Al-Farghani's Mistake

While expanding the urban settlements along the Tigris, the 9th century Abbasid caliph al-Mutawakkil needed a canal to bring water to a new city, al-Jaʻfariyya (after the caliph's birth name). He gave two courtiers the job of finding an engineer to design and build the canal. Two brothers, Muhammad and Ahmad ibn Musa, went for the expertise of al-Farghani (c.800 - 870).

al-Farghani was one of the most famous astronomers in the Muslim world in the 9th century. He had written a summary of Ptolemy's Almagest called Elements of astronomy on the celestial motions with more accurate data. Columbus used al-Farghani's calculations on his voyages across the Atlantic. Under a previous Abbasid caliph, al-Ma'mun, al-Farghani and a team had used the curvature between two points to calculate the diameter of the Earth.

He had also worked as an engineer, and for al-Mutawakkil had successfully created the New Nilometer in Old Cairo (more on that later). So designing a canal was not outside of his abilities. There was something wrong with the construction, however. A miscalculation made the entrance to the canal too deep. Water entering it would have to be abnormally high to be able to enter the rest and flow to its destination.

al-Mutawakkil was angered, and sent someone to figure out how culpable the two brothers were for the error. The investigator was not keen to see Muhammad and Ahmad punished, so he kept delaying his report. In fact, he delayed it long enough that it became a non-issue after the assassination of al-Mutawakkil, saving the lives of the brothers.

al-Farghani was not known for making mistakes, and tomorrow we'll look at some of his accomplishments, as well as explain the New Nilometer, which is exactly what you think it might be based on the name. See you then.