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

Syrian Syrians () are the majority inhabitants of Syria, indigenous to the Levant, most of whom have Arabic, especially its Levantine and Mesopotamian dialects, as a mother tongue. The cultural and linguistic heritage of the Syrian people is a blend ...

Arab astronomer and geometer. Born circa 1200, presumably (from the nisba ''al‐ʿUrḍī'') in the village of ''ʿUrḍ'' in the

Syrian desert The Syrian Desert ( ''Bādiyat Ash-Shām''), also known as the North Arabian Desert, the Jordanian steppe, or the Badiya, is a region of desert, semi-desert, and steppe, covering about of West Asia, including parts of northern Saudi Arabia, ea ...

between

Palmyra Palmyra ( ; Palmyrene dialect, Palmyrene: (), romanized: ''Tadmor''; ) is an ancient city in central Syria. It is located in the eastern part of the Levant, and archaeological finds date back to the Neolithic period, and documents first menti ...

and

Resafa Resafa (), sometimes spelled Rusafa, and known in the Byzantine era as Sergiopolis ( or , ) and briefly as Anastasiopolis (, ), was a city located in the Roman province of Euphratensis, in modern-day Syria. It is an archaeological site situated so ...

, he moved to

Damascus Damascus ( , ; ) is the capital and List of largest cities in the Levant region by population, largest city of Syria. It is the oldest capital in the world and, according to some, the fourth Holiest sites in Islam, holiest city in Islam. Kno ...

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. In 1259 he moved to Maragha in northwestern Iran, after being asked by

Nasir al-Din al-Tusi Muḥammad ibn Muḥammad ibn al-Ḥasan al-Ṭūsī (1201 – 1274), also known as Naṣīr al-Dīn al-Ṭūsī (; ) or simply as (al-)Tusi, was a Persians, Persian polymath, architect, Early Islamic philosophy, philosopher, Islamic medicine, phy ...

to help establish the Maragha observatory under the patronage of Hulagu.
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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 and Qutb al-Din al-Shirazi, in addition to being quoted by Ibn al-Shatir. 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. ...

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 (; , ; ; – 160s/170s AD) was a Greco-Roman mathematician, astronomer, astrologer, geographer, and music theorist who wrote about a dozen scientific treatises, three of which were important to later Byzantine science, Byzant ...

's ''

Almagest The ''Almagest'' ( ) is a 2nd-century Greek mathematics, mathematical and Greek astronomy, astronomical treatise on the apparent motions of the stars and planetary paths, written by Ptolemy, Claudius Ptolemy ( ) in Koine Greek. One of the most i ...

''. Saliba (1979) identified Bodleian ms.

Marsh In ecology, a marsh is a wetland that is dominated by herbaceous plants rather than by woody plants.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p More in genera ...

621 as a copy of Al-Urdi's ''Kitāb al-Hayʾa'', based on which he argued that Al-Urdi's contributions predated 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 Nicolaus Copernicus (19 February 1473 – 24 May 1543) was a Renaissance polymath who formulated a mathematical model, model of Celestial spheres#Renaissance, the universe that placed heliocentrism, the Sun rather than Earth at its cen ...

. 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 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, ...

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

See also

References

Further reading