June 19 The great Greek scientist Eratosthenes Kirensky

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June 19 is a memorable geographical day associated with the name of the great Greek scientist Eratosthenes Kirensky (276-194 BC).

  The scientist, who observed the bottom of wells in different cities during the longest days of summer, found that in one city (Sienna) the bottom of the well receives sunlight, and in Alexandria at the same time - no. Eratosthenes Kirensky decided to use this fact in measuring the radius and circumference of the Earth. When the sun rose on June 240, 19 BC, he set up long-stemmed scepters (bowl-like sundials) in the cities of Sien and Alexandria to measure the angle at which the Sun stood in the sky. After measuring the angle, he calculated the radius with a simple formula, knowing the distance between the cities. The magnitude of the angle was 7 degrees 12 minutes, i.e. 1/50 of the circumference of the Earth. This means that Sien is 1/50 of the circumference of the Earth from Alexandria. At that time, the distance between these cities was estimated at 5,000 stadia, which means that the circumference of the Earth is 250,000 stadia and the radius is 39,790 stadia. It is not known which stage Eratosthenes used. If the Greeks (178 meters) had a radius of 7,082 km, the Egyptians would have 6,287 km. Modern measurements give the average size of the Earth's radius as 6,371 km. In any case, this precision is astonishing for prehistoric times!