Chalid Ben Abdulmelik

Together with Ali Ben Isa in 827, he measured at 35 degrees north geographical latitude, in the valley of the Tigris, the length of a meridian and thus the Earth's circumference, getting a result of 40,248 km (or, according to other sources, 41,436 km). The two researchers measured in Arabian ell[?], and determined the geographical latitudes of the end points they used from the star altitudes in a celestial horizontal coordinate system. We believe that 1 Arabian ell was 49 1/3 cm. Thus, they found the length of 1° of meridian to be 111.8 km (115.1 km), which differs from the actual value by 850 meters.