It was a historic moment. = an important moment in history.
It is a historical novel. = a novel about a period in history.
Compare ‘a historic event‘ (an event that changed history) and ‘a historical event’ (such as a parade in historical costumes)

It was a historic moment. = an important moment in history.
It is a historical novel. = a novel about a period in history.
Compare ‘a historic event‘ (an event that changed history) and ‘a historical event’ (such as a parade in historical costumes)