Well now Val, I am glad you asked!
Using "an" before historical has befuddled grammarians for centuries. The letter "h" is a glottal fricative, meaning it is pronounced by closing the glottis briefly. The "velar stop" is used in letters like "k". If you do not completely stop airflow when pronouncing, you get a velar fricative "ch", as used in  the Scotish "loch". It is from this sound that we developed the "h" sound.

I mention all of this because it is a little known fact that, whether a word begins with a vowel or not does not firmly require the use of "an". In English grammar, the pronounciation is also part of the rule. Take, for example, the word "the". Depending on the article, we can pronounce it "thuh" or "thee".  

It is important that speakers and writers understand the cadence or rhythm of a sentence is the key to proper speech.