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.

[1] e.g. in the UK parliament, #7927, "the resident population within the Woolwich 20-minute isochron area was about 60,000" https://publications.parliament.uk/pa/cm200607/cmselect/cmcr...