Mu'ayyad al-Din al-'Urdi

Mu’ayyad al-Din al-’Urdi (sometimes given the epithet al-ʿĀmirī al-Dimashqī; born c. 1200 probably in Urd, Syria – 1266 Maragha, Iran)[1] was a major figure in 13th-century Islamic astronomy. He worked as an engineer and teacher of geometry in Damascus, and built instruments for al-Malik al-Mansur of Hims. In the 1250s, he moved to Maragha after being asked by Nasir al-Din al-Tusi to help establish the Maragha observatory under the patronage of Hulagu.[1]

He is known for being the first of the Maragha astronomers to develop a non-Ptolemaic model of planetary motion.[2] In particular, the Urdi lemma he developed was later used in the geocentric model of Ibn al-Shatir in the 14th century and in the heliocentric Copernican model of Nicolaus Copernicus in the 16th century.

Urdi's most notable works are Risālat al-Raṣd, a treatise on observational instruments, and Kitāb al-Hayʾa, 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.[1]

As an architect and engineer, he was responsible for constructing the water supply installations of Damascus, Syria, in his time.[3]

The Urdi Lemma

"Urdi's lemma" was 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. Anythony Grafton's demonstration of Maestlin's proof to Kepler may help to visualize. You can also drag the Ibn al-Shatir and al-Tusi sliders to zero in Dennis Duke's animation to see al-'Urdi's equant-less model for Mars in operation.

See also


  1. 1 2 3 Schmidl, Petra G. (2007). "ʿUrḍī: Muʾayyad (al‐Milla wa‐) al‐Dīn (Muʾayyad ibn Barīk [Burayk]) al‐ʿUrḍī (al‐ʿĀmirī al‐Dimashqī)". In Thomas Hockey; et al. The Biographical Encyclopedia of Astronomers. New York: Springer. pp. 1161–2. ISBN 978-0-387-31022-0. (PDF version)
  2. Saliba (1979).
  3. Al-Urdi's Article on 'The Quality of Observation', FSTC Limited


External links

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