Eratosthenes was a Greek mathematician, astronomer, and geographer who lived from 276 to 194 BCE. He is best known for calculating the circumference of the Earth with remarkable accuracy using simple geometry and observations of the Sun's angles at different locations. His work laid the foundation for future studies in geography and mathematics. In addition to his Earth measurement, Eratosthenes was the chief librarian at the Library of Alexandria, one of the most significant libraries of the ancient world. He also developed the Sieve of Eratosthenes, an efficient algorithm for finding prime numbers, which is still taught in mathematics today.