Historic vs Historical - What's the difference?

Sometimes as a native teacher it is difficult to explain  to a student an idea or concept that you feel naturally.

So was the case of historic vs. historical. What's the difference?

Hat tips to Grammar Girl and Grammar Monster:

Well, both are adjectives, but their sameness diverges there.

Historic: used to describe something momentous or influential in history

Ex: 9/11 was a historic event in US history.

Historical: generally means ''from the past'' or ''relating to history,'' momentous or not

Ex: The US Constitution is a historical document.

Bonus: Notice the indefinite article ''a'' is used instead of ''an.''

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