Eratosthenes, in the 3rd century BC, was Director of the great Library of the Museum of Alexandria where he reads in a scroll that on June 21 (southeast of the sun) on the southern border of the city of Syene (Aswan), the pillars cast no shadow and the sun is reflected exactly at the bottom of a well (i.e. at the zenith of the place). As a scientist, therefore, Eratosthenes wondered if the same thing was happening at the same time in another e.g. Alexandria. But in Alexandria, at the same day and time, the vertical poles cast a shadow. If the Earth were flat, the vertical poles in the two cities would be parallel and both should cast shadows. So if this is not true, what could happen? The answer was given by Eratosthenes by claiming that the surface of the Earth is not flat but spherical. This conclusion is obviously of fundamental importance and, moreover, it allowed Eratosthenes to determine the radius and length of the Earth's circumference. Indeed, from the length of the shadow, the difference between the geographical dimensions of the two cities, equal to about 7 degrees, is immediately calculated. Since the distance between the two cities was known from stories of pacemakers and was equal to about 800 kilometers (it is speculated that Eratosthenes rented pacemeters to measure it), the circumference of the Earth was calculated as 40000 kilometers. The vernal equinox around March 20 each year marks the beginning of spring. On this day the Sun is perpendicular above the Earth's equator and the length of the day is approximately equal to the length of the night. The action entitled: "Eratosthenes' Experiment to calculate the radius of the Earth" gives the opportunity to students from schools all over Greece to calculate the circumference of the Earth by repeating the famous experiment of the ancient Greek mathematician, astronomer and philosopher Eratosthenes. The main objective of the action is to highlight the experiment as an integral component of the educational process, through the participation of students and teachers in experiential and collaborative discovery activities.