Ignoring the vowel rule, which is right?

[Haircut]

Without quotes, it doesn't work properly, and you get an equal amount of results. Now, with quotes...

http://www.googlefight.com/index.php?lang=en_GB&word1="an+historian"&word2="a+historian"

I disagree, however.

I disagree too, that's why I posted the version without quotes

 

[Nikias]

Either "an" or "a" historian is correct. If you want to be traditional and please the purists, however, then use "an".

It's from the time when the "h" at the beginning of "historian" was silent, much in the same way that it is in "honest" and "heir" nowadays.

 

[MikeTimbers]

Also, please don't rely on Google for this as we're talking about English here not American. In American, they would say "an herb" since they do not pronounce the h at the beginning. In English we would say "a herb" because we do.

Google's results will be skewed by the vast number of American mis-spellings