Eratosthenes

Eratosthenes , c.275-c.195 BC, Greek scholar, b. Cyrene. A pupil of Callimachus in Athens, he became (c.240 BC) head of the library at Alexandria. Known for his versatility, he wrote poetry and works (most of them lost) on literature, the theater (notably on ancient comedy), mathematics, astronomy, geography, and philosophy; he also drew a map of the known world and evolved a system of chronology. Especially noted as an astronomer, he is credited with measuring the circumference and tilt of the earth and the size and distance from the earth of the sun and the moon.

Eratosthenes (c.276–194 bc) Greek scholar who first measured the Earth's circumference by geometry. Eratosthenes administered the library of Alexandria and was renowned for his work in mathematics, geography, philosophy and literature.

Eratosthenes (c.276–c.194bc)Greekwriter on many subjects, born in modern Libya. He made the first known calculation of the Earth's circumference that was based on a scientifically sound method. According to tradition, he did this by measuring the Sun's altitude as seen from Alexandria at the summer solstice, when he knew the Sun to be overhead at Syene, some 1000 km to the south. However, it is possible that he may actually have used a simpler method, which is to measure the amount by which the angle from the zenith to the horizon exceeded 90° as seen from a high point, such as a lighthouse. The effect of atmospheric refraction would give a result about 15 % in excess of the true value, which is what he obtained. Eratosthenes also measured the obliquity of the ecliptic.