>>9013362One way is to get into ham radio and try out high frequency propagation - ie, sending signals around the world by bouncing them off of the ionosphere. Let's say I'm in Washington and using HF propagation to talk to a guy in Alaska. My antenna is pointed northwest, toward Alaska. There is a slight but measurable delay between our signals, since radio waves propagate near the speed of light, with some slowdown due to atmosphere and path.
Next, I turn my antenna 180° and find the same guy again, then talk to him using "long path" propagation. Instead of pointing my signals at him directly, I point away from him, and bounce the signal around the entire globe until they arrive back in Alaska.
The delay in my signal will be measurably longer, because I am taking a longer path. This is consistent with a spheroid Earth with an atmosphere capable of reflecting radio waves back to the surface of the planet.
