Aunt Spray/Shutterstock
What would the horizon look like on a disc-shaped planet? (continued)
Derek Bolton
Sydney, Australia
Several previous correspondents seem to have taken the mass centre of the disc to be its centre of gravity, the pull of gravity always being towards that point. But only spherically symmetric masses have fixed centres of gravity; for others, its apparent position changes as one navigates it.
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Although walking towards the edge of the disc would be going ‘uphill’, it wouldn’t be as steep as has been suggested
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A disc with the same volume, mass and surface area as Earth would be 25,000 kilometres in diameter and 2000 km thick. Although walking towards the edge of the disc would be going “uphill”, it wouldn’t be as steep as has been suggested. Wherever you stand, nearby ground exerts a stronger pull than that further away. As a result, the force and direction of surface gravity wouldn’t change radically until nearing the perimeter. Closer in, there would be lots of nearby ground around you.
Above the disc centre, gravity’s strength would be 90 per cent of that on Earth’s surface, falling to half that near the perimeter. Even at 80 per cent of the way to the edge, gravity would be 80 per cent of that on Earth and the apparent slope of the ground a manageable 20 degrees. Going all the way to the edge would take equivalent effort to climbing over 1000 km.
Adding spin would provide a centrifugal force, helping keep the effective gravity more “normal”, but it would still seem hilly, like a curvy “w” in profile – a central mound surrounded by a valley, rising to a peripheral ridge. The spin would reduce the possible atmosphere, any extra spilling over the ridge and into orbit.
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