Eratosthenes And The Size Of The Earth

People in our age often arrogantly assume that we are smarter than ancient people. This is probably because we live in an information age, where the the combination of technology and widespread education makes a wide range of knowledge available to just about everybody.

Additionally the flawed theory of evolution causes us to assume we must be better than people who lived a long time ago.

This story of how a man with rudimentary measuring and calculating technology is also a reminder of how God blesses people at all ages and all nations.

So about 2200 years ago there was a very bright man called Eratosthenes who came from Greece which was then the centre of learning and philosophy. He took up residence in Alexandria in Egypt, where there was a famous library which contained all of the world’s great learning.

While he was living in Alexandria, Eratosthenes heard about a well in the village of Syene in the south of Egypt where there was a well, where at midday on the summer solstice the sun shone right to the water below without casting a shadow on the walls of the well. In other words, the sun at that time was directly overhead. Today we would recognise that Syene is located on the Tropic of Cancer.

This caused Eratosthenes to wonder if this held true at Alexandria, some distance to the north. So at midday on the summer solstice, he observed a vertical post in the ground and noted that it cast a shadow. So when the sun was overhead in Syene, it was not directly overhead in Alexandria.

Measuring the length of the shadow and the post he discovered that the sun was at an angle of 7.2 degrees off the vertical. This proved that the world is not flat but is a sphere. All he needed to do was find the distance from Alexandria to Syene to find out just how big the sphere is.

Apparently in the ancient world they had people called bematists whose job was to accurately pace out the distances between locations. He discovered that the distance between the two towns was 5000 stadia or 800 km.

Now here is the clever part. A complete circle is 360 degrees, and 7.2 degrees is one fiftieth of 360. If 800 km, the distance from Alexandria to Syene, represents one fiftieth of the circumference of the earth, then the full circumference is 50 x 800 km or 40,000 km.

Was he close? Yes! The pole to pole circumference is 40,008 km while the circumference at the equator is 40,075 km. That is brilliant calculation with such limited tools as he had.

So Eratosthenes drew a map of the known world with grids of latitude and longitude, and hundreds of towns and cities. This made it much easier to estimate distances than ever before, and laid the foundations of modern geography.

God created people with great intelligence. We have not evolved into higher levels of smartness over the millennia. Despite the arrogance of our age, people in the ancient world knew more than we give them credit for.