for that disturbing difference in which month a blue moon actually occurs, the reason is more straightforward. It's the old time-zone problem again, and nothing more. If the whole world were running on UTC—or GMT if you prefer those letters—there would be no controversy; we'd all have these full moons on the same day, and at the same time:

  • May 2
  • June 1
  • June 30
  • July 30

Assuming you subscribe to the more popular two-full-moons-in-a-month definition, your blue moon would be my blue moon, and we could both celebrate the blueness at the same time, namely during the month of June. Alas, if you live in Auckland your blue moon won't arrive until July, whereas those of us who live west of Greenwich get it already in May. Locations such as London, Riyadh, Moscow, Tokyo, and Sydney receive their blue moon in June. Here again, there's a colorful chart at The Blue Moon of 2007, which is more fun to look at than the plain old words you see here.

Of course, if you're free to travel the globe at will, there's no reason you couldn't witness all three if you wanted to.


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