Both occur, but "a historic" is standard choice in modern English.Since most speakers pronounce the h, it takes "a" not "an".Contrast to "an hour", where the h is silent.
dataflow · · focus · HN ↗
andsoitis · · focus · HN ↗
Since most speakers pronounce the h, it takes "a" not "an".
Contrast to "an hour", where the h is silent.
tmtvl · · focus · HN ↗