Historic: adjective - famous or important in history; having great and lasting importance; known or established in the past; dating from or preserved from a past time or culture.
Historical: adjective - of, relating to, or having the character of history; based on history; used in the past and reproduced in historical presentations; famous in history.
As can be seen, there is a slight element of interchangeability here. However, the primary use of each of these alternatives is as follows:
If it’s historic, it’s something of note that happened in the past.
If it’s historical, it’s something based on the past.
‘The Battle of Trafalgar, culminating in the defeat of the French and the untimely death of Admiral Lord Nelson, was an historic event in the Europe of the early 19th century.’
‘Jean Plaidy is a well-known writer of historical novels.’
Pic: A field of ripening barley.