Eratosthenes

An Egyptian librarian once heard that the sun could be seen shining at the bottom of a well in the town of Syene on the longest day of the year. He surmised that to make a reflection in a well, the sun had to be directly overhead on that day. And a sun directly overhead would cast no shadows from upright columns or posts. Yet on the longest day of the year in the city of Alexandria, where he lived, he observed that straight columns did cast shadows.

So, he decided to travel the 800 kilometers to Syene himself to verify that what he had heard was true. At midday, on the longest day of the year, he looked into the well and saw the sun reflected. And sure enough, the posts in Syene cast no shadows. He reflected on that. After a while, he began to see a bigger picture of what these seemingly unconnected facts meant. Surprisingly, it went against what nearly everyone believed at the time. You see, the librarian’s name was Eratosthenes, and he lived more than 2,200 years ago.

As the director of the greatest library in the world (the library of Alexandria in Egypt was said to possess hundreds of thousands of scrolls), Eratosthenes was at the intellectual capital of the world for his time. In the third century B.C., nearly every scholar in Alexandria and around the world believed that the earth was flat. But Eratosthenes reasoned that if the sun’s light came down straight and the earth was flat, then there would be no shadows in both Alexandria and Syene. If there were shadows in one location but not the other, then there could be only one logical explanation. The surface of the earth must be curved. In other words, the world must be a sphere.

That’s a pretty impressive mental leap, although it seems perfectly logical to us today. After all, we’ve seen pictures of our planet from space. But Eratosthenes made that big-picture connection by using everyday facts and putting them together. What’s even more impressive is that he took it a step further. He actually calculated the size of the earth! Using basic trigonometry, he measured the angle of the shadows and calculated that it was approximately 7.12 degrees. That’s about 1/50th of a circle. And he reasoned that if the distance between Syene (modern-day Aswan) and Alexandria was 800 kilometers, then the earth must be around 40,000 kilometers in circumference (50 x 800 kilometers). He wasn’t far off; the actual circumference of the earth through the poles is 40,008 kilometers.

Not long ago, I had the opportunity to visit Aswan (ancient Syene) and it made me think about how often we don’t use the minds God has given us, and instead are content being mere reflectors of others men’s (and women) thoughts.

"By wisdom a house is built, and through understanding it is established; through knowledge its rooms are filled with rare and beautiful treasures." Proverbs 24:3-4