What is the circumference of earth? How was it measured?
Imagine that you are located at a point on earth where at some time of the day the length of the shadow cast by a vertical pole is zero. You also have a friend located some 500 kilometers due north of you monitoring the length of the shadow of a vertical pole of the same height. If you ask your friend over the telephone to measure the length of the shadow he is monitoring, then a simple geometric calculation would give you the circumference of the earth and hence its diameter! This is more or less what Eratosthenes seems to have done over two centuries before Christ. You would notice that all you need is the distance between two points located on the same meridian and the valid assumption that the rays of the sun at the two locations are parallel. Of course a telephone connection would be useful. Eratosthenes avoided the need of a telephone by making his measurements at the time of the year when the shadows at the two points were minimum. In other words he chose the day of the summer s
