In all main varieties of English, the use of an as the article preceding historic (an historic) is an unnecessary affectation. The rule for the indefinite article is that we use a before words beginning with a consonant sound, and an before words beginning with a vowel sound. The h at the beginning of historic is a consonant sound, soft though it may be. As far as we know, there are no modern English dialects in which the h in historic is silent (please correct us if we’re wrong), so there’s no reason for anyone to use an instead of a before the word.
The same applies with the words historical, historian, and so on. They start with consonant sound, so their article is a.
An historic appears about a third as frequently as a historic, even in some normally well-edited publications—for example:
The very possibility of it marks an historic turning point for the entire region. [New Republic]
At least one of Lord Hutton’s recommendations will mark an historic departure. [Guardian]
And, from an historical perspective, Greece’s track record as a creditor is checkered. [Wall Street Journal]
But in most edited writing, whether British, Australian, Canadian, or American, a historic is thankfully more common than an historic—for example:
The entire five-member City Council was replaced, with a historic turnout and a whopping 95 percent of voters approving the recall. [New York Times]
Maurice Manning, a historian and chancellor of the National University of Ireland who is a former Fine Gael politician, described the mood. [Financial Times]
This collection puts Orchard’s work in a historical perspective and gives an overview of the artist’s career. [Sydney Morning Herald]