is article "an" before the noun "hour" acceptable? [duplicate]

Possible Duplicate:
When should I use “a” versus “an” in front of a word beginning with the letter h?

I have seen many authors uses "an" before "hour" like "an hour" some times. Please consider the below excerpts.

"How long is your break?", I shouted. A group of labourers sat under the banyan tree near the main campus building. "It's two-thirty, lunch ended an hour ago."

and another one is below

I would take his boat for an hour, and buy him tea and biscuits in return.

one more

' I won't take more than half an hour' I promised.

I learned from my teachers that the article "an" comes before vowels and not before consonants. Here "H" is consonant, still author uses the article "an" before "hour".

My question is, is this "an" before "hour" is acceptable? If yes can you explain why?

Solution 1:

The first sound— not the first letter— is what determines which indefinite article to use. The spelling of a word is of no concern at all; if the first sound is a vowel sound, use an, and if the first sound is consonant, use a.

Since the h in hour is silent, an hour is correct, and a hour is incorrect. We also say

• an honor / an honour
• an heir
• an honest man

There are cases where the h may or may not be silent depending on the local dialect, as with history, herb, or hotel, in which case you should follow the local style.

Solution 2:

Vowels and consonants are sounds, not letters, and it is the initial sound of a noun that determines whether the indefinite article preceding it is a or an. The initial sound of hour is /aʊ/, a vowel (well, actually, a diphthong), and not the consonant /h/.