Unless your readers all say "istoric," you should write "a historic."
So many of you asked whether a historic should have been an historic that I thought I'd better answer the question here, even though it was the topic of last week's Grammar Girl podcast and there was a link to it in the newsletter.
The choice between a and an is governed by the first sound of the next word. If it's a consonant sound, choose a; if it's a vowel sound, choose an. Although there are regional variations, the standard American pronunciation of historic starts with a consonant sound (just like the words hit and hipster), so the correct choice is a historic. There's nothing special about historic that exempts it from the standard rule. (Read the whole article.)
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