When we say the word “genius” people usually recall Tesla, Einstein or perhaps Da Vinci and Sir Isaac Newton. But we usually forget about the geniuses who lived many centuries ago and whose contributions have sparked the development of the entire human race. As such, I’d like to talk to you today about Eratosthenes of Cyrene who was born around 276 BC in Cyrene and died in 196 BC in Alexandria.

**A Man of Learning and Discovering**

Eratosthenes was a Greek mathematician and geographer, but aside from this he was a poet, astronomer and music theorist. As if that doesn’t sound impressive, he was also the chief librarian at the Library of Alexandria – the capital of knowledge and learning in the ancient world. But his most valuable contribution and legacy to humanity is the fact that the invented the discipline of geography, including a lot of the terminology used today. In fact he literally invented the word *geography*.

**The Father of Geography**

A major discovery in his career was calculating the circumference of the Earth. Due to this he was able to get a lot of data about the Earth without ever leaving Egypt. He discovered quite a bit about its size and shape and he even created a few sketches. However, he also based a lot of his assumptions based on other people’s drawings and experiences. In the Library of Alexandria he had access to various travel books, which contained various items of information and representations of the world that needed to be pieced together in some organized format. These were the missing pieces of data that he needed to complete his view of the Earth.

So where does the name geography come from? Well, it comes from the word *geographia*, lit. *“earth description*” and is closely tied to his three-volume work ** Geographika**. Eratosthenes described and mapped the entire known world in this work and he also divided the world into five general climate zones.

Eratosthenes placed overlapping lines over the surface of his map. Thanks to this grid of parallels and meridians he was able to link every place in the world and roughly calculate the distance between any two points. In his Geographika over 400 cities and their locations were shown, something that nobody had done before him. This is a very important document, especially for historians later down the line as they were able to locate the remains of many cities that were believed to be lost.

Not all of his calculations were correct of course, but they were not horribly wrong either, especially once we consider the obvious lack of technology that he had to deal with.

*The True Pentathlos*

Historians and philosophers of ancient times describe Eratosthenes as a “pentathlos” which pretty much means that he was an “all-rounder”. Looking at his contributions I decided to single out the following:

- He was able to roughly calculate the distance between the Earth and Sun.
- He was able to roughly determine the size of the stars on the night sky. While he was wrong by many magnitudes he was still ahead of many people. In fact his understanding of the galaxy was much better than many of the people who lived in the medieval times.
- But did you know that the Eratosthenes calculated that there are 365 days in a year and that every fourth year there would be 366 days? Yep, he created the first calendar!
- He was a poet who wrote many books.
- He was also a very good mathematician and his admiration for Prime numbers was quite interesting.

*Exploring Prime Numbers*

Eratosthenes created a simple algorithm for finding prime numbers. This algorithm is known in mathematics as the *Sieve of Eratosthenes* and it took quite a while for mathematicians to optimize it and discover a more efficient way to find primer numbers. This algorithm is one of the first things that computer scientists learn because prime numbers are such an interesting topic and there are a lot of fun problems out there.