a historic / an historic
Which article to use before historic and historical.
Use a historic and a historical. Both words are pronounced with an audible h, so they take a, not an. The same rule covers a hotel, a hypothesis, and a historian.
Examples
Use The city named the old train depot a historic landmark.
Use She wrote a historical novel set in 1863.
Avoid The city named the old train depot an historic landmark. (Change an to a.)
An historic is an older form that survives in some British and ceremonial prose. It is not the default in current English. For the general rule that governs this pair, see a / an.