I was wondering why we pronounce the vowels in the words south and southern. They seem to be very closely related to each other. Both refer to the same direction. South is a noun and Southern is an adjective, which I don't think caused them to have a different vowel.
Oxford Dictionary shows that south is /saʊθ/ and southern is /ˈsʌð(ə)n/. In short, south has a long vowel and southern has short.
'South' and 'southern' have different vowels because they were affected by a phonological rule in Middle English called Trisyllabic Laxing (hereafter TSL). It was a process whereby a long vowel in a stressed syllable was shortened when it was followed by two or more syllables.
Trisyllabic Laxing was a phonological rule in Middle English (and perhaps Old English) that shortened (laxed) a long vowel1 in a stressed syllable when it was followed by two or more syllables. It only applied to derived words (like divinity from divine and -ity) rather than underived words (like nightingale). At one point, it affected all the relevant words. Later on, it ceased to be a part of English phonology, though its remnants are still highly visible in Modern English. Some examples are as follows:
As you can see, there are at least two syllables after the stressed long vowels, so they got shortened because of TSL.
There's a fairly regular pattern of the vowel change in the words I mentioned above. It can be summarised as:
In Modern English, there are many exceptions such as words ending in -ness (e.g. mindfulness, loneliness etc) or later borrowings such as obese/ obesity etc.
The above changes happen in trisyllabic (three-syllable) or polysyllabic words, but 'southern' is not even a three-syllable word, why then is it pronounced with a short vowel?
The history of southern is more interesting. Wikipedia also mentions disyllabic laxing (when one syllable follows the stressed long vowel as in 'southern'), though the vowel in 'southern' is not the because of that. It is because of TSL.
The anomaly here is caused by historical sound changes. 'Southern' used to be a three-syllable word when Trisyllabic Laxing applied.
'South' was sūþ (or sūð)2 in Old English and it was pronounced /suːθ/. While 'southern' was sūþerne3 (or sūðerne) pronounced /ˈsuː.θer.ne/ ([ˈsuː.ðer.ne])
In Middle English, 'southern' was pronounced /ˈsuː.ðər.nə/. Then Trisyllabic Laxing applied to it and reduced the long vowel /uː/ to /ʊ/ (the vowel in put).
Then in Early Modern English, there was another sound change that caused the unrounding of the round vowel /ʊ/ in most varieties of English (exceptions being most accents of Northern England4 and Midlands) and changed it to /ʌ/ (vowel in strut). So the round vowel of 'southern' changed to /ʌ/.
'South' was pronounced /suːθ/ until the Great Vowel Shift, which changed the /uː/ vowel to /aʊ/ (vowel in mouth).
Void has an amazing and thorough answer. I would add one further point, which is that the weird patterns of the laxing make a whole lot more sense if recognize that it occurred in English before the Great Vowel Shift, which kept the short vowels sort of similar, and the utterly messed up the long vowels. In Old and Middle English, a "long vowel" was literally that, a vowel that was held longer. So the letter "e" was pronounced like "/e/" regardless of whether it was long (/e:/) or short (/e/), excepting that it was different in length.
However, after the GVS, the long /e:/ sound shifted to /i:/ (now the name of the letter). So words like serene and serenity would have been closer to /se're:n/ and /se're ni ti/. However, when the long /e:/ shifted, you ended up with /se'ri:n/ and /se're ni ti/, plus some other shifting.
Basically in summary, the "shortening" was due to slightly less emphasis placed on a vowel after two more syllables were added to it. And then the GVS came along and really messed up the pronunciation of the long version, making the sounds seem way more different than they originally were.
Coda: A similar phenomenon happened with the pair wind/windmill (currently /wɪnd/ and /'wɪnd mɪl/), with an extra twist. In Middle English, the pair was /wind/ and /'wind mill/. With the GVS, the first one became /waind/ (the same sound you get for "wind a watch"). So you had the slightly odd pair /waind/ and /wɪnd mɪl/. And English speakers sort of unanimously rejected that pairing some point in the later 18th century, since the words were so different.
But its legacy can be found in Shakespeare (As you like it), where the rhyme goes along with "kind":
"Blow, blow, thou winter wind (/waind/), Thou art not so unkind (/kaind/)"
