Comments by "possumverde" (@possumverde) on "Professor Dave Explains" channel.

There's a very simple test which hardly counts as scientific. Take a spirit level to the beach, look down the top of it, line it up with the horizon, and note the bubble position. Then repeat from higher elevation(s.) As you increase elevation, the tilt of the level will also increase. That means you're having to look increasingly downwards to keep the horizon centered. On a flat Earth, the horizon should appear centered with you looking straight ahead from any reasonable elevation. (Eyesight limitations will obviously come into play at some point.)
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