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Sentences involving the word "historic" sometimes use "a historic" and sometimes use "an historic".
This has something to do with British English versus American English, and should be expanded in this article.
Actually it's to do with Semi vowels.

Can someone go ahead and expand this? My understanding is that "An" is followed by a vowel sound, and "A" is followed by a consonant sound. The "H" in history is not silent, therefore "A" is correct. I'm sure that there's some reason that people use "An", other than simply parroting others, but it should be explained in the article. --Scott McNay 02:34, 4 March 2007 (UTC) (if you wish to contact me, please do so on Wikipedia, where I have an account)

Although, officially the ‘h’ sound is aspirant in many words, there are various accents (typically non-standard or regional) which remove the aspiration completely: therefore historic, hospital, heart, etc are pronounced [‘istoric] [‘ospital] [‘eart] etc. Some words do begin with a silent‘h', especially those of French origin: heir, hour, honour (US. Honor), etc. There are cases of words such as ‘hotel’ where because of their French origin the ‘h’ is dropped and people say, “an hotel”. However, this form is dying out quite quickly. J.Harrison