Al-Ma'mun's Mesurement
Around 830 AD, Caliph Al-Ma'mun commissioned a group of Muslim astronomers and Muslim geographers 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 meridian arc distance between them to be 662⁄3 miles and thus calculated the Earth's circumference to be 24,000 miles.
Another estimate given by his astronomers was 562⁄3 Arabic miles (111.8  km per degree), which corresponds to 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.
Read more about this topic: Arab Mile