
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 accuratelydated 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. 