The instructions tell you to to to a local area where the horizon extends beyond eyesight to the viewer's left and right. They hold up the long clear plastic ruler that has a slight curve at the top and a flat line below it. They line up the curved part and compare it to the flat part.

Make a pleasant day out of deprogramming a flat Earth person. Maybe have a picnic on the beach after.

I know that people don't like having their beliefs questioned though, but you could put these out there.

And I'd be surprised if somebody hasn't done this already but I'm too lazy to look for it.
-- doctorremulac3, Nov 22 2023

Here's a guy who spent a lot time talking about it. https://www.youtube...watch?v=mxhxL1LzKww
[doctorremulac3, Nov 22 2023]

Eratosthenes https://en.wikipedi...g/wiki/Eratosthenes
[doctorremulac3, Nov 22 2023]

Gonna add this: A website that shows 5 yard tall poles at various sites around the world where you can see their shadows in real time explaining how that's how some olden times dude figured out how the Earth was round in the first place.

Be a whole kit you could buy and give out for Christmas or something.
-- doctorremulac3, Nov 22 2023


Obviously fake.
-- pocmloc, Nov 22 2023


The way it works is you watch all the different posts at the same time seeing that their shadows are different lengths. (link)
-- doctorremulac3, Nov 22 2023


What if the ruler is just really big?
-- sninctown, Nov 22 2023


Which is worse? Flat earthers, solipsists, determinists or indeterminists?
-- pashute, Nov 27 2023



random, halfbakery