Case in point. I'm not an etymologist, nir should we expect people to have to research the words they use. If a word is that old and not in common parlance, it is a failure of a word and it really shouldn't be used.
> not in common parlance, it is a failure of a word
Harsh but not entirely unfair (it is, after all, how language evolves.) Although I'd counter that "isochron" isn't uncommon in travel-related situations (cf [1]) and "isochronous" is easily understandable by extension.
