Eratosthenes and His Experiment

Contrary, to popular opinion, Columbus was not the first person to proclaim that the Earth is round. The Greeks, among others, had deduced the shape of the Earth more than a thousand years earlier, and the ancient Greek scholar, Eratosthenes, calculated the circumference of the world with surprising accuracy.

Eratosthenes was born in 276 BC in a region of northern Africa that is now part of Libya. He studied in Athens, and was the third head librarian of the great library of Alexandria. In addition to calculating the Earth's circumference, Eratosthenes is remembered for his method of finding prime numbers (the Sieve of Eratosthenes), measuring the distance from the Earth to the Sun and the Moon, and numerous, detailed maps of the world as it was known at the time.

Although Eratosthenes was a brilliant scientist, he lived at a time of many notable Grecian scholars, including his good friend Archimedes, who was one of the greatest mathematicians in history. As a result, Eratosthenes earned the nickname Beta, the second letter in the Greek alphabet, from some of his envious contemporaries who claimed that he was second best among his peers in everything. But history shows that he was a leader in numerous fields including astronomy, geography, literature, poetry, philosophy, and mathematics. Even if he was only second best at so many things, in an era of amazing progress in the sciences and arts, Eratosthenes is clearly among the foremost geniuses of all time.

The World Year of Physics project "Measure the Earth with Shadows," commemorates Eratosthenes' many outstanding accomplishments by giving high school students a chance to duplicate the brilliant scientist's most famous experiment, on the largest scale ever attempted. In 2005, participating students will use shadows to measure the position of the sun at high noon. By comparing their measurements with those of students at other latitudes, they will be able to calculate the size of the Earth, just as Eratosthenes did more than two thousand years ago.

Modern scholars are uncertain of the precise value of the Earth's circumference that Eratosthenes calculated because he recorded his result in units called stadia. We don't know how long Eratosthenes' stadia were, but current estimates suggest that his value of 252,000 stadia is equivalent to a circumference somewhere between 39,690 and 46,620 kilometers. Can 21st century high school students measure the size of our planet any better than a Greek scientist living two millennia ago did? Thanks to the World Year of Physics, we'll find out in 2005.

Are you a high school teacher, or the parent of a high school level, home-schooled child? If so, consider adding the World Year of Physics project "Measure the Earth with Shadows" to your lesson plans next year.

Learn more about Eratosthenes' measurement of the Earth and other experiments on Michael Fowler's webpage at the University of Virginia.

Download our Eratosthenes Project Teacher's Guide—it contains an illustrated Student Guide, plus additional activities and suggestions for conducting the experiment.

Need Help? If you have questions or need help with the Eratosthenes experiment, please email us at shadows@aps.org.