Longitude and latitude, how sailing ships navigated in 18th and 19th century. There is a tower in my town. The ball dropped at 1 pm to allow ships to set their clocks . The time and the position of the stars / sun allowed the ships to plot where they were. Not possible if the earth was flat.
I was so sad when "Russianvids" went "flat earth". It brings in to question everything he has done. I guess he was just a "psyop" after all. There must be a lot fewer transgender celebrities than I thought. I am glad that Ivanka Trump is not a bloke, quite a relief actually!