Both a historic and an historic are used in English, and both have a long history. In modern usage, a historic is more common, especially in everyday writing and speech.
The basic rule for a and an depends on sound, not just spelling. We use a before a consonant sound and an before a vowel sound. Because most speakers pronounce the h in historic, they say a historic moment.
Some speakers, however, use an historic, often because the word begins with a weaker or less stressed h in their speech, or because they are following an older style. That is why you may still see phrases like an historic occasion in formal writing.
- Common today: a historic victory
- Also seen: an historic event
- Key point: choose the article that fits the pronunciation you use
If you want the most natural choice for current general English, a historic is usually the safest option.