Normally “an” is used before an “h” word only when the “h” is silent, such as in “an honour”. Otherwise “a” is used as with any word starting with a consonant (eg “a hotel”).
My guess is this usage of “an historical” is an unconscious device to avoid confusion with the word “ahistorical”. Formally, it should definitely be “a historical”.
Some people aspirate the “h,” some people don’t. The former would say “a historical,” the latter “an historical.” Most Americans aspirate the ‘h,’ so the correct form should be “a historical.”
“An historical” is a bit outmoded in contemporary English, although you will still see it in certain contexts. Personally, I think it sounds stupid and pretentious.
There are a few words, of which historical and historic are the most common, which begin with a sounded H but have the primary accent on the second syllable, and as a result the /h/ sound gets very short shrift. They take “a” and “an” interchangeably.
Some dialects elide or eliminate the /h/ phoneme, as they do with “humble” – think of Uriah Heep. KJV rendering of Psalm 51 and traditional prayers derived from it refer to “An humble and a contrite heart…”
I have no cite at the moment, but I would wager good money (if I had any) that this goes back to classical education in England, specifically the study of Greek. In ancient Greek, words that start with vowels have breathing marks that indicate whether you pronounce them with with an initial H sound or not. For instance, the Greek word from which “history” is derived is ιστορια (istoria). A rough breathing mark ( ‘ ) over the initial iota means that you pronounce it his’ tor i’ a. If it had a smooth breathing mark ( ’ ) over the iota, it would be pronounced like a normal short i. When it migrated into English, ιστορια naturally became history with an H.
But since many of the most pedantic (and largely disregarded) rules of grammar ultimately derive from analogies to classical languages (e.g. you shouldn’t split an infinitive in English because in Latin infinitives are one word and can’t be split), my WAG is that the fact that istoria starts with a vowel led some pedant to proclaim that history should be preceded by “an.”
If we’re talking Brits, there’s quite a few accents in England, Scotland, Wales and Northern Ireland. I can imagine Michael Caine saying “istoric”, but dropping h-sounds is not universal.
I’m confident that it covers the majority of Britain. And I agree with Cisco, that ‘an historic’ seems to be a mis-transcription of this pronunciation.
I should have thought to check my Fowler’s first thing, but it was late last night. The very first entry in the 1926 edition of A Dictionary of Modern English Usage is (of course):
*Emphasis mine. *
So the folks who thought it had to do with Britons dropping their aitches have it partly right, but it was that plus an unaccented first syllable that led to the practice. When it was proper to say an historian, it would have been wrong to say an history, because in the former case the accent is on the second syllable and in the latter it’s on the first.
I guess I’m the only one who remembers the brouhaha surrounding Time magazine’s cover story in 1984, titled “A Historic Choice”, regarding the selection of Ferarro as the Democrat’s VP choice. A lot of people complained that it should have been “An”.