[titzer]

It's not that hard. Watch a lunar eclipse. Watch the sun rise, follow an arc, set, and then watch the stars rise, follow their arcs, and then set. Then travel to a few degrees south and see the whole thing again, except the pole around which everything revolves shifts by exactly the amount you moved south.

[eru]

Yes, that's good evidence. Though I think the flat earthers had some convoluted explanation for it, too.

What's also easy to do in today's world is calling up someone in a different time zone that you trust, and ask them for when their sunset is. (The interactive calling is important, so that it's harder to have a conspiracy delay your messages or fake some news.)

[peoplewindow]

Flat earthers? Who has ever thought the Earth was flat? Even medieval people didn't think that, it's obvious to anyone who ever watched a ship sail over the horizon that the planet is not flat. They used to think the sun revolved around the Earth, which is a lot harder to check with common sense logic.

[eru]

Oh, I didn't want to imply any one particular society though the earth was flat. Flat earthers are just an example to show that it's not trivial to argue against someone with a weird believe that they are motivated to defend.