Doesn't everyone know that in the 15th century, people thought the Earth was flat? And that brave Christopher Columbus sailed westward from Spain to get to Asia to show that the Earth was round? Once I'd have answered “Yes” to both of these questions, but it turns out that they are myths.

The Greeks
The Greeks took a lead in accepting a spherical Earth, the earliest known reference being in the sixth century BCE. However it was Aristotle (384-322 BCE) who set out evidence for it:

• Ships seem gradually to disappear over the horizon, the top of the mast being last to go.
  
• During lunar eclipses, the Earth's shadow is always curved.
  
• The Sun's elevation (the angle it makes with the horizon) changes with latitude.
  
• The elevation of the stars changes with latitude and there are stars you can only see by traveling north or south.
  

Aristotle's cosmology (structure of the Universe) was based on concentric spheres with a spherical Earth at the center. The Sun is between Venus and Mars.

By the time of Eratosthenes (276-195 BCE) the spherical Earth was a given. Eratosthenes was interested in its size, and his calculation was surprisingly close to the modern value.

Ptolemy (c. 90-168 CE)
In the second century CE, Claudius Ptolemy wrote the Almagest, which remained an authoritative astronomical work for well over a thousand years. In this work Ptolemy defined the 48 classical constellations. He also built on Aristotle's cosmology with its geocentric view, in which the center of the cosmos was the spherical Earth.

The Middle Ages
The Middle Ages are often portrayed as an intellectual void between the classical period and the Renaissance. The void was filled by a tyrannical Church which decreed that the Earth was flat.

We'll ignore this ridiculous portrait of several centuries of history, except to make it clear that the cosmic view of the Church and other scholars was based on Aristotelian principles, as developed by Ptolemy. Even though some biblical language seems to describe a flat Earth, educated people didn't think the Earth was flat any more than they do today.

The definitive medieval astronomy text, De sphaera mundi (On the Sphere of the World) by Johannes de Sacrobosco (1195-1256), described Earth as a sphere. It was based on Ptolemy's Almagest and the work of Arabic astronomers. Appearing around 1230, it was copied by hand for over two hundred years until the first printed copy appeared. De sphaera then remained a university textbook for another two hundred years.

Columbus
Around the middle of the fifteenth century, the Silk Road to India and China had become quite dangerous. Countries that traded with the East were interested in a sea route. In 1488, a Portuguese explorer had found the Cape of Good Hope, which seemed to promise such a route.

But what about just sailing west to Japan, as suggested by prominent Florentine Paolo Toscanelli? Columbus liked it. As it happened, no one doubted that theoretically you could get from Europe to the Orient by sailing west. But Toscanelli couldn't get any official recognition for his idea because most people didn't accept his figures. His calculation led Columbus to a calculation of the circumference of the Earth that was too small. Yet Columbus expected to find a new route to the treasures of the east that would make him rich and famous.

The rulers of Spain referred Columbus's proposal to experts at a meeting in Salamanca. The commission advised against it, but not because anyone thought Columbus would drop off the edge of the Earth. Their discussion concerned the size of the Earth, and therefore the distance from Spain to Japan. According to the measurement of Eratosthenes, Japan was much farther away than Columbus thought it was.

As it happened, the scholars were right to question the distance, but Columbus lucked out. Although Japan wasn't where he thought it was, there was a Caribbean island. Therefore the crews of the Niña, the Pinta and the Santa Maria didn't starve to death about a quarter of the way across the Atlantic.

Columbus made four trips to the region, and he continued to insist it was somewhere in Asia.

The myth of the flat Earth
But why were medieval scholars and educated people of the fifteenth century accused of a bigoted belief about the shape of the Earth? That's not certain. But when an idea is persuasively published, it becomes a reference for others who pass it on without further research. Eventually, even a mistake becomes something “everybody knows.”

The flat Earth notion may have begun when American writer Washington Irving got overly creative in spicing up his biography of Columbus. He makes Columbus a heroic figure battling prejudice and religious bigotry. There are even hints that his espousal of the round Earth might lead him to the Inquisition. Irving described in detail the alleged arguments from scripture and references to obscure texts about a flat Earth. And he wrote, “One great difficulty was to reconcile the plan of Columbus with the cosmography of Ptolemy.” Yet, in reality, this wasn't a problem.

Evolutionary biologist Stephen Jay Gould and historian Jeffrey Burton Russell have cited the perceived battle between science and religion as a motivation for some writers. This battle heated up towards the end of the nineteenth century over the controversy surrounding evolution, as presented by Charles Darwin, and independently by Alfred Russel Wallace. Accusations of flat Earth orthodoxy was a good stick for beating theologians.

Columbus never set foot on the American continents, and to his dying day, maintained he had sailed to the Orient.