Eratosthenes was born in Cyrene. There he read about a vertical well near Syene. At noon once a year, the well was totally lit up by the sun's rays. He measured the angle of it's shadow by placing a post on the shadow at that time and date. He finally made an assumption that the Earth is round and that the sun's rays are parallel. Eratosthenes remembered from geometry that the size of the Earth's center between Alexandria and Syrene equaled the size of the angle he had measured. The result he got was 250,000 stadia (about 46,250 km).
Eratosthenes also determined the obliquity of the Ecliptic, measured the tilt of the earth's axis with great accuracy, prepared a star map containing 675 stars, suggested that a leap day be added every 4th year and tried to construct an accurately dated history.
He created the "Sieve of Eratosthenes" method of finding prime numbers smaller than any given number, which is still an important tool in number research today.
Describing the fundamentals of astronomy in verse, he came up with a poem called "Hermes".
Eratosthenes finally died when he was 80 years old. Scientists say "he voluntary starved himself to death".