The idea that the Earth is a sphere was all but settled by ancient Greek philosophers such as Aristotle (384–322 BC), who obtained empirical evidence after travelling to Egypt and seeing new constellations of stars. Eratosthenes, in the third century BC, became the first person to calculate the circumference of the Earth. Islamic scholars made further advanced measurements from about the 9th century AD onwards, while European navigators circled the Earth in the 16th century. Images from space were final proof, if any were needed.
As with any fringe movement there are disagreements and several different flat-Earth models exist to choose from. Some models propose that the Earth’s edges are surrounded by a wall of ice holding in the oceans. Others suggest our flat planet and its atmosphere are encased in a huge, hemispherical snow globe from which nothing can fall off the edges. To account for night and day, most flat-Earthers think the Sun moves in circles around the North Pole, with its light acting like a spotlight. The most recent “US model”, for example, suggests that the Sun and Moon are 50 km in diameter and circle the disc-shaped Earth at a height of 5500 km, with the stars above this on a rotating dome. Many flat-Earthers also reject gravity, with the “UK model” suggesting that the disc is itself accelerating up at 9.8 m/s2 to give the illusion of gravity.