‘A’ and ‘An’ both are indefinite articles different forms enunciation of words matter a lot such as when you mention a word that has “h” to start off with but is silent in enunciation you use ‘an’. Example: ‘Hour,’ ‘Honor’ & ‘Honest’ When words that start off with consonant ‘H’ and also ‘H’ gets enunciated in pronunciation. Example: ‘Hat,’ ‘Hotel,’ ‘Hard’ you should use “A” which means for the word historic it is better to use the article “A.”