Circumference is the distance around a curved object like a tennis ball or an orange. Measuring the circumference of everyday objects is not difficult. To measure the circumference of a grapefruit, for example, you might wrap a measuring tape or piece of string around it. But what if you wanted to measure the circumference of the Earth? Would you have to wrap a piece of string around the entire Earth? If you happen to have around 25,000 miles of string and don’t mind walking for eleven straight years, you might want to give it a try! But fortunately, there is a simpler way.

It is possible to measure the Earth’s circumference using only simple tools and a friend in a distant city. That’s exactly how the Greek astronomer Eratosthenes calculated the circumference of the planet over 2,000 years ago. Eratosthenes put a stick in the ground and measured the length of its shadow. He then asked a friend to conduct the same experiment at the same time of day in a city around 600 miles away. Because the Earth is round, the sun cast shadows of different sizes in the two cities. Eratosthenes used mathematics to compare the sizes of the two shadows, and from this he was able to calculate Earth’s circumference.

How close was Eratosthenes’ calculation? Today we know that he was only off by around 10%, or 2,500 miles. Considering the simple tools that Eratosthenes used, his measurement was incredibly accurate. Eratosthenes’ experiment shows that we can accomplish incredible things with little more than creativity and mathematics.