You can prove the earth is round if you have a friend living several thousand miles away, a cell phone and a yard stick. Plant your sticks in the ground, at the same time, measure the shadows and text each other the results. You can then use math to determine the curvature of the earth. We don't need NASA to say it's so.
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If you putthe sun at about 3000 miles away, the math still work if it was a lane. Eratostene didnt prove it was round. He came up with what the circumference would be IF it was round and IF the sun was 91 millions miles away.