written by Todd Timberlake and Mario Belloni

The Eratosthenes JS model displays the shadows cast by two gnomons (sticks) at different locations on Earth. For one gnomon, Sun is directly overhead (as would be the case if the gnomon was on the Tropic of Cancer at the summer solstice). The other gnomon is due north of the first gnomon. The sizes of the gnomons are greatly exaggerated for visibility. This simulation can be used to help illustrate how Eratosthenes was able to measure the diameter of Earth using the shadows cast by two gnomons, one situated due north of the other, on a day when the southerly gnomon cast no shadow at all. The distance between the two gnomons (along Earth's surface) can be adjusted. The length of the shadow is given, and this length can be used to determine the angle between the gnomon lines and from that the circumference (and diameter, radius, etc) of Earth. Earth can be hidden to give a better view of the relevant geometry.

The Eratosthenes 2D JS Model was developed using the Easy Java Simulations (EJS) version 5.  It is distributed as a ready-to-run html page and requires only a browser with JavaScript support.

• Download ejss_model_eratosthenesJS.zip - 114kb Compressed File

Published March 8, 2014
Last Modified December 30, 2014

This file has previous versions.

An EPUB file containing the Eratosthenes simulation,
a general description, and the features of the simulation.

• Download ejss_model_eratosthenes.epub - 117kb Compressed File

Published June 27, 2014
Last Modified July 28, 2018

This file is included in the full-text index.
This file has previous versions.

This file contains the EjsS source code for the Eratosthenes JS Model.

• Download ejss_src_eratosthenesJS.zip - 39kb Compressed File

Published April 8, 2014
Last Modified December 30, 2014

This file is included in the full-text index.
This file has previous versions.