Stand on the beach and the sea seems to go on for ever. It does not. You can work out exactly where it stops. The answer is much closer than it feels.
I can see all the way to the end of the sea.
You cannot.
You can see to a line, and the line is much closer than it looks.
How close?
That depends entirely on how tall you are. Slide the handle below from your own height up to a hilltop and watch the number move.
The curve here is drawn far rounder than real Earth. Draw it truly and it looks flat, and there is nothing to see. The numbers, though, are the real ones for the height you picked.
Now go and stand somewhere with a long view, and pick the furthest thing you can still make out. Everything past it is hidden by the curve of the planet you are standing on — not by haze, and not by your eyes giving up.
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