Eratosthenes (c. -276 to -194) was the Alexandrian scientist from which the West gets:
- the word “geography”,
- the musical scale,
- leap days,
- the idea of prime numbers,
- dating events in history,
- dates of many events of ancient history.
And that was not even his day job.
Ptolemy III of Egypt had put him in charge of the Library of Alexandria and made him tutor to his son, the future Ptolemy IV.
Like Columbus, Eratosthenes said the Earth was round and that you could sail west from Spain to reach India – some 1700 years before Columbus did.
In our time he is best known for his amazingly accurate measurement of the circumference of the Earth. But no one knew how right he was till 1522, after the voyage of Magellan.
In his own time he was best known for solving the Delian problem: doubling the cube. It was one of three great problems of Greek mathematics, along with squaring the circle and trisecting an angle.
Round Earther: Eratosthenes believed the Earth was round because its shadow was round during an eclipse of the Moon. And, from living in Alexandria, he knew you could see a whole ship from the top of the Pharos lighthouse while on the ground you might only see the sails peeking above the horizon. That made sense on a round Earth, not a flat one.
Measuring the Earth: He did it with a stick, a well, and a royal pacer. At noon on the first day of summer, the rays of the sun reach the bottom of a well in Syene (now Aswan) in the south of Egypt. That meant the sun was directly overhead. At that very same moment a stick to the north in Alexandria casts a shadow of 7 degrees. That meant 7 degrees of the Earth’s 360 degrees lay between Alexandria and the well. Once the royal pacer he hired had walked back from the well to Alexandria, pacing out the distance, all Eratosthenes had to do was divide the distance by 7 to get the length of a degree. From that he got a circumference of 252,000 stades or about 39,700 km. The right value is 40,008 km.
Other measurements:
- tilt of the Earth’s axis: 23.85 degrees (our value: 23.77)
- distance to the Sun: 125.5 million kilometres (our value: 150.0)
- distance to the Moon: 122,300 km (our value: 384,000)
Notice he gets at least the scale right, which seemed way too big to people back then.
His books: He wrote at least 12 books, among them:
- “Geographica” – where the word “geography” comes from.
- “On Ancient Comedy”
- “Erigone” – the story of Virgo.
- “Hermes” – epic poem about the god Hermes.
- “Hesiod” – epic poem about the death of the poet Hesiod.
- “Chronologica” – history with dates! Introduces the leap day (did not catch on till 200 years later).
- “Platonicus” – includes the Sieve of Eratosthenes for finding prime numbers.
- a star catalogue of 675 stars
- a book of constellations and the stories behind them
All are lost – all we have are bits and pieces of them quoted in other books.
– Abagond, 2018.
Source: mainly Google Images; “The Rise and Fall of Alexandria” (2006) by Justin Pollard and Howard Reid.
See also:
- Alexandria
- Archimedes – a friend
- Hipparchus – carried on some of his work
- Rosetta Stone – from his time
- Columbus
- Africans in the Greek and Roman world
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@ Abagond
His measurement of the distance to the moon seems more widely off than his other measurements. Any idea why he was so far off on this one when the others are astoundingly close?
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Considering that people build on the knowledge that came before them, you have to wonder what someone like this would have done if born in our day and age. What could Eratosthenes have accomplished with the knowledge base of Alfred Einstein or Stephen Hawkings, for instance?
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@abagond
You seem to have exagerated his deeds…
For exemple, he didn’t invent chronology. He wasn’t even the first greek to do it. The babylonians has a chronology since about 900 BC, they used it to register astronomical observations and study its correlation with mundane events. This “Age of Nabonassar” system used the egyptian year of 365 days. Probably the egyptians had a similar system.
As for the greeks, Timaeus was mentioned by Diodor of Sicily as the inventor of chronology, based on the 4-year cycle of olympiads
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Greek chronology:
http://penelope.uchicago.edu/Thayer/E/Roman/Texts/secondary/SMIGRA*/Chronologia.html
Age of Nabonassar:
http://penelope.uchicago.edu/Thayer/F/Roman/Texts/Censorinus/text*.html
Egyptian calendar:
https://www.britannica.com/science/Egyptian-calendar
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As for the doubling of the cube, he wasn’t the first:
http://www.takayaiwamoto.com/Greek_Math/Delian/Greek_Delian.html
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As for prime numbers, there is an african prehistorical stick with marks, these marks count 11, 13, 17, and 19. Some historians intérpreted that as a record of prime numbers
https://en.m.wikipedia.org/wiki/Ishango_bone
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@ Alberto Ribeiro
I did not say he invented those things or was the first person ever to do them, but “was the Alexandrian scientist from which the West gets” them. The Greeks and Alexandria were often a conduit, especially to Egypt.
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When one realizes that communication was not like today it is understandable why one person living somewhere would have thoughts that someone ten miles away would never hear or read”. Very few people read!
We are still finding out what people realized many years ago. For those that read, we know that Columbus had “charts”!
Like the many individuals today who know nothing about politics the majority of people in 1492 and before and long after lived day to day with little knowledge!
We have the advantage of looking back on history with all of the modern methods of interpretation and communications. They burned people at the stake for saying things in the past.
For true thinker, they realized how long it took the day to last where ever they lived, therefore, the could calculate some type of answer. Because the earth is round it probably depended on how many miles they lived from the equator. They may not been aware of that information. (East to west – north to south)
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Eratosthenes deserves more recognition. Why aren’t Hellenistic thinkers as famous as the earlier Greeks?
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Flat-earthers are weird…
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[…] “Eratosthenes Teaching in Alexandria” (c. 1635) by Bernardo Strozzi at the Montreal Museum of Fine Arts. Note that books in Eratosthenes’ time were all in scroll form. (ht Abagond) […]
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