Eratosthenes’ method

Eratosthenes of Cyrene was born around 280BC. He was a Greek mathematician and geographer but studied many disciplines. He was the chief librarian at the Library of Alexandria. He invented the discipline of geography, including some of the terminology used today

He is best known for being the first person to calculate the distance all the way around the Earth but he also developed a simple method to establish which numbers are prime numbers.

Let’s use his method to find all the primes up to 100.

First, write down all of the numbers from 2 up to the 100

Start with the first prime number 2 and mark off all multiples of 2 but leave 2 unmarked because it is a prime number but all the other multiples of 2 are not.

Then mark off all the multiples of 3 that aren’t already marked off.

5 is the next prime number. Mark off any multiples of 5 that haven’t been marked off yet.

Now sevens

There are no multiples of 11 that aren’t marked off and so we can stop. The unmarked numbers are all primes.

2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97 are the prime numbers up to 100.