Teachers use 2,000-year-old method to determine Earth's circumference

Friday, June 22, 2007
Christina Brands, a North Pemiscot Junior High teacher, marked the length of the shadow cast by a pole every three minutes as timed by Amy Kimbrell, a teacher from Crystal City, Mo., High School. The group was using shadows to determine the circumference of the Earth.

Science teachers in a summer physics class at Southeast Missouri State University relied on shadows Thursday to calculate the Earth's circumference during the summer solstice, the longest day of the year.

The low-tech calculations were pioneered more than 2,000 years ago by Greek mathematician Eratosthenes.

Three high school and junior high teachers participated in the project Thursday, and another three teachers were scheduled to perform the same task today.

The teachers in the class measured shadows cast by two metal poles placed on a sidewalk outside the university's Magill Hall around 1 p.m.

Because of daylight-saving time, 1 p.m. is actually noon, when the sun is directly overhead, said Dr. Peggy Hill, associate professor of physics at Southeast and instructor of the class.

The classroom teachers joined a network of about 60 groups around the world that calculated the Earth's size based on the ancient Greek mathematician's method.

"Science is more about trying to understand how we know rather than what we know," Hill said. "Any time we can do something hands-on, it helps us understand how we arrived at an answer."

Hill said the project makes sense in her physics class.

"Physics is a mathematical science and is based on experiment," she said. "Finding the size of the Earth involves measurement and the application of mathematics as a tool to aid our understanding."

Secondly, she said, the project forces students to look more closely and critically at the world and to question things they have taken for granted.

"It is amazing to think of how ancient people were able to measure the size of the Earth with only sticks, the length of their stride and a knowledge of basic geometry," Hill said. "Hopefully, this is a project the teachers can take back to their classroom to perform at a later date."

Eratosthenes is credited with calculating the Earth's circumference in about 240 B.C., using his knowledge of the angle of elevation of the sun at noon in two Egyptian cities, Alexandria and Syene, now Aswan.

According to his calculations, the Earth's circumference was 46,620 kilometers. That's actually about 16 percent too large. The polar circumference, the distance around the Earth through the poles, is 40,008 kilometers, or nearly 25,000 miles.

Hill's students came up with two totals: 38,800 kilometers and 39,900 kilometers.

Initially, the teachers had some errors in their calculations. But with the aid of calculators and the Internet, they corrected their equations and came up with totals close to the correct distance.

Amy Kimbrell, a teacher at Crystal City, Mo., High School, said the experiment showed just how hard it was to come up with an accurate number. "There are a lot of places where you can make a mistake," she said.

Christina Brands, a teacher at North Pemiscot Junior High School in the Bootheel, observed that the biggest challenge may have been finding a level spot on the hilly campus from which to measure the shadows and calculate the angle of the sun.

Brands said she had a new appreciation of Eratosthenes' calculations, which were made without computers or calculators. "It amazes me," she said.


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