Earth science
Eratosthenes of Cyrene [b. Cyrene (Shahhat, Libya), c. 276 bce, d. Alexandria, c. 194 bce] calculates Earth's circumference, finding a value 15 percent higher than present estimates or even closer, depending on what one assumes about the actual length of the Greek measure called a stadium. See also 450 bce Astronomy.
Eratosthenes gives the first correct explanation for the annual flood of the Nile, relating it to rains over lakes in East Africa that are the Nile's source.
MathematicsEratosthenes invents the sieve of Eratosthenes, a method that discovers all prime numbers (whole numbers with no whole-number factors other than one and themselves) up to a given prime factor. He also develops a mechanical device that can be used to double the cube. See also 420 bce Mathematics.
