276-195 B.C.E.

It can be said that Eratosthenes is most widely known as a famous Greek mathematician. What most people probably do not know is that Eratosthenes is not only a famous mathematician but also a well known geographer, astronomer and historian.

Before I get into a few of his accomplishments, let me tell you a little about his personal history. Eratosthenes was born in Cyrene, Greece, which is now known as Libya, in North Africa, in 276 B.C.E.. It is believed that he starved himself to death in 195 B.C.E. due to the fact that he became blind and could no longer work. As a young man, Eratosthenes studied in Athens. Eventually, he made such a name for himself in his many fields that he caught the attention of the ruler of Egypt, Ptolemy III. Ptolemy III invited Eratosthenes to Alexandria, Egypt for two reasons; to tutor his son and to be the librarian for the great Alexandrian University. Eratosthenes jumped at the chance. At the University, he was able to at most interested him and associate with other scholars. Now on to his accomplishments. . .

One of his major accomplishments in mathematics is his creation of a sieve that determines prime numbers up to any given limit. This sieve, which is called, the Sieve of Eratosthenes, is still important today in number research theory. Prime numbers are natural numbers greater than 1 that can be divided without remainder only by itself and by 1. Eratosthenes figured out that if you were to write down all the natural numbers from 2 to infinity and "sieve out" every second number after two (or multiples of two), then move to the next available number (3) and continue to "sieve out" every multiple of 3 and so on, one would end up with a list of prime numbers.

Eratosthenes is also known for his achievement in astronomy. Several astronomers and mathematicians before and after Eratosthenes tried to accurately measure the circumference of the Earth, but is was Eratosthenes that came through. He found the circumference of the Earth to be nearly 250,000 stadia (25,000 miles). Eratosthenes observed that the sun shone directly down a well at high noon on the day of the summer solstice in Syene and that it cast a shadow in Alexandria, directly south of where the well was. To calculate the circumference of the Earth, Eratosthenes measured the angle of the shadow to the Earth. Until he realized this, Eratosthenes believed that the sun was so far away that its rays were parallel. It is also believed that Eratosthenes made a star catalog with approximately 675 stars and created a calendar that included leap years.

As a historian, Eratosthenes decided to work on giving a systematic chronography of the known world by figuring out the dates of literary and political events from the siege of Troy up until his time. However, this was only a beginning. Others built on his foundation.

There is still more. Eratosthenes also contributed to geographic source of the river. Many scholars that preceded Eratosthenes in the study of the Nile river, tried to figure out the reason why parts of the river flooded while other parts did not. It was not until Eratosthenes that a correct answer was proposed. He believed that heavy rains near the source of the Nile was the cause of Many of Eratosthenes' peers nicknamed him "Beta" which is the second letter of the Greek alphabet, indicating that he just fell short of first place. Eratosthenes contributed greatly to many different areas of knowledge, more than I could cover in this short paper. Maybe in his time period, his peers did not feel that he contributed enough in one area or maybe they were jealous that he had contributed so much to so many areas. For a man who was nicknamed Beta, it is pretty impressive that so much of his work in these areas is still discussed today, so many years later.

Contributed by Courtney Ast


  1. DeTemple, Duane and Calvin Long. Mathematical Reasoning for Elementary Teachers. Addison-Wesley. New York; 1996.
  2. Eratosthenes of Cyrene.


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