Al-Urdi (full name: Moayad Al-Din Al-Urdi Al-Amiri Al-Dimashqi) () (d. 1266) was a medieval Syrian Arab astronomer and

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 * ...

. Born circa 1200, presumably (from the

nisba The Arabic language, Arabic word nisba (; also transcribed as ''nisbah'' or ''nisbat'') may refer to: * Arabic nouns and adjectives#Nisba, Nisba, a suffix used to form adjectives in Arabic grammar, or the adjective resulting from this formation **c ...

''al‐ʿUrḍī'') in the village of ''ʿUrḍ'' in the Syrian desert between

Palmyra Palmyra (; Palmyrene: () ''Tadmor''; ar, تَدْمُر ''Tadmur'') is an ancient city in present-day Homs Governorate, Syria. Archaeological finds date back to the Neolithic period, and documents first mention the city in the early secon ...

and Resafa, he came to Damascus at some point before 1239, where he worked as an engineer and teacher of geometry, and built instruments for al-Malik al-Mansur of

Hims ar, حمصي, Himsi , population_urban = , population_density_urban_km2 = , population_density_urban_sq_mi = , population_blank1_title = Ethnicities , population_blank1 = , population_blank2_t ...

. In 1259 he moved to

Maragha Maragheh ( fa, مراغه, Marāgheh or ''Marāgha''; az, ماراغا ) is a city and capital of Maragheh County, East Azerbaijan Province, Iran. Maragheh is on the bank of the river Sufi Chay. The population consists mostly of Iranian Azerba ...

in northwestern Iran, after being asked by Nasir al-Din al-Tusi to help establish the

Maragha observatory The Maragheh observatory (Persian: رصدخانه مراغه), also spelled Maragha, Maragah, Marageh, and Maraga, was an astronomical observatory established in the mid 13th century under the patronage of the Ilkhanid Hulagu and the directorship ...

under the patronage of

Hulagu Hulagu Khan, also known as Hülegü or Hulegu ( mn, Хүлэгү/ , lit=Surplus, translit=Hu’legu’/Qülegü; chg, ; Arabic: fa, هولاکو خان, ''Holâku Khân;'' ; 8 February 1265), was a Mongol ruler who conquered much of West ...

.
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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 Gregory Bar Hebraeus ( syc, ܓܪܝܓܘܪܝܘܣ ܒܪ ܥܒܪܝܐ, b. 1226 - d. 30 July 1286), known by his Syriac ancestral surname as Bar Ebraya or Bar Ebroyo, and also by a Latinized name Abulpharagius, was an Aramean Maphrian (regional primat ...

and

Qutb al-Din al-Shirazi Qotb al-Din Mahmoud b. Zia al-Din Mas'ud b. Mosleh Shirazi (1236–1311) ( fa, قطب‌الدین محمود بن ضیاالدین مسعود بن مصلح شیرازی) was a 13th-century Persian polymath and poet who made contributions to a ...

, in addition to being quoted by

Ibn al-Shatir ʿAbu al-Ḥasan Alāʾ al‐Dīn ʿAlī ibn Ibrāhīm al-Ansari known as Ibn al-Shatir or Ibn ash-Shatir ( ar, ابن الشاطر; 1304–1375) was an Arab astronomer, mathematician and engineer. He worked as ''muwaqqit'' (موقت, religious t ...

. 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 Celestial globes show the apparent positions of the stars in the sky. They omit the Sun, Moon, and planets because the positions of these bodies vary relative to those of the stars, but the ecliptic, along which the Sun moves, is indicated. Th ...

in 1279. 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 Claudius Ptolemy (; grc-gre, Πτολεμαῖος, ; la, Claudius Ptolemaeus; AD) was a mathematician, astronomer, astrologer, geographer, and music theorist, who wrote about a dozen scientific treatises, three of which were of importance ...

's '' Almagest''. Saliba (1979) identified Bodleian ms.

Marsh A marsh is a wetland that is dominated by herbaceous rather than woody plant species.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p Marshes can often be found a ...

621 as a copy of Al-Urdi's ''Kitāb al-Hayʾa'', based on which he argued that Al-Urdi's contributions predated

Al-Tusi Al-Tusi or Tusi is the title of several Iranian scholars who were born in the town of Tous in Khorasan. Some of the scholars with the al-Tusi title include: * Abu Nasr as-Sarraj al-Tūsī (d. 988), Sufi sheikh and historian. *Aḥmad al Ṭūsī ( ...

. Otto E. Neugebauer in 1957 argued that the works of this group of astronomers, perhaps via

, must have been received in 15th-century Europe and ultimately influenced the works of

Copernicus Nicolaus Copernicus (; pl, Mikołaj Kopernik; gml, Niklas Koppernigk, german: Nikolaus Kopernikus; 19 February 1473 – 24 May 1543) was a Renaissance polymath, active as a mathematician, astronomer, and Catholic canon, who formulated ...

. This concerns the "Urdi lemma" in particular, an extension of

Apollonius' theorem In geometry, Apollonius's theorem is a theorem relating the length of a median of a triangle to the lengths of its sides. It states that "the sum of the squares of any two sides of any triangle equals twice the square on half the third side, t ...

that allowed an

equant Equant (or punctum aequans) is a mathematical concept developed by Claudius Ptolemy in the 2nd century AD to account for the observed motion of the planets. The equant is used to explain the observed speed change in different stages of the plane ...

in an astronomic model to be replaced with an equivalent

epicycle In the Hipparchian, Ptolemaic, and Copernican systems of astronomy, the epicycle (, meaning "circle moving on another circle") was a geometric model used to explain the variations in speed and direction of the apparent motion of the Moon, S ...

that moved around a deferent centered at half the distance to the equant point.

See also

References

Further reading