Proofs that Earth is not flat? [duplicate]

Well, I do not believe that Earth is flat, but I met some conspirationistas believing that all physics we learn in school is twisted, all the Earth's pictures from space are photoshopped by NASA, etc, etc.

So those guys wanted to put a satellite into orbit, take some pics, and see for themselves.

Of course, a satellite won't stay above a flat Earth, but what other very simple means to prove/check that earth is round (well, geoid) do we have?

I am looking for simple means, available to anyone, that do not need any external tool or knowledge that a naysayer would reject.

Some things I've considered:

• Meteorological balloon with camera and a phone with GPS, with a parachute to recover it. It has been done by amateurs, and in good conditions, you can see the Earth's curvature.

• Measure the Sun's height above horizon at noon at two different latitudes. The difference, along with some simple geometry would give you Earth's diameter.

• Possible duplicate: physics.stackexchange.com/q/26427/2451 Related: skeptics.stackexchange.com/q/453 Aug 12 '13 at 15:00
– user20250
Aug 12 '13 at 15:02
• Why don't you buy an air ticket and travel from Europe to America, then to Asia, Australia, Africa, Latin America... and try to draw the continents in a space so that their mutual distances correspond to the time spent by the flights? You will see that they have to be arranged on a globe. Or at least ask some credible sources who have flown in these flights how much time it took. Aug 12 '13 at 15:25
• Isaac Asimov wrote a book (naturally!) entitled "How Did We Find Out the Earth Is Round?" Aug 12 '13 at 16:18
• @LubošMotl unless the sceptics are flying the planes themselves, I'd anticipate an argument like "clearly the airline operators are in on the conspiracy and adjust travel speeds to make the Earth look round. :/ Aug 12 '13 at 16:35

Watch a boat through binoculars as it sails away from you. Much easier.

• Be sure to do this from many points, looking in many different directions. We could be living on a cylinder! Aug 12 '13 at 16:24
• Correct... It also does not address any concerns about localized bulging of the planet that does not mimic the general shape. Aug 12 '13 at 17:05
• After having participated in an interview with a flat-Earther, I can tell you that this argument doesn't work. For some reason that I never really understood, they are able to convince themselves that the vanishing point on a flat Earth lies below ground, so that sufficiently distant objects appear to go below a "horizon".
– pela
May 8 '17 at 20:43
• The vanishing point is called "horizon"! Jul 6 '17 at 13:37

Collect observations of lunar eclipses from all over the world.

Note that earth's umbra, as made visible on the surface of the moon, always has a circular cross-section.

Consider what shape casts a round shadow in every direction...