Afon-abona vowel length

[IuliaPyrrha]

Hello, I'm not a Welsh speaker, but I've got a Welsh linguistics question: If I understand it correctly, the Welsh word for river is afon (whence every other river in England seems to be called Avon), which in term derives from the Brythonic abona. Now I was wondering about the length of the vowel o. On the internet I found various bits of more or less useful information, according to which it seems that it is important whether a syllable is stressed or not, and also whether it is final or not, and I'm just not sure whether this would count as final since it dropped the ending, so I'd be grateful for any information.

(Background of the question: I'm currently living in Bristol. Now in the area of the modern Bristol suburb Sea Mills there used to be a Roman port of the name Abona, which is quite obviously derived from the Celtic-Brythonic word, and I was wondering how to stress that. Latin stresses the paenultimate if long, but in a Celtic loanword the vowel length will obviously be related to/derived from the vowel length in the original language.)

[linguoboy]

Short in both languages. Proto-Celtic lacked *ō (although West Brythonic developed *ɔ̄ from earlier *ā; the subsequent Welsh development is /aw/ in stressed syllables and *ɔ in unstressed syllables, generally yielding modern Welsh /o/ ). The modern pronunciation of afon is ['avɔn].

Late Brythonic (c400 CE) had penultimate stress, although I'm not sure exactly how far back this can be traced.

[IuliaPyrrha]

Cool, thanks! So Brythonic would probably have had a short, but stressed o, with the stress shifted to the initial a later on after the final a had been dropped?
For Latin purposes, I live prope Ábonam then, I guess. Thank you for the info!

[linguoboy]

Cool, thanks! So Brythonic would probably have had a short, but stressed o, with the stress shifted to the initial a later on after the final a had been dropped?

According to the best of our ability to reconstruct the historical developments, that's exactly what happened.

[IuliaPyrrha]

Thanks again! That answers my question!