Knowing also that the arc of an angle this size was 1/50 of a circle, and that the distance between Syene and Alexandria was 5000 stadia, he multiplied 5000 by 50 to find the earth's circumference.
His result, 250,000 stadia (about 46,250 km), is quite close to modern measurements.
Eratosthenes also determined the obliquity of the Ecliptic, measured the tilt of the earth's axis with great accuracy obtaining the value of 23° 51' 15", prepared a star map containing 675 stars, suggested that a leap day be added every fourth year and tried to construct an accurately-dated history.
He developed the “Sieve of Eratosthenes” method of finding prime numbers smaller than any given number, which, in modified form, is still an important tool in
number theory research.
He also wrote a poem called “Hermes” which described the fundamentals of astronomy in verse!
Although most of Eratosthenes' writings are lost, many are preserved through the writings of commentators.
At the age of 80, blind and weary, he died of voluntary starvation.