The view from your house may seem flat, but ascend significantly in altitude and you'll observe the edges of your visual field begin to curve. Scientists estimate that this curvature becomes noticeable at about 20 kilometers above the ground. There are other clues too, subtly hinting at Earth's spherical shape.
Long before the advent of the Space Age, early explorers, particularly seafaring ones, had deduced the roundness of the Earth. These mariners would watch ships sail away into the distance over the seemingly flat ocean surface. They noticed that the masts of these ships were the first to appear over the horizon, followed gradually by the ship's hull. This optical phenomenon, which you might have experienced yourself, would be impossible if the Earth were indeed flat.
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