Date registered: Sep 2004
Vehicle: '87 300TD/'90 300D/'94 Quattro/'89 Vanagon TDI/'01 EV Weekender VR6
Mentioned: 6 Post(s)
Quoted: 1541 Post(s)
Both usages are sufficiently common to be considered correct in modern English.
You probably know the grammar rule that says you use an before vowel sounds (e.g. an accident, an item, an hour) and a otherwise; e.g. a book, a report, a hotel.
Following this rule, we would say "a historic", not "an historic".
Words of three or more syllables that start with h are treated differently by some speakers, though. For example, which of these pairs of sentences sounds correct to you?
* It is a historic occasion.
* It is an historic occasion.
* We can't agree on a hypothesis.
* We can't agree on an hypothesis.
A quick bit of Googling reveals that â€” as of August 2007 â€” the phrase "a historic" is used on 2.47 million pages (51%), and "an historic" on 2.39 million pages (49%).
This is a very even split and supports the view that which form you use is little more than a personal preference. Both usages are thus sufficiently common to be considered correct in modern English.