Eratosthenes Mathematician Geographer Astronomer 276 BC 194 BC

Eratosthenes Mathematician, Geographer, Astronomer 276 BC- 194 BC

• Born in Cyrene; Died in Alexandria • Known under the name β (Beta) because he was considered to be the second most knowledgeable person in the world on many subjects

Contributions: Earth’s Circumference Around 240 BC, at noon on the summer solstice, Eratosthenes measured the altitude of the sun in Alexandria and Syene. He concluded that the circumference of the Earth was 252, 000 stadia or 42, 000 km.

Contributions: The First Globe In 255 BC, Eratosthenes created the Armillary Sphere. It was what we would now call a globe. Eratostenes’ version had a large ball in the center representative of the Earth.

Contributions: Sieve The Sieve of Eratosthenes is a way to find prime numbers. Eratosthenes's method: 1. Take the square root of the integer you want to check and round it to the nearest whole number 2. List all numbers smaller than the integer and delete all even numbers 3. Check numbers up to the rounded square root to see if the rest of the numbers in the list are prime

Other Contributions • Measured distance from Earth to the Sun and to the Moon during a lunar eclipse: Earth → Sun = 804, 000 stadia Earth → Moon = 780, 000 stadia • Measured tilt of the Earth’s axis 11/83 of 180 = 23. 855º

Contributions Not Preserved • Map of the Nile • Star catalogue containing 675 stars • Poem “Hermes” about Astronomy

Bibliography • • • http: //en. wikipedia. org/wiki/Eratosthenes http: //www-groups. dcs. st-and. ac. uk/~history/Mathematicians/Eratosthenes. html http: //astrosun 2. astro. cornell. edu/academics/courses//astro 201/eratosthenes. htm