I was recently in Hawaii on Kauai. I wondered if I could see any of the other Hawaiian islands (besides the small local ones). Can you see Oahu from Kauai (70 miles away)? Rarely, and only if you get to a high enough elevation and even then you can only see the highest peaks. On the other hand, Maui is easily visible from Hawaii from sea level. Simple, practice, repeatable.

Likewise, flying in a plane for any distance and seeing the course is identical to if you pulled a string tight against a globe (give or take). I suppose pilots could choose to fly in an arch to keep the secret, but eventually some capitalist has got to spill the beans to cut costs, right?

Also: the southern hemisphere.