Okay, no one has yet properly explained what is going on with "a historian" versus "an historian." Here it is properly explained:
http://www.oxforddictionaries.com/words/...oric-event
Basically, "an" is used when a word starts with a vowel sound, "a" is used when it does not start with a vowel sound. Notice, it is the sound, not the spelling, that dictates this.
At the link above, it explains why there is confusion about words like "historic." It is because the common pronunciation has changed, and so when you look at old books, you will see it one way, when now we should change to conform to the newer pronunciation.
http://www.oxforddictionaries.com/words/...oric-event
Basically, "an" is used when a word starts with a vowel sound, "a" is used when it does not start with a vowel sound. Notice, it is the sound, not the spelling, that dictates this.
At the link above, it explains why there is confusion about words like "historic." It is because the common pronunciation has changed, and so when you look at old books, you will see it one way, when now we should change to conform to the newer pronunciation.
"A wise man ... proportions his belief to the evidence."
— David Hume, An Enquiry Concerning Human Understanding, Section X, Part I.