Man, even pagan man, has always known that the earth was a sphere! The Hebrews knew it first, because God told them six times in the Old Testament that the earth was a sphere. That was sufficient for believers!
Pagan man also knew that the earth was a sphere. Too many people seem to think that ancient man was illiterate, ignorant or stupid. I would remind you, however, that ancient man was able to do works of engineering that we have not been able to duplicate in modern times.
The ancient Sumerians have been well documented as having realized that the earth was spherical in shape. Their astrological charts that show constellations BELOW the region's view provet his fact!
First, men were quite capable of reasoning that the earth was round. The shadow of the earth upon the moon was quite sufficient to prove the point. Only a sphere will cast a round shadow regardless of orientation. If the earth were a flat circular shaped object, like a pie pan, it would cast an elliptical shadow during part of the night.
Second, men were quite aware that as one traveled from near the equator of the earth toward the north pole that the North Star would appear higher and higher in the night sky. In addition, the rotation of the earth was quite obvious, as all stars made a circular path around the North Star during the night. The angle that the North Star makes with the horizon is roughly equal to the latitude of observation. The farther north one travels the more noticeable this is.
Third, as one travels north and south over great distances, different stars become visible. This fact made it obvious that the traveler was traveling along an arc. These things were clearly known in ancient times. Everyone knew that the earth was a sphere.
Fourth, there is the weaker argument made by analogy. The lunar phases can only occur if the moon is a sphere. This would lead to a conclusion that by analogy the earth is also a sphere.
Now, we switch from reason to mathematics. Eritosthenes was born about BC 276 in Cyrene. He was well educated in the schools of Athens and Alexandria. He was a poet, philosopher and astronomer. He became the curator of the Library at Alexandria, the most famous of the ancient world.
He made measurements of the shadow of objects at the summer solstice near Aswan and at Alexandria. The difference is seven degrees. This means that the distance from Aswan to Alexandria is 7/360 of the circumference of the earth! Eritosphenes and Aristotle taught that the earth was a sphere.
The well known problem of Galileo and a church which had adopted the Greek worldview was over whether or not the earth or the sun was the center of the solar system. No one argued over whether or not the earth was a sphere. Everyone knew that the earth was a sphere.
The sole argument at the time of the sailing of Columbus was not over whether the earth was a sphere or flat; the argument was over the size of the sphere and whether the ships of the time could carry enough supplies for a voyage of that distance (thinking that the entire distance from Europe to the Orient would be over water). Period!
Make no mistake. I hold Columbus in very high regard. Columbus was a Christian missionary. His stated purpose in sailing to the Orient was to take the Gospel to an unsaved world. Where he went wrong was in arguing that it would be shorter to sail from Europe west to get to the Orient. The truth is that he was wrong. Anyone looking at a modern globe will see that it is closer to sail east!
Who then invented the flat earth concept which has been incorporated in so many of our modern day American history textbooks? Over the centuries there have been men who taught a flat earth, notably Lactantius (AD 245-325) and Cosmas Indicopleustes (AD Sixth Century). But, these men were discredited by their peers in their own times.
The story which has become so much a part of American history textbooks; the story that Columbus was warned that he would fall off the edge of the earth if he sailed west: well, that story is the invention of a struggling American author of fiction - Washington Irving (1783-1859). Yes, the story of the flat earth and Columbus is the work of the author of the Sketch Book (1820), which included his stories of The Legend of Sleepy Hollow (The Headless Horseman) and Rip Van Winkle.
Today we find it hard to believe, but in the early years of our country there were few national heroes. When we broke our ties to England we also broke our ties to the heroes of centuries past. One of the persons who helped to fill that void was Columbus. Note the large number of capitol cities named after Columbus - Columbia, SC; Columbus, Oh; and the District of Columbia.
Irving was born in New York City. In 1815, he moved to Europe. He lived in France and Spain until 1832. While in Spain he translated a collection of manuscripts concerning Columbus voyage. In 1828, he published a largely fictional work entitled The Life and Voyages of Christopher Columbus.
Irving described himself as "apt to indulge in the imagination." The obvious problem being that readers of The Life and Voyages of Christopher Columbus could not possibly tell what was fact and what was fiction. For example, his description of the Council of Salamanca was a complete fabrication.
In 1834, Antoine-Jean Letronne (1787-1848), produced a more scholarly work attempting to tie the belief in a flat earth with the early church fathers, Revue des Deux Mondes. He was educated in the teachings of Mentelle and Voltaire. In 1798, he wrote a book claiming that Jesus Christ was an imposter. In Revue he wrote that the flat earth was the majority view of the early Christian writers. His statements were untrue, but liberal scholars have quoted him for almost two centuries. Between Irving and Letronne the flat earth myth became a "well-known fact."
Two more men have served to popularize this myth. In 1874, John W. Draper wrote an antichurch diatribe entitled History of the Conflict Between Religion and Science. In 1865, Andrew D. White became cofounder of the first explicitly secular university in the U. S., Cornell University. In 1897, he wrote History of the Warfare of Science with Theology in Christendom. Both men were attempting to discredit creationists in the battle over origins.
J. B. Russell wrote Inventing the Flat Earth: Columbus and Modern Historians, 1991. He is no creationist, but he thoroughly documents how the invention of the flat earth was a direct attack against God, the history of the Church and truth in general by secular humanists who were attempting to defend the idea of Christian ignorance versus enlightened rationalists (evolutionists).