Khālid ibn ʿAbd al‐Malik al‐Marwarrūdhī

Khālid ibn ʿAbd al‐Malik al‐Marwarrūdhī (Arabic: خالد بن عبدالملك‎‎) 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 (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 metres.