Khālid ibn ʿAbd al‐Malik al‐Marwarrūdhī
Khālid ibn ʿAbd al‐Malik al‐Marwarrūdhī was a Zanji slave who was taken to Persia in the 9th century.
Together with ʿAlī ibn ʿĪsā al-Asṭurlābī in 827, he measured at 35 degrees north latitude, in the valley of the Tigris, the length of a meridian arc and thus the Earth's circumference, getting a result of 40,248 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, which differs from the actual value by 850 metres.
