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The Eratosthenes Experiment
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International collaboration
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A short History

• The size of the Earth was determined already
  more than 2000 years ago. This is quite
  impressive, but was due to a clever observation.
  The director of the famous library in Alexandria
  in Egypt, the geographer Eratosthenes (about
  276-195 BC.) did the historic achievement.

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Eratosthenes had learned a fascinating fact about
the city of Syene in southern Egypt, not far from
Aswan When the Sun was at its highest in the sky
in this city on the longest day of the year
(today we call it summer solstice), the Sun did
not cast any shadows. Actually, the Sun shined
into deep wells. The story says that nobody dared
to stare into the wells since they could become
blinded by the intense light reflected by the
water deep down.
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In Syene the Sun therefore had to be in the
zenith at this time. In Alexandria, further north
in Egypt, Eratosthenes knew from his own
experience that the shadow at the same time was
7.2 degrees (see drawings to the right). He could
then calculate the circumference of the earth by
this simple ratio
Distance betweenSyene and Aleksandria
measured anglecircumference of the earth
360
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Eratosthenes map
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He could not measure the distance between the
cities directly, but estimates of people
travelling between the cities and the time they
spent, gave a distance of about 5000 stadions.
The circumference of the Earth therefore had to
be 250 000 stadia. circumference of the earth
5000360/7.2 250000 A stadium (plural
stadia) is a Greek measure of distance equal to
600 Greek feet. Today we don't know the exact
length of one stadium. Opinions vary about how
long it actually was, with estimates varying from
154 to 215 metres. If we choose 160 m we get that
250 000 stadia correspond to 40000 kilometers.
This is close to the correct value, which is 40
074 kilometers! Anyway, the method is correct
and can be applied even today to estimate the
size of our Earth
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How do we do it today?
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You measure the solar shadow angle at solar noon
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The GLOBE way!
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And use the result from another school and do the
calculations
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Both schools with shadow to the North D/C (a -
ß)/360 Find C. Both schools with shadow to the
South D/C (ß - a)/360 Find C. One school
with shadow to the North and one school with
shadow to the South D/C (a ß)/360 Find C.
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When and how
Register for the experiment Find a partner school
Conduct Experiment at Wednesday May 25 - 2
days Wednesday September 21 -2 days After the
Experiment you register your results
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Results

• 28 countries
• 98 schools
• Average value 41179 km

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Hungary
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Romania
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Deutsche Schule Abu Dhabi Vereinigte Arabische
Emiratea
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Puerto Rico
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The Czech Republic
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South Korea
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Norway
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Where to register
www.naturfagsenteret.no/fysikk/eratosthenes/index.
htmlorwww.globe-europe.org
Join the project in September!
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Who said physics is not fun?