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The Northern, Southern, Eastern, and Western Hemispheres are just four of the infinite hemispheres of a sphere.

By "canonical definition of hemisphere" I'm talking about the surface of any half-sphere that covers half of the Earth; the "joke" in my proof is that you can choose any one.

For any two continents there's always a hemisphere that contains both of them (assuming you don't worry about the semicircle in the border of the hemisphere). If you put three equidistant continents with the maximum possible distance between them (so that they form an equilateral triangle whose centre is the centre of the sphere) then you can't do anything about it.

Think it like shining a light from some point in space that lights 80% of the land in Earth.




I see, that joke went over my head.

That is what I initially meant by having "freedom to define hemispheres however you want", eg set arbitrary centers.

That certainly makes it a more interesting problem and increases the chances substantially.




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