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) 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.

He is known for being the first of the Maragha astronomers to develop a non-Ptolemaic model of planetary motion. 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.

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

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

• Zij-i Ilkhani

References

• George Saliba (1979). "The First Non-Ptolemaic Astronomy at the Maraghah School", Isis 70 (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.
• (PDF version)

External links

• Islamic astronomy