A CIESE Collaborative Project

Project Overview

When did people first realize that the earth might be round rather than flat? What evidence did they use? By the third century BC the scholars of that time believed that the earth was a sphere. One empirical piece of evidence is observing ships as they approach the horizon. The sails appear to dip into the ocean. But no one knew for sure how big this sphere was. It took a curious & ingenious librarian named Eratosthenes to discover a remarkably simple method for measuring the circumference of the earth. This unit is the story of how Erastosthenes measured the earth.

Suggestion: Show the students the book "The Librarian Who Measured the Earth" by Kathryn Lasky (Little, Brown and Co., 1994). (You can find out more about the book at Amazon.com) As you tell them the story, show them the illustrations in the book. You can also coordinate your talk with the pages on this website. The suggested pages will be indicated below.

When to Implement the Project

Biannually during the months of March & September


Time Frame: March 7th – April 11th
Target Measurement week(s): March 17-28


Time Frame: September 7th – October 11th
Target Measurement week(s): September 17-28

Project Details

Purpose: To recreate the remarkable measurement of the circumference of the earth that was done over 2000 years ago. Using only simple tools such as rulers, protractors, and meter sticks, students will measure shadows cast by a meter stick at different locations on the earth.

Subjects: Mathematics (geometry, ratios, scale drawing, measurement, introduction to trigonometry (optional)), science, social studies, geography & history.

Grade Level: Grades 5-12

Summary: Over 2,000 years ago Eratosthenes made a remarkably accurate measurement of the earth's circumference. This project requires collaboration of students in places at different latitudes of the earth to make some simple measurements, share data, use a spreadsheet to make comparisons, and then replicate and share their results. Here is a summary of the steps involved in making the measurements:

  1. At least two sites must collaborate whose latitudes (north-south distance) are different enough to give a significant difference in measurements.
  2. On the given date students will conduct their measurements outdoor at high noon, local time.
  3. Using a standard meter stick each team of students will:
    1. Lay out a piece of paper flat on the ground.
    2. Hold the meter stick perfectly vertical. (It may be taped to a metal book end, set in sand inside a liter plastic bottle, etc.)
    3. Mark on the paper the end of the shadow at one minute intervals over a ten to twenty minute period.
    4. Several measurements should be made by several different students or teams of students the more the better.
    5. Measure the length of the shadow cast by the meter stick to the nearest centimeter and these measurements will then be analyzed by the students.
    6. Using statistical computations the class should arrive at what they feel is the length of the shadow cast at local noon (which should be the time of the shortest shadow).
  4. This length of the shadow at local high noon and the date on which the measurement is taken will be posted on the Web.
  5. This data along with the latitude for each site is enough information to use a simple proportion to make a fairly accurate calculation of the Earth's circumference as determined by each pair of sites.

Number of participants: Unlimited