Eratosthenes - Determination of the circumference of earth

Students of a school should come in contact with the students of other school of the same longitude. Both schools should measure the length of the shadows of two objects 1 meter high at the same day and time. From the length of the shadow they should calculate the tangent of the angle. Then the students should calculate the distance between the cities of the two schools on a straight line (using a map or an internet site). They they should procced according to the calculations shown on the file eratosthenes.ggb