A historic or an historic?

Summary

A historic is more common in both American and British English, but both usages are sufficiently common to be considered correct.

A well known grammar rule says that we should use an before vowel sounds; for example, an accident, an item, an hour. We use a otherwise: a book, a hotel, a university.

Notice that we say an hour, not a hour. The choice of a or an is based upon the sound of the word, not the spelling. Hour sounds as if it starts with a vowel sound (ow); hence, we use an.

Following this rule, we would say a historic, not an historic because (for most speakers) historic doesn't start with a vowel sound.

Words of three or more syllables that start with h are treated differently by some speakers, though. (This may be because of the tendency of some regional accents to drop initial Hs.)

Google's N-gram viewer allows us to see how this usage has changed over time. Here's the use of "a historic" and "an historic" for US English over the last century:

'a historic' versus 'an historic'

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It's clear that American English has shown a strong preference for "a historic" since the forties.

Here's the chart for British English:

'a historic' versus 'an historic'

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British English also shows a preference for "a historic", although not as strong. This preference is also more recent, perhaps reflecting the influence of American English brought about by the Internet.