Atmospheric Duct

An atmospheric duct is a horizontal layer in the lower atmosphere in which a thermal inversion causes light rays to be trapped and continuously guided near Earth’s surface. They tend to follow Earth’s curvature without escaping to space.

Water heats up slower than air and will be cooler than air most of the time. In turn, the colder water cools down the layer of air just above it, creating a thermal inversion that can form a duct.

It is the reason flat-Earthers like to perform observations from very close to the water’s surface. It will be easier for light to bend, revealing objects that are geometrically behind the curvature. Then, they will incorrectly attribute it to “the lack of curvature.”

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Distance to the Horizon & the Black Swan Observation

The distance to the horizon depends on the height of the observer and atmospheric refraction. The variability in atmospheric refraction can make the horizon appear in front of a distant object sometimes, and behind it in other times.

Some flat-Earthers raised a case —dubbed the “black swan” case— in which the horizon appears to be behind a distant object which is farther than the distance to the horizon according to calculation, and they use the fact to “disprove” Earth’s curvature. In reality, they did not account for atmospheric refraction, and that the amount of refraction varies.

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