He Measured the Earth

By John Hudson Tiner

Are people of today better thinkers than people who lived a long time ago? Has man today somehow developed a better brain? Is he more advanced? For a long time modern scientists believed that people who lived when they did were capable of clearer thinking because they had a higher intelligence than ancient people.

But this isn’t necessarily true. Old manuscripts of ancient Greece tell of a talented Greek scholar named Eratosthenes who lived more than two thousand years ago.

Eratosthenes grew up in Cyrene, a Greek town on the coast of Libya in Northern Africa. He studied grammar in Egypt at Alexandria, philosophy at Athens in Greece, and he traveled widely.

Later, Eratosthenes became librarian at a large museum in Alexandria where he studied and wrote on mathematics, astronomy, and geography. Eratosthenes made many exciting discoveries. But his most astonishing achievement was his calculation of the distance around the earth. The ancient Greeks understood that the earth was shaped like a ball. Later, other civilizations refused to believe this fact.

If the Earth is a ball, thought Eratosthenes, then how big is it? From his studies, Eratosthenes learned about a well in Syene (now Aswan) in southern Egypt. On June 21, the longest day of the year, the sun is at its highest position. Each year at noon on that day in Syene the sun’s rays reflected from the water at the bottom of the deep well. Consequently, the sun had to be directly overhead.

At the same hour on the same day in Alexandria the sun never shone directly overhead. Tall buildings there cast small shadows, and the length of the shadows showed that the sun was seven degrees south of being directly overhead.

Eratosthenes reasoned that this difference in position of the sun could only be due to the curving of the earth’s surface from Syene to Alexandria. Seven degrees is about one-fiftieth of a complete circle. Therefore, the distance from Syene to Alexandria is one-fiftieth of the total distance around the earth.

Eratosthenes could calculate the distance around the earth by measuring the distance from Alexandria to Syene and multiplying this distance by fifty.

But what is the north-south distance between the two cities? he wondered. His result could be no more accurate than the accuracy with which he knew that distance. He checked with camel drivers whose caravans followed the same route between Alexandria and Syene that had been used for hundreds of years. He learned that the trip took exactly fifty days to cover the 5,000 stadia (Greek measure of distance) or about 500 miles.

Through his calculations Eratosthenes reached the conclusion that the earth’s circumference was about 24,540 miles.

He said, “If the great distance were not an obstacle, we might easily pass by sea from Spain to India!” This idea went unheeded until 1492 when Columbus made a voyage to America.

Eratosthenes’ measurement of the earth’s circumference was accurate to within 1 percent. Modern scientists simply would not believe it. “How could an ancient Greek use a well to determine the earth’s diameter?” they asked. When most scientists perform an experiment for the first time, they expect their results to be off by a much larger percentage.

To the scientists of only a few years ago it seemed almost improper that Eratosthenes could achieve such a wonderfully accurate result. They were embarrassed by his precise calculations and tried to prove he had only been lucky. But this wasn’t luck. He had performed other experiments, and his results for them were just as accurate.

Scientists now are forced to admit that men of the remote past were every bit as competent as we are, even though today we have sophisticated equipment to help with our calculations.

Q. How can you get closer to the moon without leaving the earth?

A. Just stay at home and watch the moon on the horizon climb up the sky until it is directly overhead. (It takes about six hours.) While you are watching the moon change position, the earth is turning one quarter of the way around, bringing you about four thousand miles closer to the moon.

Q. What’s always falling but never lands?

A. The moon is always falling and yet it never hits the earth. It revolves in its orbit (path of travel) about 250,000 miles away from the earth. The earth’s gravity is constantly pulling on the moon so that the moon falls about three inches every second. But because the moon is moving so fast at a forward speed, about two-thirds of a mile a second, it isn’t pulled down to the earth. The earth’s gravity is just enough to bend the moon’s path into a circular orbit. Instead of falling onto the earth, the moon falls around it.

Illustrated by Sherry Thompson