But is there only one indefinite article -- or are there two? We say "a" (usually pronounced uh, but for emphasis becomes eh) before a word beginning with a consonant, but "an" before a word beginning with a vowel. So: a chicken; an egg. That extra n thrown in there to separate vowel sounds is a process called "nunnation." It's very common in English.
For instance, we often add an N to a name beginning with a vowel to make it a diminutive. So: Ned for Edward or Edgar; Nell and Nan for for Eleanor and Anne. Noakes looks like a similar kind of name, but in fact, it's a contraction, not a diminutive. In Middle English, the phrase atten Oakes meant "at the oaks." So "Robert [who lived] at the oaks" became Robin Noakes. But the process by which atten oakes could become at 'n'oakes, and then just 'n'Oakes, where the n detached from the end of one word and attached itself to the front of the next, is pure nunnation.
We see this transfer of n in words like "apron," "adder," and "auger." These words started out as napron, nædere, and naugre, from Old French, Old English, and Old French, respectively. So they used the indefinite article, "a." But over time, the n detached from the noun in each case and attached to the article. a napron became "an apron," a nadder became "an adder," and a nauger became "an auger."
If you want to see how the n moves between words, try saying the following sentence three times, very fast: A noisy noise annoys a noisy oyster.
So, why doesn't the n show up between the vowel in "the" and a following vowel? Probably because the th sound is very strong, whereas the bare, usually unaccented "a" tends to get swallowed up in the next word. If English used more glottal stops, like German, we might not need nunnation, but since we tend to run words together in our speech, the n helps separate words -- but with the tendency to lose contact with its original word and drift to the one next to it.