al-Farghani's Accomplishments

Despite the potentially reputation-damaging error in calculation made by al-Farghani in the case of a canal, he is better known for other accomplishments.

Abū al-ʿAbbās Aḥmad ibn Muḥammad ibn Kathīr al-Farghānī (c.800 - 870) was known as Alfraganus in Western Europe (see illustration, from a 1493 astronomical work). He was described as Arab and as Persian; there is a suggestion that his name comes from being born in Farghana in Uzbekistan.

His best-known work was Kitāb fī Jawāmiʿ ʿIlm al-Nujūm (Elements of astronomy on the celestial motions), a summary of Ptolemey's astronomical Almagest with revised calculations. He concentrated less on the mathematics and more on explaining the concepts in ways that were easy to understand. This work reached the West in translations by John of Seville and Gerard of Cremona. Dante's knowledge of astronomy came from al-Farghani.

One of his first recorded acts is being involved in a team that calculated the diameter of the Earth. This work influenced Columbus in his voyage across the Atlantic. Columbus, however, misunderstood the translation of al-Farghani's use of "mile." Columbus assumed al-Farghani was using the 4856-foot Roman mile; actually, al-Farghani used the 7091-foot Arabic mile. Columbus thought the diameter of the Earth was smaller.

In Cairo, al-Farghani wrote a treatise on the astrolabe earlier than al-Ashraf Umar II and Abd al-Rahman al-Sufi. Also while in Cairo he supervised the New Nilometer. The Nilometer, as you might guess, was designed to measure the height and clarity of the water of the Nile River.

The annual flooding of the Nile was crucial to Egypt's agricultural cycle, but it was unpredictable. Too heavy a flood was destructive; too light could lead to famine. Knowing what was coming was important. Nilometers come in different designs, but the simplest was a vertical column submerged in the river with markings to denote height of the water. Later, more elaborate versions involved shafts with steps that led down. Noting the height and comparing it to previous years helped predict whether the crops would be successful.

Speaking of flooding...next I want to tell you about a flood that killed thousands in more than one country. See you tomorrow.

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.