I was recently working on a little translation project and my intuition and memory suggested that "evening comes" is vesperescit. Checking dictionaries corrected me: it is vesperascit instead. Why is that? Should I have been able to guess that there is an -a- instead of an -e- before the incohative -sc-?
More generally, what determines the vowel before the incohative -sc-? My understanding was that if no vowel is already there (assuming my verb comes from vesper instead of vespera), one should expect an -e-. There is clearly a hole in my intuition when it comes to the incohative derivatives.