Khālid ibn ʿAbd al‐Malik al‐Marwarrūdhī ( ar, خالد بن عبدالملك المرو الروذي) was a 9th-century

Baghdad Baghdad (; ar, بَغْدَاد , ) is the capital of Iraq and the second-largest city in the Arab world after Cairo. It is located on the Tigris near the ruins of the ancient city of Babylon and the Sassanid Persian capital of Ctesiphon. I ...

astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either ...

. In 827, Marwarrūdhī, together with the astronomer ʿAlī ibn ʿĪsā al-Asṭurlābī and a party of surveyors, measured the length of a meridian arc of one degree of

latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...

. The party travelled to the Nineveh Plains in the valley of the

Tigris The Tigris () is the easternmost of the two great rivers that define Mesopotamia, the other being the Euphrates. The river flows south from the mountains of the Armenian Highlands through the Syrian and Arabian Deserts, and empties into the ...

, at 35 degrees north

. The measurement they obtained enabled the astronomers to obtain a value of for the

circumference In geometry, the circumference (from Latin ''circumferens'', meaning "carrying around") is the perimeter of a circle or ellipse. That is, the circumference would be the arc length of the circle, as if it were opened up and straightened out t ...

of the Earth, (or, according to other sources, ). The two researchers measured in Arabian , and determined the geographical latitudes of the end points they used from the star altitudes in a celestial horizontal coordinate system. As it is thought that one Arabian represented , they found the length of 1° of meridian to be , which differs from the true value by . Marwarrūdhī was chosen by the

geometer A geometer is a mathematician whose area of study is geometry. Some notable geometers and their main fields of work, chronologically listed, are: 1000 BCE to 1 BCE * Baudhayana (fl. c. 800 BC) – Euclidean geometry, geometric algebra * ...

Al-Abbās ibn Said al-Jawharī to organize a new observatory on

Mount Qasioun Mount Qasioun ( ar, جَبَل قَاسِيُون, transliterated as Jabal Qāsiyūn) is a mountain overlooking the city of Damascus, Syria. It has a range of restaurants, from which the whole city can be viewed. As the city has expanded over the ...

. Despite encountering technical difficulties caused by the distortion of the astronomical instruments, in he spent a year obtaining of solar and lunar observations of the Sun and the Moon. He played a role in the project determine the length of the spring season by means of astronomical observations. Marwarrūdhī was the first of three generations of astronomers.

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