Fisheye lens effect on ball
The main problem with flat-earth theory is that their claims are never followed up with experimentations to test the claim. The most prime example is the claim that Earth only looks spherical because all photos are done with a fish eye lens which warps the photo into a sphere.

Retrograde Motion of Mars
Of all the discussions I've encountered in the stream of flat Earth theory, I assumed that the retrograde motion of Mars would be one topic that flat-earthers would have left alone. My assumption was based on the fact that basic geometry can easily show that Mars' retrograde motion is impossible on the flat Earth model.

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It's been surmised by flat-earthers that if the Earth is moving then we should be able to lift off of the ground in a balloon and the ground should travel below us. Others have suggested that flights going with the rotation of the globe should be impossible because the globe rotates at 1000 mph but planes only fly at about 500 mph. These theories are both part of the same false premise which is that as soon as we leave the ground we should cease to travel in the same direction and speed as the Earth.

D. Marble Spirit Level test

It's hard to believe some of the silly demonstrations thought up to prove the Earth is flat. One recent demonstration that has been filmed by a number of guys is bringing a spirit level (bubble level) on a plane and looking for the device to show the nose of the plane pointing down. Roll your eyes all you want, but this is a real test that some people take seriously. Below I will describe exactly why this test won't work.

Centrifugal Force

I've heard from a number of flat-earthers that the spinning globe doesn't make sense because the objects would fly off of it. Just like a merry-go-round, the spinning creates a centrifugal force. They usually follow that up with irreverent numbers like the fact that the Earth has a rotational speed of around 1000 mph. But they fail to understand that linear distance traveled is not related to centrifugal force. Example video below.

View of the clouds over earth

Spend enough time talking to flat-earthers and you'll find that they are easily fooled by their own senses. For example, in the live feeds from the ISS and some satellites, it's hard to see the clouds move, so naturally the images/videos must be fake, right?

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If you talk to enough flat-earthers you will discover that two theories dominate the gravity discussion. The first theory is Universal Acceleration which has already been discussed by WTC. The second theory is less developed but claims that gravity does not exist and that objects rise and fall because of density. This density theory is what this post is about.

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Here... the curve is here. In fact, smart people have already calculated the curvature. You can use this calculator to conduct your own experiments.

Sun setting over the beach

The curve of the Earth can be observed by experimenting with the horizon over varying distances. However, we have additional proof of Earth's roundness in the rising and setting of the sun. If the Earth was flat and the sun was always above the plane of the Earth, then why is it's light not always visible and why can we not see it?

Speed of Light Testing Apparatus (Fizeau)

At this time (2019) flat Earth theorists have devised two wide-spread theories about what causes "gravity", or at least what causes the effect that we call gravity. The first theory, which is fading among the older flat-earthers, is that the flat earth is accelerating upwards which causes a downward force on the surface of the earth. The second is density theory. We will address universal acceleration in this post.