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Old 06-01-2017, 11:51 AM
Chronos Chronos is offline
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Join Date: Jan 2000
Location: The Land of Cleves
Posts: 72,140
At any height, from earthworm-level to the Moon or beyond, the horizon will form a perfect circle (assuming a perfectly spherical Earth). What changes is the angular size of that circle. From ground level, the circle is 180 across. From the Moon, it's only a couple of degrees across. At any height in between, it'll be somewhere in between those. How high you would have to be, then, to "see the curvature of the Earth" depends on how small a deviation from 180 you're able to discern. 42,000 feet or 50,000 feet is either one probably not enough. On the other hand, at 50,000 feet you'll definitely see some difference from being above so much of the atmosphere.