When the Earth was measured with a stick

         There was a time when our planet seemed huge. Its true size was first revealed by a simple but ingenious way by a man who lived in Egypt in the third century BC. His name was Eratosthenes and he was an astronomer, historian, geographer, philosopher, poet, theater critic and mathematician.

         One day, while reading a papyrus in the library, he came across a curious note. "Far to the south, at the last borders of Siena, a remarkable thing could be seen on the longest day of the year. On June 21, the shadows of the columns of the temples or a vertical stick diminish as noon approaches. At noon the sun's rays slide to the depths of a well, where, on other days, there is shade. And then, exactly at noon, the columns no longer have shade, and the Sun shines directly in the water of the well. "

         Eratosthenes wondered how it is possible that at the same time a stick from Siena should not have a shadow and a stick from Alexandria, located 800 km north, should show a very clear shadow? The only answer was that the Earth's surface is curved. Not only that, the greater the curvature, the greater the difference in length between the shadows. The sun is so far away that its rays are practically parallel when they touch the Earth.

         The sticks at different angles to the Sun will have different lengths of shadows. For the difference observed between the lengths of the shadows, the distance between Alexandria and Siena should be 7 degrees at the Earth's surface. If you could imagine these sticks stretching towards the center of the Earth, they would intersect at an angle of 7 degrees. Well, 7 degrees means about 50th of the entire circumference of the Earth, 360 degrees.

         Eratosthenes knew that the distance between Alexandria and Siena was 800 kilometers. How did he find out? He hired a man to walk and measure the distance, being able to perform the calculation we are talking about. So, 800 kilometers multiplied by 50, results in 40,000 kilometers. This was to be the circumference of the Earth. Eratosthenes was able to measure the circumference of the Earth using only sticks, eyes, feet and mind, with high accuracy.

       Today we know that the Earth has a circumference of 40,075.017 km at the Equator and a southern circumference of 40,007.86 km.

      If you want to redo the Eratosthenes Experiment, you can do it with 105 other countries, 5877 schools and 36,000 students, by participating in the Eratosthenes Experiment contest https://eratosthenes.ea.gr/.

 Presentasion of experiment

https://youtu.be/Mw30CgaXiQw?list=PL3KYzGGAjjbQoR0YLsCWJyhY-I2EYDH84

                                                                                Consatntina R./ Florinela B./ LTDM Bacau/ Romania