Personal Life and Story: Eratosthenes was born in Cyrene (modern day Libya) in 276 BC, which was at this point in time the capital of Pentapolis, the Greek states of Northern Africa. However, cities of this region were far removed from the centers of Greek and learning and power such as Athens, and it wasn't until Alexander the Great’s conquest of Pentapolis in 332 BC that the region flourished (according to the Greek humanistic standards of education and knowledge).
Early Life:
Growing up in Cyrene, Eratosthenes studied poetry, writing, reading, writing, and arithmetic while also practicing physical activities and social discourse in a local gymnasium. He eventually moved to Athens, where he continued studies in philosophy and poetry. Notable tutors of Eratosthenes during his time in Athens include Zeno of Citium, Ariston of Chios, Callimachus, and the head of the Plutonian Academy. Eratosthenes developed a passion for Plato’s work, and his first scholarly writing, Platonikos, provided mathematical proof to validate Plato's ideas.
Alexandria:
His continued work in mathematics, historical writings on the chronology of time, and his widely popular poetry all gained the eye of Pharaoh Ptolemy III Euergetes, who commissioned him as a librarian in Alexandria’s great library in 245 BC. He quickly became chief librarian, and during this time he greatly expanded Alexandria’s collection, replicated many texts, and devoted a huge section to Homer (acquiring many of his original tragedies). Eratosthenes also tutored the Pharaoh’s children, and during his time in Alexandria he accomplished the majority of his great contributions to mathematics and geography. Eratosthenes was also known for his controversial views on society, thinking of every society as both civilized and uncivilized in its own way, very different from the popular belief in Greek superiority and foreign barbarism held by many other respected philosophers, such as Aristotle.
End of Life:
Eratosthenes developed Ophthalmia, rendering him blind, and he starved himself to death rather than live without experiencing the nature he loved. He died in Alexandria in 194 BC, at age 82.
Contributions to math/science/history: Eratosthenes made several major contributions to math, science, and history in his lifetime that carry into our modern theories in all of these topics and how we interpret the world around us.
In Math:
Eratosthenes discovered a reliable method to discover prime numbers, called the Sieve of Eratosthenes. It works by generating the multiples of a given prime number as a sequence of numbers starting from that prime, with constant difference between them that is equal to that prime. This is the sieve's most important distinction from using division to guess and check each possible number for divisibility by each prime. In an updated form, this is still important in modern number theory.
In astronomy:
Eratosthenes assumed Earth was a sphere, and calculated its size with amazing accuracy, based on the angle of the sun’s rays in a well on the summer solstice. This was a huge accomplishment for human intellect in general. First to realize our planet was a sphere, then to use the powers of observation, deduction and mathematics to calculate its circumference, especially in c. 240 BCE, was an incredible feat for mankind. Eratosthenes also knew that Earth rotated once a day on its axis, which formed a line from the North Pole to the South Pole, through the center of the earth. With this knowledge, he calculated the tilt of Earth’s axis with great accuracy. Using this information, he drew the first map of the world which included meridian lines and parallel lines. He marked the equator and its size, while considering the size of the polar zones and the distance of these zones from the tropics.
In geography:
Eratosthenes was the first person to explain why and how the River Nile flooded every year, with the suggestion that it was caused by the heavy seasonal rains near the source of the river, resulting in the predictable annual flood in Egypt.
In history:
Eratosthenes made a timeline of all the scientific achievements and political/literary events of the world he could find, starting with the siege of Troy.
-Samson M. & Charlotte L.
Eratosthenes was born around 276 B.C., which is now Shahhat, Libya. He studied in Athens at the Lyceum. Around 240 B.C., King Ptolemy III of Alexandria appointed him chief librarian of the library of Alexandria. Known as one of the foremost scholars of the time, Eratosthenes produced impressive works in astronomy, mathematics, geography, philosophy, and poetry. His contemporaries gave him the nickname “Beta” because he was very good, though not quite first-rate, in all these areas of scholarship.
Geniusofasmall step, one giant leap formankind
Eratosthenes had heard from travelers about a well in Syene (now Aswan, Egypt) with an interesting property: at noon on the summer solstice, which occurs about June 21 every year, the sun illuminated the entire bottom of this well, without casting any shadows, indicating that the sun was directly overhead. Eratosthenes then measured the angle of a shadow cast by a stick at noon on the summer solstice in Alexandria, and found it made an angle of about 7.2 degrees, or about 1/50 of a complete circle.
He realized that if he knew the distance from Alexandria to Syene, he could easily calculate the circumference of Earth. But in those days it was extremely difficult to determine distance with any accuracy. Some distances between cities were measured by the time it took a camel caravan to travel from one city to the other. But camels have a tendency to wander and to walk at varying speeds. So Eratosthenes hired bematists, professional surveyors trained to walk with equal length steps. They found that Syene lies about 5000 stadia from Alexandria.
Eratosthenes then used this to calculate the circumference of the Earth to be about 250,000 stadia. Modern scholars disagree about the length of the stadium used by Eratosthenes. Values between 500 and about 600 feet have been suggested, putting Eratosthenes’ calculated circumference between about 24,000 miles and about 29,000 miles. The Earth is now known to measure about 24,900 miles around the equator, slightly less around the poles.
Eratosthenes had made the assumption that the sun was so far away that its rays were essentially parallel, that Alexandria is due north of Syene, and that Syene is exactly on the tropic of cancer. While not exactly correct, these assumptions are good enough to make a quite accurate measurement using Eratosthenes’ method. His basic method is sound, and is even used by schoolchildren around the world today.