In all main varieties of English, using an as the article preceding historic (an historic) is pretentious and should be avoided. 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. No one pronounces it istoric, so a is appropriate, not an.
The same applies with the words historical, historian, and so on. An creates a highfalutin tone that is inappropriate for most types of writing. No matter how often you hear or read an in these contexts, don’t give in to the temptation.
Examples
An historic appears about a third as frequently as a historic, even in some high-standards 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 most respected publications, whether British, Australian, Canadian, or American, use a historic more often 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]
