EratosthenesEratosthenes

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 = 804,000,000 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://astrosun2.astro.cornell.edu/academics/courses//astro201/eratosthenes.htm