The library of Eratosthenes

september 25

Tuesday 25 September 2007 by perbosc

Lafrançaise, New York, Wrocław


  • Date: 25/09/2007
  • Shadow/Ombre: 100cm
  • Angle: 45°
  •  New York, Brooklyn, USA

  • Date: 25/09/2007
  • Shadow/Ombre: 86cm
  • Angle: 42°
  • Greetings from Brooklyn Friends School! We are located in Brooklyn, New York in the U.S. Our Latitude is 40.41.36 N and our longitude is 73.59.17 W. We are a Quaker school and we have around 600 students from family center to 12th grade. We have around 130 faculty members. New York is home to many important buildings and structures like the Empire State building and the Statue of Liberty. And we are not far from ground zero; where the World Trade Center used to be.

    Middle Schoolers Perform the World’s Greatest Experiment

    by Jeffrey Stanley

    Janet Villas’ eighth grade Earth Science class is studying the dimensions and movement of the Earth right now, so the special “Noon Day” project undertaken by kids around the world fit in perfectly with her lesson plans.

    “Our measurement at Clearpool was a one second reading with the sun darting in and out of clouds,” she said, “so we used our Brooklyn backup measurement instead. Our students did a great job and had to take it home and try to do the calculations themselves.”

    The measurement in question, using the sun to determine the Earth’s circumference, was part math, part science, part historical reenactment.

    “Eratosthenes was the librarian at the Great Library in Alexandria, Egypt,” explained Janet. “He devised a way using simple shadows to calculate the circumference of the Earth with astonishing accuracy about 2300 years ago.”

    In what the New York Times once called the number one experiment of all-time, Eratosthenes did pretty much this: he looked up at the noon day sun directly overhead in a town he was visiting, and he knew that at noon back in his hometown of Alexandria the sun was 1/50th of a full circle away from directly overhead at the same time of day. He put two and two together and figured that the geographic distance between the two towns must therefore equal 1/50th of the Earth’s circumference, so he calculated the distance between the two towns and multiplied by 50. His result, about 25,200 miles, was pretty accurate (the Earth’s average circumference is 24,860 miles or 40,008 km).

    Janet’s students reenacted Eratosthenes’ discovery by participating in the Noon Day Project, an international event which can only happen twice a year at the equinoxes.

    Created by Stevens Institute of Technology in Hoboken, the project links schools and students together to share data and calculate the Earth’s circumference together. “Students can easily recreate this experiment, but they need to partner with another school somewhere far away to get results,” Janet explained.

    “We had to construct a gnomen, or large sundial, to cast a shadow, then read the shadow at precisely noon and calculate the angle and length of the shadow.” They took the main reading during the eighth grade’s leadership retreat at Clearpool Education Center in Carmel, New York on September 27th but the uncertainty caused by cloud cover made them opt to go with a previous in-class reading instead.

    They then picked Miami Dade County School’s data from the Noon Day Project’s website because they’re in the same hemisphere as BFS and gathered their data on nearly the same day. “Our answer was 39, 960 km which is darned close to the actual answer of 40,008.”

    One might question the necessity or value of determining the circumference of the Earth when students can easily look it up in a textbook but Janet sees it differently. “The real excitement is that it’s possible to find out such a big thing on your own with a simple shadow stick." She also stressed the communal value of the Noon Day Project. “It’s exciting to think that others just like you are also pondering this experiment. It proved that the Earth must indeed be a sphere.”

     Wrocław, Pologne

  • Date: 25/09/2007
  • Gnomon: 102cm
  • Shadow/Ombre: 118,5cm
  • Angle: 49°20
  • Liceum Ogólnokształcące : 51,11°N 17,11° E

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