This is slightly beyond what an average layman can do, but it's not terribly hard or expensive to receive images of our globe from NOAA weather satellites [1]. Generally the most costly part is a ~$120 USD antenna. It's a super fun and rewarding project if this is the kind of thing that interests you.

[1] https://en.wikipedia.org/wiki/Geostationary_Operational_Envi...

The problem is that then you aren't verifying it yourself, and a flat earther can claim you're just using one of the propaganda sources created by the government.

You can also use the ESA earth observation program, which provides free downloads of earth observation data every ~5 days. Look for Sentinel-2. It still won't prove that earth is round though :).

A flat earther would tell you the "satellite" photos are faked, and could provide you with faked photos of their own.

The question is how you distinguish the two: always go mainstream? Always counter the old-fashioned ways? Trust the best credentialed person? Distrust credentials achieved during brainwashing, go with independent self-taught thinkers? Independently verify every claim, replicating experiments from first principles?

But they can always say anything is faked. They can say that my eyes distort what I see and that math can’t be trusted either.

The only way you could really prove it is to define a set of axioms that you some how convince them to agree to. And then get them to agree to logic. And then construct an experiment using those rules whereby if the Earth is round then they die. And see if they are willing to test it. And if they don’t then you win. And if they do it and die then you also win.

There is a simple experiment to show curvature that was actually done by some flat earthers, and it proved curvature, but then they somehow classified it as an interesting but not conclusive result. Here's the experiment: take 3 wood planks, make a hole at the same eight in each of them. Then put them standing in the soil making sure that all have the same height (meaning that the hole is at the same height for all of them). Now use a laser to have a straight line of light (you can observe by other means that a laser beam is a straight line). If the wooden planks are far enough form each other, the laser will not e able to pass trough all 3 holes, despite them being at the same height from heart's surface. This implies curvature.

That's too prone to terrain irregularities, and I can't imagine that there's a single area of land on earth that's remotely as smooth as the surface of an sphere. Further, even if you assume perfect instrumentation, how would one account for gravity?

> I can't imagine that there's a single area of land on earth that's remotely as smooth as the surface of an sphere

Salar de Uyuni - "As a result, the variation in the surface elevation over the 10,582-square-kilometer (4,086 sq mi) area of Salar de Uyuni is less than 1 meter (3 ft 3 in) normal to the Earth's circumference".

That's less than 1 in 6371000 - pretty smooth, no?

> That's too prone to terrain irregularities

Nah, it's still doable, especially on that^ salt flat - if you have two close holes and a 3rd one 55km away, a flat earth will let you line up the laser through all three with less than 2m of adjustment. By my rough calculations, on a spherical earth, the laser will be ~250m above ground at 55km - pretty obviously proving the hypothesis incorrect, no?

That's a great example, thanks!

Sorry, I fail to see what would be the problem with gravity here. Do you mean we should consider that gravity curves the laser beam?

Yeah I think they mean the latter. Satellite footage could be doctored after all, unless you're launched the satellite.

It's not really that hard to estimate the circumference of the earth and prove that it has to be spherical. But most methods I know of require quite a bit of driving, or friends that are far away and can take a measurement at exactly the same time.

I'm not a flat earther but I do want to do my own measurement of the earth's circumference one day for fun.

> and prove that it has to be spherical

To be precise, the earth is not spherical. It's unfortunately not even an ellipsoid: I live well above the sea level yet my home is like 20 meters below the surface of the WGS84 reference ellipsoid.

More like a geometric method used in https://news.ycombinator.com/item?id=29975544

The simplest way I could come up with while discussing the issue with my kids, was to call a grandma on the other side of the globe and ask here when did she have a sunset. It still could mean that the earth is a hemisphere, but at least it's not pancake-style flat.

Or that there is more than one sun. Once you are willing to throw all science out the window there are a lot of options.