Supposing we consider two concentric circles of 1000 ft and 40000 ft, and travel along them at a speed of x kts or km/h or m/s (any speed unit).
Surely the arc length covered by them in equal time would be equal, but if we look down vertically from 40000 ft, at the end of 1 hr, we would see that the lower plane is ahead, right? I'm really confused.