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atmospheric refraction could greatly extend that distance. See http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/mirage.html...

"The crew of a Canadian ship in the Arctic reported the mirage of a sailing ship off in the distance, upside down. The image was so sharp that the crew could make out the ropes in the rigging and could see people moving about on the deck. Two months later, the ships actually met. When the captains compared their ships' logs, they found that at the time of the sighting the two ships had been 80 miles apart!" (Gallant) The suggested explanation for a visible mirage at such a great distance is the observation of light traveling a long distance when a layer of cooler and denser air is sandwiched between two layers of warmer air (a temperature inversion).



Except the distances here are twice that, and the farther you get apart the more the curvature matters.


Yes true, but presumably this report is for observers standing less than 10m off the surface, not on tall mountains or towers




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