Eratosthenes (born 276 B.C.) was a man of letters in the time of the Ptolemys, being a native of the African city Cyrene. He was librarian at Alexandria, and did much for science. He discovered the obliquity of the ecliptic, made researches concerning the size of the earth, made a system of mathematical geography, and wrote a work on the constellations. Fragments of his geographical works, besides his work on the constellations, have been published.