It’s an hourglass. The word hourglass begins with a silent "h", so it takes the article "an".

It’s an hourly. Choosing between "a" or "an" is determined by the initial sound of the following word. "Hourly" starts with a silent "h" (it sounds like "ourly"), so it takes the article "an".

It’s an hour. Because the word "hour" begins with a silent "h" ("hour" and "our" have the same pronunciation), we use the article "an".

It’s an honorarium. The rule for using "a" or "an" is based on the initial sound of the following word, and not how it is spelled. Since the word "honorarium" begins with a vowel sound (the "h" is silent), we use the article "an".

It’s an honorable. The choice between "a" and "an" is determined by the initial sound of the following word. Since "honorable" starts with a vowel sound (the "h" is silent), it takes the indefinite article "an".

It’s an honorary. The rule for choosing between “a” and “an” is based on the pronunciation of the word that follows, and not on how it is spelled.

It’s an honor. The rule for choosing between “a” and “an” is based on pronunciation, not spelling. Since the word honor begins with a silent "h", it takes the indefinite article "an".

It’s an honest. Since “honest” starts with a silent “h”, we use the indefinite article “an”.

Both "a historical" and "an historical" are used, but the article "a" is far more common, particularly in American English.