Forget about the improbabilities of actually manufacturing such a thing, but let's assume someone made a sphere, say, the size of the moon, but hollow. Sorta like a soap bubble. And let's assume it's made out of a very dense (e.g., lots of mass) material, with a wall thickness of maybe 200 feet. Assuming it were not close enough to any other large body that their gravity were a significant factor, if you were standing on it, where would its gravity pull you? I.e., where would the "center" of its gravity be?
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