777236ER From , joined Dec 1969, posts, RR:
Reply 2, posted (11 years 11 months 1 week 4 days 9 hours ago) and read 3362 times:
It's just a line on a map. Like an isobar which connects points of equal pressure and isochrone connects points where things happen at the same time. A circle marking the distance in each direction a plane could travel in an hour is an icochrone.