Eratosthenes was off by just a few hundred miles!Ĭolumbus, however, didn’t get the memo. His measurement was about 24,600 miles, and today we know that the Earth actually measures 24,902 miles around at the equator. What’s more, Eratosthenes could use the length of the shadow to calculate the size of the entire Earth, without ever leaving Egypt.

The shadows were different lengths, which meant the sticks weren’t parallel-the Earth was round after all! In late June, on the longest day of the year, he had two sticks placed straight in the ground in two different cities, five hundred miles apart, and measured their shadows. Around 240 BC, Eratosthenes devised a very clever experiment to measure the Earth. If you really want to celebrate the discovery of round Earth, don’t celebrate Columbus Day on October 12-celebrate Eratosthenes (“air-uh-TOSS-thuh-neez”) Day on June 21! Eratosthenes was the Greek who invented the word “geography” and a very smart guy-in fact, he was the head librarian at the ancient world’s largest library, in Alexandria. Thales thought it was a round, flat disk floating in water, like a pancake that’s fallen overboard at sea.Īnaximander thought the Earth was a cylinder, while Anaximenes (no relation) believed it was a flat rectangle floating on compressed air.īut by 500 BC or so, most people agreed with the philosophers Pythagoras and Aristotle: The Earth was round, like a ball. The earliest Greek thinkers disagreed about the shape of the Earth. This is not even close to true! By Columbus’s time, scientists had known the Earth was round for almost two thousand years.

You might have heard or read that Columbus proved the Earth was round in 1492, when he sailed from Spain to the Caribbean. It is our home, unless any of you are aliens who have secretly invaded our planet for your own purposes.

“Geography” comes from the Greek word for “description of the Earth.” “Geo-” means “Earth,” like in “geology.” The “-graphy” part means “to write,” like in “graphic” or “biography.” So geographers study and describe the Earth.

Ken jennings geography book