If you learned in school that Christopher Columbus sailed from Spain in 1492 and crossed the Atlantic Ocean, disproving a common belief in those days that the Earth was flat, then the lesson was wrong.

Historians say there is no doubt that the educated in Columbus’s day knew quite well that the Earth was not flat but round. In fact, this was known many centuries earlier.

As early as the sixth century B.C., Pythagoras — and later Aristotle and Euclid — wrote about the Earth as a sphere. Ptolemy wrote “Geography” at the height of the Roman Empire, 1,300 years before Columbus sailed, and considered the idea of a round planet as fact.

“Geography” became a standard reference, and Columbus himself owned a copy. For him, the big question was not the shape of the Earth but the size of the ocean he wanted to cross.

During the early Middle Ages, it is true that many Europeans succumbed to rumor and started believing that they lived on a flat Earth.

But Islamic countries knew better and preserved the Greek learning. By the late Middle Ages, Europe had caught up and in some cases surpassed the knowledge of ancient Greece and medieval Islam.

Several books published in Europe between 1200 and 1500 discussed the Earth’s shape, including “The Sphere,” written in the early 1200s, which was required reading in European universities in the 1300s and beyond. It was still in use 500 years after it was penned.

So how did it become common thought in the 20th century that people in the 15th century believed the Earth was flat?

In a 1991 book, “Inventing the Flat Earth,” retired University of California professor Jeffrey Burton Russell explains how the myth was perpetuated in the 1800s by writers including Washington Irving and Antoinne-Jean Letronne.

In 1828, Irving wrote “The Life and Voyages of Christopher Columbus,” which sounds like a biography but is mostly fiction. It says that Europeans learned from Columbus’s trips to the New World that the planet was round.

Letronne insisted that early Christian writers thought the Earth was flat. Though they did not, he was widely quoted for many years.

Others, too, helped perpetuate the myth.

The 1995 book “Poetry of the Universe: A Mathematical Exploration of the Cosmos,” by Robert Osserman, professor emeritus of mathematics at Stanford University, makes clear that Columbus did not worry that he would fall off the Earth’s edge.

There was, though, some concern about what would happen if he got to the bottom of the spherical planet.

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