While it still depends on where you live, emphasis has shifted away from saying an historic.
It used to be that the h was silent, so an was used. But people have starting pronouncing the h more, so it's starting to switch over to just an a.
I actually grew up pronouncing the h in historic but still using an an—although I've recently changed that for a couple of reasons:
- I like to follow common guidance, even if it's different from what I was used to in the past.
- I can't really reconcile saying an historic while also saying a history.
The Chicago Manual of Style (17th ed.), 7.32, says this about it:
The indefinite article a, not an, is used in English before words beginning with a pronounced h. (British English differs from American English in not pronouncing the h in many cases; when in doubt, check a standard dictionary.)
a historical study
As for the h being silent in the UK, here is what Oxford Dictionaries says in the blog post "'A historic event’ or ‘an historic event'?":
Let’s go back to those three words that tend to cause problems: historic, horrific, and hotel. If hotel was pronounced without its initial letter ‘h’ (i.e. as if it were spelled ‘otel’), then it would be correct to use an in front of it. The same is true of historic and horrific. If horrific was pronounced ‘orrific’ and historic was pronounced ‘istoric’ then it would be appropriate to refer to ‘an istoric occasion’ or ‘an orrific accident’. In the 18th and 19th centuries, people often did pronounce these words in this way.
Today, though, these three words are generally pronounced with a spoken ‘h’ at the beginning and so it’s now more logical to refer to ‘a hotel’, ‘a historic event’, or ‘a horrific accident’.