Mu'ayyad al-Din al-'Urdi
Mu’ayyad al-Din al-’Urdi (died 1266) was an Muslim astronomer, mathematician, architect and engineer working at the Maragheh observatory. He was born in Aleppo,todays Syria, and later moved to Maragheh, Azarbaijan, to work at the Maragha observatory under the guidance of Nasir al-Din Tusi.
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.
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 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.
- Saliba (1979), p. 576.
- Saliba (1979).
- Al-Urdi's Article on 'The Quality of Observation', FSTC Limited
- 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)
- Islamic astronomy
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