An experiment carried out 2,200 years ago by ancient Greek mathematician, geographer and astronomer Eratosthenes to calculate the circumference of the Earth will be repeated this Saturday by amateur astronomers in the northern Greek cities of Alexandroupolis and Thessaloniki and in Warsaw.

The experiment will be part of a simulation game called "The Sun measures the Earth" organised by the Thrace Amateur Astronomers' Society, the Astrophysics and Astronomy department of Thessaloniki's Aristotelian University in collaboration with the Friends of Astronomy Club and the Polish Amateur Astronomers Society.

Duplicating the ancient scientists' experiment, astronomers in the three cities will measure the shadow cast by a metre-long bar placed near the Alexandroupolis Light House, the Thessaloniki University Observatory and the Vistula River crossing Warsaw.

The measurements will begin at 11:00 in the morning and end at 16:00 in the afternoon (Greek time) and the results will be fed into a linked computer system in the three cities in real time, which will carry out the necessary calculations.

Organisers said the experiment was a game designed to promote the public's involvement in amateur astronomy and to show that this was not just observing things through telescopes.

The simulation is taking place seven days earlier than the summer solstice on June 21, which is the day chosen by Eratosthenes to carry out his measurements and the longest day of the year, but will seek to confirm the precise measurements calculated by the Cyrenean mathematician concerning the Earth's diameter.

Eratosthenes and the Aswan well

The mathematician and scientist Eratosthenes was born in Cyrene (276-194 B.C) in present-day Libya. He studied in Alexandria and Athens and became chief librarian of the Great Library of Alexandria, where he learned of a deep, vertical well near the ancient Egyptian city of Swenet (known in Greek as Syene and near the present-day Aswan Dam) in southern Egypt where the sun's rays fell on the water once a year at noon on the summer solstice without casting a shadow.

Setting up a vertical pole in Alexandria, he measured the angle formed by its shadow at exactly the same hour and day.

He found from his measurement that, in his hometown of Alexandria, the angle of elevation of the Sun would be 1/50 of a full circle (7°12') south of the zenith at the same time.

Assuming that Alexandria was due north of Syene he concluded that the distance from Alexandria to Syene must be 1/50 of the total circumference of the Earth. His estimated distance between the cities was 5000 stadia (about 500 geographical or nautical miles). He rounded the result to a final value of 700 stadia per degree, which implies a circumference of 252,000 stadia. There is debate about the exact size of the stadion he used. The common Attic stadion was about 185 m, which would imply a circumference of 46,620 km, i.e. 16.3% too large. However, if he used the "Egyptian stadion" of about 157.5 m, his measurement turns out to be 39,690 km, an error of less than 1%.