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# Biography Eratosthenes of Cyrene

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Prime numbers have fascinated scholars of mathematics since their invention. Well, maybe not fascinated, but at least they have generated interest and still do.  What are prime numbers?  Basically, a number is prime if it is a natural number greater than 1 which has no divisors except itself and 1. So the smallest prime number is 2, and there is an infinite number of them. That was determined by Euclid way back around 300 BC. There is no formula to determine all prime numbers, but there is a simple way for even a beginning math student to discover which numbers are prime and which are not. To illustrate this method, write the numbers from 2 to 100. Circle the 2 and cross out every multiple of 2. Circle the 3 and cross out every multiple of 3 which remains. Continue this process and you will have circled all prime numbers up to 100. Of course, it doesn’t stop there, but a student’s patience only extends so far. This method of determining prime numbers is known as The Sieve of Eratosthenes.

Who was Eratosthenes? Born in 276 BC in Cyrene, Greece, now known as Libya, in North Africa, Eratosthenes as a young boy showed promise as a scholar of many interests. He was surrounded by the outstanding teachers of his day, and was able for a time to travel to study in Athens. It was Ptolemy III of Egypt who heard of this exceptional young man, and invited him to be the tutor of his son Philopator and the librarian at the great university of Alexandria. This was an exceptional opportunity and Eratosthenes lost no time in accepting the offer and heading to start a new life in Alexandria, Egypt.  By now he was in his mid-thirties. In this new posting, he relished the contact with a circle of interesting and well-educated peers. As the third librarian at the most famous university of his day, Eratosthenes was able to thrive and explore new areas of study.

These studies included both astronomy and geography. Eratosthenes is best known for his measurement of the circumference of the Earth. Others before him had tried to get this done, but he was the first to succeed. He compared the shadows created by the sun at summer solstice both in Syene and Alexandria. He calculated the circumference would be nearly 250,000 stadia (25,000 miles). He also worked on a catalogue of stars in the universe and a calendar which included leap years. As a historian, he worked on a listing of events in the known world from the time of the siege of Troy. As a geographer, he made new discoveries about the Nile River and its sources, information which had eluded scholars up till then.

So what kind of man was Eratosthenes?  A curious one, one can assume. He never married, so one can presume maybe his social life was restricted. He loved learning, and spent his life in that pursuit. It is alleged that toward the end of his life, he became blind and starved himself to death when he was unable to earn a living. He was in his early eighties at the time.

How was his work recognized during his time?  It seems that his fellow scholars did acknowledge his ability in a wide range of scientific and other fields. They nicknamed him Beta, because, although he had ability in all, he was not the leader of any. This seems unnecessarily harsh when so much of his work is recognized today.

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