I'm just back from two weeks in England [...] We were over for WINGS 2009, which stands for Windsor INternational Guide and Scout camp. The festivities included the kick-off for the centenary of Guiding (I think it's a one-year celebration and that the actual anniversary is next year).Some good observations from Ann there. The centennial/centenary divide works as well for multiples thereof, so in Massachusetts, they've had the Darwin 2009 Bicentennial Project and at Down House, Darwin's home in Kent, they've celebrated his bicentenary.
"Centenary" isn't the word we would use in the US; we would say "centennial". And we would pronounce the second syllable with a short 'e', while they pronounce it with a long 'e'.
Because the second 'e' in centennial is followed by a double consonant, it's fairly clear that it should be pronounced as a short 'e'--i.e. the second syllable is ten. But if Americans were to say centenary, they would expect to pronounce it with a ten there too. In fact, you can hear the American Heritage's pronunciation of it on WordNik. I'm very bad at explaining pronunciation differences, and I can't explain this one (phonologists...help!). It's not down to different stress patterns, as both dialects usually stress the second syllable. I thought at first that it was the same as in plenary,
So, BrE speakers who say 'plennary' and 'centeenary' are following the rule for plenary and not for centenary, but AmE speakers who say 'pleenary' and 'centennary' go the other way (though, I have to say, I don't think I've ever said PLEEnary in AmE, no matter what the dictionaries tell me). And it's probably not surprising that AmE tend not to follow the rule for centenary, since it's a word we're unlikely to encounter and so rely on rules and analogy with centennial (which people of my age know well from our country's bicentennial in the 70s...and lots of other bicentennials of towns and other institutions since then).This, I think, is just randomly assigning one of two possible pronunciations of the letter "e" in a borrowing from Latin; it's orthography-based pronunciation anyway, and I think the letter is ambiguous between a short and a long vowel. [...] to be a bit more confusing: there is an old (and now mostly unproductive) rule of "trisyllabic laxing": a long stressed vowel becomes short if followed by two short vowels, hence sereene but serenity, sayne but sanity etc. Applying TSL should give you pleenum but plennary, for example, but this "rule" is pretty much only in the lexicon now and has acquired lots of exceptions.