{{Short description|Arab astronomer (c1200–1266)}}
[[File:Mu'ayyas al-Din al-'Urdi's (d. 1260 or 1266) Risala fi Kayfiyyat al-Arsad wa-ma Yuhtaju Ila 'ilmihi Wa-'Amalihi Min al-Turuq al-Mu'addiya ila Ma'rifat 'Awdat al-Kawakib, Iran, 15th century.jpg|thumb|right|Manuscript of al-Urdi's ''Risala fi Kayfiyyat al-Arsad wa-ma Yuhtaju Ila 'ilmihi Wa-'Amalihi Min al-Turuq al-Mu'addiya ila Ma'rifat 'Awdat al-Kawakib''. Copy created in Iran, dated 15th century]]
'''Al-Urdi''' (full name: '''Moayad Al-Din Al-Urdi Al-Amiri Al-Dimashqi''') <ref name="BEA" /> ({{Langx|ar|مؤيد الدين العرضي العامري الدمشقي}}) (d. 1266) was a medieval [[Syrians|Syrian]] [[astronomer|Arab astronomer]] and [[geometry|geometer]].<ref>{{cite web |title=Urdi |url=https://islamsci.mcgill.ca/RASI/BEA/Urdi_BEA.htm |website=islamsci.mcgill.ca}}</ref>

Born circa 1200, presumably (from the [[Nisba (onomastics)|nisba]] ''al‐ʿUrḍī'') in the village of ''ʿUrḍ'' in the [[Syrian Desert|Syrian desert]] between [[Palmyra]] and [[Resafa]], he moved to [[Damascus#Seljuq and Ayyubid periods|Damascus]] at some point before 1239, where he worked as an engineer and teacher of geometry, and built instruments for [[Al-Mansur Ibrahim|al-Malik al-Mansur]] of [[Homs|Hims]]. In 1259 he moved to [[Maragheh#Mongol rule|Maragha]] in northwestern Iran, after being asked by [[Nasir al-Din al-Tusi]] to help establish the [[Maragha observatory]] under the patronage of [[Hulagu]].<ref name=BEA>{{cite encyclopedia | editor = Thomas Hockey | display-editors = etal | last = Schmidl | first = Petra G. | title=ʿUrḍī: Muʾayyad (al‐Milla wa‐) al‐Dīn (Muʾayyad ibn Barīk &#91;Burayk&#93;) al‐ʿUrḍī (al‐ʿĀmirī al‐Dimashqī) | encyclopedia = The Biographical Encyclopedia of Astronomers | publisher = Springer | date = 2007 | location = New York | pages = 1161–2 | url=http://islamsci.mcgill.ca/RASI/BEA/Urdi_BEA.htm | isbn=978-0-387-31022-0}} ([http://islamsci.mcgill.ca/RASI/BEA/Urdi_BEA.pdf PDF version])</ref> Al-Urdi's most notable works are ''Risālat al-Raṣd'', a treatise on observational instruments, and ''Kitāb al-Hayʾa'' (كتاب الهيئة), a work on theoretical astronomy. His influence can be seen on [[Bar Hebraeus]] and [[Qutb al-Din al-Shirazi]], in addition to being quoted by [[Ibn al-Shatir]].<ref name=BEA />

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's ''Kitāb al‐Hayʾa'' and also constructed a [[celestial globe]]  in 1279.<ref>This globe was bought by [[Augustus, Elector of Saxony]] in 1562 and since then has been kept in [[Dresden]] (now in [[Mathematisch-Physikalischer Salon]]). Drechsler, Adolph (1873). ''[https://archive.org/details/bub_gb_dqiCIQaBRCEC Der Arabische Himmels‐Globus angefertigt 1279 zu Maragha von Muhammed bin Muwajid Elardhi zugehörig dem Königl. Mathematisch‐physikalischen Salon zu Dresden]'' Dresden: Königl. Hofbuchhandlung von Hermann Burdach. (2nd edition reprinted in Sezgin, ''Astronomische Instrumente'', Vol. 4, pp. 215–241; (reprinted in Sezgin, ''School of Marāgha'', Vol. 1, pp. 261–289).</ref>

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 in [[Ptolemy]]'s ''[[Almagest]]''. Saliba (1979) identified [[Bodleian Library|Bodleian ms.]] [[Narcissus Marsh|Marsh]] 621 as a copy of Al-Urdi's ''Kitāb al-Hayʾa'', based on which he argued that Al-Urdi's contributions predated [[Nasir al-Din al-Tusi|Al-Tusi]]. [[Otto E. Neugebauer]] in 1957 argued that the works of this group of astronomers, perhaps via [[Ibn al-Shatir]], must have been received in 15th-century Europe and ultimately influenced the works of [[Copernicus]].
This concerns the "'''Urdi lemma'''" in particular,  an extension of [[Apollonius' theorem]] that allowed an [[equant]] in an astronomic model to be replaced with an equivalent [[epicycle]] that moved around a deferent centered at half the distance to the equant point.<ref>{{cite book|last=Saliba|first=George|title=Islamic Science and the Making of the European Renaissance|year=2007|url=https://archive.org/details/islamicsciencema00sali_967|url-access=limited|publisher=MIT Press|publication-date=2007|via=EBSCOHost|page=[https://archive.org/details/islamicsciencema00sali_967/page/n214 203]}}</ref>

==See also==
*[[Zij-i Ilkhani]]

==References==
{{Reflist}}

==Further reading==
*[[George Saliba]] (1979). "The First Non-Ptolemaic Astronomy at the Maraghah School", ''[[Isis (journal)|Isis]]'' '''70''' (4), p.&nbsp;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.

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==External links==
*[http://faculty.kfupm.edu.sa/phys/alshukri/PHYS215/Islamic_astronomy.htm Islamic astronomy (kfupm.edu.sa)]

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{{Islamic astronomy}}
{{Islamic mathematics}}
{{Authority control}}

{{DEFAULTSORT:Urdi}}
[[Category:Year of birth unknown]]
[[Category:1266 deaths]]
[[Category:13th-century astronomers]]
[[Category:Medieval Syrian astronomers]]
[[Category:Medieval Syrian mathematicians]]
[[Category:13th-century Syrian people]]
[[Category:13th-century educators]]