<P> Eratosthenes made several important contributions to mathematics and science, and was a friend of Archimedes . Around 255 BC, he invented the armillary sphere . In On the Circular Motions of the Celestial Bodies, Cleomedes credited him with having calculated the Earth's circumference around 240 BC, using knowledge of the angle of elevation of the Sun at noon on the summer solstice in Alexandria and on Elephantine Island near Syene (modern Aswan, Egypt). </P> <P> Eratosthenes believed there was good and bad in every nation and criticized Aristotle for arguing that humanity was divided into Greeks and barbarians, and that the Greeks should keep themselves racially pure . As he aged he contracted ophthalmia, becoming blind around 195 BC . Losing the ability to read and to observe nature plagued and depressed him, leading him to voluntarily starve himself to death . He died in 194 BC at 82 in Alexandria . </P> <P> Eratosthenes calculated the circumference of the Earth without leaving Egypt . He knew that at local noon on the summer solstice in Syene (modern Aswan, Egypt), the Sun was directly overhead . (Syene is at latitude 24 ° 05 ′ North, near to the Tropic of Cancer, which was 23 ° 42 ′ North in 100 BC) He knew this because the shadow of someone looking down a deep well at that time in Syene blocked the reflection of the Sun on the water . He then measured the Sun's angle of elevation at noon in Alexandria by using a vertical rod, known as a gnomon, and measuring the length of its shadow on the ground . Using the length of the rod, and the length of the shadow, as the legs of a triangle, he calculated the angle of the sun's rays . This turned out to be about 7 °, or 1 / 50th the circumference of a circle . Taking the Earth as spherical, and knowing both the distance and direction of Syene, he concluded that the Earth's circumference was fifty times that distance . </P> <P> His knowledge of the size of Egypt was founded on the work of many generations of surveying trips . Pharaonic bookkeepers gave a distance between Syene and Alexandria of 5,000 stadia (a figure that was checked yearly). Some historians say that the distance was corroborated by inquiring about the time that it took to travel from Syene to Alexandria by camel . Some claim Eratosthenes used the Olympic stade of 176.4 m, which would imply a circumference of 44,100 km, an error of 10%, but the 184.8 m Italian stade became (300 years later) the most commonly accepted value for the length of the stade, which implies a circumference of 46,100 km, an error of 15% . It was unlikely, even accounting for his extremely primitive measuring tools, that Eratosthenes could have calculated an accurate measurement for the circumference of the Earth . He made five important assumptions (none of which is perfectly accurate): </P>

Who measured the earth with sticks and shadows