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Eratosthenes teaches in Alexandria, in a painting by Bernardo Strozzi, 17th century
There are still those who don’t believe the earth is round, despite all the scientific advances from geography to astronomy. But for one ancient sage, Eratosthenes of Cyrene (276 BC 194 BC), it only took a stick to ascertain this fact and still be able to estimate the size of the planet’s circumference with good estimate accuracy.
Eratosthenes started from the knowledge of an important phenomenon: the summer solstice, that is, the day when one of the poles of the earth is at its maximum inclination towards the sun. When he held the post of Director of the Library of Alexandria, he found a scholarly manuscript which stated that in what was then the city of Siena now called Aswan, in southern Egypt on that particular date of the year the midday sun was perfectly perpendicular to the ground, at the socalled zenith, so it was possible to see it easily at the bottom of a well.
This gave him a flash, a breakthrough. If he were to measure the inclination of the sunlight somewhere else around midday on the solstice, with a basic knowledge of mathematics he would be able to calculate the circumference of the earth if he knew the distance from one point to the other. To do this, it would suffice to use a trigonometric relationship.
Of course, it required preparation. Eratosthenes did what was customary at the time: he hired a wanderer. They were professionals trained to walk long distances with regular strides to accurately measure distances between cities. However, before resorting to practice, the thinker thought that it would be possible to calculate this distance using mathematics. “He intended to discover the distance between Siena and Alexandria based on the time traveled by camels,” says geographer Leandro Sales Esteves, professor at the Universidade Presbiteriana Mackenzie (UPM). “But he scrapped the idea due to the lack of accuracy of this method.”
According to the geographer, in order to hire traveling surveyors, Eratosthenes needed the approval of the Egyptian government.
It is about 800 kilometers as the crow flies today the shortest route on the road is 1011 kilometers and can be covered on foot in 204 hours. In the unit of measurement at that time (stadium, that was just over 157 meters), the distance was determined with 5040 stadiums. On Midsummer’s Day in Alexandria, Eratosthenes fastened a stick perpendicular to the ground. The idea was to measure the length of the shadow cast by the stick on the ground at noon and thus find the angle of inclination.
It reached the number 7.2 degrees, that is, the total circumference (360º) divided by 50. In this way, it was enough to multiply the distance between the two cities by 50 to carry out the mathematical triangulation already known the total size of the earth. Eratosthenes came up with the equivalent of 39,750 kilometers. Let’s be honest, very close to what is known today: the circumference of the earth is 40,075 kilometers.
“Eratosthenes also calculated the radius of the earth with great accuracy and came up with a measure of 6,366 kilometers,” says Esteves. “Currently, this measurement is known to be 6,371 kilometers.”
who was
Eratosthenes was a wise man of ancient Greece. And being wise then meant accumulating knowledge, which today is spread across different areas. Although usually classified as a philosopher, he was a mathematician, grammarian, poet, geographer, librarian, and astronomer.
He was born in Cyrene, a city in North Africa, in presentday Libya, which was then part of the Greek world. “Ancient Greece was not a single country, but a group of citystates,” explains geographer Eliseu Savério Sposito, professor and researcher at São Paulo State University (UNESP). “The cities were independent of each other, but maintained common customs.”
According to the teacher, this meant that they built up contact networks and cultivated similar practices of religiosity, language, artistic practices and political models.
According to ancient records, Eratosthenes was brought to Athens as a young man to study with the leading philosophers of his day. There he finally drew attention to himself with his skills. Then the ruler of Egypt, Ptolemy 3rd Evergetes (280 BC 221 BC), ordered him to be brought to Alexandria. Originally he was to be the tutor of the son of the powerful Ptolemy.
But some time later, Eratosthenes took on a position that would make the eyes of every intellectual of the day light up: he became librarian and director of the Library of Alexandria, the great repository of ancient Greek knowledge.
It was at this post that he had the idea for the experiment that would anchor him. But this was not his only contribution to universal knowledge. He is also considered the founder of geography, as a field of human knowledge. This is because he published a work entitled Geográfica, in which he coined his own vocabulary of technical terms.
For Eratosthenes, the title of first geographer should be attributed to the poet Homer (928 BC 898 BC), as he wrote a number of climatic and topological descriptions. Written in three volumes, Geográfica eventually had excerpts quoted by several scholars over the following centuries, including the Roman naturalist Gaius Pliny II (2379), better known as Pliny the Elder.
Geográfica was lost over time and today only 155 fragments of the work are known precisely because of these quotations in other works.
flat earth vs. round earth
For contemporary researchers, however, a relevant point of Eratosthenes’ discovery is that it shows that the notion of a spherical earth was already in effect. After all, for someone contemplating measuring the circumference of the planet, it was first necessary to start with the understanding that there was a circumference to be measured.
“The achievement of Eratosthenes proves beyond any doubt that the idea that the earth is round has existed since ancient times,” comments historian Vítor Soares, who oversees the podcast História em Meia Hora. “This proof question is interesting because we have both a philosophical and a mathematical question in it.” Eventually, the wise man of antiquity used a trigonometric method to achieve his purpose.
“It has been known since the Greeks that the earth is round,” says Sposito. “When the Greeks developed astronomy, a branch of mathematics, they created threedimensional models to explain the apparent motion of the planets. That was still in the 4th century BC.”
The geographer mentions several names in addition to Eratosthenes. The philosopher and astronomer Heraclides of Pontus (390 BC 310 BC) proposed that the earth rotates on its own axis. The astronomer and mathematician Aristarchus of Samos (310 BC 230 BC) presented the theory of the heliocentric system, in which the earth revolves around the sun.
“The cosmology developed in ancient Greece has important thinkers who provided evidence for the spherical Earth model we know today,” comments geographer Esteves. “Although the flatEarth model was used by some ancient civilizations, the spherical Earth model was extended to other regions by Greek cosmology, particularly due to the influence of thinkers such as Pythagoras, Aristotle, Ptolemy, and Eratosthenes.”
In this sense, Eratosthenes did science the same way science is done today: he used to build on the research of his peers and make progress. In your case through a concrete experiment.
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Eratosthenes of Cyrene was a philosopher, mathematician, grammarian, poet, geographer, librarian and astronomer
“It has the task of empirically proving an idea already put forward by other Greek thinkers,” points out the geographer Claudio Eduardo de Castro, professor and researcher at the State University of Maranhão (Uema). “In addition, there are maps of Han Dynasty China at almost the same time on a scale of 1:90,000 that show an orthogonal spatial grid.”
According to Castro, “Thanks to our ignorance of Eastern cartography, and in spite of it, we can ask whether the progress of knowledge about the Earth in the West was based on knowledge from the East, which so often used this cartography in its voyages to Europe”. Or, alternatively, ” when the cartography of the spherical, western earth was rapidly assimilated and practiced in the east”.
For the historian Soares, this Flat Earth vs. Round Earth debate is “interesting because it is fundamentally a dispute over narratives” since the scientific knowledge about it is very old.
“Many people believe that in the Middle Ages the church spread the idea that the earth was flat or something. But that’s a myth,” he defends himself. “We can prove this with art. If you look at different paintings depicting religious figures or even members of the church, there are images of them with a globe in their hands, symbolizing the world.”
The screenwriter of the podcast História em Meia Hora, history professor Victor Alexandre, cites science historian Christine Garwood’s book, Flat Earth: The History of an Infamous Idea, to contextualize that in the 18th and 19th centuries, philosophers and thinkers emerged who they were interested in “smearing” the image of the Middle Ages by valuing both classical antiquity and modern times.
“As part of this attempt to suggest that the Middle Ages was a time of darkness, these thinkers inserted the idea that people during that time believed the earth was flat,” says Alexandre. “The goal was to reinforce the notion that modernity was a time of cultural and scientific salvation.”
Even flat earths, if they still exist today, apparently existed both in antiquity and in the Middle Ages. The historian Soares quotes the thinker, encyclopedist and later considered a saint by the Catholic Church Isidore of Seville (560636). “But these thinkers were a minority compared to the existing consensus,” Soares points out. “The problem is how to apply these individual cases to an entire period. Even in modern times, every flatearth theory crumbled when the first navigators succeeded in navigating around the earth.”
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World map according to Eratosthenes of Cyrene
flat cards
If scientific knowledge is not fully accessible even today, in older societies this knowledge was obviously the privilege of a minority, an intellectual elite. “We know that ancient societies were generally deeply stratified and that access to knowledge and academic training was restricted to a smaller fraction,” recalls the geographer Esteves.
“In spaces where the knowledge of Greek thinkers circulated, the idea of the sphericity of the earth became dominant. In addition, the scientific foundations presented by these thinkers became an important reference for scientists in the following periods,” he points out.
The geographer Castro points out that the ancient world was still “very much linked to daily work in the countryside, where the needs imposed were linked to the cycles of nature, to the places practiced, rudimentary, agriculture and livestock, extractivism and wars over these resources”.
“In this context, almost universal in societies of all sizes at the time, walking on the territory made him believe in the flatness of the earth, and this, incidentally, was limited to what he lived,” he defines. “In this ancient world cartography, an indispensable tool in the practice of location, fulfilled precisely this role: planning directions on maps, essential places for the goals of vital functions.”
The planet is naturally spherical. But after all, the map has always been flat.
“Part of this cartography may reach our days through Mesopotamian records of the Sumerian, Assyrian, Babylonian peoples, but we cannot help but highlight the Orientals, who may have predated the West and, influencing them, precise cartography already used with the purpose of delineating borders , water protection areas and military purposes,” says Castro. “Unfortunately, almost all of the cartography was lost first because it was in general use and because it was created on fragile substrates such as clay.”
For the geographer everything leads to the assumption that according to the experiment of Eratosthenes and the whole context of the time “antiquity knew of this sphericity”, but this knowledge was limited to small groups. “And that this knowledge had little impact on the daily life of societies, since such knowledge contributed little to livelihood. The cartography of a flat earth was enough for that,” he adds.
History professor Victor Alexandre points out that the current debate being sought by Flacherden is “characterized by a major rejection of scientific consensus,” in narratives often fueled “by the way social networks work.” become.
“Through the internet, people who believe the earth is flat can connect and form affective relationships that supersede any scientific truth. With that, I believe it will be necessary to become aware, to convince someone that the earth is actually spherical, and try to develop a close relationship with these people, as much as joking can be the most fun at the moment,” defended he himself.