You could prove this on Earth by watching a boat sail away, using binoculars or a telescope as it gets further. The bottom of the boat will disappear first as it goes around the Earth.

@WoarnaA You’ll be proven wrong soon enough

https://www.telegraph.co.uk/tv/2019/07/16/space-cadets-cruel-reality-tv-prank-turned-ipswich-soviet-space/

Hmm, this is concerning. I'll go to great care to avoid being fooled in this way, especially given the improvements in technology since then.

@NasaLies This might be a market you're interested in. :)

If you're in space, all you see is that the earth is round — like a circle, or a pancake. How will you confirm that it is round like a ball?

@Mqrius As we orbit I should be able to see the ball rotate and the continents in my view change as they would on a sphere.

You could also find out by measuring the angles in a large triangle on the Earth's surface.

@tailcalled Infeasible without trusting maps or GPS.

@IsaacKing Just find a mountain that you keep sight of the whole time

@xyz I suspect that triangle would be too small to measure the curvature to a statistically significant level? Not sure though.

Also even with a single reference point, would be challenging to be exact enough.

What if the spaceship has fake windows and shows counterfeit videos of a round Earth instead? Will you require independent auditors to check for that? Or will you leave the spaceship?

@GustavoMafra Hmm. I hadn't considered that! I think if I can inspect the spaceship windows from the outside beforehand, that would be good enough. Leaving it in a spacesuit would be even better though.

In some sense you can confirm the answer with your own eyes today by viewing the Earth from space as captured by various cameras from the past. Though this interpretation would real be torturing the expression.

I presume you mean that you yourself must have the opportunity to be in space.

There’s also a sense in which you do have “the opportunity” to do this today if you’re willing to spend around 20 million, even though you haven’t done it, and that you can confirm the answer with your own eyes by watching ships sail over the horizon

@JimHays Photos can be doctored, and there are other potential explanations for phenomena like objects dissapearing over the horizon. Best to be sure. :)

@IsaacKing so you are saying all photos on Google maps are doctored? lol then the guy who came over to my house to draw it from up top did an amazing job 😂