I've heard it numerous times, that in the old days people thought that the world was flat. Most commonly I heard this in connection with the voyage of Columbus, that he believed the earth to be a sphere while most other people thought he must fail to reach India as the world is flat.
Is there any direct evidence, e.g. written documents, that shows when people first knew that the earth is a sphere? And since when was this common knowledge in the educated population?
According to Dr Karl, it was known that the earth was a sphere as far back as Aristotle. The three reasons were:
The scientists thought Columbus's voyage would fail, not because they thought the earth was flat, but rather because he would have to travel 20000 nautical miles, rather than 5000.
So, for the last 2500 years, in Europe and in the Middle East, the flat-earthers were in a very small minority. At least, this is what the historian Jeffrey Burton Russell, of the University of California at Santa Barbara, reckons.
His book, In Inventing the Flat Earth, claims that since the third century BC, practically all educated people in the western world believed in a spherical earth.
Looking as a historian into the historical record, he found tens of thousands of Christian theologians, poets, artists and scientists who believed that the earth was a sphere.
On the other hand, he could find only five Christian authorities who believed in a flat earth.
The Straight Dope also discussed this.
According to historian James Hannam, the trope that the people in the Middle Ages believed that the Earth was flat was created in the 19th Century.
It is not difficult to see how the story of Columbus was adapted so that he became the figure of progress rather than a lucky man who profited from his error. According to Jeffrey Burton Russell here, the invention of the flat Earth myth can be laid at the feet of the nineteenth century writer Washington Irving, who included it in his historical novel on Columbus, and the wider idea that the everyone in the Middle Ages was deluded has been widely accepted ever since.
The Real Flat Earthers
The myth that Christians in the Middle Ages thought the world was flat was given a massive boost by Andrew Dickson White's weighty tome The Warfare of Science with Theology published in 1896. This book has become something of a running joke among historians of science and it is dutifully mentioned as a prime example of misinformation in the preface of most modern works on science and religion. The flat Earth is discussed in chapter 2 and one can almost sense White's confusion that hardly any of the sources support his hypothesis that Christians widely believed in it. He finds himself grudgingly admitting that St Clement, Origen, St Ambrose, St Augustine, St Isodore, St Albertus Magnus and St Thomas Aquinas all accepted the Earth was a globe - in other words none of the great doctors of the church had considered the matter in doubt. Although an analysis of what White actually says suggests he was aware that the flat Earth was largely a myth, he certainly gives an impression of ignorant Christians suppressing rational knowledge of its real shape.
Here is the link that Hannam is referring to. In it Jeffrey Burton Russell writes:
In my research, I looked to see how old the idea was that medieval Christians believed the earth was flat. I obviously did not find it among medieval Christians. Nor among anti-Catholic Protestant reformers. Nor in Copernicus or Galileo or their followers, who had to demonstrate the superiority of a heliocentric system, but not of a spherical earth. I was sure I would find it among the eighteenth-century philosophes, among all their vitriolic sneers at Christianity, but not a word. I am still amazed at where it first appears.
No one before the 1830s believed that medieval people thought that the earth was flat.
The idea was established, almost contemporaneously, by a Frenchman and an American, between whom I have not been able to establish a connection, though they were both in Paris at the same time. One was Antoine-Jean Letronne (1787-1848), an academic of strong antireligious prejudices who had studied both geography and patristics and who cleverly drew upon both to misrepresent the church fathers and their medieval successors as believing in a flat earth, in his On the Cosmographical Ideas of the Church Fathers (1834). The American was no other than our beloved storyteller Washington Irving (1783-1859), who loved to write historical fiction under the guise of history. His misrepresentations of the history of early New York City and of the life of Washington were topped by his history of Christopher Columbus (1828). It was he who invented the indelible picture of the young Columbus, a "simple mariner," appearing before a dark crowd of benighted inquisitors and hooded theologians at a council of Salamanca, all of whom believed, according to Irving, that the earth was flat like a plate. Well, yes, there was a meeting at Salamanca in 1491, but Irving's version of it, to quote a distinguished modern historian of Columbus, was "pure moonshine. Washington Irving, scenting his opportunity for a picturesque and moving scene," created a fictitious account of this "nonexistent university council" and "let his imagination go completely...the whole story is misleading and mischievous nonsense."
You can see Aquinas - who lived in the 13th Century - referring to the sun lighting up the "hemisphere" at Question 67 of the Summa Theologica, which is evidence that it was a commonplace understanding that the Earth was a sphere at that time. [Interestingly, Aquinas disputes that light is a body on the grounds that the movement of light cannot be discerned at large distances. His reasoning is impeccable; his measuring instruments were insufficient; his conclusion was wrong.]
In case the quotes from Aquinas seem too indirect, here is a far less indirect quote from a respected authority, Bede the Venerable, from more than 500 years earlier:
The Earth is a sphere, set in the middle of the universe. It is not circular like a shield or spread out like a wheel, but looks round, like a ball.
This plaque with that quote has been on open display in the British Library: 
Bede was a well known Benedictine Monk, teacher and scholar in 7th and 8th century Northumbria, known as "the father of English history" for writing Ecclesiastical History of the English People. He was very well known across Europe in his lifetime so it is unlikely that his view on the shape of the earth was unknown or aberrant for the time.
Eratosthenes (276–195 BCE) is famous, among other things, for calculating the circumference of the Earth – which of course implies that the knowledge about Earth’s shape was available in his days.
In fact, his method of measuring Earth’s circumference is routinely taught in school today1 so it’s a bit surprising how many people still think that people in the middle age believed Earth to be flat.
1 At least in France and Germany it’s part of the standard curriculum of physics.
There is an essay by the late Stephen Jay Gould, “The Late Birth of a Flat Earth”, that discusses this very question. In a nutshell, Gould traces back the myth that people in the Middle Ages thought the earth was flat to a 19th-century origin in the form of a fabricated textbook passage. The text is included in Gould’s 1995 collection of essays, Dinosaur in a Haystack.
Gould recounts a reconstruction of the flat-earth myth from a book by historian J.B. Russell, who traces the myth back to two 1870s books, by John Draper and Andrew D. White (founder of Cornell University), respectively, which argued for a continual, and ultimately winning, struggle of reason and science against the obscurantism of the Church. But, as Gould vividly explains, scholars from the Venerable Bede to Roger Bacon and Thomas Aquinas took a spherical earth entirely for granted. The upshot: neither Dark nor Middle Ages were quite as benighted as some stories would have them.
About the fact the (educated) Middle Age people knew well that the Earth is spherical, let me add that the Earth Dante had in mind was indeed spherical, and in his Divine Comedy he describes precisely the moment in which, when he reaches the bottom of the Inferno, which is an enormous conical empty space having its vertex in the centre of the Earth, Dante goes through the centre of gravity of the planet and his "up" become his "down":
The Guide [Virgil] with labour and with hard-drawn breath
Turned round his head where he had had his legs,
And grappled to the hair [of Satan, who stands at the centre of the Earth], as one who mounts,
So that to Hell I thought we were returning.
(Inferno, XXXIV, 78--81, translation by Longfellow; see also the following lines).
Myth of the Flat Earth I found this to be a very good source
We all know that Christopher Columbus encountered stiff resistance about his idea of sailing off West to try and reach the East Indies. Many of us have laboured under the impression that people were concerned that he would sail off the edge of the Earth which was widely believed to be flat. History is thought to have vindicated Columbus against those filled with the Christian superstition of a flat Earth who held on to old fashioned beliefs. A minority of people are even under the impression that Galileo's trial centred on the subject rather than whether the Earth orbited the sun.
It comes as some surprise, therefore, to find that Columbus was wrong and his critics were right - not because the world is actually flat after all, but because at the time everyone knew it was a globe and were arguing about how big it was. The idea that the uncouth people of the Middle Ages thought the Earth was flat is an example of the myth that has been propagated since the nineteenth century to give us a quite unfair view of this vibrant and exciting period."
[...]
In particular a committee set up in Salamanca examined the plans and rejected them on the grounds that Columbus had underestimated the distance he would have to travel.
Eratosthenes of Cyrene (Wikipedia)
My man Carl Sagan tipped me off to this one in the Cosmos. If you haven't seen it, go watch it now! Youtube! Free and available to everyone because he was a beast!
I do think it was a generally accepted concept among those who cared to think about it before Eratosthenes, but he gets the credit for proving it.
Eratosthenes had proved the Earth was round and calculated its circumference to within 1% of its actual circumference (he calculated 1°, i.e. 1/360, of circumference to be 59.5 nautical (6076 ft) miles) in ~200BC and his writings were well known among educated people at the time (And since universities were still primarily religious and geared towards theology, this included the Church leadership). Columbus actually massively underestimated how far west Asia due to 2 mistakes:
In combination, this made sailing West to Asia (without the Americas in the way) a workable proposition from a 101% suicide mission. After been repeatedly told that he's an idiot by people in the know, Columbus famously convinced queen Isabella of his idea and the rest, as they say, is history.
That's why, to his last breath, he refused to believe he had discovered a new continent, because that would make him a lucky fool and the continent is named America and not Columbia.
Aryabhata (476-550 AD), an Indian mathematician and astronomer, in his Aryabhatiyam used the word "Bhoo Gola (भूगोल)" to refer to the Earth in which the word "भू (Bhoo)" means the earth and "गोल (Gola)" means a sphere or the globe.
In the introduction of the book, source[1] mentions that
The "गोलपाद (Gola Paada)" section of the Aryabhatiyam deals with the motion of the Sun, Moon, and the planets on the celestial sphere. It describes the various circles of the celestial sphere and indicates the method of automatically rotating the sphere once in twenty-four hours ; explains the motion of the Earth, Sun, Moon, and the planets; describes the motion of the celestial sphere as seen by those on the equator and by those on the north and south poles, and gives rules relating to the various problems of spherical astronomy. It also deals with the calculation and graphical representation of the eclipses and the visibility of the planet.
Aryabhata believed that the Earth rotates about its axis and calculated the period of one sidereal rotation of the Earth equal to be 23:56:4.1 while the modern value is 23.56:4.091
Sources :
Aryabhatiya of Aryabhata by Kripa Shankar Shukla,1976, Indian National Science Academy, New Delhi.
Aryabhata - Wikipedia article.
