> That all of earth is round, comes not from direct observation (except for the very few people who have been in space).

or anyone who's been in a plane?

Square has.

https://en.wikipedia.org/wiki/Flatland

Wow, that is a weird and interesting plot, thanks for that, I take it as a recommendation.

What kind of planes are you flying?

I mean, I just checked, it seems under ideal conditions one could see the curvature of earth at 10.5 km height, but to me it was not really a convincing curvature last time I did a long flight. Your experience was different?

10.5km (34.4kft) is an ordinary cruising altitude for an ordinary commercial airliner.

The curvature of the earth can be quite apparent on a clear day at that altitude when flying over water. Or the midwestern US.

Well thanks, then this is what I will recommend the next time I engage with flat earthers.