Al-Urdi (full name:Moayad Al-Din Al-Urdi Al-Amiri Al-Dimashqi)^[1] (Arabic:مؤيد الدين العرضي العامري الدمشقي) (d. 1266) was a medievalSyrianArab astronomer andgeometer.^[2]

Born circa 1200, presumably (from thenisbaal‐ʿUrḍī) in the village ofʿUrḍ in theSyrian desert betweenPalmyra andResafa, he moved toDamascus at some point before 1239, where he worked as an engineer and teacher of geometry, and built instruments foral-Malik al-Mansur ofHims. In 1259 he moved toMaragha in northwestern Iran, after being asked byNasir al-Din al-Tusi to help establish theMaragha observatory under the patronage ofHulagu.^[1] Al-Urdi's most notable works areRisālat al-Raṣd, a treatise on observational instruments, andKitāb al-Hayʾa (كتاب الهيئة), a work on theoretical astronomy. His influence can be seen onBar Hebraeus andQutb al-Din al-Shirazi, in addition to being quoted byIbn al-Shatir.^[1]

Al-Urdi contributed to the construction of the observatory outside of the city, constructing special devices and water wheels in order to supply the observatory, which was built on a hill, with drinking water. He also constructed some of the instruments used in the observatory, in the year 1261/2.Al-Urdi's son, who also worked in the observatory, made a copy of his father'sKitāb al‐Hayʾa and also constructed acelestial globe in 1279.^[3]

Al-Urdi is a member of the group of Islamic astronomers of the 13th and 14th centuries who were active in the criticism of the astronomical model presented inPtolemy'sAlmagest. Saliba (1979) identifiedBodleian ms.Marsh 621 as a copy of Al-Urdi'sKitāb al-Hayʾa, based on which he argued that Al-Urdi's contributions predatedAl-Tusi.Otto E. Neugebauer in 1957 argued that the works of this group of astronomers, perhaps viaIbn al-Shatir, must have been received in 15th-century Europe and ultimately influenced the works ofCopernicus.This concerns the "Urdi lemma" in particular, an extension ofApollonius' theorem that allowed anequant in an astronomic model to be replaced with an equivalentepicycle that moved around a deferent centered at half the distance to the equant point.^[4]

• Zij-i Ilkhani

• George Saliba (1979). "The First Non-Ptolemaic Astronomy at the Maraghah School",Isis70 (4), p. 571-576.
• George Saliba (1990).The Astronomical Work of Mu'ayyad al-Din al-'Urdi (d. 1266): A Thirteenth Century Reform of Ptolemaic Astronomy, Markaz dirasat al-Wahda al-'Arabiya, Beirut.

• 1266 deaths
• 13th-century astronomers
• Medieval Syrian astronomers
• Medieval Syrian mathematicians
• 13th-century Syrian people

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• Year of birth unknown