Eratosthenes by e-mail

Ted Davis (
Fri, 12 Jun 1998 11:35:52 -0400

I am looking for a college teacher who wishes to have their students
collaborate with mine on a simple experiment that requires observations from
two locations separated by a at least a hundred miles (more is better)
North-South. Longitude does not matter.

We will be repeating the classic measurement of the earth's circumference,
carried out by Eratosthenes in ancient Alexandria. The method I have
developed calls for students to measure the lengths of shadows cast by a
vertical object (such as a pole or fence post) every ten minutes, bracketing
local noon -- in our case, they do this from 12:30 to 1:30 pm, EDT. These
are used to find the sun's maximum height above the horizon, expressed as an
angle. Results are exchanged with observers at another location, who do the
same thing on the same day. We give them the actual North-South distance
between campuses and then students can calculate the earth's polar
circumference. I've done this each of the past three years, and the WORST
result is off by only 1-2 percent.

I seek a teacher who wants to collaborate. They would need to plan on
having various students (who can be assigned different days) carry out the
measurements over a two-week period, in order to ensure that we get a few
days of sunshine in both locations that will give data. In the past I've
done this in mid-September, but another period during the fall term would
also work.

The experiment is ideal for basic physical science courses. The students
are usually very surprised at how simple this is, and yet how much they
learn from it.

Any takers?

Ted Davis
Messiah College