More about the 2005 Year of Physics Eratosthenes Project

Prelude Why I Developed this Website by Ihor Charischak

The Story The Librarian who Measured the Earth - Details of how Eratosthenes (275-194 BC) the chief librarian of the finest library of his time made his remarkable measurement.

Some ideas for teaching this unit Strategies for teaching and learning this "unit".

Doing the measurements Helpful suggestions for doing the project.

The angle measurement you need is the one formed by the meter stick and the sun's rays (the acute angle opposite the base angle of the triangle formed). In the photo its the angle that is covered by the protractor.

An Example of measuring the circumference from two locations on the earth.

Using Trigonometry An introduction to the ideas used by Ken in his spreadsheet explanation below.

Using Spreadsheets Ken Cole's explanation from the 1997 project.

Some Useful Tools

Download A Sketchpad Noon Day project. (requires GSP 4)

A Java Sketchpad model which lets you explore the angles formed by the sun's rays at Alexandria and Syene. 

A Java Sketchpad model which lets you explore when there are shadows at both locations. 

Finding your latitude and longitude
USA (only)

The World

Another way to latitude and longitude is to use Google earth. For downloading information go to

Latitude-longitude converter
Need to convert one degree/minute form to another?

Go to

The formulas are as follows:

Degrees Minutes Seconds to Degrees Minutes.m
Degrees = Degrees Minutes.m = Minutes + (Seconds / 60)

Degrees Minutes.m to Decimal Degrees
.d = M.m / 60
Decimal Degrees = Degrees + .d

Finding your distance between sites
You don't need to necessarily partner with someone on or near your longitudinal (north-south) line. But you do need to find the north-south (or perpendicular) distance between your latitude and your partner's latitude. (You can also partner with the equator! See how Ken Cole did it.)

Tools for finding Local Noon

  • Why Local (or solar) noon?
  • Here you can find Astronomical data such as  he sunrise & sunset data for your location.
  • The NOAA Solar Calculator can provide the time of solar noon, sunrise and sunset for avariety of locations around the world for any date from 1000 B.C. to A.D. 3000.

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