Mu'ayyad al-Din al-Urdi

Al-Urdi (full name: Moayad Al-Din Al-Urdi Al-Amiri Al-Dimashqi) [1] (Arabic: مؤيد الدين العرضي العامري الدمشقي) (d. 1266) was a medieval Syrian Arab astronomer and geometer.

[2] Born circa 1200, presumably (from the nisba al‐ʿUrḍī) in the village of ʿUrḍ in the Syrian desert between Palmyra and Resafa, he moved 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.

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'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.

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.