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On Thursday evening, May 31st, a full Moon will rise for the second time this month (the first time was on May 2nd). Many people call the second full Moon in a calendar month a "blue Moon" and use the expression "once in a blue Moon" to mean something that occurs only rarely. While the latter meaning can be traced back centuries, the former definition is much newer and it's wrong!
It is rare to have two full Moons in a single month. The reason is simple: the average time between full Moons is 29½ days. Thus February, with at most 29 days, can never accommodate two full Moons. To squeeze a pair into a month with 30 days, the first must occur on the 1st of the month. Months with 31 days can have two full Moons only if the first one occurs by the 2nd of the month, as happened a few weeks ago.
It turns out that the next full Moon occurs at 9:04 p.m. Eastern Daylight Time on May 31st but that's 1:04 a.m. Greenwich Mean Time on June 1st. So our friends in Europe won't experience a blue Moon in May. But another full Moon is due on June 30th. So for Europe it’s June, not May, that has two full Moons in 2007.
If you want to tell your readers, listeners, or viewers that Thursday's full Moon is a blue Moon, go right ahead. Countless other newspapers, radio and TV stations, and websites will certainly do so. But be aware that, technically, every one of these reports will be in error!
According to Canadian folklorist Philip Hiscock, the term "blue Moon" has been around for more than 400 years, but its modern calendrical meaning has become widespread only in the last 25. And as discovered eight years ago, it can be traced to a mistake published in Sky & Telescope in the 1940s!
Sky & Telescope admitted to its "blue Moon blooper," an error that had crept onto the magazine's pages 53 years earlier, in its May 1999 issue, page 36; see also March 1999, page 52, and/or follow the links at the end of this press release. Hiscock and Texas astronomer Donald W. Olson helped the magazine's editors figure out how the 1946 mistake was made, and how the erroneous meaning of blue Moon (as the second full Moon in a month) eventually spread around the world.
Before 1946, a blue Moon always meant something else. For example, says Hiscock, sometimes it referred to an obvious absurdity. Quite a few old songs use it as a symbol of sadness and loneliness. There's even a cocktail called a blue Moon; it's a mix of curaçao, gin, and perhaps a twist of lemon. And, exceedingly rarely, the Moon actually does turn blue in our sky when powerful volcanic eruptions, fires, or storms throw huge quantities of dust into Earth's atmosphere.
Our 1946 writer, amateur astronomer James Hugh Pruett (18861955), made an incorrect assumption about how the term had been used in the Maine Farmers' Almanac, where it consistently referred to the third full Moon in a three-month season containing four. (By this definition there is no blue Moon in May or June 2007, and the next one happens in May 2008.)
There's no turning back now. The concepts of a blue Moon as the second full Moon in a month and the third full Moon in a season containing four are listed as definitions 1a and 1b, respectively, in the American Heritage Dictionary (Houghton Mifflin Co., 4th edition, 2000).
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Sky Publishing (a New Track Media company) was founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, the original editors of Sky & Telescope magazine. In addition to Sky & Telescope and SkyandTelescope.com, the company publishes two annuals (Beautiful Universe and SkyWatch), as well as books, star atlases, posters, prints, globes, and other fine astronomy products.