There is quite a lot of confusion about this pair, with them being used pretty interchangeably. But, as is often the case, there is a distinction and there is a rule, and it’s nice to maintain those separate meanings and subtle points of difference. Isn’t it?
Historic, or, for that matter, geographic, pinpoints a particular time or example. So historic means famous or important in history: a historic event. “The historic moment when the Titanic sank”. Similarly, geographic refers to a specific thing, a specific point to do with geography: the geographic north pole.
Historical, on the other hand, is a more general term, getting across the idea of something as concerning history, for instance, historical evidence. It also means “belonging to or set in the past”, so a “historical re-enactment”. And geographical means concerning geography, so you have a Geographical Society.
“Historical evidence points to this historic event being a turning point in the War.”
“The Geographical Society undertook an expedition to the Geographic North Pole.”