I've read in various sources that the verb nosco 'know' had a long vowel in the first syllable in Classical Latin pronunciation: nōscō [noːskoː]. I'm wondering what the linguistic evidence is for the vowel being long before -sc- in this word.
The most direct type of evidence would probably be a statement from a Classical author: do any of them talk about the length of this vowel? The next most direct type of evidence that I can think of would be the quality of the vowel in Romance descendants, but I'm not sure that I know all of the relevant sound changes. Italian has close o in coˈn[o]scere, which seems to support the long vowel reconstruction (based on the correspondence Latin ō = Italian close [o], Latin ŏ = Italian open [ɔ] or [wɔ]), but I know that Italian vowel qualities sometimes went through more complicated changes. Old French conoistre > French connaȋtre is possibly also evidence for Latin ō, since if I'm reading this Wikipedia table right, the word should have developed to connuitre instead if it had had short ŏ in Latin. Is this an accurate summary of what these two forms tell us about the Latin word? Do these reflexes, or those in any other Romance language, suffice to establish the length of the vowel in Latin?
The comparative or etymological evidence is even harder for me to understand. De Vaan says that in Proto-Indo-European, the present would have been a zero-grade form *ǵnh₃-sḱé-, which would have developed in Latin to *gnāscō. The actual form gnōscō is explained by de Vaan 2008 as resulting from the introduction of a full-grade vocalism *ǵneh₃-sḱé-, taken from either the aorist or the perfect. That makes sense, but it's still a bit unclear to me when this substitution is supposed to have taken place (did it affect any other Indo-European languages, or just Latin?), and whether it is an isolated case of substitution or an example of some more generally applicable process of analogy that affected the present-tense forms of other verbs.