History of Geodesy - Islamic World

Islamic World

The Muslim scholars, who held to the spherical Earth theory, used it to calculate the distance and direction from any given point on the earth to Mecca. This determined the Qibla, or Muslim direction of prayer. Muslim mathematicians developed spherical trigonometry which was used in these calculations.

Around AD 830 Caliph al-Ma'mun commissioned a group of astronomers to measure the distance from Tadmur (Palmyra) to al-Raqqah, in modern Syria. They found the cities to be separated by one degree of latitude and the distance between them to be 66⅔ miles and thus calculated the Earth's circumference to be 24,000 miles. Another estimate given was 56⅔ Arabic miles per degree, which corresponds to 111.8 km per degree and a circumference of 40,248 km, very close to the currently modern values of 111.3 km per degree and 40,068 km circumference, respectively.

Muslim astronomers and geographers were aware of magnetic declination by the 15th century, when the Egyptian Muslim astronomer 'Izz al-Din al-Wafa'i (d. 1469/1471) measured it as 7 degrees from Cairo.