the science of geodesy
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What is Geodesy?
Geodesy is the scientific discipline that deals with the measurement and representation of earth. What was Biruni's contribution?
Biruni introduced techniques to measure the earth and distances on it using triangulation. He found the radius of the earth to be 6339.6 km, a value not obtained in the West until the 16th century. His Masudic Canon contains a table giving the coordinates of six hundred places, almost all of which he had direct knowledge. How did he do it? He first 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. He made both the measurements using an astrolabe. He then used the trigonometric formula (figure 2) to determine the height of the mountain. He then stood at the highest point of the mountain, where he measured the dip angle using an astrolabe. He applied the values he obtained for the dip angle and the mountain's height to a trigonometric formula (figure 3) in order to calculate the Earth's radius. 
figure 3
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figure 1
figure 3
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