This is from infoplease.com:

"This is a contested question.
Let's start with the basics. A word starting with a vowel sound is preceded by 'an,' no matter how it's spelled. Thus, one would write 'an hour' and 'an honest woman.' A word starting with a strongly stressed 'h' sound is preceded by 'a.' Thus, to most Americans, it's 'a hanger' and 'a helicopter.'
There are two questions at issue here. The first is what one ought to do with a weakly pronounced 'h.' Some—this editor included—would precede it with 'an'; others—and this is increasingly preferred—precede it with 'a.'"

I second that, "a historian".

Hi. I belive it's "a historian."