We had a Great day yesterday with the Astronomy amateur
club measuring the Earth’s radius and see some sun spots,
The Eratosthenes’ measurement of Earth’s radius is one of
the most popular and fantastic experiences in the history of Science. Over the decades,
it has always been admired not only because of its apparent simplicity but also
due to the different domains of knowledge that it involves: Geography,
Mathematics and Physics.
Let’s remember the story: Eratosthenes had heard that in
the summer solstice (the longest day of the year), at noon, the Sun was
straight above the town called Syene (modern Aswan). The travelers reported
that a vertical pole cast no shadow at that time in that town.
In Alexandria, where Eratosthenes lived, he observed that
in the same day, at the same time, the Sun cast a shadow.
He measured the angle of the Sun rays in that conditions, and according to
Kleomedes, Eratosthenes obtained 7º 12' (1/50th
of a circle, e. g. 7.2º).
Observing a map of ancient Egypt is relatively easy to
see that Syene is closer to the Tropic of Cancer and Alexandria and Syene lay
apparently on a direct north67 south line.
At that time, the distance from Alexandria to Syene was
known to be about 5000 stadia. Eratosthenes' original writings on the measurement
of the Earth were lost, although legend has it that he had someone walk from
the two towns. It is hard to know for sure what corresponds exactly the unit
stadia. Despite the controversy discussion about the ancient units1, it is more
or less consensual that one stadia would correspond to one-tenth statute mile1.
In this case, 5000 stades would correspond at 500 miles (about 804.6 km).
