What is the difference between historic and historical?
Historic means important in history — significant enough to be remembered. Historical means relating to history, or belonging to the past, with no judgement about importance.
A historic decision changed the course of events. A historical decision is simply one made at some point in the past.
So every historic event is historical, but the vast majority of historical events are not historic. Most of the past was unremarkable.
Historic carries evaluation: a historic victory, a historic building, a historic moment. Historical is neutral and descriptive: historical records, historical fiction, historical context, a historical figure.
The error usually runs one way. People reach for historic when they mean merely of the past, presumably because it sounds weightier — historic documents for what are just old documents. The reverse mistake is rarer.
A separate question attaches to both: a or an? An historic was standard when the h was not pronounced, following French. In modern English the h in historic is pronounced by most speakers, so a historic is the logical choice and is now the more common recommendation. An historic survives as a formal variant and is not wrong, but it sits oddly if you then pronounce the h.
The same pair-pattern appears elsewhere: economic and economical, where economical means thrifty; classic and classical; electric and electrical. In each case the -al form is broader and more neutral, and the shorter form is more specific or more loaded.
When unsure, ask whether you are claiming significance. If not, historical is safer.
A further pair on the same model: continual means repeatedly, with breaks; continuous means without interruption. A continual noise stops and starts; a continuous one never does. That one is more often confused than either historic pair.