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Worldbuilding

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    $\begingroup$ I agree, assuming that all other factors are the same (gravity, atmosphere, smell, etc.), sailors would probably notice first if nobody's directly looking or using scientific instrumentation $\endgroup$ Commented Jan 23, 2020 at 13:36
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    $\begingroup$ Assuming normal person height, the horizon is about 5 km away on Earth. The haze and distance might well make unaided eyes unable to see a ship at the point it goes behind the horizon. The horizon would be something-like 100 times as far. You would need EXTREMELY good eyes to see a ship at the about-500 km away. A smallish telescope would be required. But they would certainly be able to see it for much more than the roughly 5 km to the horizon on the Earth. And probably for a couple of days instead of the hour or less for a ship on Earth. $\endgroup$ Commented Jan 23, 2020 at 14:36
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    $\begingroup$ Flatter Earth would definitely make sea navigation much easier, if you can keep a visual on your destination. Two spires could denote a ship lane at close range, and then you'd use the horizon shape as you get further away. $\endgroup$ Commented Jan 24, 2020 at 14:30
  • $\begingroup$ "Similar triangles" is trigonometry is it not? $\endgroup$ Commented Jan 24, 2020 at 17:16
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    $\begingroup$ @JohnO No, it's simple geometry. You don't need any tables, and don't need to know the value of any of the angles in degrees, radians, or what have you. Just need to know the lengths of the legs and have a way to be sure the triangles are similar, i.e. provably have the same angles. $\endgroup$ Commented Jan 24, 2020 at 17:17