it's not the letter the word beings with which decides if you use a or an, e.g. i don't go to a university
"an historian" is correct. "An" is to be used when the first letter of a word is phonetically pronounced with a starting vowel. Not the word itself. Thus "h" appears to be a special case, as it is pronounced "aitch". I do not care for those who improperly pronounce it "haitch" like the window licking autisms they are.
If somebody doesn't know when to use a or an i would like to know what they got in GCSE english