Who was Eratosthenes of Cyrene?
Eratosthenes of Cyrene (276-194 BC)

Eratosthenes was born in Cyrene which is now in Libya. After studying in Alexandria and Athens, he became the director of the Great Library in Alexandria.
It was there where he read about a deep vertical well near Syene (now Aswan) in southern Egypt, which, once a year at noon (on the day of the Summer Solstice), was entirely lit up to the bottom by the sun, that is, the sun was directly overhead, with its rays shining directly into it. 

He then placed a vertical post at Alexandria (which was almost due north of Syenne) and measured the angle of its shadow on that exact date and time.

Making the assumptions that (a) the earth is round and that (b) the sun's rays are essentially parallel, Eratosthenes knew from geometry that the size of the measured angle equaled the size of the angle at the earth's center between Syene and Alexandria.
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.