‘An’ is correct because ‘historic’ has a ‘soft’ consonant sound instead of a ‘hard’ consonant sound..............................
An high expectation?
An hippopotamus?
"S" is a soft consonant, "z," so too with "t" vs. "d." Should the same distinction be made there?
"Historic" is the only word I consistently hear used this way in this country (I could be wrong about that), so while it is fairly common I'm not familiar with any English grammar "rule" prescribing "an" as the indefinite article. My feeling, and that is all it is, is that in Britain "an" is sometimes used. They do say, I think, "an hippopotamus." But I've never heard Americans say that.
The h is adpirated it is not soft
See post 23