Erathosthenes is known as an astronomer and a mathematician. The two things he is famous for are calculating the circumference of the Earth and the development of a method for calculating all prime numbers. His calculation for the circumference of the Earth was around 46,000km, which is remarkably close to the current measured value.

In mathematical circles Eratosthenes is famous for the method of finding prime numbers, known as the ‘Sieve of Eratosthenes’. The best way to explain this is to take a specific example. We look at an example on the next page where the task is write down the prime numbers between 2 and 20.

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