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Wednesday, September 2, 2009

Where did that saying come from - "once in a blue moon"?


We, as a species, often use words, sayings, phrases, idioms, quotes, clichés, proverbs, maxims, quips, adages, epigrams and epithets without actually knowing what they mean. Sometimes this happens so widely and consistently as to butcher the original meaning, and billions of people end up wandering around saying things that are wildly inaccurate and linguistically nonsensical.

I digress; back to the subject. I was researching my latest post over at Blogstronomy* and came across the phrase 'blue moon,' or, more usually 'once in a blue moon.' Of course, I know what it 'means': 'once in a blue moon' is used to describe something that doesn't happen very often. For example "TeaKay's Blog is read once in a blue moon." But where did it come from?

According to an admittedly small amount of research, the term 'blue moon' relates to full moons. Specifically, it refers to a full moon that is outside of the regular pattern: Most years will have twelve full moons, one in each month. A lunar month (that is, the time it takes for the moon to orbit the Earth once) is a bit less than 28 days. Even accounting for the fact that it's a bit longer than this between full moons, that's quite plainly more than 12 cycles in each year (it is, apparently, 12.37 cycles), so every so often we get a 'leap**' full moon. According to my FWSE skillz, an extra full moon slots itself in every 2.7 years. But who's to say which full moon is the 'extra' one?
  • Back in olden times***, if there were more than the traditional three full moons in any season (a season being three months, of course), then the third full moon would be the 'blue' one. The next blue moon using this definition will be on November 21st, 2010, followed by August 21st, 2013.
  • More recently, if there are two full moons in any one month, it's the second one that is deemed to be blue. This is thought to have stemmed from a misinterpretation of the Farmer's Almanac by a 1946 edition of Sky & Telescope magazine. The next blue moon using this definition will be on December 31st, 2009 (following the first full moon on the 2nd), followed by August 31st, 2012 (following the first full moon on the 2nd).

Can moons ever really be blue?
In the right conditions, our moon can (and has) appear(ed) blue to the naked eye. If there is the right concentration of smoke or dust particles of the right size (about one micrometre in diameter), light at longer wavelengths (that's the red end of the visible spectrum) may be scattered more than the shorter wavelengths, meaning that the shorter wavelengths of light are more likely to reach your eyes, shifting the observed colour of the moon towards the bluer end of the visible light spectrum.

As for any other moons... I guess that depends on what they're made of.



* It's all about where moons come from. I really think you'd rather enjoy it.
** As in 'leap' year or 'leap' second. I made up that terminology. It's not official, so if you don't get it, ignore it.
*** A standard phrase that means 'I don't know when, I can't be bothered to look it up, but it was quite a while ago now.'