Great day measuring the Earth’s radius ‪

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).