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Mu'ayyid al-Din al-'Urdi
Mu'ayyid al-Din al-'Urdi was an astronomer and engineer who was asked by Tusi to come to Marāgha and help design and construct the observatory. He played a key part in the building of the observatory by producing large water wheels to bring water towards the hill where the observatory would be. Al-'Urdi was noted as one of the instrument makers in the observatory and he wrote the treatise ‘Urḍī's Risāla fī Kayfiyyat al‐arṣād, which is preserved in a manuscript in Paris, and details the instruments in the observatory that were constructed before 1262 AD. Some notable instruments documented in this treatise include the Mural quadrant, an armillary sphere which was used to measure the ecliptic longitude and latitude, a solstitial armilla which was used to determine the obliquity of the ecliptic, and an equinoctial armilla which was used to determine when the sun entered the equatorial plane as well as the path of the sun. Mu'ayyad al-Din al-Urdi tracked 3 lunar eclipses while at the Maragheh observatory, which he then compared to Ptolemy's observations of lunar eclipses to calculate the average lunar longitudinal, anomalous, and retrograde motions.