If something is "historic" it is of importance in history.
If it is "historical" it is simply something that happened in the past.
The two can have completely different implications -- if something is historical it is in the past -- gone, and of no further significance. But an historic event is still of great importance or significance.
The OED gives this as the current sense of historic:
"esp. Forming an important part or item of history; noted or celebrated in history; having an interest or importance due to connexion with historical events. (The prevailing current sense.)"