The ancients were well aware the world was a sphere. Pythagoras (6th century B.C.) is generally credited with having first suggested a round Earth. Aristotle (4th century B.C.) agreed and supported the theory with observations such as that the southern constellations rise higher in the sky when a person travels south. He also noted that during a lunar eclipse, the Earth’s shadow is round.  Eratosthenes (3rd century B.C., head librarian at the Library of Alexandria) built on their ideas and calculated the circumference of the Earth with remarkable accuracy at about 252,000 stadia.  Depending on which stadion measurement he was using, his figure was either just 1% too small or 16% too large; many scholars think it likely that he was using the Egyptian stadion (157.5 m), being in Egypt at the time, which would make his estimate about 1% to small… remarkable.

St. Augustine (354–430) wrote,  “it be supposed or scientifically demonstrated that the world is of a round and spherical form.” (De Civitate Dei, Book XVI, Chapter 9) 

The monk Bede (c. 672–735) wrote in his influential treatise on computus, The Reckoning of Time, that the Earth was round. He explained the unequal length of daylight from "the roundness of the Earth.” The large number of surviving manuscripts of The Reckoning of Time, copied to meet the Carolingian requirement that all priests should study the computus, indicates that many, if not most, priests were exposed to the idea of the sphericity of the Earth. Ælfric of Eynsham paraphrased Bede into Old English, saying, "Now the Earth's roundness and the Sun's orbit constitute the obstacle to the day's being equally long in every land."

Most of the material is from "Spherical Earth"

Dante's Divine Comedy, written in Italian in the early 14th century, portrays Earth as a sphere, discussing implications such as the different stars visible in the southern hemisphere, the altered position of the sun, and the various timezones of the Earth. Also, the Elucidarium of Honorius Augustodunensis (c. 1120), an important manual for the instruction of lesser clergy, which was translated into Middle English, Old French, Middle High German, Old Russian, Middle Dutch, Old Norse, Icelandic, Spanish, and several Italian dialects, explicitly refers to a spherical Earth. Likewise, the fact that Bertold von Regensburg (mid-13th century) used the spherical Earth as an illustration in a sermon shows that he could assume this knowledge among his congregation. The sermon was held in the vernacular German, and thus was not intended for a learned audience.

The idea that Columbus sailed to the “New World” against the wisdom of his day is a complete myth, if a very persistent one. So then where did this myth actually come from? The earliest source I've been able to pinpoint is Washington Irving (author of The Legend of Sleepy Hollow) in his four-volume series titled A History of the Life and Voyages of Christopher Columbus. This work is a mixture of fact and fiction. There is a scene depicted in the book where shadowy Catholic clergymen warn Columbus that he might sail off the end of the Earth. This, of course, is not supported by any real historical data, but it nevertheless provides good fodder for Internet memes. (Jon Sorensen)

Columbus’ Error

Columbus did indeed face resistance while searching for sponsors for his voyage, but the issue of contention was not whether the earth was flat or round, but over the size of the earth. Those who opposed Columbus believed that the circumference of the earth was too great for ships to sail around to the other side. There was no talk about “falling off the edge of the world.” Columbus had calculated that the distance for his trip from the Canary Islands to Japan would be about 4,450 km, which is one-fifth the actual distance of 22,000 km. If not for the placement of the Americas in between, Columbus and his crew would have surely perished, as his critics predicted. Columbus’ voyage—and later explorations by others—did not change the perception of the shape of the earth, but merely added new land masses to the Middle Age maps of the world.

Rather than being a bold triumph of science over superstition, Columbus’ voyage is proof that sometimes even dumb blind luck can make you famous.