He was an astronomer, historian, geographer, philosopher, poet, theater critic and mathematician. He studied in Alexandria and Athens. Around the year 255 a. C was the third director of the Alexandria Library.
Eratosthenes was born in Cyrene (Libya) in the year 276 BC. He worked with math problems, such as cube duplication and prime numbers. He wrote many books of which there is only news by bibliographical references of other authors.
One of his main contributions to science and astronomy was his work on the measurement of the earth. Eratosthenes in his studies of the papyri of the library of Alexandria, found a report of observations in Siena, about 800 km southeast of Alexandria, in which it was said that the sun's rays falling on a stick at noon on the summer solstice (the current June 21) did not produce shadow.
Eratosthenes then made the same observations in Alexandria on the same day at the same time, discovering that the sunlight hit vertically in a waterhole on the same day at the same time. He assumed correctly that, if the Sun was at a great distance, its rays upon reaching the earth should arrive in parallel, if it was flat as it was believed in those times, and no differences should be found between the shadows cast by the objects at the same time of the same day, regardless of where they were.
However, by demonstrating that if they did (the shadow left by the Sienna tower formed 7 degrees with the vertical), he deduced that the earth was not flat and, using the known distance between the two cities and the measured angle of the shadows , calculated the circumference of the earth in approximately 250,000 stadiums (about 40,000 kilometers, quite accurate for the time and its resources).
He also calculated the distance to the Sun in 804,000,000 stadiums and the distance to the Moon in 780,000 stadiums. He almost measured the inclination of the ecliptic at 23º 51 '15 ". Another astronomical work was a compilation in a catalog of about 675 stars.
He created one of the most advanced calendars for his time and a chronological history of the world since the Trojan War. He conducted geography research drawing maps of the known world, large areas of the Nile River and described the Eudaimon region (now Yemen) in Arabia.
Eratosthenes at the end of his life was affected by blindness and starved to death of his own free will in 194 B.C. in Alexandria.
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