The ancient Greek scholar Eratosthenes (c. 276 BCE to c. 195 BCE) is commonly called the "father of geography," due to the fact that he essentially invented it as a scholarly discipline. Eratosthenes was the first to use the word *geography *and other terms that are still in use today, and he also had a small-scale notion of the planet within a greater view of the universe that paved the way for our modern understanding of the cosmos. Among his accomplishments was his uncannily accurate calculation of the circumference of the earth.

### Brief Biography of Eratosthenes

Eratosthenes was born around 276 BCE at a Greek colony in Cyrene, territory lying in what is present-day Libya. He was educated at the academies of Athens and was appointed to run the Great Library at Alexandria in 245 BCE by Pharoah Ptolemy III. While serving as head librarian and scholar, Eratosthenes wrote a comprehensive treatise about the world, called *Geography*. This was the first use of the word, which in Greek literally means "writing about the earth." *Geography* also introduced the concepts of torrid, temperate and frigid climate zones.

In addition to his fame as a mathematician and geographer, Eratosthenes was a very gifted philosopher, poet, astronomer and musical theorist. As a scholar in Alexandria, he made several significant contributions to science, including the recognition that a year is slightly longer than 365 days and therefore requires an extra day every four years to allow the calendar to remain consistent.

In old age, Eratosthenes became blind and died of self-induced starvation in either 192 or 196 BCsE. He had thus lived to be about 80 to 84 years old.

### Eratosthenes' Famous Experiment

A very famous mathematical calculation in which Eratosthenes determined the circumference of the earth is a key part of why we remember and celebrate his contribution to science.

Having heard of a deep well at Syene (near the Tropic of Cancer and modern-day Aswan) where sunlight only struck the bottom of the well on the summer solstice, Eratosthenes worked out a method by which he could calculate the circumference of the earth using basic geometry. (Greek scholars knew that the earth was indeed a sphere.) The fact that Eratosthenes was a close friend of the famous Greek mathematician Archimedes is perhaps one reason for his success in this calculation. If he did not collaborate directly with Archimedes in this exercise, he must have certainly been helped by his friendship with the great pioneer in geometry and physics.

To calculate the circumference of the earth, Eratosthenes needed two critical measurements. He knew the approximate distance between Syene and Alexandria, as measured by camel-powered trade caravans. He then measured the angle of the shadow in Alexandria on the solstice. By taking the angle of the shadow (7°12') and dividing it into the 360 degrees of a circle (360 divided by 7.2 yields 50), Eratosthenes could then multiply the distance between Alexandria and Syene by 50 to determine the circumference of the earth.

Remarkably, Eratosthenes determined the circumference to be 25,000 miles, just 100 miles over the actual circumference at the equator (24,901 miles). Although Eratosthenes made mathematical errors in his calculations, these fortunately canceled each other out and yielded an amazingly accurate answer that still causes scientists to marvel.

A few decades later, the Greek geographer Posidonius insisted that Eratosthenes' circumference was too large. He calculated the circumference on his own and obtained a figure of 18,000 miles--7,000 miles too short. During the middle ages, most scholars accepted Eratosthenes' circumference, though Christopher Columbus used Posidonius' circumference to convince his supporters that he could quickly reach Asia by sailing west from Europe. As we now know, this was a critical error on Columbus' part. Had he used Eratosthenes' figure instead, Columbus would have known he was not yet to Asia when he landed in the New World.