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Eratosthenes Crater, the Moon © Apollo 17, NASA

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Eratosthenes (276 BC - 194 BC)

Eratosthenes came from a town called Cyrene, which was situated in an area that is now Libya, in North Africa. Back in 276 BC, this area was a Greek colony. Eratosthenes was sent to Athens to receive his education. He studied mathematics and philosophy at the famous Athenian academies. When he was in his thirties he was invited by Ptolemy III, the king of Egypt at that time, to come to Alexandria to tutor the king's son. Eratosthenes stayed in Alexandria and, in 240 BC, he was appointed third librarian at the famous Library of Alexandria.

Eratosthenes was a philosopher, a mathematician, an astronomer, a literary critic and even a poet! He wrote many treatises but, unfortunately, very few of these have survived through the ages. We only know about them because other writers at the time referred to Eratosthenes's work in their own writings. Eratosthenes was the first person to use the word geography, which means "writing about the Earth", as a title for one of his papers. His many achievements include drawing a map of the world, making a star catalogue of 675 stars, devising a calendar that included leap years, starting a chronology of world events from the siege of Troy and developing a way of finding prime numbers, called the Sieve of Eratosthenes.

Eratosthenes : Public Domain Image

He is, perhaps, most remembered for calculating the circumference of the Earth. He did this by measuring the angle of the sun at Alexandria during the summer solstice. He calculated this at 7° to the vertical, using an upright pole to cast a shadow. He knew that, 800 kilometres to the south of Alexandria there was a city called Syene (we know it as Aswan) where the sun only reflected water from the bottom of a well on the summer solstice. This meant that the sun was directly overhead, so the angle of the sun was 0°. It was amazing that Eratosthenes's calculation of around 40232 km was only 160 km more than the actual circumference of the Earth! Apparently, he did make mistakes in his calculation but, miraculously, his mistakes cancelled each other out.

Despite all of his achievements, Eratosthenes was never considered at the best in any one field of study. His nick-name, rather cruelly, was "Beta", meaning second best. In his old age he became blind and this sent him into a depression. He died in his early 80's from voluntary starvation. Considering all his achievements in his life-time, he well deserves to have a Moon crater named after him!

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