Eratosthenes reasoned that if two sticks of equal height are placed on the Earth’s surface and aligned with the direction of the Sun, then at any given point in time, assuming the Earth is flat, they will cast shadows of equal length. In other words, the angle between the sticks and rays of Sunlight would be the same. However, when he conducted an experiment with the help of friends in Assouan, he found that this was not so.
In his experiment, sticks were placed vertically in the ground at Alexandria and at Assouan. At noon the angle made by the Sun’s rays and the top of the sticks was measured it was found that there was a difference of 7.2o between the two measurements. Now, Alexandria and Assouan lie on the same meridian of longitude and so noon always occurs at exactly the same instant in those places, which means that they must be in line with the direction of the Sun at that moment.