I was going to write, "isn't it *an* historic moment?". But before doing so, I googled, and came up with:
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 — as of March 2008 — the phrase "a historic" is used on 5.1 million pages (68%), and "an historic" on 2.34 million pages (32%).
There is a clear preference here in favour of "a historic". Even so, roughly one-third of the usage is for the other form. This supports the view that which form you use is little more than a personal preference.
Both usages are sufficiently common to be considered correct in modern English.
How fitting that both are acceptable.