Both are being used and neither is incorrect. “An historic” used to be more prevalent in British English until around 1990, but “a historic” has become the stan

Both are being used and neither is incorrect. “An historic” used to be more prevalent in British English until around 1990, but “a historic” has become the standard choice now: https://books.google.com/ngrams/graph?content=a+historic%2Can+historic&year_start=1800&year_end=2022&corpus=en&smoothing=3&case_insensitive=false https://books.google.com/ngrams/graph?content=a+historic%2Ca...