Why dividing Earth into two separate hemispheres is even harder than one might assume

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Dear Evil Engineer,

We hope you are able to receive this message, which we are transmitting from a communications outpost located a few light-years from Earth.

Our observations suggest that your planet remains unaware of the ongoing scramble for the planets of the Orion Arm between ourselves and our rival empires. Territorial claims have largely been settled with limited planetary destruction – however, the planet Earth, with its rich resources of water, oxygen and coffee beans, remains subject to competing claims. We have therefore signed a treaty agreeing to the partition of Earth between ourselves and those cretins from Scutum-Centaurus. We will decide afterwards who will take the northern hemisphere and who will take the southern hemisphere with a simple waveform collapse... we hope this news is not excessively distressing.

Despite our being two galaxy-spanning empires with all the unimaginably advanced...