Both are acceptable depending on how strongly you pronounce the 'h' in historian.

a historian =)

It should be a historian. You use "an" if the next word starts with a vowel sound. If not, then use "a".

It's "a historian". You would only use "an" if you are in the habit of dropping your H's (which would mean you speak a very specific dialect).

an употребляется перед гласной ( an alephant, an umbrella) a употребляется перед согласной (a historian, a monkey).