<P> Sieve of Eratosthenes </P> <P> Eratosthenes of Cyrene (/ ɛr əˈtɒsθəniːz /; Greek: Ἐρατοσθένης ὁ Κυρηναῖος, IPA: (eratosthénɛːs); c. 276 BC--c. 195 / 194 BC) was a Greek mathematician, geographer, poet, astronomer, and music theorist . He was a man of learning, becoming the chief librarian at the Library of Alexandria . He invented the discipline of geography, including the terminology used today . </P> <P> He is best known for being the first person to calculate the circumference of the Earth, which he did by comparing altitudes of the mid-day sun at two places a known North - South distance apart . His calculation was remarkably accurate . He was also the first to calculate the tilt of the Earth's axis (again with remarkable accuracy). Additionally, he may have accurately calculated the distance from the Earth to the Sun and invented the leap day . He created the first map of the world, incorporating parallels and meridians based on the available geographic knowledge of his era . </P> <P> Eratosthenes was the founder of scientific chronology; he endeavored to revise the dates of the chief literary and political events from the conquest of Troy . Eratosthenes dated The Sack of Troy to 1183 BC . In number theory, he introduced the sieve of Eratosthenes, an efficient method of identifying prime numbers . </P>

How did they figure out the circumference of the earth