Somewhere between a Lunar Eclipse, Time, and Gravity, basically if all are consistent yet happening in different time zone in different places simoutaneously, quasimodo my friend, the earth is round!
However Ringworld is an entirely different scenario ;)
Some Greeks named Erathostene and Ptolemeus (sp?) woke one day and decided that it was so. 1,500 years of Christianity tried to deny it, sending dissidents to their death.
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