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.

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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.