Biruni (973 - 1048) developed a new method using trigonometric calculations to compute earth's radius based on the angle between the horizontal line and true horizon from a mountain top with known height. He calculated the height of the mountain by going to two points at sea level with a known distance apart and then measuring the angle between the plain and the top of the mountain for both points. Biruni's estimate of 6,339.9 km for the Earth radius had an error of 0,002 and was 16.8 km less than the current value of 6,356.7 km. The idea came to him when he was on top of a tall mountain near Nandana in India. He measured the dip angle using an astrolabe and he applied to the law of sines formula. He also made use of algebra in his calculation. Assuming the highest point of mountain is 968m and dip angle is 1 degree, what is the diameter of your earth. How Biruni Solved it? Answer at Wikipedia: http://bit.ly/Biruni
