It's a common misunderstanding to think that the way a dictionary divides headwords is supposed to indicate the syllabification of the word. It doesn't; it's related to the syllabification, but not the same. What it's meant to indicate is where a word can be broken for hyphenation.
You can find more explanation of this in the answer to the following ELU question: Different syllabic boundaries in various dictionaries?
Actual phonetic syllabification is also complicated. Certain words are easy to divide into syllables (e.g. "hangnail" should clearly be divided as /hæŋ.neɪl/) but many others are more difficult. Different scholars have different theories about how to divide words like "barrel", "mattress", "later" and "selfish". John Wells proposed syllabifying these as /bær.əl/, /mætr.əs/, /leɪt.ə/ and /sɛlf.ɪʃ/ respectively.
TLDR
'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.
Explanation
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:
- insane /ɪnˈseɪn/ → insanity /ɪnˈsæn.ə.ti/
- profane /prəˈfeɪn/ → profanity /prəˈfæn.ə.ti/
- sincere /sɪnˈsɪə/ → sincerity /sɪnˈsɛr.ə.ti/
- serene /səˈriːn/ → serenity /səˈrɛn.ə.ti/
- impede /ɪmˈpiːd/ → /ɪmˌpɛd.ə.mənt/
- divine /dɪˈvaɪn/ → divinity /dɪˈvɪn.ə.ti/
- derive /dɪˈraɪv/ → derivative /dɪˈrɪv.ə.tiv/
- pronounce /prəˈnaʊns/ → pronunciation /prəˌnʌn.siˈeɪ.ʃ(ə)n/
- provoke /prəˈvəʊk/ → provocative /prəˈvɒk.ə.tɪv/
- holy /ˈhəʊli/ → holiday /ˈhɒlɪdeɪ/
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:
- /eɪ/ → /æ/
- /ɪə/ → /ɛ/
- /iː/ → /ɛ/
- /aɪ/ → /ɪ/
- /aʊ/ → /ʌ/
- /əʊ/ → /ɒ/ (AmE: /ɑ/)
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 short vowel in 'southern'
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).
- The long vowels in the base words were then diphthongised by the Great Vowel Shift in Early Modern English.
- Both ⟨þ⟩ (þorn—thorn) and ⟨ð⟩ (eth) were used interchangeably to represent the th sounds in Old English (read this answer for details). And ⟨ū⟩ represented the long U sound /uː/ (as in moon).
- The final e was pronounced up until late Middle English.
- That's why most northerners rhyme but with put.
References:
Best Answer
𝑇𝐿;𝐷𝑅
'Wind' (n) and 'wind' (v) had the same vowel in Old English. Both had a short vowel /i/ which was lengthened in Late OE due to a sound change triggered by consonant clusters such as /nd, ld, mb, rd/ etc. The vowel in 'wind' (n) got shortened in the seventeenth century for some reasons. The short and long vowels in 'child' and 'children' can also be attributed to the same sound change.
𝐻𝑜𝑚𝑜𝑟𝑔𝑎𝑛𝑖𝑐 𝐿𝑒𝑛𝑔𝑡ℎ𝑒𝑛𝑖𝑛𝑔
By about the tenth century (Late Old English), there was a sound change called Homorganic Lengthening (HL) through which original short vowels were lengthened in certain words in the environment of a following voiced homorganic cluster. To be precise, before a cluster of Sonorant (Nasal or Liquid) + voiced homorganic obstruent.
Homorganic means having the same place of articulation, so vowels before clusters like /nd/, /ld/, /mb/, /rd/, /rn/, /ŋɡ/ etc., were lengthened in certain words. The change also applied to some clusters with /r/ as their first element such as /rd/, /rn/, /rl/, /rð/ (Donka Minkova) so the vowel in words like board, hoard, yearn, earl, earth etc., were also lengthened due to HL.
Examples
Influence of a third consonant
However, the lengthening didn't take place when the homorganic cluster in question was followed by a third consonant. It means HC+C ('HC' being the homorganic cluster and 'C' another consonant) cluster was impervious to HL. This can be illustrated by the following example:
The vowel in 'grind' is long due to HL and that of 'grindstone' (ModE: /ˈɡraɪn(d)stəʊn/) should've been short because the /nd/ is followed by a third consonant, but it isn't. Otto Jespersen says that 'grindstone' was formerly always /ˈgrinstən/ and the long vowel in ModE is by analogy with 'grind'.
This point also accounts for the long and short vowels in 'child' and 'children'.
Child vs children
Don't let the w[ɪ]nd w[aɪ]nd you up
There are different assumptions as to why the vowels in 'wind' (v) and 'wind' (n) are different:
This was a sporadic sound change and only affected certain words. There are many, many words that have a 'voiced homorganic cluster' at the end, yet they have short vowels: band, hand, land, dumb, lamb, sand, send, held, bend, blend, end, rend, send, spend, wend etc. (Minkova)
Notes
Abbreviations
References