Why did so many people believe the world is flat when it is obviously a sphere?
Answers
The earth is not a sphere. The earth is an oblate spheroid with a diameter at the equator that is 43 km larger than at the poles, according to the National Oceanic and Atmospheric Administration. This bulging at the equator is caused by the spinning motion of the earth, much like how spinning pizza dough in the air causes it to flatten due to centrifugal force. The centrifugal force is very real in the non-inertial frame, and is not imaginary or fictional, but is simply non-fundamental in that it arises from the spinning motion of the frame itself. Furthermore, because of mountains, oceans, hills, canyons, etc., the earth is not really an oblate spheroid. But these deviations from a perfectly smooth oblate spheroid shape are only 0.001% of the earth's radius, so the earth has the shape to a very good approximation.
bulges of earth
The earth is not a sphere. It is an oblate spheroid. The flattening in this image is exaggerated in order to make the earth's non-spherical shape obvious. The bulging at the equator is caused by the centrifugal force due to the spinning motion. Public Domain Image, source: Christopher S. Baird.
Also, virtually nobody has ever believed the earth is flat. This is a myth according to historian Jeffrey Burton Russell, as explained in his book "Inventing the Flat Earth". As far back as the ancient Greeks, man has known the earth is round. Around 240 BC, Eratosthenes calculated the Earth's circumference to a fairly accurate value without ever leaving Egypt using shadows and geometry. Christopher Columbus did not sail on his famous voyage in order to prove the earth was round