I would be curiious if any sailors out there have anything to add. It seems to me that sailing over long distances would drive home the sphericity of the earth. For that matter 'hull down' is a very old term, and offers a direct demonstration of the sea's (earth's) shape. I know that in the days of tall sail the rough distance to another ship far off was judged by how much of it stuck up over the horizon.

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The Devil offered me power. I told him I preferred aperture.