A group of people who believe the Earth is flat have announced their “biggest, boldest, best adventure yet”: a Flat Earth cruise scheduled for 2020. The cruise, organized by the Flat Earth International Conference, promises to be a lovely time. Flat earthers – who include the rapper B.o.B. and reality television person Tila Tequila – will be able to enjoy restaurants, swimming pools and perhaps even an artificial surf wave. There’s just one problem for those seeking to celebrate the flatness of the Earth. The navigational systems cruise ships, and other vessels, use rely on the fact that the Earth...