Without being in the sky, it is impossible to see the curvature of the Earth. However, you can always see a demonstration of this if you visit a harbor or any place with a wide-open view of the water.
If you are able to watch a ship sail off to sea, watch its mast and flag as it fades off into the distance. You will notice that, in fact, it does not "fade off into the distance" at all; instead, you will see its mast and flag appear to slowly sink. The ship sailed beyond the point at which you would see it. Just to be sure, bring a pair of binoculars with you so that you can see even farther off into the distance.
It's as if you're watching it go over to the other side of a hill. This phenomenon can only be explained by a sphere-shaped planet.
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